Abstract: Attenuation of a surface acoustic wave is used as a highly sensitive and noninvasive probe of persistent photoconductivity effects in ZnCdSe/ZnSe quantum wells. These effects are observed over long time-scales exceeding several minutes at low temperatures. By varying the optical excitation energy and power and temperature we show that these effects arise from carriers photogenerated by interband excitation which are trapped in random potential fluctuations in the quantum wells related to compositional fluctuations. Effects related to defect levels in the band gap can be excluded and a transition of the conduction mechanism with temperature from a hopping to a percolation regime is observed. The transition temperature observed for our quantum well material is strongly reduced compared to bulk crystals. This indicates a superior structural quality giving rise to only weak potential fluctuation of less than or similar to 3 meV. (C) 2010 American Institute of Physics. [doi:10.1063/1.3373415]
Abstract: A conventional microscope produces a sharp image from just a single object-plane. This is often a limitation, notably in cell biology. We present a microscope attachment which records sharp images from several object-planes simultaneously. The key concept is to introduce a distorted diffraction grating into the optical system, establishing an order-dependent focussing power in order to generate several images, each arising from a different object-plane. We exploit this multiplane imaging not just for bio-imaging but also for nano-particle tracking, achieving similar to 10 nm z position resolution by parameterising the images with an image sharpness metric. (C)2010 Optical Society of America
Abstract: Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3428960]
Abstract: During epitaxial lift-off of II-VI semiconductors a sacrificial layer of MgS is dissolved by acid. Here we show that the etching speed of this process-varies inversely as the square root of the layer thickness, following a model developed previously for III-V lift-off where the rate limiting step in both cases is transport of insoluble product gases from the etching layer. We also propose a model to explain why sacrificial layer etching fails when strong cohesive forces resist the lifting of the epilayer. This occurs when the sacrificial layer is too thin or when it contains more than a critical amount of an insoluble component, cohension arising from dispersion forces or chains of insoluble atoms, respectively (C) 2010 WILEY-VCH GmbH & Co. KGaA, Weinheim
Abstract: The spin states of single Mn atoms embedded in InAs quantum dots (QD) were investigated by optical means. In (In, Ga) As, the Mn impurity acts both as acceptor and a magnetic moment. In the low density limit, which is relevant here, the acceptor is in the neutral state A(0) with an effective spin J = 1. Using magneto-photoluminescence experiments, we probed the exchange interaction between the dot confined carriers and the Mn spin. Peculiar properties are found such as a large influence of the QD strain field on the acceptor spin states, a ferromagnetic hole-Mn spin coupling and a very small interaction with electrons. The Mn atoms were deposited during the QD growth by molecular beam epitaxy. Although the substrate temperature favored large segregation of the Mn atoms, we could measure a few dots containing single magnetic impurities. The zero-magnetic field exciton of a Mn-doped single dot shows a complex spectrum consisting of two doublets and a singlet. They correspond to the excitonic recombination of electron-hole pairs, affected by the exchange coupling with the Mn magnetic moment in the J(z) = +/- 1, 0 states. The fine structure of each doublet is due to the QD in-plane anisotropy which partially mixes the J(z) = +/- 1 states. The magnetic field dependence exhibits even more striking modifications, with several crossing and anti-crossing again linked to the QD strain field. We developed a model taking into account the spin interactions between the Mn impurity and the carriers in the dot. A very good agreement is found with the experimental data.
Abstract: Semiconductors have uniquely attractive properties for electronics and photonics. However, it has been difficult to find a highly coherent quantum state in a semiconductor for applications in quantum sensing and quantum information processing. We report coherent population trapping, an optical quantum interference effect, on a single hole. The results demonstrate that a hole spin in a quantum dot is highly coherent.
Abstract: Structure-spectra relationship in semiconductor quantum dots (QDs) is investigated by subjecting the same QD sample to single-dot spectroscopy and cross-sectional scanning tunneling microscopy (XSTM) structural measurements. We find that the conventional approach of using XSTM structure as input to calculate the spectra produces some notable conflicts with the measured spectra. We demonstrate a theoretical "inverse approach" which deciphers structural information from the measured spectra and finds structural models that agree with both XSTM and spectroscopy data. This effectively "closes the loop" between structure and spectroscopy in QDs.
Abstract: By using extreme numerical-aperture solid-immersion microscopy at 1553 nm we demonstrate, under certain circumstances, polarization-sensitive imaging with resolution values approaching 100 rim which substantially surpass the classical scalar diffraction-limit embodied by Sparrow's resolution criterion.
Abstract: We present temperature dependent high resolution resonant optical spectroscopy on a single, negatively charged InGaAs quantum dot. We performed laser transmission measurements yielding the natural linewidth of the excitonic ground state transition of a quantum dot in a temperature range from 4.2 K up to 25 K. Here we describe the linewidth evolution and the temperature induced red shift of the resonance energy with simple models based on the exciton-photon coupling in the quantum dot. The resonant spectroscopy measurements are complemented with results from non-resonant PL measurements on the very same quantum dot. Here we observe a simple linear behaviour of the linewidth according to an effect of a fluctuating environment. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: By making use of epitaxial lift-off, ZnCdSe quantum wells are transferred onto a LiNbO3 substrate in order to employ its enhanced piezoelectric properties. The photoluminescence emission of this hybrid structure is characterized and the influence of a surface acoustic wave on the free exciton and bound exciton emission is investigated. Finally, two counterpropagating surface acoustic waves are launched leading to a decrease in the acoustic wave mediated exciton dissociation.
Abstract: An optical write-store-read process is demonstrated in a single InGaAs quantum dot within a charge-tunable device. A single dark exciton is created by nongeminate optical excitation allowing a dark exciton-based memory bit to be stored for over similar to 1 mu s. Read-out is performed with a gigahertz bandwidth electrical pulse, forcing an electron spin-flip followed by recombination as a bright neutral exciton, or by charging with an additional electron followed by a recombination as a negative trion. These processes have been used to determine accurately the dark exciton spin-flip lifetime as it varies with static electric field.
Abstract: The wide bandgap alloy Zn0.2Mg0.8S0.64Se0.36 has recently been grown by molecular beam epitaxy (MBE) and been shown to be oxidation and acid resistant. This makes it attractive either as a replacement or adjunct to MgS in II-VI multilayers. In this paper we compare the structural and optical properties of MBE grown multilayer structures containing Zn0.2Mg0.8S0.64Se0.36 to those grown with the quaternary alloy replaced by MgS. Cross-sectional high-resolution transmission electron microscopy (HRTEM) and X-ray interference spectra of ZnSe/Zn0.2Mg0.8S0.64Se0.36/ZnSe multilayers show the Zn0.2Mg0.8S0.64Se0.36 layers are of good crystal quality and do not phase segregate. Layer interfaces are seen to be flat and Zn0.2Mg0.8S0.64Se0.36 does not introduce defects into the overlying ZnSe. Atomic force microscopy shows the surface of a 30 nm Zn0.2Mg0.8S0.64Se0.36 layer is atomically flat, in contrast with similar MgS layers, which show pronounced I D surface ridges, indicating that the Zn0.2Mg0.8S0.64Se0.36 layers have not started to relax. ZnSe quantum wells grown with Zn0.2Mg0.8S0.64Se0.36 barriers show 77K photoluminescence comparable in wavelength and intensity to ZnSe wells of similar thickness grown with MgS barriers. This has allowed us to demonstrate the use of the quaternary alloy, which resists oxidation in place of MgS in multilayer structures. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: An epitaxial lift-off technique for removing wide bandgap II-VI heterostructures from GaAs substrates has previously been demonstrated using lattice-matched MgS as the sacrificial layer. However, using MgS as an etch release layer prevents its use as a wide bandgap barrier in the rest of the structure. Here, we describe the use of the etch-resistant alloy Zn.2Mg.8S.64Se.36 which we have developed as a replacement for MgS. We demonstrate that this alloy can be grown by MBE together with MgS in heterostructures and used as a barrier for ZnSe. A ZnSe quantum well with Zn.2Mg.8S.64Se.36 barriers shows no decrease in photoluminescence intensity after the etching process but shows a shift in emission wavelength associated with the changing strain state. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: We observe dressed states and quantum interference effects in a strongly driven three-level quantum dot ladder system. The effect of a strong coupling field on one dipole transition is measured by a weak probe field on the second dipole transition using differential reflection. When the coupling energy is much larger than both the homogeneous and inhomogeneous linewidths an Autler-Townes splitting is observed. Significant differences are observed when the transitions resonant with the strong and weak fields are swapped, particularly when the coupling energy is nearly equal to the measured linewidth. This result is attributed to quantum interference: destructive or constructive interference with modest visibility is observed depending on the pump/probe geometry. The data demonstrate that coherence of both the bi-exciton and the exciton is maintained in this solid-state system, even under intense illumination, which is crucial for prospects in quantum information processing and nonlinear optical devices.
Abstract: The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dot is observed in resonance transmission spectroscopy. We use a pump laser excitation to drive strongly the unpolarized trion transition in a quantum dot and detect its modified absorption spectrum with a second weak probe laser. By tuning the pump laser near resonance, we observe an ac-Stark effect dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two level system. Although the pump and probe laser fields are resonant with the same transition, we do not observe all features in the Mollow spectrum. We combine the results of pump probe with saturation spectroscopy data to deduce the individual contributions to the low power linewidth.
Abstract: The Fano effect(1) is ubiquitous in the spectroscopy of, for instance, atoms(1,2), bulk solids(3,4) and semiconductor heterostructures(5-7). It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport(5,6) and optical spectra(7,8) in semiconductors. Whereas Fano's original theory(1) applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions(9). Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes(2), the original test-bed of Fano's ideas(1), occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross- section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid- state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two- level quantum system ( qubits), which should ideally be decoupled from all other states.
Abstract: The spin of an electron is a natural two- level system for realizing a quantum bit in the solid state(1-16). For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon- related spin relaxation(1-7). However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds(8-14). Spin- echo techniques have been used to mitigate the hyperfine interaction(14,15), but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this(16,17) but is very difficult to realize in practice(12,18,19). Exploring materials with zero- spin nuclei is another option, and carbon nanotubes(20), graphene quantum dots(21) and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin- 3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak(22,23). We demonstrate here high fidelity ( about 99 per cent) initialization of a single hole spin confined to a self- assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid- state quantum networks(24) that can intra- convert the spin state with the polarization of a photon.
Abstract: We describe the design and performance of a fiber-based confocal microscope for cryogenic operation. The microscope combines positioning at low temperatures along three space coordinates of millimeter translation and nanometerprecision with high stability and optical performance at the diffraction limit. It was successfully tested under ambient conditions as well as at liquid nitrogen (77 K) and liquid helium (4 K) temperatures. The compact nonmagnetic design provides for long term position stability against helium refilling transfers, temperature sweeps, as well as magnetic field variation between -9 and 9 T. As a demonstration of the microscope performance, applications in the spectroscopy of single semiconductor quantum dots are presented. 0 2008 American Institute of Physics.
Abstract: Structures containing Zn1-xMgxS have been grown lattice matched to GaAs by using molecular beam epitaxy (MBE) with ZnS as the source of S. The composition of the alloy produced has been determined using double-crystal X-ray spectroscopy and X-ray interference measurements. Both techniques indicate that 0.88 <= x <= 0.93. This result is confirmed by both secondary ion mass spectroscopy and an Auger analysis carried out on the material. These results show that the crystalline quality of the material produced is excellent and that it has been grown coherently to the GaAs substrate. Photoluminescence spectroscopy shows a high intensity emission with a narrow full width half maximum, confirming the suitability of this alloy as a high-bandgap barrier material.
Abstract: We report high-resolution resonant two-color laser spectroscopy on single self-assembled InGaAs quantum dots. The negatively charged exciton in a quantum dot can decay radiatively into both electron-spin ground states, the states with an electron in the quantum dot with its spin parallel or antiparallel to an applied magnetic field. The two decay paths can be used for optical spin alignment via optical pumping. We apply two laser fields at two different colors resonant with the two Zeeman split optical transitions to study the properties of the spin of the resident electron in the quantum dot in both the Faraday and Voigt geometries. The resonant two-color signal is monitored as a function of the laser power as well as the applied magnetic and electric field allowing us to determine spin decay and dephasing rates. We find the rate at which the optical spin alignment can be performed depending on the direction and the magnitude of the applied magnetic field. Finally we demonstrate the feasibility of performing coherent all-optical spin manipulation of an electron spin in a quantum dot.
Abstract: Samples containing ZnMgSSe alloy were grown by using molecular beam epitaxy at 240 degrees C and were analyzed by using X-ray interference. The alloy composition was found to be Zn0.20Mg0.80S0.64Se0.36. The surfaces of these layers were found to be extremely flat, unlike MgS layers of similar thickness grown under identical conditions, which show pronounced ridges. Structures with Zn0.20Mg0.80S0.64Se0.36 barriers were grown with ZnSe quantum wells and showed good quantum confinement with a sharp PL peak. Calculations of the phase stability of ZnMgSSe alloys suggest that an alloy of this composition should phase separate. However, samples with this composition are demonstrably single phase, and the discrepancy with the calculation can be removed if the enthalpy of formation of zinc-blende MgS is reduced by less than 2 % to -231 kJ mol(-1).
Abstract: We report the observation of strong and weak exciton-photon coupling in a variety of ZnSe-based microcavities fabricated using epitaxial liftoff. Molecular-beam-epitaxy grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one-wavelength optical length at the exciton emission were transferred to a SiO2/Ta2O5 mirror with a reflectance of 96 %. Three experiments are presented, evidencing strong exciton-photon coupling in both a fixed and tunable microcavity and lasing at room temperature in a monolithic device.
Abstract: By using extreme numerical-aperture solid-immersion microscopy at 1553nm we demonstrate, under certain circumstances, polarization-sensitive imaging with resolution values approaching 100nm which substantially surpass the classical scalar diffraction-limit embodied by Sparrow's resolution criterion. (C) 2008 Optical Society of America
Abstract: Solid immersion lens (SIL) microscopy combines the advantages of conventional microscopy with those of near-field techniques, and is being increasingly adopted across a diverse range of technologies and applications. A comprehensive overview of the state-of-the-art in this rapidly expanding subject is therefore increasingly relevant. Important benefits are enabled by SIL-focusing, including an improved lateral and axial spatial profiling resolution when a SIL is used in laser-scanning microscopy or excitation, and an improved collection efficiency when a SIL is used in a light-collection mode, for example in fluorescence micro-spectroscopy. These advantages arise from the increase in numerical aperture (NA) that is provided by a SIL. Other SIL-enhanced improvements, for example spherical-aberration-free sub-surface imaging, are a fundamental consequence of the aplanatic imaging condition that results from the spherical geometry of the SIL. The theory of SIL imaging exposes the unique properties of SILs that provide advantages in applications involving the interrogation of photonic and electronic nanostructures. Such applications range from the sub-surface examination of the complex three-dimensional microstructures fabricated in silicon integrated circuits, to quantum photoluminescence and transmission measurements in semiconductor quantum dot nanostructures.
Abstract: We present results on the charge dependence of the radiative recombination lifetime, tau, and the emission energy of excitons confined to single self-assembled InGaAs quantum dots. There are significant dot-to-dot fluctuations in the lifetimes for a particular emission energy. To reach general conclusions, we present the statistical behavior by analyzing data recorded on a large number of individual quantum dots. Exciton charge is controlled with extremely high fidelity through an n-type field effect structure, which provides access to the neutral exciton (X-0), the biexciton (2X(0)), and the positively (X1+) and negatively (X1-) charged excitons. We find significant differences in the recombination lifetime of each exciton such that, on average, tau(X1-)/tau(X-0)=1.25, tau(X1+)/tau(X-0)=1.58, and tau(2X(0))/tau(X-0)=0.65. We attribute the change in lifetime to significant changes in the single particle hole wave function on charging the dot, an effect more pronounced on charging X-0 with a single hole than with a single electron. We verify this interpretation by recasting the experimental data on exciton energies in terms of Coulomb energies. We directly show that the electron-hole Coulomb energy is charge dependent, reducing in value by 5%-10% in the presence of an additional electron, and that the electron-electron and hole-hole Coulomb energies are almost equal.
Abstract: The complete control of the electron occupation of a single InGaAs dot is shown to produce highly antibunched single photon emission with nonresonant optical excitation. Intensity correlation measurements show g((2))(0) values of 3% (50%) at low (high) excitation power. A distinct double peak structure is shown at time zero, demonstrating that although two photons may be emitted per excitation pulse, they are not simultaneously emitted. We interpret this feature as a hole recapture process from the wetting layer into the dot after initial recombination. The recapture dynamics is shown to be adjustable through engineering the valence potential. (c) 2008 American Institute of Physics.
Abstract: We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few mu T. In one case, the spin resonance behaves as a driven 3-level lambda system with weak damping; in another one, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes.
Abstract: We report the observation of power law dynamics on nanosecond to microsecond timescales in the fluorescence decay from semiconductor nanocrystals and draw a comparison between this behavior and power law fluorescence blinking from single nanocrystals. The link is supported by comparison of blinking and lifetime data measured simultaneously from the same nanocrystal. Our results reveal that the power law coefficient changes little over the nine decades in time from 10 ns to 10 s, in contrast with the predictions of some diffusion based models of power law behavior. (C) 2008 American Institute of Physics.
Abstract: We present an optical signature of a hybridization between a localized quantum dot state and a filled continuum. Radiative recombination of the negatively charged trion in a single quantum dot leaves behind a single electron. We show that in two regions of vertical electric field, the electron hybridizes with a continuum through a tunneling interaction. The hybridization manifests itself through an unusual voltage dependence of the emission energy and a non-Lorentzian line shape, features which we reproduce with a theory based on the Anderson Hamiltonian.
Abstract: We performed high resolution resonant laser spectroscopy on a single self-assembled quantum dot (QD) at liquid He temperatures. We explore the two-level nature of the QD excitonic transition through the investigation of its behavior as a function of the laser power. The quantum ground state exciton absorption peak size diminishes with increasing power while its width increases. Fitting these dependencies to the predictions of a two-level atom model we extract unambiguously a radiative lifetime of 660ps, a residual collisional broadening about 0.18 mu eV as well as the spectral fluctuation 1.3 mu eV. We find that at high power the line width of the exciton absorption is essentially given by the Rabi frequency. (c) 2007 Elsevier B.V. AD rights reserved.
Abstract: It has been known since 1959 that the focal-plane intensity distribution produced by focusing polarized light with a high-numerical-aperture lens should be highly asymmetric(1). Remarkably, the consequences of this fundamental effect in direct image acquisition have remained unexploited, although vectorial effects have been observed in the contexts of free-space focusing(2), molecular fluorescence(3) and photolithography(4). By using extreme-numerical-aperture ( values of 3.5), solidimmersion microscopy(5-7) we have obtained images of a silicon integrated circuit showing, for the first time, the dramatic influence of polarization on their spatial resolution, with values from 100 nm to 250 nm. Our data show that polarization-sensitive imaging can substantially surpass the scalar diffraction limit embodied by classical formulae such as Sparrow's criterion. Such performance will have an impact on activities such as integrated-circuit failure analysis, where optical inspection faces serious challenges from the sub-100-nm feature sizes routinely used in production devices.
Abstract: The fine structure splitting of neutral excitons in InGaAs quantum dots is investigated using polarization sensitive photoluminescence. The QDs were grown with an in situ annealing step to shift the emission wavelength to similar to 950nm. Statistics of the fine structure reveal a large spread in magnitude and little preferential orientation of the polarization axis. We speculate that these findings are due to the redistribution of QD material during the annealing step. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Entangled photons can be generated "on demand" in a novel scheme involving unitary time reordering of the photons emitted in a radiative decay cascade. The scheme yields polarization entangled photon pairs, even though prior to reordering the emitted photons carry significant "which path information" and their polarizations are unentangled. This shows that quantum chronology can be manipulated in a way that is lossless and deterministic (unitary). The theory can, in principle, be tested and applied to the biexciton cascade in semiconductor quantum dots.
Abstract: We have performed resonant interband transmission spectroscopy on the transitions between the s-shells and the p-shells of a single charge tunable InGaAs quantum dot (QD). In contrast to the s-shell spectroscopy, investigating p-shell transitions allows the study of a QD charged with up to four electrons. The exciton charging state is clearly identified as a function of gate voltage ranges. In contrast to the s-shell, the p-shell electronic states show a strong tunnel coupling to the Fermi sea of the back contact. (C) 2008 American Institute of Physics.
Abstract: We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few mu T. In one case, the spin resonance behaves as a driven 3-level lambda system with weak damping; in another one, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes.
Abstract: We have performed resonant interband transmission spectroscopy on the transitions between the s-shells and the p-shells of a single charge tunable InGaAs quantum dot (QD). In contrast to the s-shell spectroscopy, investigating p-shell transitions allows the study of a QD charged with up to four electrons. The exciton charging state is clearly identified as a function of gate voltage ranges. In contrast to the s-shell, the p-shell electronic states show a strong tunnel coupling to the Fermi sea of the back contact. (C) 2008 American Institute of Physics.
Abstract: Entangled photons can be generated “on demand†in a novel scheme involving unitary time reordering of the photons emitted in a radiative decay cascade. The scheme yields polarization entangled photon pairs, even though prior to reordering the emitted photons carry significant “which path information†and their polarizations are unentangled. This shows that quantum chronology can be manipulated in a way that is lossless and deterministic (unitary). The theory can, in principle, be tested and applied to the biexciton cascade in semiconductor quantum dots.
Abstract: We report the observation of power law dynamics on nanosecond to microsecond timescales in the fluorescence decay from semiconductor nanocrystals and draw a comparison between this behavior and power law fluorescence blinking from single nanocrystals. The link is supported by comparison of blinking and lifetime data measured simultaneously from the same nanocrystal. Our results reveal that the power law coefficient changes little over the nine decades in time from 10 ns to 10 s, in contrast with the predictions of some diffusion based models of power law behavior. (C) 2008 American Institute of Physics.
Abstract: We present an optical signature of a hybridization between a localized quantum dot state and a filled continuum. Radiative recombination of the negatively charged trion in a single quantum dot leaves behind a single electron. We show that in two regions of vertical electric field, the electron hybridizes with a continuum through a tunneling interaction. The hybridization manifests itself through an unusual voltage dependence of the emission energy and a non-Lorentzian line shape, features which we reproduce with a theory based on the Anderson Hamiltonian.
Abstract: The fine structure splitting of neutral excitons in InGaAs quantum dots is investigated using polarization sensitive photoluminescence. The QDs were grown with an in situ annealing step to shift the emission wavelength to similar to 950nm. Statistics of the fine structure reveal a large spread in magnitude and little preferential orientation of the polarization axis. We speculate that these findings are due to the redistribution of QD material during the annealing step. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: We report the observation of strong and weak exciton-photon coupling in a variety of ZnSe-based microcavities fabricated using epitaxial liftoff. Molecular-beam-epitaxy grown ZnSe/Zn$_{0.9}$Cd$_{0.1}$Se quantum wells with a one-wavelength optical length at the exciton emission were transferred to a SiO$_2$/Ta$_2$O$_5$ mirror with a reflectance of 96 \%. Three experiments are presented, evidencing strong exciton-photon coupling in both a fixed and tunable microcavity and lasing at room temperature in a monolithic device.
Abstract: The complete control of the electron occupation of a single InGaAs dot is shown to produce highly antibunched single photon emission with nonresonant optical excitation. Intensity correlation measurements show g((2))(0) values of 3% (50%) at low (high) excitation power. A distinct double peak structure is shown at time zero, demonstrating that although two photons may be emitted per excitation pulse, they are not simultaneously emitted. We interpret this feature as a hole recapture process from the wetting layer into the dot after initial recombination. The recapture dynamics is shown to be adjustable through engineering the valence potential. (c) 2008 American Institute of Physics.
Abstract: It has been known since 1959 that the focal-plane intensity distribution produced by focusing polarized light with a high-numerical-aperture lens should be highly asymmetric(1). Remarkably, the consequences of this fundamental effect in direct image acquisition have remained unexploited, although vectorial effects have been observed in the contexts of free-space focusing(2), molecular fluorescence(3) and photolithography(4). By using extreme-numerical-aperture ( values of 3.5), solidimmersion microscopy(5-7) we have obtained images of a silicon integrated circuit showing, for the first time, the dramatic influence of polarization on their spatial resolution, with values from 100 nm to 250 nm. Our data show that polarization-sensitive imaging can substantially surpass the scalar diffraction limit embodied by classical formulae such as Sparrow’s criterion. Such performance will have an impact on activities such as integrated-circuit failure analysis, where optical inspection faces serious challenges from the sub-100-nm feature sizes routinely used in production devices.
Abstract: We performed high resolution resonant laser spectroscopy on a single self-assembled quantum dot (QD) at liquid He temperatures. We explore the two-level nature of the QD excitonic transition through the investigation of its behavior as a function of the laser power. The quantum ground state exciton absorption peak size diminishes with increasing power while its width increases. Fitting these dependencies to the predictions of a two-level atom model we extract unambiguously a radiative lifetime of 660ps, a residual collisional broadening about 0.18 mu eV as well as the spectral fluctuation 1.3 mu eV. We find that at high power the line width of the exciton absorption is essentially given by the Rabi frequency. (c) 2007 Elsevier B.V. AD rights reserved.
Abstract: The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dot is observed in resonance transmission spectroscopy. We use a pump laser excitation to drive strongly the unpolarized trion transition in a quantum dot and detect its modified absorption spectrum with a second weak probe laser. By tuning the pump laser near resonance, we observe an ac-Stark effect dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two level system. Although the pump and probe laser fields are resonant with the same transition, we do not observe all features in the Mollow spectrum. We combine the results of pump probe with saturation spectroscopy data to deduce the individual contributions to the low power linewidth.
Abstract: We describe the design and performance of a fiber-based confocal microscope for cryogenic operation. The microscope combines positioning at low temperatures along three space coordinates of millimeter translation and nanometerprecision with high stability and optical performance at the diffraction limit. It was successfully tested under ambient conditions as well as at liquid nitrogen (77 K) and liquid helium (4 K) temperatures. The compact nonmagnetic design provides for long term position stability against helium refilling transfers, temperature sweeps, as well as magnetic field variation between -9 and 9 T. As a demonstration of the microscope performance, applications in the spectroscopy of single semiconductor quantum dots are presented. 0 2008 American Institute of Physics.
Abstract: The Fano effect(1) is ubiquitous in the spectroscopy of, for instance, atoms(1,2), bulk solids(3,4) and semiconductor heterostructures(5-7). It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport(5,6) and optical spectra(7,8) in semiconductors. Whereas Fano’s original theory(1) applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions(9). Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes(2), the original test-bed of Fano’s ideas(1), occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross- section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid- state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two- level quantum system ( qubits), which should ideally be decoupled from all other states.
Abstract: The spin of an electron is a natural two- level system for realizing a quantum bit in the solid state(1-16). For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon- related spin relaxation(1-7). However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds(8-14). Spin- echo techniques have been used to mitigate the hyperfine interaction(14,15), but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this(16,17) but is very difficult to realize in practice(12,18,19). Exploring materials with zero- spin nuclei is another option, and carbon nanotubes(20), graphene quantum dots(21) and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin- 3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak(22,23). We demonstrate here high fidelity ( about 99 per cent) initialization of a single hole spin confined to a self- assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid- state quantum networks(24) that can intra- convert the spin state with the polarization of a photon.
Abstract: Samples containing ZnMgSSe alloy were grown by using molecular beam epitaxy at 240 $^\circ$C and were analyzed by using X-ray interference. The alloy composition was found to be Zn$_{0.20}$Mg$_{0.80}$S$_{0.64}$Se$_{0.36}$. The surfaces of these layers were found to be extremely flat, unlike MgS layers of similar thickness grown under identical conditions, which show pronounced ridges. Structures with Zn$_{0.20}$Mg$_{0.80}$S$_{0.64}$Se$_{0.36}$ barriers were grown with ZnSe quantum wells and showed good quantum confinement with a sharp PL peak. Calculations of the phase stability of ZnMgSSe alloys suggest that an alloy of this composition should phase separate. However, samples with this composition are demonstrably single phase, and the discrepancy with the calculation can be removed if the enthalpy of formation of zinc-blende MgS is reduced by less than 2 \% to $-231$ kJ mol$^{-1}$.
Abstract: Structures containing Zn$_{1-x}$Mg$_x$S have been grown lattice matched to GaAs by using molecular beam epitaxy (MBE) with ZnS as the source of S. The composition of the alloy produced has been determined using double-crystal X-ray spectroscopy and X-ray interference measurements. Both techniques indicate that 0.88 $\leq$ x $\leq$ 0.93. This result is con rmed by both secondary ion mass spectroscopy and an Auger analysis carried out on the material. These results show that the crystalline quality of the material produced is excellent and that it has been grown coherently to the GaAs substrate. Photoluminescence spectroscopy shows a high intensity emission with a narrow full width half maximum, con rming the suitability of this alloy as a high-bandgap barrier material.
Abstract: By implementing two-photon optical-beam-induced-current microscopy using a solid-immersion lens, imaging inside a silicon flip chip is reported with 166nm lateral resolution and an axial resolution capable of resolving features only 100nm in height.
Abstract: Coulomb interactions between electrons lead to the observed multiplet structure and breakdown of the Aufbau principle for atomic d and f shells(1). Nevertheless, these effects can disappear in extended systems. For instance, the multiplet structure of atomic carbon is not a feature of graphite or diamond. A quantum dot is an extended system containing similar to 10(6) atoms for which electron-electron interactions do survive and the interplay between the Coulomb energy, J, and the quantization energy, Delta E, is crucial to Coulomb blockade(2-5). We have discovered consequences of Coulomb interactions in self-assembled quantum dots by interpreting experimental spectra with an atomistic calculation. The Coulomb effects, evident in the photon emission process, are tunable in situ by controlling the quantum dot charge from + 6e to -6e. The same dot shows two regimes: J <= Delta E for electron charging yet J similar to Delta E for hole charging. We find a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole-hole interactions; promotion-demotion processes in the final state of the emission process, effects first predicted a decade ago(6); and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics.
Abstract: Two- and three-dimensional sub-surface optical beam induced current imaging of a silicon flip-chip is described and is illustrated by results corresponding to 166 nm lateral resolution and an axial performance capable of localising feature depths to around 100 nm accuracy. The experimental results are compared with theoretically modelled performance based on analytic expressions for the system point spread functions valid for high numerical apertures, and are interpreted using numerical geometric ray tracing calculations. Examples of depth-resolved feature profiling are presented and include depth cross-sections through a matrix of tungsten vias and a depth-resolved image of part of a poly-silicon wire. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: The authors perform transmission spectroscopy on single quantum dots and examine the effects of a resident carrier's spin, the incident laser spot size, polarization, and power on the experimental contrast. They demonstrate a factor of 5 improvement in the maximum contrast by using a solid immersion lens to decrease the spot area. This increase yields a maximum signal to noise ratio of similar to 2000 Hz(-1/2), which will allow for megahertz detection frequencies. The authors anticipate that this improvement will allow further investigation of spectral fluctuation and open up the feasibility for an all-optical readout of an electron spin in a quantum dot. (C) 2007 American Institute of Physics.
Abstract: We report the observation of strong exciton-photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO2/Ta2O5 mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle-resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of 23.6 meV at 20 K, composite evolution of the lower and upper polaritons and narrowing of the lower polariton linewidth near resonance. The heavy-hole exciton oscillator strength per quantum well is also deduced to be 1.78 x 10(13) cm(-2).
Abstract: By implementing two-photon optical-beam-induced-current microscopy using a solid-immersion lens, imaging inside a silicon flip chip is reported with 166nm lateral resolution and an axial resolution capable of resolving features only 100nm deep. (C) 2006 Optical Society of America
Abstract: We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10x10 mu m(2)). Using a confocal microscope configuration and solid immersion lens, we achieve a spot size of 320 nm full width at half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: The higher detection efficiency device gives a uniform response, whereas the lower detection efficiency device is limited by a single defect or constriction. (C) 2007 American Institute of Physics.
Abstract: We present high resolution modulation spectroscopy on single quantum dots and discuss briefly the differences to other spectroscopy techniques. We use this technique to study the excitonic fine structure while charging the quantum dot and applying mechanical strain to it. We also show that the fine structure can be used as a polarization analyzer. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A(0) whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mu eV, but vanishingly small for electrons.
Abstract: We present key features of the strong coupling regime in a ZnSe-based microcavity; anticrossing with a normal mode splitting of 23.6 meV at 20 K; narrowing of the lower polariton linewidth near resonance; and composite evolution of the cavity-polaritons. The heavy-hole exciton oscillator strength, f(osc) = 1.78x10(13) cm(-2) is also deduced.
Abstract: We demonstrate level-repulsion of exciton-polaritons in ZnSe/Zn0.9Cd0.1Se quantum wells transferred to SiO2/Ta2O5 mirrors using epitaxial liftoff to fabricate our microcavities. The heavy-hole exciton oscillator strength is calculated to be 5.7 x 10(12) cm(-2). (C)2007 Optical Society of America
Abstract: Three-dimensional subsurface imaging through the back side of a silicon flip chip is reported with a diffraction-limited lateral resolution of 166 nm and an axial performance capable of resolving features only 100 nm deep. This performance was achieved by implementing sample-scanned two-photon optical beam induced current microscopy using a silicon solid immersion lens and a peak detection algorithm. The excitation source was a 1530 nm erbium:fiber laser, and the lateral optical resolution obtained corresponds to 11% of the free-space wavelength. (c) 2007 American Institute of Physics.
Abstract: The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. (c) 2007 American Institute of Physics.
Abstract: We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A(0) whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mu eV, but vanishingly small for electrons.
Abstract: We report the observation of strong exciton-photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO2/Ta2O5 mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle-resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of 23.6 meV at 20 K, composite evolution of the lower and upper polaritons and narrowing of the lower polariton linewidth near resonance. The heavy-hole exciton oscillator strength per quantum well is also deduced to be 1.78 x 10(13) cm(-2).
Abstract: Two- and three-dimensional sub-surface optical beam induced current imaging of a silicon flip-chip is described and is illustrated by results corresponding to 166 nm lateral resolution and an axial performance capable of localising feature depths to around 100 nm accuracy. The experimental results are compared with theoretically modelled performance based on analytic expressions for the system point spread functions valid for high numerical apertures, and are interpreted using numerical geometric ray tracing calculations. Examples of depth-resolved feature profiling are presented and include depth cross-sections through a matrix of tungsten vias and a depth-resolved image of part of a poly-silicon wire. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: The small physical size of self assembled quantum dots gives rise to pronounced Coulomb interactions within the dots. By studying different excitons in the same quantum dot we show that the Coulomb interactions significantly alter the radiative recombination lifetime. The lifetime changes are larger upon charging from the neutral exciton to the positively charged exciton than from charging from the neutral exciton to the negatively charged exciton. This is attributed to a frozen electron wavefunction and a non-frozen hole wavefunction, leading to a non-perturbative hole-hole Coulomb interaction. Theoretical calculations based on a path integral quantum Monte-Carlo approach show good agreement between experiment and theory.
Abstract: The authors perform transmission spectroscopy on single quantum dots and examine the effects of a resident carrier’s spin, the incident laser spot size, polarization, and power on the experimental contrast. They demonstrate a factor of 5 improvement in the maximum contrast by using a solid immersion lens to decrease the spot area. This increase yields a maximum signal to noise ratio of similar to 2000 Hz(-1/2), which will allow for megahertz detection frequencies. The authors anticipate that this improvement will allow further investigation of spectral fluctuation and open up the feasibility for an all-optical readout of an electron spin in a quantum dot. (C) 2007 American Institute of Physics.
Abstract: We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10x10 mu m(2)). Using a confocal microscope configuration and solid immersion lens, we achieve a spot size of 320 nm full width at half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: The higher detection efficiency device gives a uniform response, whereas the lower detection efficiency device is limited by a single defect or constriction. (C) 2007 American Institute of Physics.
Abstract: We present new understanding of excitonic fine structure in close-to-symmetric InAs/GaAs and InGaAs/GaAs quantum dots. We demonstrate excellent agreement between spectroscopy and many-body pseudopotential theory in the energy splittings, selection rules and polarizations of the optical emissions from doubly charged excitons. We discover a marked difference between the fine structure of the doubly negatively and doubly positively charged excitons. The features in the doubly charged emission spectra are shown to arise mainly from the lack of inversion symmetry in the underlying crystal lattice.
Abstract: Three-dimensional subsurface imaging through the back side of a silicon flip chip is reported with a diffraction-limited lateral resolution of 166 nm and an axial performance capable of resolving features only 100 nm deep. This performance was achieved by implementing sample-scanned two-photon optical beam induced current microscopy using a silicon solid immersion lens and a peak detection algorithm. The excitation source was a 1530 nm erbium:fiber laser, and the lateral optical resolution obtained corresponds to 11% of the free-space wavelength. (c) 2007 American Institute of Physics.
Abstract: We present new understanding of excitonic fine structure in close-to-symmetric InAs/GaAs and InGaAs/GaAs quantum dots. We demonstrate excellent agreement between spectroscopy and many-body pseudopotential theory in the energy splittings, selection rules and polarizations of the optical emissions from doubly charged excitons. We discover a marked difference between the fine structure of the doubly negatively and doubly positively charged excitons. The features in the doubly charged emission spectra are shown to arise mainly from the lack of inversion symmetry in the underlying crystal lattice.
Abstract: The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. (c) 2007 American Institute of Physics.
Abstract: This paper reports the imaging of a silicon flip-chip with high resolution by detection of the photocurrent generated by the two-photon absorption of 1530nm light from a femtosecond Er:fiber laser. High resolution imaging was made possible by the inclusion of a silicon solid immersion lens, which increased the numerical aperture of the microscope. Using this technique, features on a sub-micron scale are clearly resolvable with excellent contrast, and the resolution of the system was found to be 325nm.
Abstract: We show that a piezoelectric actuator can be used to apply uniaxial stress to a layer of self-assembled quantum dots. The applied stress leads to a change of the quantum dot's ground state exciton energy by up to a few hundred mu eV. This approach allows the possibility of an in situ and continuous tuning of the stress at temperatures down to 4 K and offers an alternative to tuning by temperature and Stark effect. We measure the relative change in the charging energy to the n-doped back contact by capacitance and the change in the exciton energy by photolumiescence. By tuning the uniaxial stress we are able to perform reflection spectroscopy on a single dot. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The development of a fiber-based, tunable optical cavity with open access is reported. The cavity is of the Fabry-Perot type and is formed with miniature spherical mirrors positioned on the end of single- or multimode optical fibers by a transfer technique, which involves lifting a high-quality mirror from a smooth convex substrate, either a ball lens or microlens. The cavities typically have a finesse of similar to 1000 and a mode volume of 600 mu m(3). The detection of small ensembles of cold Rb atoms guided through such a cavity on an atom chip is demonstrated. (c) 2006 American Institute of Physics.
Abstract: The effect of increasing annealing times on the emission energies and dot densities of ZnSe/CdSe quantum dots has been studied. Ensemble PL carried out at 77 K demonstrates the energy shift of the main peak attributed to fractional monolayer islands, which is due to annealing. mu PL conducted at 4 K was then used to study the increased spectral separation of the emission energies from SK dots. Finally, ex-situ AFM measurements of two samples, showed a reduction in the Stranski-Krastanow dot density of approximately one order of magnitude due to annealing.
Abstract: The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of an applied uniaxial stress. The spectrum of the excitonic Rayleigh scattering was measured in reflectivity using high-resolution laser spectroscopy while the sample was submitted to a tunable uniaxial stress along its [110] crystal axis. We show that using this stretching technique, the quantum dot potential is elastically deformable such that the exciton fine structure splitting can be substantially reduced.
Abstract: We have determined the direct and exchange electron-electron and electron-hole Coulomb energies in CdSe/ZnSe quantum dots. The experiments are based on single dot photoluminescence at low temperature. By controlling the charging with a vertical transistor structure and by applying a symmetry-breaking magnetic field, we show how we can determine all the Coulomb energies. The direct Coulomb energies are responsible for large, similar to 20 meV, red-shifts of the emission on charging. The exchange Coulomb energies lead to a very pronounced fine structure splitting, up to 2.6 meV, for the neutral exciton. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: We report electron and hole tunnelling phenomena in a single self-assembled quantum dot as a function of the applied electric field. We use absorption spectroscopy which allows us to measure excitonic transitions under conditions where optical recombination cannot be observed due to the high, ionizing, electric field. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We report on the decay dynamics of positively charged excitons confined to single InAs quantum dots embedded in an n-type field-effect structure. The positively charged exciton's dynamics are found to be strongly dependent on device dimensions. With a large (small) dot capping layer the decay is dominated by hole tunneling (radiative recombination). The hole tunneling is successfully modeled with a WKB-like zero-dimensional to three-dimensional tunneling approximation. Hole tunneling is not observed in the dynamics of the neutral exciton negatively charged exciton, or biexciton, an effect we attribute to an increase in barrier height through the interdot Coulomb interactions. (c) 2006 American Institute of Physics.
Abstract: The authors have determined both the real and imaginary parts of the dielectric polarizability of a single quantum dot. The experiment is based on the observation and the manipulation of Rayleigh scattering at photon frequencies near the resonance of an optical exciton transition in single self-assembled InAs and InGaAs quantum dots. The interference between the narrow-band laser field and the weak electromagnetic field coherently scattered by the quantum dot is detected with a cryogenic Fabry-Perot setup by combined differential transmission and reflectivity measurements. (c) 2006 American Institute of Physics.
Abstract: We present photoluminescence spectra from single InAs/GaAs quantum dots embedded in a Schottky diode heterostructure. The pronounced Coulomb blockade can be exploited to change the exciton charge in discrete steps from +6e to -8e via an applied gate voltage. The results demonstrate a number of effects. The spectra of highly negatively charged excitons are dominated by configuration interactions in the final state, a viewpoint supported by few-electron calculations based on a harmonic oscillator potential. The positively charged excitons on the other hand show a very different behavior that can only be explained by taking strong spin-orbit coupling for the holes into account. Particularly, a hole charging sequence is revealed that defies Hund's rule as well as the Aufbau principle. Finally, photoluminescence from excited states offers direct access to the hole quantization energy. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.
Abstract: We report high resolution laser absorption spectroscopy of a single InGaAs/GaAs self-assembled quantum dot embedded in a field-effect structure. We show experimentally that the interband optical absorption to the lower Zeeman branch of the singly charged exciton is strongly inhibited due to spin (Pauli) blockade of the optical transition. At high magnetic fields the optical absorption to the upper Zeeman branch dominates the absorption spectrum. We find, however, that the spin blockade is not complete and a 10% leakage remains at high magnetic fields. Applying a gate voltage to empty the dot of its resident electron turns the spin blockade off. This effect is observed at 1.5 K and up to 9 T. (c) 2005 American Institute of Physics.
Abstract: We report high resolution solid-immersion sub-surface imaging of a flip-chip by detecting the two-photon photocurrent generated by a 1530nm femtosecond Er:fiber laser. Features show high contrast and a resolution of 325nm. (c) 2005 Optical Society of America.
Abstract: Magnetic field and temperature dependent photoluminescence studies on neutral and charged excitons in individual InAs quantum dots allow us to uncover different mechanisms by which the discrete quantum dot states are coupled to delocalized continuum states in a quantum well (the wetting layer). The behavior of the neutral and singly charged excitons can be explained taking only discrete quantum dot states into account. For doubly and triply charged excitons we have to consider spin dependent coherent and incoherent interactions between discrete quantum dot states and delocalized. wetting layer states.
Abstract: By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron-electron exchange interaction, and the electron-hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L=2 becomes a bright exciton with L=1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.
Abstract: We have performed detailed photoluminescence (PL) and absorption spectroscopy on the same single self-assembled quantum dot in a charge-tunable device. The transition from neutral to charged exciton in the PL occurs at a more negative voltage than the corresponding transition in absorption. We have developed a model of the Coulomb blockade to account for this observation. At large negative bias, the absorption broadens as a result of electron and hole tunneling. We observe resonant features in this regime whenever the quantum dot hole level is resonant with two-dimensional hole states located at the capping layer-blocking barrier interface in our structure.
Abstract: We report high-resolution subsurface imaging of a silicon flip chip by detection of the photocurrent generated by the two-photon absorption of 1530-nm light from a femtosecond Er:fiber laser. The technique combines the focal sensitivity of two-photon excitation with the enhanced optical resolution of high-numerical-aperture solid-immersion imaging. Features on a sub-1-mum scale are clearly resolvable with high contrast, showing a resolution of 325 nm. (C) 2005 Optical Society of America.
Abstract: We report on a spin-flip process in a charge tunable quantum dot in which a non-radiative dark exciton, with angular momentum L = 2, becomes a radiative bright exciton, L = 1, through interactions with a Fermi sea in an n-doped back contact. This process presents itself by a strongly bias dependent secondary lifetime in radiative lifetime measurements of the neutral exciton. By studying dots with different emission energies the spin-flip process is shown to have a strong dependence on the tunneling rate between the dot and the back contact. An Anderson-like Hamiltonian is used to model a coherent spin swap process between an electron in the dot and electrons in the back contact and gives very good agreement with the experiment.
Abstract: Epitaxial liftoff is a post-growth process by which the active part of a semiconductor heterostructure, the epitaxial layer, is removed from its original substrate and deposited onto a new substrate. This is a well established technique in GaAs-based heterostructures where epitaxial liftoff can be achieved by exploiting the contrast in the etch rates of GaAs and AlAs in hydrofluoric acid. We report here successful epitaxial liftoff of a ZnSe-based heterostructure. We find that a metastable layer of MgS acts as a perfect release layer based on the huge contrast in the etch rates of ZnSe and MgS in hydrochloric acid. Epitaxial liftoff of millimeter-sized ZnSe samples takes a fraction of the time required for GaAs liftoff. Photoluminescence experiments confirm that the liftoff layer has the same optical characteristics as the original wafer material. (C) 2005 American Institute of Physics.
Abstract: An annealing process has been applied during the growth of CdSe/ZnSe quantum dots (QDs). The annealing was able to spectrally separate the photoluminescence (PL) emission of two types of dots in the sample by as much as 160 meV. In a μ-PL study we found that the spectral separation between the emission peaks from individual QDs ill the spectral region corresponding to the low energy ensemble PL feature had been significantly increased by the annealing. Despite not having directly affected the dot density, by defining a spectral window of 50 meV, based on the full-width at half-maximum (FWHM) of the ensemble PL of the normally grown sample, we have reduced the number of dots in the window from several thousand to approximately 10 making it possible to isolate the emission of single QDs for further study. © 2005 Elsevier B.V. All rights reserved.
Abstract: We report measurements of the exciton decay dynamics of a single self-assembled quantum dot following non-resonant excitation. The singly charged exciton, the trion, exhibits a simple one component exponential decay corresponding to radiative recombination. Conversely, the neutral exciton exhibits a two component exponential decay. We argue that the secondary component arises from dark exciton creation and subsequent conversion to a bright exciton through a spin flip. The spin flip time is a strong function of the bias applied between a Fermi reservoir close to the dot and a Schottky gate electrode as a result of a Kondo-like spin swap process.
Abstract: Epitaxial Lift-Off of ZnSe/ZnCdSe and MgS/ZnCdSe quantum well structures from their GaAs substrate has been achieved by using highly reactive MgS as the sacrificial layer. This technique has proved possible in II-VI semiconductor materials due to the huge contrast in the etch rates between the metastable MgS release layer and the II-VI quantum well materials. In this paper, we outline the epitaxial lift-off technique used and confirm the success of the new method using photoluminescence experiments taken before and after lift-off. © 2005 Elsevier B.V. All rights reserved.
Abstract: We report the generation of neutral, negatively charged and positively charged excitons in the same single InAs quantum dot in a controlled manner. The control parameters of the respective excitons are a vertical electric field, applied to a capacitor-like structure in which the quantum dots are embedded, and optical pump power. We have already established that Coulomb blockade can be exploited to control the charge of excitons containing one hole, the neutral exciton, X-0, and the singly negatively charged exciton, K1-. We extend this concept to excitons; containing 2 holes, the biexciton, 2X(0), and significantly the single positively charged exciton, X1+. We support these assignments with a Coulomb blockade (CB) model. The emission from X1- is always redshified from X-0, but surprisingly we observe blue- as well as redshifts for X1+ from X-0.
Abstract: We report the controlled generation of neutral, negatively-charged and positively-charged excitons in the same single InAs quantum dot. The control parameters are a vertical electric field applied to a capacitor-like structure, in which the quantum dots are embedded, and optical pump power. The strong Coulomb blockade in quantum dots can be exploited to control the charge of excitons containing one hole, the neutral exciton, X-0, and singly negatively charged exciton, X1-. We show here how this concept can be extended to excitons containing two holes, the biexciton, 2X(0), and significantly the singly positively charged exciton, X1+. We support all these assignments with a Coulomb blockade model. For all dots, the emission from the X1- is redshifted relative to the neutral exciton, but surprisingly we observe blueshifts as well as small redshifts for X1+. (c) 2005 American Institute of Physics.
Abstract: New optical transitions in charged self-assembled InAs quantum dots have been investigated. The main optical emission is due to fast recombination of holes and electrons from their respective s levels. But photoluminescence (PL) experiments on quantum dots embedded in a vertical tunneling structure reveal a new group of weak lines blueshifted from the main s-s PL by 15 meV. These lines arise as the electronic p shell is first populated for the doubly negatively charged exciton, X2-, induced by the vertical electric field. We therefore attribute these lines to s-p transitions, which are only possible to observe due to the breaking of the rotational symmetry in a real dot and provide a measure of its extent. These transitions show two remarkable effects. First, the population of the wetting layer splits the PL and accordingly the p sub-shells into two branches analogous to a magnetic field. Secondly the s-p PL retains in contrast to the s-s emission a small linewidth up to X6- after the wetting layer is populated. This can be explained by an intra-dot interaction triggered by the s shell vacancy present in the s-s final state but not in the s-p final state.
Abstract: We report temperature-dependent photoluminescence on neutral and charged excitons in individual InAs quantum dots. We find narrow emission lines for temperatures up to 30 K for exciton transitions where only the electron ground state is occupied. In contrast, for doubly charged excitons where the excited electron state is occupied, we observe a drastic increase of the ground state transition linewidth even at 30 K. We interpret this as evidence that the excited electron state is degenerate with the low energy tail of continuum states.
Abstract: Magnetic field and temperature dependent photoluminescence studies on neutral and charged excitons in individual InAs quantum dots allow us to uncover different mechanisms by which the discrete quantum dot states are coupled to delocalized continuum states in a quantum well (the wetting layer). The behaviour of the neutral and singly charged excitons can be explained taking only discrete quantum dot states into account. For doubly and triply charged excitons we have to consider spin dependent coherent and incoherent interactions between discrete quantum dot states and delocalized wetting layer states.
Abstract: We show how the optical properties of a single semiconductor quantum dot can be controlled with a small dc voltage applied to a gate electrode. We find that the transmission spectrum of the neutral exciton exhibits two narrow lines with similar to2 mueV linewidth. The splitting into two linearly polarized components arises through an exchange interaction within the exciton. The exchange interaction can be turned off by choosing a gate voltage where the dot is occupied with an additional electron. Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system. Our experiments show that the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a mueV energy scale.
Abstract: The emission of a neutral exciton in a symmetric semiconductor quantum dot is split by the isotropic electron-hole exchange interaction into dark and bright states. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton, the singly and doubly charged biexcitons. By measuring these splittings we can access the isotropic exchange energies for holes in the ground and electrons ground and excited states.
Abstract: Time-resolved photoluminescence of single charge tuneable quantum dots allows us to probe the differences in recombination dynamics between neutral and negatively charged excitons. We find that the luminescence decay from a neutral exciton contains a second lifetime component of several nanoseconds that is not present in the luminescence from singly or doubly charged excitons. We attribute the slowly decaying component to excitation cycles in which the initial exciton formed in the dot is dark, with angular momentum M = 2, and which subsequently scatters into the bright state with M = 1. The nature of the scattering mechanism is revealed by the dependence of the lifetime on the electrical bias applied across the charge-tuneable device. That the lifetime changes by an order of magnitude within a short bias range implies that the dark-to-bright transmutation does not occur through a simple spin flip. Rather it appears to come about by the dot briefly entering a higher energy charging state which allows exchange of the existing electron with another from the n-type contact region. We model the lifetimes and relative intensities of the two decay components using a simple rate equation analysis.
Abstract: We compare and contrast CdSe quantum dots grown on ZnSe and MgS. We find significant differences in the optical and structural properties of the two systems. Through an annealing process we have been able to separate the PL energy of the two peaks inherent in the photoluminescence of ZnSe/CdSe quantum dots. Temperature dependent photoluminescence was performed from 250 K to 77 K to explore the thermally induced carrier transfer in the two systems. We interpret the striking differences between the PL from CdSe quantum dots grown on MgS and those grown on ZnSe to the intermixing and alloying at the ZnSe, CdSe interface.
Abstract: We report on optical spectroscopy of self-assembled InAs quantum dots in a magnetic field. We describe how we measure the emission characteristics of a single quantum dot (QD) in high magnetic fields at low temperature using a miniature, fiber-based confocal microscope. Example results are presented on a QD whose charge can be controlled using a field-effect device. For the uncharged, singly and doubly charged excitons we find a diamagnetism and the spin Zeeman effect. In contrast, for the triply-charged exciton we find a fundamentally different behavior. Anti-crossings in magnetic field imply that confined states of the QD are hybridized with Landau-like levels associated with the two-dimensional continuum. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The luminescence spectroscopy of charge tunable self-assembled InAs quantum dots is reviewed. We discuss that for confined excitons charged with more than three electrons in excess the quantum dot state can be resonantly coupled to the two-dimensional continuum of electronic states of the wetting layer. This coupling manifests itself in a magnetic field and is triggered by photon emission. In contrast the neutral, singly and doubly charged exciton have optical spectra consistent with the artificial atom picture. (C) 2004 Published by Elsevier B.V.
Abstract: Excitonic transitions of single InAs self-assembled quantum dots were directly measured at 4.2 K in an optical transmission experiment. We use the Stark effect in order to tune the exciton energy of a single quantum dot into resonance with a narrow-band laser. With this method, sharp resonances in the transmission spectra are observed. The oscillator strengths as well as the homogeneous line widths of the single-dot optical transitions are obtained. A clear saturation in the absorption is observed at modest laser powers. (C) 2003 Published by Elsevier B.V.
Abstract: We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: We use a plane-wave technique to study the electron and hole wave functions in self-assembled InGaAs quantum rings, investigating the influence on the electron-hole overlap of variations in the ring shape and composition profiles, and including the effects of built-in strain and the piezoelectric potential. Lateral variation of the ground-state electron and hole wave functions in a perfect ringlike structure is observed as a consequence of the piezoelectric potential, which has minima and maxima on the (110) and (1(1) over bar 0) planes passing through the center of the ring. We find for rings with reflection symmetry in the vertical direction that although the average electron and hole positions are equal, there is a significant vertical separation of the electron and hole peak probability densities in the (110) and (1(1) over bar 0) planes. A net vertical separation of the electron and hole may occur either through the piezoelectric potential when the (110) and (1(1) over bar 0) planes are no longer equivalent, or through strain effects when there is a shape or composition asymmetry in the [001] direction. Comparison of the theoretical predictions with experimental data confirm the presence of a large asymmetry in the ring profile both in the vertical direction, where the ring is deduced to be volcanolike, and also in the growth plane, where the rings are elongated along the [1(1) over bar 0] direction.
Abstract: We show how the resonant absorption of the ground state neutral exciton confined in a single InGaAs self-assembled quantum dot can be directly observed in an optical transmission experiment. A spectrum of the differential transmitted intensity is obtained by sweeping the exciton energy into resonance with laser photons exploiting the voltage induced Stark-shift. We describe the details of this experimental technique and some example results which exploit the similar to1 mueV spectral resolution. In addition to the fine structure splitting of the neutral exciton and an upper bound on the homogeneous linewidth at 4.2 K, we also determine the transition electric dipole moment. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as 'artificial atoms', because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons(1) (bound electron - hole pairs) or with electrons(2). The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter(3-6). At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots(7-9), motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently(10-12). We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.
Abstract: An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.
Abstract: We investigate the influence of optical pump power on a series of charging events of individual quantum dots embedded in a field effect structure. Increasing the optical pump powers moves all charging events towards more negative voltages and charged biexciton states in addition to single exciton states are observed. The linear shift in voltage with pump power is explained by the accumulation of photogenerated holes within the structure. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: In this paper we report the growth by atomic layer epitaxy (ALE) and optical properties of ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10(9) cm(-2) were measured by atomic force microscopy (ATM). In capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and a spectrally broad emission at 2.15 eV from the internal Mn2+ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. At high Mn concentrations the temporal change in magnetization causes a broadening of the FWHM of lines from single dots of up to 4 meV. However, for low concentrations single dot PL linewidths were resolution limited at < 0.2 meV. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons.
Abstract: We describe excitons in quantum dots by allowing for an interaction with a Fermi sea of electrons. We argue that these excitons can be realized very simply with self-assembled quantum dots, using the wetting layer as host for the Fermi sea. We show that a tunnel hybridization of a charged exciton with the Fermi sea leads to two striking effects in the optical spectra. First, the photoluminescence lines become strongly dependent on the vertical bias. Second, if the exciton spin is nonzero, the Kondo effect leads to peculiar photoluminescence line shapes with a linewidth determined by the Kondo temperature.
Abstract: We show how the mechanical rigidity of a slightly detuned miniature Fabry-Perot cavity can be modified with light. We use a microcavity in which one of the mirrors is a soft compliant microlever optimized to detect bolometric forces. The static compliance can either be decreased to zero or increased considerably depending on the detuning of the light with respect to the cavity resonance. (C) 2003 American Institute of Physics.
Abstract: We report on low temperature (4.2 K) magneto-luminescence measurements performed on charged tuneable GaInAs self-assembled quantum dots. We mapped the magnetic field dispersion (0-9 Tesla) of the exciton with excess electron charges set from 0 to 3. For the doubly and triply charged excitons the emission line shows a doublet corresponding to the singlet and triplet configuration of the quantum dot in the final state after photon emission. We map here for the first time the dispersion of the singlet state of the doubly charged exciton and show that the triply charged exciton undergoes a transition resulting from a magnetic field induced frustration of the Hund's rule.
Abstract: In this paper we report the growth and optical properties of ZnSe/CdSe: Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 10(9) cm(-2) were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn2+ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at < 0.2 meV.
Abstract: CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and mu-PL measurements also show evidence of quantum dot structures. Power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K. This value is larger than both the CdSe bulk LO phonon energy and the thermal energy at 300 K All rights reserved (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Excitonic interband optical transitions within single InAs self-assembled quantum dots have been directly observed in a transmission experiment at 4.2 K. Using Stark shift, the excitonic energy levels of a single quantum dot are tuned into resonance with a narrow-band laser line. The Stark shift is also modulated at low frequencies. Relative changes in transmission can be detected this way down to one part per million. The oscillator strength as well the homogeneous linewidth of the transition is obtained. (C) 2003 American Institute of Physics.
Abstract: An analytical formulation of the interband optical transmission and reflectivity spectra of a single quantum dot embedded in a semiconductor is presented. We consider the effect of the sample surface as well as other reflecting surfaces on the shape of the spectra near the ground state exciton resonance. The saturation of the transmission and reflectivity spectra due to the quantum optical saturation of the transition at higher light power is presented. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Photoluminescence and time-resolved photoluminescence measurements of charge tunable quantum-dot heterostructures reveal that by appropriate biasing of the device, about 90% of photogenerated holes can be stored at an interface near to the nanostructures and subsequently transferred into the nanostructures in a controlled fashion. The capture dynamics are sensitive to the form of the valence band potential in the layer that caps the Stranski-Krastanow dots. The dependence of the capture rate on applied electric field suggests that the valence band confinement potential is "soft'' in the capping layer, with a spatial extent of around 14 nm. (C) 2003 American Institute of Physics.
Abstract: The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between "bright" exciton ground states and novel "dark" states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.
Abstract: In this paper we review the recent progress in the growth and spectroscopy of CdSe quantum dots. In particular, Atomic Layer Epitaxy (ALE) has been used to grow ZnSe/CdSe and ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10(9) cm(-2) were measured by Atomic Force Microscopy (AFM). In the capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and in the high Mn concentration samples a spectrally broad emission at 2.15 eV from the internal Mn2+ transition. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. Also, CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. Unlike the ZnSe/CdSe dots the PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and mu-PL measurements also show evidence of quantum dot structures and power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K.
Abstract: Quantum dots are nanometre-sized clusters of semiconductor material which confine electrons in all three directions. The physics of quantum dots are dominated by quantization : there are discrete energy levels, as in real atoms. Quantum dots can now be self-assembled directly in the growth of inorganic semiconductors, and this discovery has fuelled an explosion in the interest in this field. A review of some of this work is presented, concentrating on the optical properties of quantum dots, and possible applications for photonic devices.
Abstract: We have measured the vertical Stark effect of excitons confined to individual self-assembled ring-shaped quantum dots. We find that the excitons have very large permanent dipole moments corresponding to electron-hole separations up to 2.5 nm, comparable to the nanostructures' physical height. We find a trend of both permanent dipole moment and polarizability on the emission energy, but a very strong correlation between the permanent dipole moment and the polarizability.
Abstract: In this paper we present calculations of the magneto-excitonic absorption spectrum for two different quantum ring geometries. Calculations are performed using a direct diagonalization of the single particle wavefunctions. Two different behaviours including the novel appearance of a ground state exciton with a finite angular momentum are described and calculations of the conditional correlation function are used to explore the nature of the exciton peaks.
Abstract: We report on measurements of the magneto-optical properties of excitons confined in ring-shaped self-assembled semiconductor quantum dots. The rings are embedded in a field-effect structure that allows the number of confined electrons to be set electrostatically. In addition, electron-hole pairs are generated optically. The resulting photoluminescence spectra of neutral, singly and doubly charged excitons were measured at 4.2 K as a function of the applied magnetic field (0-9 T). The emission energy shows a diamagnetic shift as well as a Zeeman splitting. We measured the emission of different charge states of the exciton in many individual dots, In a few of the measured rings, a new behavior was observed, namely a clear departure from the low field diamagnetic dispersion for fields larger than 6 T. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We report measurements of the vertical permanent dipole moment and the polarizability of excited quantum rings by using single-dot spectroscopy. The rings are self-assembled using a modified Stranski-Krastanow growth procedure. The permanent dipole moment is surprisingly large, corresponding to electron-hole separations comparable to the quantum rings' vertical height, and opposite in sign to that of more conventional InAs quantum dots. The polarizabilities are consistent with vertical confinement lengths of about 3 nm for both electrons and holes, We find large fluctuations in both the excitonic dipole moment and polarizability from ring to ring, but a linear relationship between these two quantities. (C) 2002 Elsevier Science B,V. All rights reserved.
Abstract: We present results on the influence of a magnetic field on excitons in semiconductor quantum dots, concentrating on the diamagnetic curvature. We use samples with a bimodal ensemble photoluminescence (PL) and we find that for the low-energy PL branch, the diamagnetic curvature is independent of charge, yet for the high-energy branch, the diamagnetic curvature is strongly reduced with excess charge. Guided by model calculations, we interpret the two classes as typical of the strong and intermediate confinement regimes. In the light of this, we predict that in the weak confinement regime the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons.
Abstract: We have succeeded in preparing excitons with a specific charge in single semiconductor quantum rings. Buried InAs quantum rings are loaded with electrons from a reservoir through a tunneling barrier and an additional electron-hole pair is generated by optical excitation. Single rings are addressed with nano-optical techniques. We observe abrupt shifts in the emission energy as electrons are added one by one. Furthermore, the experiments provide unique insights into the interaction of electrons in semiconductor nano-islands with their environment. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples. (C) 2001 American Institute of Physics.
Abstract: Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers(1-3); this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters(4), storage devices(5-7) and fluorescence markers(8). Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.
Abstract: Self-assembled quantum dots have been incorporated into a field-effect structure allowing the dots to be filled sequentially with electrons. Results of interband spectroscopy on charge-tunable structures are presented. For InAs dots in GaAs and for InAs dots in InP characterisation of the devices with capacitance and transmission spectroscopies yields detailed energy level diagrams. Furthermore, we have measured the oscillator strengths of the various interband transitions. Both these systems have a band gap in an awkward spectral range for single dot experiments. Instead, we present photoluminescence experiments on single quantum rings. Charging of the rings with single electrons is detected by pronounced shifts in the photoluminescence energy.
Abstract: We present experiments on the intersubband resonance (ISR) in InAs/AlSb quantum wells with the aim of understanding the linewidth. We find that fluctuations in the well width dominate the scattering right up to temperatures well beyond room temperature. ISR with two occupied subbands is used to gain insight into these phenomena. We find clear evidence for Landau damping and we argue generally that Landau damping represents the crucial scattering mechanism for all ISRs. The argument is strengthened by considering the intrasubband plasmon, where we also find evidence for Landau damping, in this case in a magnetic field. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: A remarkable morphological change of self-assembled InAs quantum dots takes place during growth if a pause is introduced after overgrowing the dots with a few nm of GaAs. Atomic force microscopy indicates that the shape of the dots changes lens-like to ring-like. We report here the results of capacitance and interband transmission experiments on such ring-like structures embedded in a GaAs matrix. In particular, we compare the electronic properties of conventional dots with those of the rings. Significant changes are found which qualitatively support a quantum ring model. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photoconductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov-de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data. [S0163-1829(99)51244-7].
Abstract: The electronic structure of self-assembled InAs quantum dots embedded in an InP matrix has been investigated using Fourier-transform infrared spectroscopy and capacitance spectroscopy. We observe a splitting of about 25 meV between the conduction-band excited states. We argue that this is likely to be a consequence of a strong anisotropy in the lateral size of the dots. Furthermore, we observe a replica in the absorption spectrum, shifted by about 160 meV from the fundamental, which Eve attribute to an excited heavy-hole state. The InAs/InP dots can be well described in a simple adiabatic approach with a hard quantum well-like potential for the vertical confinement and a soft anisotropic harmonic potential for the lateral confinement. [S0163-1829(99)50636-X].
Abstract: We report spectroscopic measurements of charge-tunable quantum dots. The samples contain vertically aligned double dots which we can fill with electrons from a back contact. We show how we can also accumulate holes in the dots by illuminating the samples with below band gap radiation when a large negative bias is applied. We argue that this is possible through a large disparity in the electron and hole tunneling times. Interband spectroscopy reveals a strong reduction in the quantization energy for the dots in the second layer. (C) 1999 American Institute of Physics. [S0003-6951(99)00813-X].
Abstract: We have measured the intersubband resonances of an InAs/AlSb quantum well with two occupied subbands from cryogenic temperatures to well above room temperature. The higher energy mode is very robust with increasing temperature; the lower energy mode, however, broadens above 200 K. We explain the results in terms of Landau damping and argue generally that the collective nature of the intersubband resonance is crucial for an understanding of the scattering mechanisms that determine the intersubband resonance linewidth.
Abstract: We present perturbation theory results for the Coulomb interactions between electrons, and between electrons and holes, in small quantum dots. The results are used to model both capacitance and interband spectroscopy on charge-tunable, self-assembled dots. At the level of the experimental resolution, currently limited by the inhomogeneous broadening in the dot ensemble, the model calculations reproduce the experimental results extremely well. [S0163-1829(98)051747-6].
Abstract: We have studied the interband excitations of an ensemble of InAs self-assembled quantum dots by detecting absorption directly in transmission experiments. The dots are embedded in a MISFET structure, allowing the dots' electron occupation to be controlled with a gate voltage. We show how Coulomb blockade in the device's C - V-g characteristic corresponds to Pauli blocking of optical transitions in transmission. Furthermore, the second absorption peak of the dots shifts by some 20 meV and weakens when the first electron level is filled with two electrons, evidence of an exciton-electron interaction. The results also provide a direct measurement of the oscillator strength of the dots. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: Interband excitations of an ensemble of InAs self-assembled quantum dots have been directly observed in transmission experiments. The dots are embedded in a field-effect structure allowing us to load the dots electrically. We establish an exact correspondence between Coulomb blockade in the device's vertical transport properties and Pauli blocking in the transmission spectra. We observe substantial shifts, up to 20 meV, in the energies of the higher excitations on occupation of the electron ground state. We argue that this is a consequence of an exciton-electron interaction.
Abstract: We have made a systematic investigation of the Zeeman splitting of n=1 heavy-hole excitons in a range of Al0.36Ga0.64As/GaAs and InxGa1-xAs/GaAs (x=0.08 and 0.11) quantum wells at 1.8 K and in magnetic fields of up to 6 T applied along the growth axis (001). Calculations of splitting as a function of field were made using an eight-band K . P model which reproduce all the main features of the experimental data, including the sign, and give good quantitative agreement. The observed splittings are Linear in low field (<1 T), but become nonlinear as field is increased. This behavior is attributed to a spin-dependent field-induced admixture between the light- and heavy-hole valence bands. For the GaAs/AlGaAs system agreement between experiment and theory requires a value for the Luttinger parameter kappa in bulk GaAs close to 1.2 which is the generally accepted value, and rules out a lower value (0.7) which was proposed recently. From the theoretical fits to the InxGa1-xAs/GaAs Zeeman data we find that there is significant ''bowing'' of kappa(x) which can be reproduced accurately using a perturbation theory relating the Luttinger kappa and gamma parameters, where gamma(1,2,3) are obtained from experimentally determined light- and heavy-hole effective masses.
Abstract: We present both theoretical and experimental results on the intersubband resonance in InAs/AlSb quantum wells. From a Kane (k . p) description of the band structure we investigate the effect of the large nonparabolicity and of the high Fermi wave vector on the selection rules and matrix elements. The 1-2 transition in parallel excitation (x) is shown to be very weak from simple parity arguments; in perpendicular excitation (z) the matrix element [z] is shown to be largely unaffected by nonparabolicity. The 1-3 transition turns out to be very weak in both geometries. Two band-gap engineering approaches to enhancing the parallel excitation of 1-2 are considered but the effect remains small as compared to the conventional z excitation. In z excitation the depolarization field condenses all the oscillator strength into essentially one sharp line despite the broadening expected from the nonparabolicity in the band dispersions. Inclusion of the depolarization field in the theory gives us good agreement with both the experimentally determined line shape and [z] matrix element.
Abstract: The consequences of a highly nonparabolic band structure on intersubband resonance are explored with experiments and theory on InAs/AlSb quantum wells. From a single-particle viewpoint, the intersubband resonance is expected to be broad because the intersubband spacing is highly k-dependent. Experimentally, however, we observe a relatively narrow line and this is interpreted as evidence that the intersubband resonance corresponds to a collective response of the high density electron system. Calculations taking into account resonant screening reproduce the experimental results reasonably well. Band nonparabolicity is shown from k . p theory to have no marked effects on the intersubband matrix elements both for conventional perpendicular excitation and for parallel excitation where the matrix elements, although non-zero, are small. These results are confirmed experimentally by tilting the sample with respect to the light beam. (C) 1996 Academic Press Limited
Abstract: We present cyclotron resonance measurements on electron-hole systems in the crossed gap system InAs/GaSb with additional confinement from AlSb barriers. A large, similar to 2.5 meV, splitting has been observed in the electron cyclotron resonance when holes are also present. We focus on the pronounced filling factor dependence of the effect to argue that we have a nonparabolicity-induced splitting, enhanced through an additional energy dependence of the g factor. This enhancement could be caused by a spin-orbit effect, with the holes apparently allowing the two transitions to be observed through a decoupling of the two cyclotron resonance transitions.
Abstract: The cyclotron resonance of high-mobility GaAs/AlxGa1-xAs heterojunctions displays both a temperature and Landau-level occupancy nu dependence. For occupancies below similar to 1/10, the behavior is identical to that found in high-purity bulk GaAs with spin splitting of the resonance. The position of the two peaks changes only slightly as the temperature is raised from 0.1 to 2 K. For 1/10 less than or equal to nu less than or equal to 1/6, the peak position and relative peak intensity of the two peaks shifts radically as the temperature is raised or the density varied. At integral and greater occupancies only a single cyclotron resonance peak is observed whose position changes very little with temperature. Finally, the fractional regime with occupancies between 1/6 and 1 shows only a single cyclotron resonance with a slight temperature dependence and provides no evidence that the many-body interactions responsible for the fractional quantum Hall effect influence the cyclotron resonance. The experimental cyclotron resonance behavior can be explained by a recently published theory by Cooper and Chalker which models the system in terms of the Coulomb interaction and the thermal population of both spin slates. A rigorous comparison between data taken over a wide range of densities and temperatures and the theoretical model establishes the validity of this explanation.
Abstract: Splittings of the n=1 heavy hole exciton spin states in zero and small magnetic fields for several type I quantum well systems are investigated with mu eV precision using cw photoluminescence techniques. Field-dependent spin relaxation rates are also obtained.
Abstract: We have performed magnetotransport measurements on InAs/(Ga,In)Sb heterostructures under hydrostatic pressure. The system has a cross gap band alignment which leads to the formation of co-existent 2D electron-hole gases. The amount of overlap and thus the electron and hole concentrations can be tuned by the application of hydrostatic pressure. The samples studied here have nearly equal electron and hole concentrations and show large oscillatory quantum Hall features. Measurements of the band overlap Delta and its rate of change with pressure d(Delta)/dP provides evidence that both the growth direction and also the composition of the interface layer play an important role in determining the band line-up.
Abstract: Cyclotron resonance measurements on a GaAs heterojunction in the extreme quantum limit are presented. It has been reported recently that a new splitting appears for filling factors less than 1/6. We have applied hydrostatic pressure in order to investigate the dependence on electron g-factor, since a pressure of similar to 8 kbar halves the g-factor, yet changes the effective mass by only similar to 5%. The critical filling factor, defined as the point where the two split resonances have equal intensity, is found to be strongly reduced with pressure. A general relationship is presented for the dependence of the critical filling factor on the Zeeman splitting.
Abstract: Semimetallic superlattices and double heterojunctions of the system InAs/InxGa1-xSb have been studied by cyclotron resonance and magnetotransport in the magnetic field range 10-100 T. The cyclotron resonance is used to show that there is a magnetic field induced transition from semimetallic to semiconducting behaviour, at which almost all the carriers in the system disappear. The magnetotransport shows that large oscillatory Hall voltages occur in the quantised regime, and these are strongly temperature dependent. The field positions of the Hall resistivity peaks are related to the crossing of the electron and hole Landau levels. Hydrostatic pressure is used to achieve a controlled decrease of the band overlap, and to show that the Hall peaks and minima move to zero field as the zero overlap condition is approached. Both the cyclotron resonance and the magnetotransport show that the magnitude of the band overlap is orientation dependent, being approximately 60 meV larger for (111) oriented structures. This is attributed to the presence of a large interface dipole for this orientation.
Abstract: The band overlap at the InAs/GaSb interface has been measured using electron and hole densities deduced from magnetotransport measurements, combined with self-consistent energy level calculations, for structures with both [001] and [111]A orientations. This band crossing is found to be 140 meV for the [001] orientation but increases to 200 meV for [111]A. The difference is attributed to the presence of an interface dipole for [111]A growth. In addition, the band overlap decreases, with applied hydrostatic pressure, at a higher rate for the [111]A orientation.
Abstract: We have measured optically-detected-cyclotron-resonance on In0.05Ga0.95As/GaAs superlattices with large magnetic fields applied both parallel (Faraday) and perpendicular (Voigt) to the growth direction. Cyclotron resonance in the Voigt geometry reveals a band structure in the growth direction. A semi-classical quantization of the Kronig-Penney dispersion gives good agreement with the data, providing that the cyclotron radius is larger than the superlattice period. We observe 1s-2p+ transitions from residual impurities which lie predominantly in the barrier regions. The impurity transition has a remarkable behaviour in the Voigt geometry, moving from the bulk 1s-2p+ field to close to the bulk free electron field as the barrier thickness increases, and is exactly mid-way between these two limits when the cyclotron radius equals the barrier thickness.
Abstract: We have measured optically detected cyclotron resonance using far-infrared radiation on an exceptionally pure sample of GaAs in fields up to 15.5 T. This relatively new experimental technique is shown to offer high resolution of free and donor impurity-bound electron transitions without the reproducibility problems of photoconductivity. The data confirm the existence of metastable donor states and provide a detailed picture of chemical shifts. The optically detected cyclotron resonance signal represents an interaction between the donor bound electron states which are influenced by the far-infrared radiation and the donor bound exciton states which are responsible for the photoluminescence. Attenuation of the luminescence intensity under far-infrared illumination is primarily the result of a photothermal effect. At high fields, there is indication of an interaction between the electron and excitonic energy levels.
Abstract: Magnetotransport measurements have been carried out under hydrostatic pressure on [001]- and [111]-oriented InAs/GaSb structures grown by metal-organic vapor-phase epitaxy. The system is semimetallic at zero pressure, but as the pressure is increased, electrons, located in the InAs layers, transfer back into the valence band of GaSb and a semimetal-to-semiconductor transition is observed. In double heterojunctions the zero band overlap condition occurs at 14 and 17 kbar for the [001] and [111]A orientations, respectively. The band crossing at the interface has been calculated self-consistently for the two orientations, taking into account the intrinsic and extrinsic origins of the carriers. This band overlap is found not only to be strongly orientation dependent, approximately 60 meV larger in [111]A compared with [001], but also to decrease at rates of 10 and 12 meV/kbar for the [001] and [111]A orientations, respectively. These rates are much larger than those determined from molecular beam epitaxy samples, but closer to the variation with pressure of the band gaps of the bulk materials.
Abstract: Interband magneto-optical studies have been performed on a series of strained single and coupled Ga1-xInxSb/GaSb quantum wells. We have investigated the dependence of interwell coupling on the well and barrier thicknesses and on the In content of the wells. Superlattice miniband formation is discussed where we observe a large anisotropy in carrier masses along and perpendicular to the, growth direction. A reduced in-plane mass of the highest valence band (\m(J)\ = 3/2, conventional heavy-hole band) is observed, dependent on the degree of the heavy-hole-light-hole (\m(J)\ = 1/2) decoupling. An extra Landau level transition appears between the conventional l = 0 and l = 1 Landau level transitions, increasing in intensity as the magnetic field perpendicular to the layer increases. This is interpreted as a result of mixing between the heavy-hole and light-hole states induced by the magnetic field. Interband spin splitting is observed in narrow Ga0.88In0.12Sb/GaSb single quantum wells but not wide wells. The difference arises from the valence-band spin splittings, which are very dependent on the heavy-hole-light-hole coupling, and therefore on the quantum-well width. These last two observations are explained well by Landau level calculations with the Luttinger k.p Hamiltonian.
Abstract: A helium dilution refrigerator has been modified to enable cyclotron resonance measurements to 100 mK on low-density, 2D electron systems in the ultra-quantum limit. Previous cyclotron resonance work to 300 mK indicates the presence of a phase boundary at a filling factor of nu(c) almost-equal-to 1/10, separating gas-like behavior at nu < nu(c), and a new mixed-spin liquid at nu greater-than-or-equal-to nu(c). The present experiment probes the ground state of these phases, as the temperature is lowered to the Wigner solid regime. We find that, even at 100 mK, the system is spin polarized only at low values of nu.
Abstract: Ultra-high magnetic field (> 150 T) cyclotron resonance has been performed on type II InAs/Ga(In)Sb superlattices. The electron density data reveals a dramatic depopulation at a critical magnetic field corresponding to the uncrossing of the lowest electron and hole Landau levels. The magnitude of this critical field decreases with decreasing superlattice period.
Abstract: A transition from semimetallic to semiconducting behavior induced by a magnetic field has been observed in type-II superlattices of InAs/Ga1-xInxSb by the use of very high magnetic fields, in excess of 100 T. The carrier densities and effective masses were measured by the study of cyclotron resonance using wavelengths in the region 10.6-3.39 mum. This was used to demonstrate that the zero-point energy associated with the lowest-electron and highest-hole Landau levels was sufficient to uncross the energy bands in long-period, semimetallic structures. For (100)-oriented structures this transition was found to occur in the region of 50-60 T, while for (111)A-oriented samples the uncrossing field was found to move up to the region of 100 T, due to an orientational dependence of the band offset. The effective masses, studied as a function of both photon energy and superlattice period, were found to be in good agreement with the predictions of k.p theory.
Abstract: We report a study of the quantum Hall effect and Shubnikov-de Haas oscillations in semimetallic type II heterostructures of the strained layer system InAs/Ga1-xInxSb which are almost intrinsic. In high magnetic fields up to 50 T, rho(xy) has large peaks, demonstrating the high degree of charge compensation in the system. Between these peaks, intrinsic quantum Hall minima are observed, where rho(xy) approaches zero.
Abstract: Magneto-optical experiments have been used to study a range of InGaAsP-based mutliple-quantum-well (MQW) structures containing biaxial strains, ranging from 1.6% tensile to 1.0% compressive. The observed excitonic transitions, involving both heavy and light holes, are studied in fields up to 15 T. Estimates of the hole effective masses are made, providing details of the valence-band nonparabolicities, and electronlike behavior is demonstrated for both heavy and light holes with different amounts of tensile strain. This is related to band crossings within the valence band and enables an estimate of 0.68+/-0.10 to be made of the heterojunction band offset in a strained In1-xGaxAs/InGaAsP MQW, with approximately 1.25% tensile strain in the well region. The experimental data are compared to the results of k.p Hamiltonian calculations of the in-plane valence-band dispersion.
Abstract: Cyclotron resonance has been employed to explore the effects of substrate orientation and carrier concentration on the valence band structure of strained InxGa1-xSb/GaSb quantum wells. The in-plane masses are much reduced by the strain decoupling of the heavy-hole and light-hole levels. We find that the in-plane mass is lower for [1 1 1] growth than for [0 0 1] growth, which we relate to the fact that gamma3 > gamma2 (Luttinger parameters). [1 1 1] samples were grown with high carrier concentrations (approximately 5 x 10(11) cm-2) by exploiting the modifications to the band profile induced by the piezoelectric field. For filling factors v < 3 we observe two cyclotron resonance lines, with masses differing by up to approximately 40%, corresponding to heavy-hole transitions with opposite spin, M(J) = +/-3/2. The results show that the first M(J) = +3/2 levels are more strongly coupled to the light-hole states than the first M(J) = -3/2 levels. This behaviour is verified by k . p theory calculations of the Landau levels.
Abstract: We report interband magneto-optics on a series of InxGa1-xAs/GaAs superlattices in pulsed magnetic fields up to 45 T. The experiments demonstrate that the heavy-hole spin splitting is strongly dependent on the degree of interwell coupling in the valence band. Calculations of the Landau levels in these structures give very good agreement with the experimental data and show that the heavy-hole spin splitting passes through zero at low fields for a weakly coupled superlattice, but that this moves up to approximately 40 T when strong interwell coupling occurs.
Abstract: Magneto-photoconductivity has been studied on a series of strained Ga1-xInxSb/GaSb single, double and multiple quantum wells. Coupling between the wells is investigated as a function of well width, barrier width and indium concentration. Identification of transitions, in particular that from the valence band continuum to E1, gives an accurate experimental band offset ratio of DELTA E(c) = 0.50 +/- 0.03DELTA E(g) for this system. Application of a magnetic field both perpendicular and parallel to the superlattice layer planes shows the formation of a miniband in the superlattice samples, with large anisotropy in masses and spin-splitting. Interband spin-splitting in the parallel magnetic field orientation comes only from the electrons and is due to two-dimensional spin quantization of the holes, allowing the electron g-factor to be deduced as g* = -10.4 +/- 0.6.
Abstract: Cyclotron resonance on low density, 2D electron systems in the ultra-quantum limit show split and shifted resonances. This demonstrates the presence in the ground state of both spin orientation electrons, whose relative proportion is strongly temperature and occupancy dependent. A critical occupancy of approximately 1/10 divides single-particle behavior in a gaseous phase from the quantum liquid where the resonance positions are temperature and carrier density dependent. The resonances suggest a mixed spin state for some regions of the liquid phase.
Abstract: We have detected cyclotron resonance in a series of undoped GaAs quantum wells by modulating the photoluminescence intensity with far-infrared radiation. The conduction band mass was measured for different quantum well widths, and good agreement with a simple formula based on k . p theory is achieved. An offset was observed in the cyclotron resonance energy, strongly dependent on well width. The interpretation is that monolayer width fluctuations localize the carriers, giving an additional binding energy to the cyclotron resonance transition.
Abstract: Ultra-high magnetic field (<150 T) cyclotron resonance has been studied in superlattices of InAs/GaSb with band gaps close to zero where the effects of conduction and valence bands are enhanced. Several samples exhibit a low-field structure associated with the N = 2 and 4 harmonics of the cyclotron resonance. The interband transition data is also shown.
Abstract: A variety of optical and electrical studies are described for superlattices and heterostructures based on the materials system InAs/GaSb. The crossed-band-gap alignment of this system leads to a semimetal to semiconductor transition as a function of either superlattice period, magnetic field or pressure. Cyclotron resonance is studied for both electrons and holes, and the electron resonance is observed in the magnetic field range where the field induced band crossing occurs. Studies of the pressure dependence of the band offset show that both (1 1 1)A and (1 0 0) oriented structures have a pressure coefficient of 10.7 meV/kbar, but the band crossing at zero pressure is larger for the (1 1 1)A case. Compensated quantum Hall plateaux are observed at high magnetic fields and low temperatures, and large oscillatory features are observed in the Hall voltage under a range of conditions. In very high fields we have observed the zero-resistance Hall plateaux occurring due to total compensation of the electron and hole states.
Abstract: Magnetotransport measurements of MOVPE-grown GaSb/InAs heterostructures in magnetic fields up to 50T have demonstrated novel transport effects for a system with coexisting electrons and holes. The Hall resistivity, rho(xy), has large features and strong minima, while the longitudinal magnetoresistance, rho(xx), shows plateau-like features. This is in sharp contrast with the behaviour of two-dimensional systems containing only one carrier type, where rho(xx) oscillates and rho(xy) shows quantised plateaus. The samples measured are close to the intrinsic limit (n(e)/n(h) congruent-to 1.5), and compensated quantum Hall plateaus are seen, with rho(xy) approaching zero for fields at which there is the same integral number of filled Landau levels of electrons as of holes.
Abstract: Cyclotron resonance ed high magnetic field with radiation in the far infrared has been used to measure the pressure coefficient of the effective mass, dm*/dp, of a GaAs heterojunction at pressures up to 8 kbar. Pressure is known to reduce the carrier concentration n(s). In this experiment, persistent photoconductivity was used to vary n(s) for each pressure so that dm*/dp was measured for fixed n(s). Furthermore, the energy dependence of the effective mass was determined for each n(s) and p. We find that, within our experimental error of approximately 5%, dm*/dp for the heterojunction is equal to dm*/dp for bulk GaAs, independent of n(s). This is understood from a simple k . p model. Anomalously n(s)-dependent pressure coefficients were observed for some cyclotron energies, which we relate to anomalous cyclotron mass values, perturbed from their true values by a number of effects that complicate cyclotron resonance in two dimensions.
Abstract: We have detected cyclotron resonance in a series of undoped GaAs quantum wells by modulating the photoluminescence intensity with far-infrared radiation. The conduction-band mass was measured for different quantum-well widths, and good agreement with a simple formula based on k . p theory is achieved. An offset was observed in the cyclotron-resonance energy, strongly dependent on well width. The interpretation is that monolayer-width fluctuations localize the carriers, giving an additional binding energy to the cyclotron-resonance transition.
Abstract: Cyclotron resonance measurements are reported for both electrons and holes in type II InAs/GaSb superlattices and double heterostructures (DHETS). Superlattice (multi quantum well) samples of InAs/GaSb grown by MOVPE have sufficiently high hole gas mobilities and densities to allow the first measurements of hole cyclotron resonance in this system. The measured hole masses are approximately 0.1m(e) and 0.2 m(e), indicating a large reduction over the bulk values due to the decoupling of the valence band by strain and confinement. This is in good agreement with eight-band k . p theory reported here, and previous calculations. The electron masses in both superlattices and DHETs are found to be strongly influenced by non-parabolicity and the carrier concentration, leading to considerable increases over the band edge values. Similar values are found for both electron and hole masses in structures grown along both [001] and [111]A directions.
Abstract: Calculations are presented of the valence band structure of strongly coupled semiconductor superlattices. An unusual and unexpected spin behaviour is observed when a large magnetic field is applied along the superlattice direction. The superlattice dispersion of the +3/2 state inverts at high field whereas the dispersion of the -3/2 state has a conventional Kronig-Penney dispersion. This means that the effective g-factor changes sign at the zone edge (PI) compared to the zone centre (GAMMA). The effect can be so large as to move the band edge from GAMMA to PI at high field. The effect is shown to be a subtle consequence of heavy hole-light hole mixing. Some implications of this new spin behaviour are considered.
Abstract: Magneto-optical studies have been conducted on two InxGa1-xAs/GaAs superlattices to investigate excitonic transitions at the saddle points formed at q(z) = pi/d of the superlattice miniband structure. We report on a direct observation of both the E1(PI)-HH1(PI) saddle-point exciton and the indirect E1(GAMMA)-HH1(PI) excitonic resonance in both perpendicular and parallel magnetic fields, with both features showing perpendicular field-induced (GAMMA-PI) mixing in the separate carrier minibands. Theoretical calculations demonstrate highly field- and wave-vector-dependent valence-band g factors, which, because of the small valence-band widths, lead to the inversion of the m(j) = + 3/2 E-q(z) dispersion curves at high perpendicular fields. The unusual behavior of the light-hole transitions indicates a large degree of in-plane mixing of the heavy- and light-hole states, which would appear to be closely linked to the observed wave-vector mixing and complex g factors.
Abstract: We report photoluminescence measurements made on highly strained nominally GaAs/GaSb/GaAs heterostructures (grown by MOVPE) at pressures up to 5.5 GPa in a diamond anvil cell. Two samples have been studied, a single 15 angstrom GaSb layer, and a five 15 angstrom GaSb layer heterostructure. In both cases the 1.3 eV strained layer luminescence increased in energy with pressure with a large pressure coefficient. This identifies the transition as one involving a GAMMA state in the conduction band. It is suggested that lower energy luminescence, observed only from the five-layer sample, involves a defect. A theoretical model predicts that the bands line up in a type-II configuration, and the observed behaviour at pressures beyond the GaAs GAMMA-X crossover confirms this. The possibility of incorporation of As atoms in the thin nominally GaSb layer is considered.
Abstract: We report a high-pressure study of metal-organic-vapor-phase-epitaxially-grown bulk GaSb and Ga(1-x)In(x)Sb/GaSb quantum wells. For photoluminescence experiments we have used a diamond-anvil cell with pressures up to 75 kbar, using the photoluminescence from bulk InP as a pressure gauge. For bulk GaSb we have measured the GAMMA-gap pressure coefficient as 1.64 +/- 0.03 times larger than that of InP. No bulk GaSb luminescence could be observed above the L(c)-GAMMA-c crossover. The GAMMA-gap of the quantum well is found to have the same pressure coefficient as the bulk GaSb to within our approximately 3% experimental error. This is understood essentially as a consequence of the material and electronic similarity of GaSb and Ga(1-x)In(x)Sb (x approximately 15%). At higher pressure both the quantum well L(c) and X(c) conduction-band states are unambiguously observed. There is an L(c)-GAMMA-c crossover at 16 kbar, and then an X(c)-L(c) crossover at 43 kbar. These effects are discussed in relation to the hydrostatic- and shear-strain perturbations to the band structure. The quantum-well luminescence quenched at about approximately 70 kbar, and this is interpreted as a phase change of the GaSb.
Abstract: p-type 90-angstrom In0.18Ga0.82As/GaAs quantum wells with carrier concentrations in the range p(s) = (1.5-4.3) X 1O(11) cm-2 have been studied by magneto-optics. Cyclotron resonance measures the effective mass of the (M(J)) = 3/2 holes as congruent-to 0.16 for in-plane motion. The mass is light because of the strain decoupling of the "heavy-hole" (M(J)) = 3/2 and "light-hole" (M(J)) = 1/2 states. The effective mass has been measured as a function of carrier concentration and field. The totally decoupled limit is not achieved and the residual coupling between the (M(J)) = 3/2 and 1/2 states is well described by a calculation of the Landau levels using an eight-band k.p model. Filling-factor-related anomalies in the cyclotron resonance are observed and interpreted in terms of hole-hole interactions combined with the presence of localizing potentials. Interband photoconductivity measurements determine the conduction-band structure and good agreement with the calculations is achieved. The magnetoexciton binding energy and band filling are considered in the analysis of the interband data.
Abstract: Magneto-transport at low temperature has been employed to study a series of p-type in0.18Ga0.82As/GaAs quantum wells with carrier concentrations in the range (1.5-4.3) x 10(11) cm-2. Tilting the field from the growth direction does not change the relative magnitudes of the spin-split conductivity minima. This is a remarkable effect, and confirms the recent results of Martin et al on an in0.15Ga0.85Sb/GaSb quantum well. Strain decoupling of the \M(J)\ = 3/2 and \M(J)\ = 1/2 states projects the spin onto the growth direction. We find here that this effect occurs for the nu = 3, 4 and nu = 5, 6 and nu = 7, 8 spin-split conductivity minima. The Shubnikov-de Haas oscillations in perpendicular field have been compared to the Landau levels calculated with an eight-band kappa.p model and this determines an upper limit to the value of kappa-l, the spin-magnetic-field coupling parameter.
Abstract: The miniband structure in strained In(x)Ga1-xAs-GaAs superlattices is studied using interband transmission and reflectivity in magnetic fields up to 16 T. Measured in-plane effective masses are in good agreement with an eight-band k.p envelope-function calculation. Significant coupling through barrier layers 100 angstrom thick is demonstrated by the presence of a finite miniband dispersion and a reduction in the exciton binding energy for samples with thin well layers. The superlattice Landau levels are observed to saturate in high magnetic fields near the top of the minibands, and this behavior together with the superlattice masses are fitted semiclassically suing the envelope-function model. Landau levels from the n = 2 exciton are analyzed in an uncoupled sample, and, as this state evolves into an unbound miniband, it is predicted to show very anisotropic quasi-one-dimensional behavior.
