Arran Curran

Abstract: The viscosity of a fluid can be measured by tracking the motion of a suspended micron-sized particle trapped by optical tweezers. However, when the particle density is high, additional particles entering the trap compromise the tracking procedure and degrade the accuracy of the measurement. In this work we introduce an additional Laguerre-Gaussian, i.e. annular, beam surrounding the trap, acting as an optical shield to exclude contaminating particles. (C) 2012 Optical Society of America

Abstract: Holographic optical tweezers are used to construct a static bistable optical potential energy landscape where a Brownian particle experiences restoring forces from two nearby optical traps and undergoes thermally activated transitions between the two energy minima. Hydrodynamic coupling between two such systems results in their partial synchronization. This is interpreted as an emergence of higher mobility pathways, along which it is easier to overcome barriers to structural rearrangement.

Abstract: We present a generic microcavity platform for cavity experiments on optically active nanostructures, such as quantum dots, nanocrystals, color centers, and carbon nanotubes. The cavity is of the Fabry-Perot type with a planar back mirror and a miniature concave top mirror with radius of curvature similar to 100 mu m. Optical access is achieved by free beam coupling, allowing good mode-matching to the cavity mode. The cavity has a high Q-factor, reasonably small mode volume, open access, spatial and spectral tunability, and operates at cryogenic temperatures. Spectral and spatial tuning of the Purcell effect (weak coupling regime) on a single InGaAs quantum dot is demonstrated. (C) 2011 American Institute of Physics. [doi:10.1063/1.3632057]

Abstract: The hydrodynamic interactions of micro-silica spheres trapped in a variety of networks using holographic optical tweezers are measured and characterized in terms of their predicted eigenmodes. The characteristic eigenmodes of the networks are distinguishable within 20-40 seconds of acquisition time. Three different multi-particle networks are considered; an eight-particle linear chain, a nine-particle square grid and, finally, an eight-particle ring. The eigenmodes and their decay rates are shown to behave as predicted by the Oseen tensor and the Langevin equation, respectively. Finally, we demonstrate the potential of using our micro-ring as a non-invasive sensor to the local environmental viscosity, by showing the distortion of the eigenmode spectrum due to the proximity of a planar boundary. The eight particle ring that may lend itself to a micro-listening device. Superimposed are the eigenvectors of one of the hydrodynamic modes.

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: 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 hydrodynamic interactions of micro-silica spheres trapped in a variety of networks using holographic optical tweezers are measured and characterized in terms of their predicted eigenmodes. The characteristic eigenmodes of the networks are distinguishable within 20-40 seconds of acquisition time. Three different multi-particle networks are considered; an eight-particle linear chain, a nine-particle square grid and, finally, an eight-particle ring. The eigenmodes and their decay rates are shown to behave as predicted by the Oseen tensor and the Langevin equation, respectively. Finally, we demonstrate the potential of using our micro-ring as a non-invasive sensor to the local environmental viscosity, by showing the distortion of the eigenmode spectrum due to the proximity of a planar boundary. The eight particle ring that may lend itself to a micro-listening device. Superimposed are the eigenvectors of one of the hydrodynamic modes.

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: The hydrodynamic interactions of micro-silica spheres trapped in a variety of networks using holographic optical tweezers are measured and characterized in terms of their predicted eigenmodes. The characteristic eigenmodes of the networks are distinguishable within 20-40 seconds of acquisition time. Three different multi-particle networks are considered; an eight-particle linear chain, a nine-particle square grid and, finally, an eight-particle ring. The eigenmodes and their decay rates are shown to behave as predicted by the Oseen tensor and the Langevin equation, respectively. Finally, we demonstrate the potential of using our micro-ring as a non-invasive sensor to the local environmental viscosity, by showing the distortion of the eigenmode spectrum due to the proximity of a planar boundary.

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: 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: 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: The wide bandgap alloy Zn(0.2)Mg(0.8)S(0.64)Se(0.36) 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 Zn(0.2)Mg(0.8)S(0.64)Se(0.36) 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/Zn(0.2)Mg(0.8)S(0.64)Se(0.36)/ZnSe multilayers show the Zn(0.2)Mg(0.8)S(0.64)Se(0.36) layers are of good crystal quality and do not phase segregate. Layer interfaces are seen to be flat and Zn(0.2)Mg(0.8)S(0.64)Se(0.36) does not introduce defects into the overlying ZnSe. Atomic force microscopy shows the surface of a 30 nm Zn(0.2)Mg(0.8)S(0.64)Se(0.36) layer is atomically flat, in contrast with similar MgS layers, which show pronounced I D surface ridges, indicating that the Zn(0.2)Mg(0.8)S(0.64)Se(0.36) layers have not started to relax. ZnSe quantum wells grown with Zn(0.2)Mg(0.8)S(0.64)Se(0.36) 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(.2)Mg(.8)S(.64)Se(.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(.2)Mg(.8)S(.64)Se(.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: 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: 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: 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: 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 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: 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: 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: 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 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: 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: 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 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: 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: 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. &COPY; 2005 Elsevier B.V. All rights reserved.

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: We present experimental evidence of hydrodynamic assisted escape from a potential well. Holographic optical tweezers are used to landscape a bistable system composed of two optical traps, separated by 400nm as seen by a Si colloid of radius 400nm. We observe thermally activated transitions between the two metastable states in the system with transition rates that are in agreement with Kramers theory. Introducing a second bistable system into our experiment allows us to study the behaviour of thermally activated transitions in the presence of hydrodynamic interactions. The two bistable systems are placed in a line separated by a few micrometers. Using camera tracking technologies we track each of the two beads as they hop back and forth within their respective system. The escape events are recorded and any correlation between the two systems are then computed. We consistently find that the number of observed correlations are as expected and that the number of correlations having a positive coefficient are greater than the number of correlations having a negative coefficient. The hydrodynamic interactions assist in the escape from a metastable potential. Our results are particularly relevant in the context of concentrated colloidal suspensions where hydrodynamic interactions could lead to the formation of higher mobility paths along which it is easier to overcome barriers to structural rearrangement.

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/Zn(0.9)Cd(0.1)Se quantum wells transferred to SiO(2)/Ta(2)O(5) 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