Abstract: We experimentally demonstrate the first method to focus light inside disordered photonic metamaterials. In such materials, scattering prevents light from forming a geometric focus. Instead of geometric optics, we used multi-path interference to make the scattering process itself concentrate light on a fluorescent nanoscale probe at the target position. Our method uses the fact that the disorder in a solid material is fixed in time. Therefore, even disordered light scattering is deterministic. Measurements of the probes fluorescence provided the information needed to construct a specific linear combination of hundreds of incident waves, which interfere constructively at the probe. (c) 2008 Optical Society of America.
Abstract: We have studied the effect of the boundaries on the local density of states (LDOS) in one-, two-, and three-dimensional finite size photonic structures. The LDOS was calculated with the help of the local perturbation method (LPM) and a new LPM-Bloch method using periodicity of the system. The methods are applicable for the clusters made of small (relative to the incident wavelength) particles or for the clusters which can be considered as made of such particles. It was demonstrated that the LPM- Bloch method is an accurate numerical tool for calculation of the LDOS in the finite size photonic structures with weak interference. (C) 2008 Optical Society of America.
Abstract: We report on a new experimental method for enhanced backscattering spectroscopy (EBS) of strongly scattering media over a bandwidth from 530-1000 nm. The instrument consists of a supercontinuum light source and an angle-dependent detection system using a fiber-coupled grating spectrometer. Using a combination of two setups, the backscattered intensity is obtained over a large angular range and using circularly polarized light. We present broadband EBS of a TiO2 powder and of a strongly scattering porous GaP layer. In combination with theoretical model fits, the EBS system yields the optical transport mean free path over the available spectral window. (c) 2008 Optical Society of America.
Abstract: We present time-resolved measurements of light transport through strongly scattering macroporous gallium phosphide at various vacuum wavelengths between 705 nm and 855 nm. Within this range the transport mean free path is strongly wavelength dependent, whereas the observed energy velocity is shown to be independent of the wavelength. We conclude that microscopic resonances, which can strongly slow down the diffusion process in, e.g., granular TiO2, are absent in macroporous gallium phosphide in the wavelength region of concern.
Abstract: We demonstrate a method for fully characterizing diffuse transport of light in a statistically anisotropic opaque material. Our technique provides a simple means of determining all parameters governing anisotropic diffusion. Anisotropy in the diffusion constant, the mean free path, and the extrapolation length are, for the first time, determined independently. These results show that the anisotropic diffusion model is effective for modeling transport in anisotropic samples, providing that the light is allowed to travel several times the transport mean free path from the source. (C) 2008 Optical Society of America.
Abstract: A theory is presented which incorporates the effect of dielectric anisotropy in random multiple scattering media. It predicts anisotropic diffusion, and a deflection of the diffuse energy flow in anisotropic slabs in the direction parallel to the slab. The transmittance integrated over all incoming and outgoing directions scales with the transport mean free path along the surface normal. The escape function in anisotropic dielectrics is no longer bell shaped. In this model anisotropy facilitates Anderson localization.
Abstract: We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.
Abstract: Using a density matrix approach, we study the simplest systems that display both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The other atoms supply feedback through multiple scattering of light. We show that, if the atoms are in each other's near field, the system exhibits large gain narrowing and spectral mode redistribution. The observed phenomena are more pronounced if the feedback is enhanced. Our system is to our knowledge the simplest exactly solvable microscopic system which shows the approach to laser oscillation.
Abstract: We prescribe the minimal set of experimental data and parameters that should be reported for random-laser experiments and models. This prescript allows for a quantitative comparison between different experiments, and for a criterion whether a model predicts the outcome of an experiment correctly. In none of more than 150 papers on random lasers that we found these requirements were fulfilled. We have nevertheless been able to analyze a number of published experimental results and recent experiments of our own. Using our method we determined that the most intriguing property of the random laser (spikes) is in fact remarkably similar for different random lasers. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: The local density of states is studied with the help of the local perturbation method for vector and scalar wave fields propagating in finite size photonic structures. The local density of states is numerically calculated for one-, two-, and three-dimensional finite size photonic structures considering them as made of small particles. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: We show that time-independent scattering coefficients calculated from the standard extrapolation of Mie theory to the gain regime have physical meaning up to the laser threshold. The theoretical width of a resonance decreases linearly with increasing gain and becomes zero at the laser threshold. We performed experiments on dielectric microspheres with gain, trapped with optical tweezers. The width of the mode was measured to narrow as a function of the gain up to the lasing threshold, confirming both the validity of the extrapolation of Mie theory to the gain regime below threshold and our interpretation of its point of divergence as the laser threshold. (c) 2006 Optical Society of America
Abstract: We present a quantitative experimental and theoretical study of intensity fluctuations in the emitted light of a random laser that has different realizations of disorder for every pump pulse. A model that clarifies these intrinsic fluctuations is developed. We describe the output versus input power graphs of the random laser with an effective spontaneous emission factor (beta factor).
Abstract: We present an extension of the T-matrix approach to scattering of light by a three-level system, using a description based on a Master equation. More particularly, we apply our formalism to calculate the T matrix of a pumped three-level atom, providing an exact and analytical expression describing the influence of a pump on the light-scattering properties of an atomic three-level system.
Abstract: We present an experimental study of the propagation of quantum noise in a multiple scattering random medium. Both static and dynamic scattering measurements are performed: the total transmission of noise is related to the mean free path for scattering, while the noise frequency correlation function determines the diffusion constant. The quantum noise observables are found to scale markedly differently with scattering parameters compared to classical noise observables. The measurements are explained with a full quantum model of multiple scattering.
Abstract: We predict a new spatial quantum correlation in light propagating through a multiple scattering random medium. The correlation depends on the quantum state of the light illuminating the medium, is infinite in range, and dominates over classical mesoscopic intensity correlations. The spatial quantum correlation is revealed in the quantum fluctuations of the total transmission or reflection through the sample and should be readily observable experimentally.
Abstract: Unavoidable variations in size and position of the building blocks of photonic crystals cause light scattering and extinction of coherent beams. We present a model for both two- and three-dimensional photonic crystals that relates the extinction length to the magnitude of the variations. The predicted lengths agree well with our experiments on high-quality opals and inverse opals, and with literature data analyzed by us. As a result, control over photons is limited to distances up to 50 lattice parameters (similar to 15 mu m) in state-of-the-art structures, thereby impeding applications that require large photonic crystals, such as proposed optical integrated circuits. Conversely, scattering in photonic crystals may lead to different physics such as Anderson localization and nonclassical diffusion.
Abstract: The dynamics of a collection of resonant atoms embedded inside an inhomogeneous nondispersive and lossless dielectric is described with a dipole Hamiltonian that is based on a canonical quantization theory. The dielectric is described macroscopically by a position-dependent dielectric function and the atoms as microscopic harmonic oscillators. We identify and discuss the role of several types of Green tensors that describe the spatio-temporal propagation of field operators. After integrating out the atomic degrees of freedom, a multiple-scattering formalism emerges in which an exact Lippmann-Schwinger equation for the electric field operator plays a central role. The equation describes atoms as point sources and point scatterers for light. First, single-atom properties are calculated such as position-dependent spontaneous-emission rates as well as differential cross sections for elastic scattering and for resonance fluorescence. Secondly, multiatom processes are studied. It is shown that the medium modifies both the resonant and the static parts of the dipole-dipole interactions. These interatomic interactions may cause the atoms to scatter and emit light cooperatively. Unlike in free space, differences in position-dependent emission rates and radiative line shifts influence cooperative decay in the dielectric. As a generic example, it is shown that near a partially reflecting plane there is a sharp transition from two-atom superradiance to single-atom emission as the atomic positions are varied.
Abstract: The propagation of light in nonperiodic quasicrystals is studied by ultrashort pulse interferometry. Samples consist of multilayer dielectric structures of the Fibonacci type and are realized from porous silicon. We observe mode beating and strong pulse stretching in the light transport through these systems, and a strongly suppressed group velocity for frequencies close to a Fibonacci band gap. A theoretical description based on transfer matrix theory allows us to interpret the results in terms of Fibonacci band-edge resonances.
Abstract: Measurements of the angular-resolved-optical transmission through strongly scattering samples of porous gallium phosphide are described. Currently porous GaP is the strongest-scattering material for visible light. From these measurements the effective refractive index and the average reflectivity at the sample interface can be obtained. These parameters are of great importance for an accurate interpretation of optical experiments, and are for the first time determined in strongly scattering samples. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We discuss the optical properties of one-dimensional complex dielectric systems, in particular the time-resolved transmission through thick porous silicon quasiperiodic multi-layers. Both in numerical calculations and experiments we find dramatic distortion effects, i.e. pulse stretching and coherent beatings, when band-edge states are resonantly excited. Numerical simulations and experiments are in good agreement. We argue that porous silicon can be used conveniently for the fabrication of one-dimensional complex photonic structures.
Abstract: Macroscopic field quantization is presented for a nondispersive photonic dielectric environment, both in the absence and presence of guest atoms. Starting with a minimal-coupling Lagrangian, a careful look at functional derivatives shows how to obtain Maxwell's equations before and after choosing a suitable gauge. A Hamiltonian is derived with a multipolar interaction between the guest atoms and the electromagnetic field. Canonical variables and fields are determined and in particular, the field canonically conjugate to the vector potential is identified by functional differentiation as minus the full displacement field. An important result is that inside the dielectric a dipole couples to a field that is neither the (transverse) electric nor the macroscopic displacement field. The dielectric function is different from the bulk dielectric function at the position of the dipole, so that local-field effects must be taken into account.
Abstract: Two-dimensional near-field images of light transmitted through a disordered dielectric structure have been measured for two probe wavelengths. From these data, the 2D spatial dependence of the intensity correlation function, C(Delta(R) over right arrow), has been extracted. We observe that the spatial dependence of C is dominated by a rapidly varying feature determined by the wavelength of the probe light and the average refractive index of the material, as expected by theory. Finally, we deduce the absolute value of the effective refractive index by fitting the theoretical spatial dependence of C to our experimental results.
Abstract: We present the first experiments that demonstrate strong angle-independent modification of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. We show that both the fluorescence quantum efficiency and weak disorder play a key role in interpreting the experimental data. We compare the angle-independent emission spectra of dye in photonic crystals with spectra from such crystals with much smaller lattice spacings, for which emission is in the long wavelength limit. The ratio of emission power spectra shows inhibition of emission up to a factor similar to5 over a large bandwidth of 13% of the first order Bragg resonance frequency. The inhibition shifts to increasing wavelength with the lattice parameter, confirming the photonic nature of the phenomenon. The center frequency and bandwidth of the inhibition agree with the calculated total density of states, but the measured inhibition of the vacuum fluctuations is much larger. This result is confirmed by experiments using different dyes. We likely probe the strongly modulated local photonic density of states, due to the spatially nonuniform distribution of dye molecules over the unit cell.
Abstract: We observed for the first time a strong angle-independent modi cation of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. Comparison with spectra from such crystals with much smaller lattice spacing, for which emission is in the long wavelength limit, reveals inhibition of emission up to a factor similar to5 over a large bandwidth of 13% of the first order Bragg resonance frequency. The center frequency and bandwidth of the inhibition agree with calculated total density of states, while the measured inhibition of vacuum fluctuations is much larger. Because of the specific location of the dye molecules, we likely probe the strongly modulated local photonic density of states.
Abstract: Light transport in a strongly scattering, strongly anisotropic material is studied experimentally using both static and time-resolved techniques. Both the static and the dynamic results are well characterized by a diffusion equation with an anisotropic diffusion tensor and a scalar absorption term. Light diffuses 4.0 times faster along the uniaxial axis of the material compared with diffusion in the orthogonal directions.
Abstract: We have performed enhanced backscattering experiments in a high gain random laser, under circumstances where a stationary theory predicts the laser intensity to diverge. Above the laser threshold the observed backscatter cone changes only gradually, not showing any signs of the divergence. We present measurements and theory-generalized laser equations with a diffusive transport term for pump and laser light-to explain the observed behavior. The population dynamics prevent an indefinite growth of the intensity. Time dependence is essential for a theory of random lasers above the laser threshold.
Abstract: We have studied enhanced backscattering of both polystyrene opals and strongly photonic crystals of air spheres in TiO2 in the wavelength range of first and higher order stop bands. The shape of the enhanced backscattering cones is well described by diffusion theory. We find transport mean free paths of the order of 15 mu m both for opals and air spheres. Close to the stop band the cone width is decreased due to internal reflections generated by the photonic band structure. Widening occurs due to attenuation of the coherent beam by Bragg scattering. We present a model that incorporates these effects and successfully explains the data. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: A theory is presented for propagation of waves in bounded media near the mobility edge, based on the self-consistent theory for localization. It predicts a spatially inhomogeneous diffusion constant that leads to scale dependence in enhanced backscattering and transmission.
Abstract: The local-field corrections to the spontaneous emission rate of an impurity atom in an otherwise homogeneous dielectric are calculated by making use of a macroscopic approach. An extension of a method introduced by Onsager and Bottcher is presented. By distinguishing between the substitutional and interstitial character of impurities, the well-known empty-cavity and Lorentz local-field factors are obtained, respectively. For fluids, the substitutional case is predicted to occur prevalently. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We present measurements on the luminescence of Eu(fod)(3) dissolved in a series of (deuterated) alcohols and mixtures hereof, to test local-field corrections on spontaneous emission. The alcohols used are CH3(CH2)(m)OH, with m ranging from 0 for methanol to 9 for 1-decanol. Accordingly, the refractive index varies from 1.33 to 1.44. To test the influence of the intrinsic molecular vibrations of the alcohols on the luminescence properties, also deuterated alcohols are used: CH3(CH2)(m)OD, with m = 0,1,3. Although the chemically homologic series of alcohols was carefully selected, small variations in the local environment of Eu(fod)(3) complex upon changing the host (alcohol), blur most of the local-field effect on spontaneous emission. (C) 2000 American Institute of Physics. [S0021-9606(00)03332-8].
Abstract: We show that ominidirectional reflection is not a sufficient signature of a photonic bandgap. Although dramatic angular redistribution takes place, the mode density of the electromagnetic field is hardly altered within the ominidirectional reflection range but rather has characteristics typical of a waveguide. The strikingly large polarization anisotropy is due to the huge dielectric contrast but not to a photonic bandgap. (C) 2000 Optical Society of America OCIS codes: 230.1480, 230.4170, 230.7390.
Abstract: We have measured the optical fluorescence spectra of dye incorporated in high-quality photonic crystals made from colloids. The spectra reveal a stopgap that is due to Bragg reflection with strikingly reduced attenuation compared with plane-wave transmission. The modified attenuation is independent of the position of the sources in the sample and is brought about by diffuse scattering from defects near the surface. In the presence of a photonic bandgap, the diffuse component would disappear. Thus we have found a simple, unambiguous probe for the presence of photonic bandgaps. (C) 1999 Optical Society of America S0740-3224(99)02909-4].
Abstract: We investigate the influence of the excitation spot diameter on the laser threshold of a scattering amplifying medium. Fluorescence spectra are recorded from a suspension of TiO2 scatterers in Sulforhodamine B dye. The threshold pump intensity becomes larger by a factor of 70 if the excitation beam diameter gets close to the mean free path l. This increase is explained by use of a simple model describing diffusion out of the amplifying volume and is confirmed by a Monte Carlo simulation. (C) 1999 Optical Society of America.
Abstract: We have measured spectrally resolved fluorescence lifetimes of dye incorporated in high-quality photonic crystals, made of colloidal spheres. The emission spectrum shows a pronounced Bragg notch. In contrast, the fluorescence lifetime does not depend on the interaction between light and the photonic crystal. The results are explained with a simple model of an atom in a cavity. The effects of homogeneous and inhomogeneous broadening of the emission spectrum of dye inside photonic crystals are discussed.
Abstract: We studied enhanced backscattering of light from anodically and photoanodically etched, macroporous GaP networks. The most strongly scattering material for visible light reported to date, photoanodically etched GaP, features anomalous rounding of the top of the backscatter cone. The phenomenon cannot be attributed to finite sample size or absorption and is most likely the onset of Anderson localization.
Abstract: We have performed optical reflectivity experiments and band structure diffraction calculations on fee photonic crystals, consisting of air spheres in titania (TiO2). The spectra reveal Bragg peaks whose large widths agree well with the calculations. The peaks overlap in many directions, causing the propagation of light to be limited by Bragg reflection to 45% of the 4 pi solid angler This illustrates that the air-sphere crystals interact so strongly with light that an unprecedented limitation in the directions of propagation of light in photonic crystals is achieved.
Abstract: We have used phase-sensitive ultrashort-pulse interferometry to study the modification of optical pulse propagation near the photonic band edges in colloidal crystals consisting of polystyrene spheres in water. A strong suppression of the group velocity is found at frequencies near the L gap of the fee lattice. The roup velocity dispersion diverges at the band edges and shows branches of both normal dispersion and anomalous dispersion, which can be interpreted as large changes in the effective mass, both positive and negative. We obtain excellent agreement with the dynamical diffraction theory.
Abstract: We present near infrared total transmission measurements through samples of randomly packed silicon powders. At different wavelengths we analyze in detail the scattering properties and the effects of residual absorption. The loa est value of kl(s), where k is the wave vector and l(s) is the scattering mean free path is 3.2. We also observe that kl(s) is nearly constant over a wide wavelength range. This phenomenon is associated with the high polydispersity of the particles. We use the energy density coherent potenial approximation to explain our measurements.
Abstract: We present results of experiments on the spontaneous emission rate of a europium complex in dense supercritical CO2. The refractive index of the supercritical gas is varied from 1.00 to 1.27 by increasing the pressure up to a 1000 bars. Accordingly, the spontaneous emission rate changes. Local-field effects on spontaneous emission are clearly observed in these experiments. The empty-cavity model for the spontaneous emission rate inside dielectrics is confirmed.
Abstract: The authors present a closed formulation of resonant point scatterers for classical-wave propagation problems. A Green's-function approach is employed in which all the small-distance singularities are regularized. Application of point scatterers considerably simplifies multiple-scattering calculations needed, for instance, for understanding the optical properties of dense cold gases and optical lattices. In the case of the vector description of light, it is shown that two different regularization parameters are required in order to obtain physically meaningful results. One parameter is related to the physical size of the pointlike scattering particle, while the other is connected to its dynamic properties. Al parameters involved are defined in terms of physical observables leading to a complete and self-consistent treatment. The applicability of the point-scatterer model to several physical models is demonstrated. We calculate the local density of states of waves in the presence of one resonant point scatterer. For the vector case, the bare polarizability is shown to enter the local density of states. For a collection of resonant point dipoles, the Lorentz-Lorenz relation for the dielectric constant is derived for cubic lattices and for disordered arrangements.
Abstract: Resonant classical light scattering by impurity atoms inside dielectric cubic lattices is investigated in the point-dipole limit. Modifications to resonance frequencies and linewidths are shown to be different for substitutional and interstitial impurities. Spontaneous emission rates inside dielectrics exhibit the well-known empty-cavity and Lorentz local-field factors for substitutional and interstitial atoms, respectively. The results are generalized to disordered dielectrics, indicating that the substitutional case occurs prevalently for impurity atoms.
Abstract: The treatment in the literature of the transfer functions for interferometers does not allow for a proper use of the Kramers-Kronig (KK) relations between amplitude and phase. We will present a systematic description of transfer functions. We show that if the full frequency dependence of all the optical parameters, including the complex index of refraction,is properly taken into account KK relations do exist. The modification of KK relations for amplitude and phase due to zeros of transfer functions is described. As an example, numerical results for the phase calculated from the amplitude of light reflected from a Gires-Tournois interferometer are presented.
Abstract: We develop a microscopic scattering theory for the electromagnetic response of dielectrics. We derive the Lorentz-Lorenz relation for a hard-sphere fluid and for hard-sphere mixtures by summing rigorously the relevant class of multiple scattering events which incorporates particle correlations. The derivation neither makes use of macroscopic concepts such as local and reaction fields nor does it invoke decoupling schemes for high-order correlation functions.
Abstract: We determined orientational relaxation times for rhodamine 700 dye in glycerol-water mixtures using time-resolved fluorescence depolarization. It appears that the orientational relaxation time varies linearly with the viscosity of the solvent between 1 and 60 cP, in accordance with the Perrin-Stokes-Einstein model with stick boundary conditions. Previously others have found that for two anionic dyes in glycerol-water and a cationic dye in glycerol-ethylene glycol mixtures, the orientational relaxation time becomes less sensitive to the viscosity at very high viscosities (>25 cP at least). We discuss the influence of dye and solvent on the relation between orientational relaxation time and viscosity, which suggests that the relaxation time as a function of viscosity can be scaled on a common curve. (C) 1997 American Institute of Physics.
Abstract: We present results of experiments on the transport of light through thin random media. Total transmission and time-resolved propagation measurements were performed using strongly scattering samples of varying thickness L. For L/l < 8, where l is the transport mean free path, the observed decay times from the long-time exponential behavior exhibit strong deviations from diffusion theory and radiative transport theory, whereas the total transmission measurements do not. For the thinnest sample (L/l similar or equal to 2) a reduction of the diffusion coefficient with a factor of 2 was observed.
Abstract: Experimental results are reported on coherent backscattering and speckle from an amplifying random medium. Also, coherent backscattering with gain is calculated completely, using diffusion theory. To conclude, the concept of a random laser is discussed.
Abstract: This educational work presents a new approach towards resonant interaction between classical light and matter. The interaction between light and matter is considered from three different points of view: the light picture (where the material degrees of freedom have been integrated out, and leaving one with scattering theory), the matter picture (where the radiative degrees of freedom have been eliminated and providing one essentially with atomic physics). In addition the polariton approach is discussed, in which the degrees of freedom of light and matter are treated on the same footing. Although the first approach will by far be given most of the attention, we will frequently emphasize the equivalence of the three methods. Much of the presented material is selfcontained. We demonstrate that in the dynamical properties of multiple scattering of light the ''matter'' properties play a dominant role. Several ''paradigms of atomic physics'' will be discussed from the view point of light scattering theory. We shall introduce the far-reaching analogy between the dielectric ''Mie'' sphere in classical optics, and the two-level atom in semi-classical atomic physics. This mapping turns out to be much more faithful than the widely used analogy between scattering theory for De Broglie waves and classical waves. In scattering theory the semi-classical two-level atom is equivalent to a point scatterer.
Abstract: With two-color picosecond infrared laser spectroscopy the dynamics of O-H and O-D stretch vibrations in zeolites are investigated. Zeolites appear to be good model systems to study transfer of vibrational energy in a solid. For the O-D vibrations, transient spectral holes are burnt in the inhomogeneously broadened absorption bands by saturating the absorption with a strong pump pulse. From the spectral hole widths the homogeneous absorption linewidths are obtained. The excited population lifetimes are determined using a time-resolved pump-probe technique, and in combination with the homogeneous linewidth the pure dephasing time is revealed as well. For high concentrations of O-H oscillators the vibrational stretch excitations are found to diffuse spectrally through the inhomogeneous absorption band. This spectral diffusion process is explained by direct site-to-site transfer of the excitations due to dipole-dipole coupling (Forster transfer). The dependences of the transient spectral signals on oscillator concentration and the results of one-color polarization resolved experiments confirm this explanation. The spectral transients are satisfactorily described by simulations in which the site-to-site transfer by dipole-dipole coupling is taken into account. (C) 1996 American Institute of Physics.
Abstract: The influence of photonic band structures on optical diffraction has been studied with colloidal crystals with large refractive index ratios up to 1.45 and polarizibilities per volume as large as 0.6. It is found that the apparent Bragg spacings are strongly dependent on the wavelength of light. The dynamical diffraction theory that correctly describes weak photonic effects encountered in x-ray diffraction also breaks down. Two simple models are presented that give a much better description of the diffraction of photonic crystals.
Abstract: We show rigorously that the coefficient for spontaneous emission of an atom placed in a dielectric is proportional to the local radiative density of states -that is only a part of the local density of the eigenmodes of the Maxwell equations. Spontaneous emission is inhibited if the atom is located at a position where this local radiative density is small, even if the total density of states is not vanishing. This radiative density of states can be obtained without having to perform a full quantum calculation of the radiation-matter system. We demonstrate this principle by solving numerically a scalar model for a dielectric that consists of a lattice of resonating dipoles.
Abstract: A calculation of the optical band structure of a three dimensional lattice of resonant two-level atoms in the dipole approximation is presented. The formation of band gaps is exhibited and confirmed by a calculation of the density of states. The band structure can be characterized by two dimensionless parameters. We find a longitudinal polarization mode as well as a class of vacuum modes that are unaltered by the interaction with matter. Numerical calculations are performed for a face centered cubic lattice; other lattices can be evaluated as easily.
Abstract: We apply elements of the self-consistent theory for localization, reciprocity and conservation of energy, in order to find a theory for the enhancement factor in Coherent Backscattering of classical waves in 3D. An application is presented for the classical Milne problem.
Abstract: We report on coherent backscattering measurements from amplifying random media using optically pumped Ti:sapphire powders. The top of the backscattering cone sharpens with increasing gain, reflecting a change in the relative contributions to the intensity from paths of different lengths. A calculation based on diffusion theory is performed on coherent backscattering from amplifying random media, which is in good agreement with the data.
Abstract: The vibrational dynamics of free and hydrogen bonded O-H and O-D groups in zeolites are investigated as a function of temperature with picosecond infrared saturation spectroscopy. From the temperature dependence of the population relaxation rate the order of the multiphonon relaxation process is obtained. It is noted that, in contrast with the conventional theory for vibrational relaxation of an oscillator in a dense medium, the absolute decay rate hardly scales with the number N of accepting modes. This leads us to conclude that, in addition to the order N of the relaxation process, the relaxation rate is also determined by the number of possible relaxation channels. From the temperature dependence of the decay rate of H-bonded oscillators we obtain insight in the role of H-bonding in vibrational relaxation in solids. It is found that the hydrogen bond efficiently accepts part of the oscillator energy in the relaxation process.
Abstract: With time-resolved vibrational spectroscopy new information is obtained about the structural environment and vibrational dynamics of acidic hydroxyls in proton loaded zeolites. From O-H vibrational population lifetimes of zeolite Y we conclude that part of the protons are hydrogen bonded. Furthermore we find that the vibrational dynamics of the O-H oscillators are influenced by a resonant coupling mechanism.
Abstract: We report the first measurement of the distribution function of the fluctuations on the total transmission of multiple scattered light. The shape of the distribution is predominantly Gaussian. A non-Gaussian contribution to the distribution function is found, caused by correlation in the cubed intensity. The scattering diagrams responsible for this new correlation are calculated without free parameters, and a good agreement is found between experiment and theory.
Abstract: Using a time-dependent scattering theory for light we obtain expressions for the time that light spends in a finite dielectric medium surrounded by vacuum, the so-called dwell time. Given an incoming wavepacket, we focus upon the light scattered in an arbitrary direction. In view of the similarity between Maxwell's equations and the Schrodinger equation, some useful results are derived for the case of Schrodinger potential scattering as well. We show that the dwell time of a Schrodinger particle is a derivative of the phase shifts with respect to the potential, rather than to energy as is the case for the phase-delay time. We indicate the relation to absorption arguments. Because the potential for classical waves is energy dependent, derivatives of the phase shift with respect to potential enter into the phase-delay time for classical waves.
Abstract: Using a single-mode glass fiber as a dispersive nonlinear medium, we observed hysteresis, period doubling and chaotic behavior of 10 ps laser pulses in a ring cavity. Some special timing logic allowed us to control all relevant parameters, and so for the first time the nonlinear dynamics were studied quantitatively. The general features were in agreement with predictions obtained from the Ikeda map, but quantitative agreement could only be obtained by extending the theory with the influence of group velocity dispersion and pulse shape. No adjustable parameters were used in the theory.
Abstract: When a small object is placed in a multiple-scattering medium the stationary diffusion equation can be used to derive the disturbance in the transmitted and backscattered light intensity. The diffusion equation will describe the intensity outside and inside the object. The object is characterized by a size, a diffusion constant, and an absorption length. In this way absorbing objects as well as nonabsorbing objects can be treated. The results are derived for two and three dimensions. Experiments are performed on suspended titanium dioxide particles in glycerine, wherein objects could be placed. There is good agreement between theory and experiment. This work shows that with the use of continuous light sources, it may be possible to recover the location of objects accurately inside a diffusive scattering medium.
Abstract: We present theoretical and experimental results of the leading order angular correlations of second harmonic light generated inside random media, for both the transmission and reflection geometries. We find the striking result that correlations in reflection of the second harmonic light scales with the sample thickness L, in sharp contrast to the corresponding short-range correlation function in reflection of the fundamental light in the linear scattering regime, which scales with mean free path l*.
Abstract: We present exact expressions for the transport velocity of scalar classical waves in random dielectric media in lowest order of the density of the scatterers (Boltzmann limit). This speed enters into the diffusion constant of multiply scattered classical waves. We discuss the relation with the Wigner phase delay time.
Abstract: We discuss various speeds of propagation that are relevant for the dynamics of classical waves in random media: phase velocity, group velocity and transport velocity. The transport velocity can be much smaller than the phase velocity due to the long dwelling of classical waves inside resonant scatterers. We show that the transport velocity of light can be obtained experimentally by studying both transient and steady-state diffusion of light.
Abstract: The influence of the refractive-index mismatch on coherent backscattering from disordered systems is experimentally studied. The contrast in the refractive index between sample and surrounding media is varied, and thereby the surface reflectivity, using different glass types. Agreement is achieved with current theoretical models. Values for the mean-index of refraction are found that are close to the index of the interparticle media, in contrast with results predicted by theoretical models on heterogeneous media.
Abstract: We show experimentally that the electron distribution of a laser-heated metal is a nonthermal distribution on the time scale of the electron-phonon (e-ph) energy relaxation time tau(E). We measured tau(E) in Ag and Au films as a function of lattice temperature (10-300 K) and laser-energy density (0.3-1.3 J cm-3), combining femtosecond optical transient-reflection techniques with the surface-plasmon polariton resonance. Calculations support the experimental result that the nonthermal distribution leads to a slower e-ph energy relaxation process below room temperature, compared to the thermalized limit.
Abstract: We report on an experimental study of wavelength-dependent intensity fluctuations in coherent light that passed through a random dielectric medium. For the intensity that is scattered into a narrow solid angle and for the total transmission, we find exponential- and power-law decay in the intensity-intensity and transmission-transmission correlation, respectively. We derive an expression for the intensity-intensity correlation in the medium under the prevailing experimental conditions: Gaussian incident beam, slab geometry, and some absorption. From this expression we calculate the short-range and long-range correlation functions, using a Langevin approach to find the latter. Two earlier calculations of the long-range correlation function disagreed by a prefactor. The present theory quantitatively describes the long-range correlation, and therefore removes the uncertainty about this factor. With the Boltzmann diffusion constant for light in the random medium as the only fitting parameter, we obtain quantitative agreement between theory and experiment.
Abstract: Using intense femtosecond pulses, we have generated phonon polaritons in the ferroelectric crystal LiNbO3. Phonon-polariton pulses consisting of almost-equal-to 8 oscillations of the electric field were generated. They were detected in a time-resolved way by diffraction of a probe pulse from the standing wave formed by these phonon polaritons. We determined their dispersion for frequencies up to 130 cm-1. The pulse width of the phonon polaritons was almost-equal-to 3 ps. In addition, we have studied their propagation in the crystal, by diffracting a probe pulse from one of the traveling phonon polaritons. We demonstrate that in this case, the diffracted signal is sensitive to the phase of the phonon polariton. Analytical calculations show that this can be explained in terms of the interference between the electric fields of the nondiffracted probe beam and the first-order diffracted probe beam.
Abstract: We give a microscopic derivation of the speed of light relevant for energy transport in media containing randomly distributed scatterers. A comparison is made with the concept of "mass-enhancement" in electron transport theory. The consequences for the Thouless criterion for strong localization are discussed. Finally, quantitative results are obtained for simple scatterers, such as semiclassical oscillators and dielectric spheres. We will also introduce some heuristic approaches and discuss the validity.
Abstract: Both for scalar and for vector waves it is shown in a simple manner how the limit of point scatterers can be achieved for spherical scattering objects in three dimensions. Applications for multiple scattering are discussed.
Abstract: We present a localization theory for scalar waves in three dimensions that incorporates off-shell contributions and repetitive scattering between the particles. We argue that localization may be difficult to reach for strong individual scattering efficiencies.
Abstract: We present results of optical experiments which demonstrate that in a strongly scattering medium containing resonant scatterers the velocity for electromagnetic energy may differ by an order of magnitude from the phase velocity. We derive a microscopic theory that yields an expression for this velocity. Discrepancies are removed, and excellent agreement is found between experiment and theory.
Abstract: The vibrational coherence relaxation of the OMEGA+ bound state of the Fermi dyad v1:2v2 in solid CO2 is studied with the time-resolved stimulated Raman technique as a function of temperature for pressures up to 4.4 GPa. The relaxation is determined by a competition between two contributions: a down-conversion process with a weak pressure dependence, and a pure dephasing process, the latter becoming dominant at higher pressures.
Abstract: In this paper we present a study of the vibrational energy relaxation processes of chloroform, bromoform, and iodoform in solution after excitation of the C-H stretch vibration. The relaxation is studied with ultrafast infrared saturation spectroscopy using intense infrared pulses with a pulse duration of 19 ps. The experiments were performed in a polar and a nonpolar solvent in order to study the effects of the polarity of the solvent on the relaxation processes. We observe that in both types of solvent the relaxation takes place via two consecutive relaxation processes and that the relaxation leads to ultrafast changes of the absorption band of the C-H stretch vibrations. We discuss the differences in the time constants of the relaxation processes of the haloforms in terms of the energy differences between the vibrational levels and the interactions with the solvent.
Abstract: In this paper we present a time-resolved study of the vibrational relaxation after excitation of the asymmetric CH2 stretch vibration of dibromomethane and diiodomethane and the C-H stretch vibration of 1,1,2,2-tetrabromoethane. The experiments were performed in a polar and a nonpolar solvent in order to study the influence of the polarity of the solvent on the relaxation. We observe that in both types of solvent the vibrational energy transfer is successively intra- and intermolecular and that the intramolecular relaxation leads to a shift of the transition frequency of the excited molecular vibration. We discuss the experimentally determined time constants of the relaxation in terms of the energy differences between the molecular vibrations and the interactions with the solvent.
Abstract: The technique of accumulated photon echo is used in combination with a high-pressure diamond anvil cell to measure the splitting of the 4A2 ground state and the one-phonon spontaneous decay rate of the 2ABAR(2E) level at 10 K in ruby up to 4.3 GPa of hydrostatic pressure. The 4A2 ground-state splitting is found to be 0.383 +/- 0.001 cm-1 at ambient pressure, and increases with a slope of +(6 +/- 1) x 10(-3) cm-1/GPa. The spontaneous decay rate of 2ABAR(2E) increases only weakly with pressure. The pressure dependences of both quantities are accounted for in terms of the trigonal-field and spin-orbit-coupling parameters.
Abstract: We present a high-power femtosecond dye-laser system based on a CPM oscillator. The pulses have an energy up to 0.6 mJ and a variable pulse duration between 70 fs and 5 ps. The wavelength of the pulses is tunable throughout the visible, and the chirp of the pulses can be varied continuously.
Abstract: We consider the similarities between short optical pulses and atomic electron wave packets. The atomic potential acts in a similar way on the electron wave function as an optical cavity does on electromagnetic waves. It is shown that effects well known in optics, such as creation of pulsed light, dispersion, and shortening of optical pulses, can also occur for matter waves.
Abstract: We discuss the influence of interference in multiple scattering of light in strongly scattering media on field (amplitude) and intensity correlations. Results of experimental measurements on the dependence of the recently found long-range intensity fluctuations on the spatial profile of the incoming beam are presented. Our results are consistent with the recently introduced concept of an energy velocity, which is substantially lower than the phase velocity. We demonstrate that an important relation exists between the redshifts introduced by Wolf and weak localization of light.
Abstract: We demonstrate that a parametric set-up consisting of three LiNbO3 crystals pumped by the output of a picosecond Nd:YAG laser can easily be extended so that wavelengths between 1.2 and 8.7-mu-m can efficiently be generated. This extension consists of an extra LiNbO3 crystal and an AgGaS2 crystal. In the LiNbO3 crystal the idler generated by the parametric set-up is frequency doubled. The doubled idler is used as a seed for parametric amplification in the AgGaS2 crystal pumped by the Nd:YAG. In this way the doubled idler between 1.2 and 1.4-mu-m is strongly amplified and a strong pulse at the difference wavelength between 4.5 and 8.7-mu-m is generated. The maximum energy-conversion efficiency is 5.4%. With this set-up we can generate pulses with a pulse duration between 15 and 20 ps that are continuously tunable from 1.2 to 8.7-mu-m.
Abstract: We discuss several interference effects in multiple scattering of light in strongly scattering media. Results of experimental measurements on the dependence of the long-range intensity fluctuations on the spatial profile of the incoming beam are presented. From the theory side we show results on the effect of the resonance properties of the individual scatterers on the scattering mean free path. We demonstrate that an important relation exists between the redshifts recently introduced by Wolf and weak localization of light.
