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Using a free electron laser we have performed midinfrared transmission and reflection measurements in strongly scattering Ge powder. We have studied the wavelength dependence of the scattering properties and the optical absorption. Results indicate that our samples are close (kl(s) approximately 3) to the Anderson localization transition but still above it. This is in contrast to the recently reported strong localization of light (kl(s)

A random laser is a disordered medium that both scatters and amplifies light. Important parameters in this system are the transport mean free path l, the size of the amplifying volume and the amplification length l amp. Properties of light that is propagating through such a medium are modified by the presence of gain, giving rise to a range of surprising phenomena. One of these is the occurrence of a lasing threshold in the output power, pulse duration and spectral width of the emission [1]. These experiments have raised a large number of questions regarding the mechanism that gives rise to these phenomena. One of the most intriguing is how to connect the multiple scattering picture with the laser picture.

In liquid nitrogen at 295 K we observe for the first time the minimum in the variation of the vibrational dephasing rate T−1 2 with density at ρ = 2.15 × 1022. cm−3. At higher densities the dephasing rate shows a steep rise with density. The observed behaviour shows good agreement with results of previously published molecular dynamics simulations.

We have measured the absorption spectrum, the emission spectrum, the emission lifetime, and the photostability of fluorescein isothiocyanate (FITC) incorporated inside colloidal silica spheres as a function of the dye concentration in the spheres, while minimizing scattering effects. Six batches of stable, monodisperse particles were synthesized with FITC up to high densities of 0.03 M. At dye concentrations above 0.001 M, we observe a large red shift of 10 nm in the absorption and the emission spectra, as well as a strong reduction of the lifetime. At the same time, the photostability of the dye is considerably improved. These effects are caused by an increased energy transfer between the dye molecules as their concentration increases. Several excitation quenching models are examined, namely annihilation quenching, surface quenching, and a fractal distribution of quenchers. None of the models that assume a homogeneous distribution of FITC provide a sufficient explanation of the observed effects. It is suggested that the dye molecules tend to form clusters during the synthesis of the colloidal spheres. It is concluded that colloids with a low dye concentration are useful for photonic applications, whereas high dye concentrations are interesting for optical experiments in colloid science.

Propagation of light in strongly scattering media displays many interesting phenomena such as weak localization and universal conductance fluctuations that are caused by interference of multiple scattered light. A tremendous simplification is obtained if the propagation can be described with transport theory, where all interference effects are neglected. If the sample size (L) is considerably larger than the transport mean free path I the coherent beam is suppressed and the equation of radiative transfer simplifies further to a diffusion equation. Recently, interest has arisen in medical imaging applications to determine optically the location of objects in strongly scattering biological tissues. Optical imaging may serve as an important substitute for X-ray-based techniques as optical radiation is nonionizing.

The propagation of sound in three dimensional periodic lattices of solid-solid and solid-liquid composites is determined by calculating the acoustic band structure. Inclusion of viscous damping in the liquid is essential in understanding the acoustical properties of the solid-liquid composite when the characteristic length scale of the system matches the viscous penetration depth in the liquid. By using complex sound velocities to model the visco-elastic properties of the liquid we show that the solid-viscous liquid composite displays acoustic band gaps. These results are consistent with the hydrodynamic analysis of viscoelastic modes in porous media, binary mixtures and with a stratified layer model. As a characteristic example the ultrasonic band structure of silica spheres in ice and of colloidal crystals of silica in glycerol/water mixture are presented. Both systems display gaps under experimentally obtainable conditions.

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.

Structural relaxation in amorphous systems is an area of much current interest. Many studies have been devoted to the sol-gel transition in systems based on polymers @1‐3#, natural gelatine @4#, and gels based on spherical colloids @5‐7#. Here we report on the sol-gel transition in a system of charged disks with an aspect ratio of 25, suspended in water. In such systems the orientational degrees of freedom play a crucial role not only in the dynamics but also in the static structure of the gel. Molecular dynamics simulations on hard disk systems reveal a rich phase diagram with nematic and cubatic liquid crystalline phases @8#. However, before these liquid crystalline phases can be formed the system enters a glassy phase or gel. Current opinion favors the ‘‘house of cards’’ structure for the gel @9#, which implies a random structure with short range orientational order. On the average the disks are oriented with their positively charged rim towards the negatively charged base of their neighbor. Within this view the orientational degrees of freedom will play a crucial role in the formation of the gel. Structure and dynamics in the neighborhood of the transition can be studied best by using the noninvasive technique of light scattering, dynamic ~DLS! and static. DLS probes the density correlation function describing the time evolution of the density fluctuations. The striking feature, largely emerging from DLS measurements around the gel point @4‐6#, is the marked similarity of the behavior of the correlation function with the scenarios given by the mode cou

We have performed optical diffraction studies on colloidal crystals with large refractive index mismatches up to 1.45 and polarizibilities per volume as large as 0.6. These conditions push colloidal crystals into the regime where strong coupling between photonic crystals and the light field occurs. It is found that the photonic band structures result in apparent Bragg spacings that strongly depend 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.

The analysis of non-stationary signals calls for speciflc tools which go beyond classical Fourier analysis. Here we present the discrete wavelet transform as a quantitative method to analyse the scaling properties of non-stationary and chaotic signals obtained by light scattering. The analysis is performed directly on the time-resolved scattered intensity. The result is the scaling exponent of the underlying power spectrum and the frequency window where the scaling can be observed. These properties are obtained with a higher accuracy and in a fraction of the time compared with the classical method of intensity correlation functions. As a test we applied the new method to the sol-gel transition of a suspension of disc-shaped colloidal particles in water.

In dynamic light scattering the intensity correlation function is calculated by means of a correlator from the measured scattered fluctuating intensity. Subsequently the intermediate scattering function, or its Fourier transform the dynamic structure factor, can be extracted from the correlation function. This by now classical method has, as any experimental technique, its own limitations. One of the limitations becomes apparent when the intensity correlation function possesses an algebraic tail. It is observed for example in colloidal or polymeric systems undergoing a glass transition1-3. Then it is possible, but difficult to extract the true intermediate scattering function of the system, even though the system is considered in equilibrium. Non-stationary systems or systems behaving chaotically in time pose a real problem. For a non-stationary system the zero frequency divergence in the power spectrum of the fluctuating intensity precludes any quantitative analysis by means of correlation functions, since an infinite time range should be covered. Examples are the formation of a glass or a gel, and turbulence.

Characterizing the streaming potential and electroosmosis properties of porous media is essential in applying seismoelectric and electroseismic phenomena for oil exploration. Some parameters such as porosity, permeability, formation factor, pore size, the number of pores, and the zeta potential of the samples can be obtained from elementary measurements. We performed streaming potential and electro-osmosis measurements for 6 unconsolidated samples made of spherical polymer particles. To check the validity of the measurements, we also used alternative analysis to determine the average pore size of the samples and, moreover, used a sample made of sand particles to determine the zeta potential.

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 ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$. We measured ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ in 45-nm Ag and 30-nm Au thin films as a function of lattice temperature (${\mathit{T}}_{\mathit{i}}$=10--300 K) and laser-energy density (${\mathit{U}}_{\mathit{l}}$=0.3--1.3 J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), combining femtosecond optical transient-reflection techniques with the surface-plasmon polariton resonance. The experimental effective e-ph energy relaxation time decreased from 710--530 fs and 830--530 fs for Ag and Au, respectively, when temperature is lowered from 300 to 10 K. At various temperatures we varied ${\mathit{U}}_{\mathit{l}}$ between 0.3--1.3 J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and observed that ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ is independent from ${\mathit{U}}_{\mathit{l}}$ within the given range. The results were first compared to theoretical predictions of the two-temperature model (TTM). The TTM is the generally accepted model for e-ph energy relaxation and is based on the assumption that electrons and lattice can be described by two different time-dependent temperatures ${\mathit{T}}_{\mathit{e}}$ and ${\mathit{T}}_{\mathit{i}}$, implying that the two subsystems each have a thermal distribution. The TTM predicts a quasiproportional relation between ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ and ${\mathit{T}}_{\mathit{i}}$ in the perturbative regime where ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ is not affected by ${\mathit{U}}_{\mathit{l}}$.Hence, it is shown that the measured dependencies of ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ on lattice temperature and energy density are incompatible with the TTM. It is proven that the TTM assumption of a thermal electron distribution does not hold especially under our experimental conditions of low laser power and lattice temperature. The electron distribution is a nonthermal distribution on the picosecond time scale of e-ph energy relaxation. We developed a new model, the nonthermal electron model (NEM), in which we account for the (finite) electron-electron (e-e) and electron-phonon dynamics simultaneously. It is demonstrated that incomplete electron thermalization yields a slower e-ph energy relaxation in comparison to the thermalized limit. With the NEM we are able to give a consistent description of our data and obtain values for the e-e scattering rate K=0.10\ifmmode\pm\else\textpm\fi{}0.05 ${\mathrm{fs}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}2}$ for Ag and Au and for the e-ph coupling ${\mathit{g}}_{\mathrm{\ensuremath{\infty}}}$=3.5\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Wm}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Ag and 3.0\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Wm}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Au.

Since the discovery of enhanced backscatter (weak localization) [1], light scattering in disordered media has drawn a lot of attention. In the pursuit of strong localization, a lot of interesting phenomena were discovered, such as fluctuations on the transmission of light through disordered slabs [2–7]. Recently, a new class of disordered media have come into focus, “non-linear disordered media.” By non-linear in this context, a non-linear response of the constituting particles of the disordered medium to the electro-magnetic field is meant, e.g., particles with a second-harmonic susceptibility. Agranovich and Kravtsov predicted interesting phenomena that can be observed in non-linear disordered media [8]. They stated that enhanced backscatter can also be observed in the second-harmonic light generated in a non-linear disordered medium, although this is not yet experimentally verified.

The relaxation dynamics of laser-heated electrons has been studied extensively in metals and (hiTc) superconductors[1] by various femtosecond thermomodulation techniques. These measurements give new insights in the electronic properties and provide direct information about the electron-phonon (e-p) coupling strength. A nearly unexplored type of dynamics is the (e-e) dynamics within the electron gas in a laser-heated metal. The extremely small e-e collision time (~ 10 fs), associated with the highly nonthermal electron distribution during excitation, makes direct observation a challenging task. We report the first experimental evidence of e-e dynamics in thermoreflection measurements by its direct effect on the e-p energy relaxation process for Ag and Au in the regime of low temperature and low laser-energy density[2]. The well-established two-temperature model cannot account for our results and a new model has been succesfully applied.

We experimentally study the effects of correlations in the propagation of ultrasonic waves in water from a multielement source to a multielement detector through a strongly scattering system of randomly placed vertical rods. Due to the strong scattering, the wave transport in the sample is in the diffusive regime. The correlation between the waves is induced when the distance between transducer elements is within the coherence region of the scattered sound. We measure the multichannel transfer matrix H, each element of which represents the signal strength between the m individual transmitters and n receivers. The observed eigenvalue distribution of the matrix HH dagger clearly shows the effect of correlations between channels and can be interpreted using random matrix theory. These results are of practical importance in many areas, e.g., for evaluating the information transfer capacity of such a complex scattering system.

In recent years, femtosecond thermomodulation experiments1-3 have been performed to study the relaxation dynamics of electrons in metals. The electrons were heated to a high (> 1000 K) non-equilibrium temperature. At these extreme conditions, the dynamics depends strongly on the electron temperature, resulting in a nonexponential decay.

Depolarization of coherent light waves in disordered media has been studied directly in the time-domain with femtosecond time resolution. The experimental results are compared with recent results from calculations for Rayleigh scattering in random media.

In liquid nitrogen at 295 K the minimum in the variation of the vibrational dephasing rate T 2 −1 with density is observed at ρ=2.15×10 22 cm −3 . The observed behaviour shows good agreement with the results of previously published molecular dynamics simulations.

Time-resolved backscattering of coherent light from a disordered medium under weak localization conditions has been observed for the first time with femtosecond resolution using a light-gating technique based on second-harmonic generation. The time dependence of the observed enhanced backscattering is consistent with simple diffusion theory.

We analyze and reinterpret experiments with electron- and nuclear-spin (''doubly'')-polarized hydrogen (Harrow-downX) in a cell with a very low concentration of magnetic impurities. The destabilizing magnetic relaxation due to remaining impurities (G/sub i//sup s/) is found to be suppressed by increasing the thickness of the helium film covering the walls of the cell. Using recent information on surface three-body recombination, we obtain good quantitative agreement with more recent experiments if we assume thermal gradients, up to ..delta..T/Tapprox. =0.1. We extract both the value and field dependence for the three-body surface recombination (L/sub 3//sup s/ = 1.5(2) x 10/sup -24/ cm/sup 4/ s/sup -1/ at B = 8 T). We also extract the magnetic field dependence of the impurity relaxation.

A gas of 99.8% electron- and nuclear-spin-polarized hydrogen has been prepared. The surface and volume nuclear relaxation rates have been measured and the magnetic field dependence (1+16.68/B)/sup 2/ has been confirmed. The ratio of the surface recombination rate constants for collisions between atoms in hyperfine states, a-a and a-b, is measured to be 2.23(25). Nuclear relaxation on the surface has been suppressed by using /sup 3/He to make an atomically flat surface.

A new technique enabled the authors to compress doubly polarized hydrogen by up to five orders of magnitude and study the sample in very small volumes at constant density up to 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$/${\mathrm{cm}}^{3}$. The first determination of the bulk three-body dipolar recombination rate was made, ${{K}_{V}}^{3\mathrm{b}}=4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}39}$ ${\mathrm{cm}}^{6}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The authors also identified a new process, bulk electronic dipolar $b\ensuremath{\rightarrow}c$-state relaxation with rate constant ${{G}_{V}}^{\mathrm{bc}}\mathrm{exp}(2{\ensuremath{\mu}}_{\mathrm{B}}\frac{B}{{k}_{\mathrm{B}}}T)=8(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$. Samples were very delicate and too rapid a compression could result in an explosion.