Ultimate degree of residual polarization of incoherently backscattered light for multiple scattering of linearly polarized light

By invoking ideas about the distribution of the optical paths of partial components of the scattered field, we obtain an expression for estimating the degree of residual polarization of light that is incoherently backscattered from a disordered multiply scattering semi-infinite medium illuminated by linearly polarized light. In the backscattering regime, the depolarization length of the linearly polarized light in the disordered medium becomes smaller with the passage from the isotropic to anisotropic scattering. Experiments with model media featuring substantially anisotropic scattering (the anisotropy parameter of 0.90 ≤ g ≤ 0.95) demonstrated that for backscattering of linearly polarized light, the depolarization length is close to the transport length of the scattering medium.     

Stimulated light emission in dense fog confined inside a porous glass matrix

We report on light amplification through stimulated emission in a dielectrically disordered medium. Liquid fragments confined in the solid matrix of porous quartz layers result in a random fluctuation of the dielectric function, and dye molecules embedded in the voids yield optical gain. The level of opacity is tunable by the ambient vapor pressure of the dielectric substance. In the multiple scattering regime, a strong intensity enhancement of the dye emission accompanied by significant spectral narrowing is observed above the threshold for a layer being in the opalescence state.     

Superfluorescence of photonic paint

We consider the cooperative decay of incoherently pumped atoms in a disordered medium, where light undergoes multiple scattering. It is shown that the cooperation number, which determines the duration and amplitude of superfluorescent impulses, is given by the number of atoms along a diffusive trajectory of the light propagating through the medium. We also consider the problem of reflection of a probe wave during cooperative emission. Zh. éksp. Teor. Fiz. 113, 816–825 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Quantum confinement and disorder in porous silicon: Effects on the optical and transport properties

Summary The striking optical properties of porous silicon (PS) show a twofold aspect typical of an ordered and a disordered material, respectively. Raman, electron microscopy, and resonant photoluminescence studies indicate that the light emission originates from crystalline regions. On the contrary, several features, like the non-exponential decay of photoluminescence (PL), the broad emission spectrum, the photoluminescence fatigue under light exposure etc. are typical of a disordered material and reminiscent of similar effects founde.g. in amorphous semiconductors. These twoapparently conflicting aspects have for a long time hindered the understanding of the basic light emission mechanism. In this paper we report new optical data showing that disorder in porous silicon leads to strong carrier localisation. Light emission in PS is suggested to occur through transitions involving localized states. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Random lasing in strongly disordered medium

Using the time-dependent theory, we calculate the random-laser emission spectra in a two-dimensional strongly disordered medium. The calculation results show that in low dimensional systems, such as thin- film disordered media and planar waveguides, the larger the difference of the refractive indices between the scattering and background media, the smaller the lasing threshold. We also reveal the existence of multi-mode survival and mode competition. We experimentally obtain the emission spectra of a dye solution with Al particles doped at different pumping energies, and the experimental results agree well with the calculated ones.     

Conductance distribution in two-dimensional localized systems with and without magnetic fields

We have obtained the universal conductance distribution of two-dimensional disordered systems in the strongly localized limit. This distribution is directly related to the Tracy-Widom distribution, which has recently appeared in many different problems. We first map a forward scattering paths model into a problem of directed random polymers previously solved. We show numerically that the same distribution also applies to other forward scattering paths models and to the Anderson model. We show that most of the electric current follows a preferential percolation-type path. The particular form of the distribution depends on the type of leads used to measure the conductance. The application of a moderate magnetic field changes the average conductance and the size of fluctuations, but not the distribution when properly scaled. Although the presence of magnetic field changes the universality class, we show that the conductance distribution in the strongly localized limit is the same for both classes.     

Magnetic field effects on coherent backscattering of light

We have studied the influence of magneto-optical Faraday rotation on coherent backscattering of light experimentally, theoretically and by computer simulations of Monte-Carlo type. The consistency of these three approaches reveals new aspects of the propagation of vector waves in turbid media with and without Faraday rotation. Experimentally, we have demonstrated that the Faraday rotation may almost completely destroy the reciprocity of light paths. However, as shown by the simulations, the cone of coherent backscattering may not only be destroyed but also shifted off the exact backscattering direction, parallel to the magnetic field, as long as the amount of circular polarization is not completely destroyed by the multiple scattering. The relationship between coherent backscattering, depolarization and Faraday rotation are explained by a simple path model of vector waves. This leads to a new characteristic correlation length required to properly describe the influence of Faraday rotation on multiple light scattering. Received 28 January 2000     

Enhanced resonant backscattering of excitons in disordered quantum wells

A clear signature of enhanced backscattering of excitons is observed in the directional resonant Rayleigh scattering of light from localized two-dimensional excitons in disordered quantum wells. Its spectral dependence and time dynamics are measured and theoretically predicted in a quantitative way. The intensity enhancement has a large momentum span extending beyond the external light emission cone. This is a consequence of the small localization length of the exciton as a massive particle probed close to the band bottom. The localization length can be controlled by the photon kinetic energy. This constitutes a qualitative difference to backscattering phenomena in other branches of physics.     

金纳米薄膜的荧光光谱特性

采用电化学方法制备了胶体盒纳米球状颗粒,并利用自组装方法在石英玻璃村底上镀制了金纳米薄膜。在室温下测得其紫外-可见吸收光谱和荧光发射光谱。在吸收光谱中观察到两个吸收峰,其中610nm、处的吸收峰来源于凝聚金纳米颗粒纵向的表面等离子体共振。在荧光发射光谱中也观察到与纵向表面等离子体共振有关的长波段的发射峰。增加激励光强度或增加薄膜中金粒子散密度都将导致新荧光发射峰的产生．这表明金纳米薄膜中存在循环多重散射,并由此引发了荧光发射峰数目和强度的变化。     

Nonlinear coherent transport of waves in disordered media

We present a diagrammatic theory for coherent backscattering from disordered dilute media in the nonlinear regime. We show that the coherent backscattering enhancement factor is strongly affected by the nonlinearity, and we corroborate these results by numerical simulations. Our theory can be applied to several physical scenarios such as scattering of light in a nonlinear Kerr medium or propagation of matter waves in disordered potentials.     

Low-frequency Raman scattering in a polycrystalline C60 film: The role of orientational disorder

Low-frequency Raman scattering in the orientationally disordered phase of a polycrystalline C₆₀ film is investigated. By analogy with disordered media (glasses), the low-frequency Raman spectra are interpreted in terms of light scattering by localized vibrational states.     

Path degeneracy and EXAFS analysis of disordered materials

Analysis of EXAFS data measured on a material with a disordered local configuration environment around the absorbing atom can be challenging owing to the proliferation of photoelectron scattering paths that must be considered in the analysis. In the case where the absorbing atom exists in multiple inequivalent sites, the problem is compounded by having to consider each site separately. A method is proposed for automating the calculation of theory for inequivalent sites, then averaging the contributions from sufficiently similar scattering paths. With this approach, the complexity of implementing a successful fitting model on a highly disordered sample is reduced. As an example, an analysis of Ti K‐edge data on zirconolite, CaZrTi₂O₇, which has three inequivalent Ti sites, is presented.     

Phase control algorithms for focusing light through turbid media

Light propagation in materials with microscopic inhomogeneities is affected by scattering. In scattering materials, such as powders, disordered metamaterials or biological tissue, multiple scattering on sub-wavelength particles makes light diffuse. Recently, we showed that it is possible to construct a wavefront that focuses through a solid, strongly scattering object. The focusing wavefront uniquely matches a certain configuration of the particles in the medium. To focus light through a turbid liquid or living tissue, it is necessary to dynamically adjust the wavefront as the particles in the medium move. Here we present three algorithms for constructing a wavefront that focuses through a scattering medium. We analyze the dynamic behavior of these algorithms and compare their sensitivity to measurement noise. The algorithms are compared both experimentally and using numerical simulations. The results are in good agreement with an intuitive model, which may be used to develop dynamic diffusion compensators with applications in, for example, light delivery in human tissue.     

Suppression of spin-orbit scattering in strongly disordered gold nanojunctions

We discovered that spin-orbit scattering in strongly disordered gold nanojunctions is strongly suppressed relative to that in weakly disordered gold thin films. This property is unusual because in weakly disordered films spin-orbit scattering increases with disorder. Granularity and freezing of spin-orbit scattering inside the grains explains the suppression of spin-orbit scattering. We propose a generalized Elliot-Yafet relation that applies to a strongly disordered granular regime.     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Anisotropic weak localization of light

We have observed angular anisotropy in weak localization of light from highly scattering, orientationally ordered, nematic liquid crystals. This demonstration of angular anisotropy in a multiple-scattering interference phenomenon was facilitated by a light scattering instrument with extraordinary angular resolution. The measured anisotropies were consistent with a simple model of coherent backscattering generalized for propagation-direction dependent mean free paths.     

Exceptional reduction of the diffusion constant in partially disordered photonic crystals

In real photonic crystals light is scattered by the imperfections of the periodic potential. We study experimentally the propagation of diffused light in silicon inverse opals and report an exceptionally reduced diffusion constant of 3.0+/-0.7 m(2)/s, in samples which are only partially disordered. Waves scattered both by the lattice planes and by their imperfections interfere and light is efficiently trapped in this hybrid scattering regime. Not only higher quality crystals, but also random materials present an order of magnitude bigger diffusion constant and hence weaker scattering.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Spectral changes of light produced by scattering from disordered anisotropic media

We investigate theoretically changes in the spectrum of polychromatic light scattered by a disordered, birefringent medium. We derive an expression for the spectrum of scattered light for ordinary and extraordinary incident waves within the accuracy of the first Born approximation. Using this result, we analyze the changes in the spectrum of light due to the combined action of disorder and anisotropy in the scattering process.