Focusing coherent light through opaque strongly scattering media

We report focusing of coherent light through opaque scattering materials by control of the incident wavefront. The multiply scattered light forms a focus with a brightness that is up to a factor of 1000 higher than the brightness of the normal diffuse transmission.     

Double focusing method for the coherent tomography of inhomogeneous media

A new method of double focusing is proposed for the tomography of inhomogeneous media which combines the approaches of active and passive coherent probing schemes. In combination with spatial filtering, this makes it possible to resolve inhomogeneities as small as half the operating wavelength in the probed medium. The satisfactory operation of the method is confirmed by the results of simulation modeling. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 87–92, April, 1997.     

Hidden black: coherent enhancement of absorption in strongly scattering media

We show that a weakly absorbing, strongly scattering (white) medium can be made very strongly absorbing at any frequency within its strong-scattering bandwidth by optimizing the input electromagnetic field. For uniform absorption, results from random matrix theory imply that the reflectivity of the medium can be suppressed by a factor ～(?(a)/?)N(-2), where N is the number of incident channels and ?, ?(a) are the elastic and absorption mean free paths, respectively. It is thus possible to increase absorption from a few percent to >99%. For a localized weak absorber buried in a nonabsorbing scattering medium, we find a large but bounded enhancement.     

Incoherent solitons in strongly nonlocal media: The coherent density theory

We study the properties of one dimension incoherent accessible solitons in strongly nonlocal media with noninstantaneous Kerr nonlinearity. Following the coherent density theory, we obtain an exact solution of such incoherent solitons. The spatial width of the incoherent solitons is related to the incoherent angular power spectrum θ₀ as well as the incident power. The evolution properties of the intensity profile and the coherence characteristics are also discussed in detail when the solitons undergo periodic harmonic oscillation.     

Density-matrix formalism for partially coherent optical fields propagating in slightly inhomogeneous media

The integral of motion method and the density matrix formalism are used to describe the propagation of partially coherent radiation in slightly inhomogeneous media. Expressions for the parameters of partially coherent gaussian beams in media with general square-law refractive-index distributions are obtained using operator algebra. New equations for the correlation function are formulated and beam invariants are obtained.     

Correlated coherent states and the problem of wave propagation in randomly inhomogeneous media

Radiophysics and Quantum Electronics -     

Multiple scattering and intensity fluctuations in optical coherent tomography of randomly inhomogeneous media

An expression for signal intensity fluctuations in optical coherent tomography has been obtained for the first time in the framework of the theory of the multiple scattering of low-coherent optical radiation in a random medium. The contribution of the multiple scattering of low-coherent radiation backscattered from a randomly inhomogeneous layer, as well as the speckles of the interference component in optical coherent tomography, has been calculated.     

Study of the time correlation function of scattered light in bounded strongly inhomogeneous media

The time correlation function of the multiply scattered light by a medium comprised of finite-size scatterers is studied theoretically and experimentally. A solution of the Bethe-Salpeter equation for the time correlation function is sought in the P ₂-approximation in the form of a series of terms of the Legendre polynomials. With allowance for the boundedness of the medium, the problem is reduced to a generalized Milne equation, which is solved by the Wiener-Hopf method. The time dependence of the correlation function is studied experimentally in a concentrated latex suspension for particles of different sizes. The results of numerical calculations are in qualitative agreement with the measured dependence of the time correlation function on the scatterer size.     

Phase transition in strongly inhomogeneous ferromagnets

A field theoretical model is proposed to describe the critical behaviour of a strongly inhomogeneous spin system with a position dependent concentration of magnetic atoms C(R) and magnetisation M(R). Assuming a finite number of $\text{n}$ Fouriermodes $\text{C}{}_{\mathrm{<mtext>Q</mtext>}}{}_{\mathrm{<mtext>v</mtext>}}\text{v}\text{= 1}$,..., $\text{n}$, to express C(R), the quenched randomness requires to interpret {$\text{Q}{}_{\mathrm{<mtext>v</mtext>}}\text{|}\text{Q}{}_{\mathrm{<mtext>v</mtext>}}\text{|}{}^2$} on a set of invariant or marginal lengths. As consequence, M(R) can be described by n Fourier-modes M_Qv, where $\text{n ?}\text{n}$. For short range spin-spin interaction, we find for strong inhomogeneity, i.e. large n, the critical exponent between those of the related homogeneous system and those of the spherical model.     

Hole-burning NMR in strongly inhomogeneous fields

Different pulse sequences for frequency-selective NMR in the highly inhomogeneous fields of single-sided NMR are explored. A modified Hahn-echo is used to burn a hole in the spectrum of the detected echo. The hole diminishes following molecular dynamics on the scale of the echo time. Preliminary experiments were performed on pure water and natural rubber with the NMR-MOUSE. The results demonstrate the feasibility of hole burning to study slow molecular dynamics by mobile NMR in strongly inhomogeneous magnetic fields.     

Diffusion in inhomogeneous media

The problem is to establish the correct diffusion equation in a medium that is inhomogeneous and whose temperature also varies in space. As a special model we study particles whose phase space distribution obeys Kramers' equation with a generalized collision operator. In the usual limit of strong collisions a diffusion equation is obtained. This equation contains additional drift terms, which depend on the form of the collision operator. They cannot be expressed as a mobility and a diffusion coefficient, unless the decay law of the velocity happens to be linear. Conclusion: no universal form of the diffusion equation exists, but each system has to be studied individually.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Singularity spectra of strongly inhomogeneous multifractals

Generalized multifractal formalism is used to study singularity spectra of strongly inhomogeneous multifractals characterized by coarse-grained probability measures with zero minimal and/or infinite maximal H?lder exponents. Due to involving two additional types of scaling indices, the generalized formalism is shown to be able to describe complex multifractal objects by families of bivariate spectra rather than familiar single spectra of singularity strengths of one type, providing a more complete and adequate characteristics of such objects. It is proved that the families of extended singularity spectra can reveal unusual forms with many maxima, reflecting complex scaling structures of strongly inhomogeneous multifractals. Received 25 April 2001 and Received in final form 26 February 2002     

Solution to the inverse scattering problem for strongly fluctuating media using partially coherent light

We investigate the inverse scattering problem for statistically homogeneous, isotropic random media under conditions of strong fluctuations of optical wavefields. We present a method for determining the spectral density of the dielectric constant fluctuations in such media from scattering of partially coherent light. The method may find applications to a wide class of turbulent media such as the turbulent atmosphere and certain turbulent plasmas where backscattering and depolarization effects are negligible.     

Selective saturation in strongly inhomogeneous magnetic fields

The possibility to produce selective saturation by nuclear magnetic resonance (NMR) sequences of low-power radio-frequency pulses in strongly inhomogeneous magnetic fields is explored. The saturation of parts of the sensitive volume is produced by a particular pulse sequence with reduced amplitude distribution and the spectrum of the recorded signal is compared with the simulated spectrum. The spectra of the recorded free induction decays and echo signals are in good agreement with the simulated spectra of the pulse sequence, which demonstrates the effect of the selective saturation. The results obtained are an important step towards the development of new mobile and lowpower NMR equipments operating with inhomogeneous magnetic fields.     

Phonon coherent states in transient phenomena

Phonon coherent states (PCS) are shown to be involved in electronic absorption and emission of light and in non radiative decay of molecules in gases and of centers in solids. The relations giving the absorption and emission coefficients and the non radiative decay rate constant are discussed.     

Stochastic reaction-diffusion phenomena in porous media with nonlinear kinetics: effects of quenched porosity fluctuations

We study the effects of quenched porosity fluctuations in the presence of nonlinear kinetics in a reaction-diffusion porous system. Adopting a weak-noise approximation and a mean-field assumption, a renormalized equation of motion for the average concentration of a chemical species is obtained. It is characterized by a smaller effective diffusion coefficient and by the presence of supplementary nonlinear reactive terms. Explicit results are given for a Gaussian porosity distribution and for a long-ranged one. Comparisons with simulations in one dimension are presented.