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.     

基于压缩感知的动态散射成像

利用基于压缩感知的成像系统可以透过静态的散射介质获得高质量的重建图像. 但是当散射介质动态变化时, 因为采样所得的测量值受到散射介质衰减系数非线性变化的影响, 重建图像质量会大大下降. 针对上述情况, 本文提出基于压缩感知成像系统的测量值线性拉伸算法, 该算法能够对所得到的非线性测量值进行分析, 根据测量值大小的不同将测量值划分成数个区域并计算补偿系数, 从而根据补偿系数进行测量值线性拉伸变换, 使测量值线性化. 最后再对变换后的测量值进行压缩感知重建计算. 通过理论分析、计算机仿真和实验证明了所提算法能够有效地应对动态的散射介质, 提高基于压缩感知成像系统在透过动态散射介质时的图像重建质量.     

Quantum magnetic flux through helical molecules in optically active media

After study of an electron moving in a loop of wire in an uniform external magnetic field with its velocity vector perpendicular to the field and quantization of the angular momentum of the moving electron in the equilibrium state, we find the quantum magnetic flux through the solenoids or loops of wire, like the quantum magnetic flux trapped in hollow superconducting cylinders. Optically active media have the helical molecular structure, which acts as the natural micro-solenoid for the electromagnetic waves passing through them. Applying the result of the quantized magnetic field in the propagation direction induced by helical molecules, we find that optic activity is the natural Faraday effect, when the optical rotation of a plane-polarized wave through an optically active medium is caused by the quantized magnetic field induced by the incident light, which has been confirmed by the experimental observations on α-quartz. Through measurements of the rotatory power and the Verdet constant of an optically active substance, we can determine the quantized magnetic field. PACS 03.70.+k; 33.55.Ad; 74.25.Ha; 78.20.Ek; 78.20.Ls     

Imaging through dynamic scattering media with stitched speckle patterns

Imaging through scattering media via speckle autocorrelation is a popular method based on the optical memory effect.However,it fails if the amount of valid information acquired is insufficient due to a limited sensor size.In this Letter,we reveal a relationship between the detector and object sizes for the minimum requirement to ensure image reconstruction by defining a sampling ratio R,and propose a method to enhance the image quality at a small R by capturing multiple frames of speckle patterns and piecing them together.This method will be helpful in expanding applications of speckle autocorrelation to remote sensing,underwater probing,and so on.     

Imaging through scattering media with the double aperture set-up

To obtain the image of an object obscured by moving scattering media, a set-up involving a lens with double aperture is employed. The condition for the minimum speed of the scattering media is first derived, the evaluation of which using the set-up parameters yields 0.038 km/h. An improved reconstructed image is then confirmed in the experiment. Advantages of the technique include the relief of the need, respectively, for a local reference beam and a mechanically very stable set-up.     

Self-switching of radiation in magnetically active and optically active media

Features of self-switching of unidirectional distributed coupled waves (UDCWs) in magnetically active and optically active media are considered. The linear coupling of these UDCWs is caused by the rotation of the plane of polarization, i.e., by the Faraday effect.     

Rayleigh and Raman scattering from optically active molecules

The intensity of Rayleigh and Raman scattering from optically active molecules is shown to be slightly different in right and left circularly polarized incident light. The circular intensity differential of the Rayleigh line is dependent on components of the optical activity tensor, and that of the Raman lines is a function of the variation of the optical activity with the vibrational coordinates. This circular intensity differential might be of the order of 10^-3 times that of the Rayleigh or Raman intensity.     

Surface plasmon-polariton waves in optically active media

A theory of surface plasmon-polariton wave propagation along an interface of an optically active medium and metal or metamaterial has been developed. It is shown that optical activity leads to changing of cross section and polarization of the surface modes, whereas the propagation constant in first approximation does not depend on the gyrotropy coefficient.     

Imaging optically scattering objects with ultrasound-modulated optical tomography

We show the feasibility of imaging objects having different optical scattering coefficients relative to the surrounding scattering medium using ultrasound-modulated optical tomography (UOT). While the spatial resolution depends on ultrasound parameters, the image contrast depends on the difference in scattering coefficient between the object and the surrounding medium. Experimental measurements obtained with a CCD-based speckle contrast detection scheme are in agreement with Monte Carlo simulations and analytical calculations. This study complements previous UOT experiments that demonstrated optical absorption contrast.     

Laser beam-induced non-linear effects in optically active media

A general equation is proposed for the dynamical electric permittivity tensor, taking into account, besides frequency dispersion, spatial dispersion describing the linear and non-linear optical activities of the medium. It permits one to determine the symmetry relations and value of the optical Kerr effect, a generalised Havelock relation, and the non-linear change in optical rotation angle for arbitrary conditions of observation. Simple examples are adduced to give a microscopic interpretation of the results derived, rendering apparent the basic mechanisms (fluctuations of density, non-linear changes in the optical polarisability tensor and gyration tensor, molecular reorientation, and various radial and angular correlations) leading to induced optical non-linearities. Measurements of non-linear changes in optical rotation angle are shown to be promising in solutions of polymers or colloids, and will permit,inter alia, direct determinations of the anisotropy of gyration properties of molecules macromolecules and colloid particles.A brief summary of the contents is to be found in the Review of the International Quantum Electronics Conference held at Miami on 14–17 May 1968, published inIEEE J. Quantum Electronics,QE-4 (1968) 330.     

Low-order light scattering in multiple scattering disperse media

Light scattering has been investigated in systems in which both single and higher order scattering occur. The Monte Carlo simulation technique for studying light scattering in randomly inhomogeneous, strongly scattering disperse media was employed. The reliability of the data obtained has been checked by comparing the results of the computer simulation with analytical calculations for the intensity of doubly scattered light. The first several scattering orders have been analyzed for different geometries of the optical experiment. It has been shown, in particular, that, depending on the detector aperture, the contribution of multiple scattering can vary by almost an order of magnitude.

     

Stereoscopic imaging through scattering media

We develop and experimentally test a method for three-dimensional imaging of hidden objects in a scattering medium. In our scheme, objects hidden between two biological tissues at different depths from the viewing system are recovered, and their three-dimensional locations are computed. Analogous to a fly's two eyes, two microlens arrays are used to observe the hidden objects from different perspectives. At the output of each lens array we construct the objects from several sets of many speckled images with a previously suggested technique that uses a reference point. The differences of the reconstructed images in both arrays with respect to the reference point yield the information regarding the relative depth among the various objects.     

Circular electrodichroism of light in isotropic optically active media

Circular electrodichroism is predicted in liquids and gases containing unequal concentrations of mirror stereoisomers of chiral molecules. A longitudinal (quasi)static electric field increases absorption for a light wave circularly polarized in one direction and decreases the absorption for a wave circularly polarized in the opposite direction. The corresponding nonlinear susceptibility is proportional to the decay constants of the excited states and is absent in a nondissipative medium. Estimates of the magnitude of the effect are presented and show that it may well be observable experimentally. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 515–520 (25 October 1997)     

Photoacoustic-guided convergence of light through optically diffusive media

We demonstrate that laser beams can be converged toward a light-absorbing target through optically diffusive media by using photoacoustic-guided interferometric focusing. The convergence of light is achieved by shaping the wavefront of the incident light with a deformable mirror to maximize the photoacoustic signal, which is proportional to the scattered light intensity at the light absorber.     

Imaging through scattering media using characteristics of the temporal distribution of transmitted laser pulses

An object consisting of small inhomogeneities embedded in a highly scattering solution was imaged using measurements of the time-resolved transmitted intensity of picosecond pulses of near-infrared light. Data acquisition involved translating the object in two orthogonal directions across the beam, and recording the temporal distribution of transmitted light at a series of discrete positions. Images were constructed from the total transmitted light, the first four moments of the temporal distribution, and from parameters derived from a comparison of the distribution with an analytical model, based on the diffusion approximation to the radiative transfer theory. The results show that the optical properties along a line-of-sight between source and detector influence some of these characteristics more than others.     

Light scattering in a layer of correlatively arranged optically soft particles

The light scattering by an ensemble of monodisperse spatially correlated optically soft spherical particles is studied in the interference approximation. A model of the interaction of particles is proposed in which the spatial correlation between particles is determined by a radius R _c exceeding the particle radius R _p. The radial distribution function is calculated in the Percus-Yevick approximation for hard spheres of the radius R _c. To simulate the radiation scattering from an individual particle of the radius R _p, the Mie equations are used. It is shown that, in a medium of correlated small nonabsorbing particles of the radius R _c > R _p, an abnormal wavelength dependence of the refractive index is possible at a low volume concentration of particles. The results obtained explain some experimentally observed features of the scattering in sodium borosilicate glasses with a small concentration of scattering centers.     

Interior radiances in optically deep absorbing media—II Rayleigh scattering

The interior radiances are calculated within an optically deep absorbing medium scattering according to the Rayleigh phase function. The accuracy of the matrix operator method is improved by many orders of magnitude through the use of accurate starting values obtained by the Runge-Kutta method rather than from the single scattering approximation. The radiance and flux are given for a range of solar zenith angles and for single scattering albedos of 1, 0.99, 0.9, 0.5 and 0.1. The development of the asymptotic angular distribution of the radiance is illustrated. It is shown that this asymptotic distribution is probably physically unobservable when ω₀ < 0.8, since the flux is less than 10^-8 of its original value at the beginning of the asymptotic region. The ratio of the upward to downward flux is calculated and is shown to be remarkably constant within the medium except very close to the boundaries. The heating rate within the medium is found to be very nearly proportional to the downward flux, except near the boundaries. When the single scattering albedo is small, a number of examples illustrate the significant contribution of the direct solar flux to the total flux even at great optical depths within the medium. The total downward flux decreases exponentially with optical depth away from boundaries when the single scattering albedo is greater than or equal to 0.9; when it is less than or equal to 0.5 only an approximate exponential fit can be obtained within the region accessible to experimental observation.     

On the theory of multiple scattering of sound waves by spherical particles in liquid and elastic media

Two independent systems of equations are derived for describing the scalar and vector potentials of the sound field in a liquid or elastic medium containing discrete inhomogeneities. One of the systems determines the sound field as the sum of the fields scattered by the particles according to the law of scattering by a single particle with the oscillation amplitudes governed by the properties of the inhomogeneous medium. The other system determines the sound field as the sum of the scattered fields formed in the inhomogeneous medium with the oscillation amplitudes of a particle in a homogeneous medium. Expressions relating the fields that occur in a medium consisting of N particles to the fields in a medium consisting of N − 1 particles are proposed. These expressions may simplify and diversify the methods used for computer simulation of sound fields with the aim to verify the calculations. The results of the study are valid for any particle concentrations under the condition that the scattering by a single particle is determined by its monopole, dipole, and rotary oscillations.