Time-resolved backscattering of circularly and linearly polarized light in a turbid medium

Time-resolved backscattering profiles of circularly and linearly polarized light were measured from a turbid medium composed of small and large polystyrene sphere particles in water. It is shown that, based on the measurements of the time-resolved backscattered copolarized and cross-polarized components of the incident polarized light, either linearly or circularly polarized light can be used to effectively image an object that is deep inside a turbid medium composed of small particles, depending on the depolarization properties of the object itself. For large particles such as in tissue, fog, and clouds, the experimentally observed polarization memory effect on the backscattering temporal profiles suggests that a significant improvement in the image contrast can be achieved by use of circularly polarized light.     

Backscattering of linearly and circularly polarized light in randomly inhomogeneous media

The scattering of linearly or circularly polarized light from a semibounded randomly inhomogeneous medium is considered. A new technique for simulating the electromagnetic radiation transport using the Monte Carlo method is proposed, which makes it possible to avoid cumbersome calculation of Muller matrices. Expressions are obtained for the co- and cross-polarized components of backscattered light for incident light of arbitrary polarization. The coherent and incoherent backscattering components are calculated for arbitrary combinations of incident and scattered light polarizations. It is shown that the main contribution to coherent backscattering is from the co- and cross-polarized components for linearly and circularly polarized light, respectively. The backscattering from an optically active random medium is calculated.     

Backscattering of circularly polarized pulses

Using numerical simulations of vector radiative transport, we examine time-resolved backscattering of circularly polarized plane waves normally incident upon a slab containing a random distribution of latex spheres in water. For large spheres the effect of polarization memory occurs a short time after first-order scattering and before depolarization. It is the result of successive near-forward-scattering events that maintain the incident wave's helicity. For moderately large scatterers it exhibits a simple dependence on the anisotropy factor. For larger spheres or those with higher refractive indices, it also depends on complicated angular and polarization characteristics of backscattering given by Mie theory.     

Multiple scattering of polarized light in a turbid medium

It is shown that multiple scattering of polarized light in a turbid medium can be represented as independent propagation of three basic modes: intensity and linearly and circularly polarized modes. Weak interaction between the basic modes can be described by perturbation theory and gives rise to “overtones” (additional polarization modes). Transport equations for the basic and additional modes are derived from a vector radiative transfer equation. Analytical solutions to these equations are found in the practically important cases of diffusive light propagation and small-angle multiple scattering. The results obtained are in good agreement with experimental and numerical results and provide an explanation for the experimentally observed difference in depolarization between linearly and circularly polarized waves.     

Three-photon ionization of the potassium atom by linearly and circularly polarized light

The measurement and calculation results of the dependence of the three-photon ionization process of potassium atoms by linearly and circularly polarized light within the frequencies from 16300 to 16600 cm^-1 are presented. The experimental data do not confirm the predictions of perturbation theory on the existence of a window of nonlinear transparency for circularly polarized light.     

Coherent backscattering of polarized light from a turbid medium

Approximate analytical expressions for the intensities of the polarized components of light reflected from a disordered medium with large discrete particles (larger than the wavelength) have been derived with the use of the method of decoupling of the vector transfer equation that is based on separate treatment of basic and additional polarization modes. The results obtained provide the relation between the peak shape in the angular distribution of the backscattered radiation with a given polarization and the optical characteristics of the medium and are in good agreement with experimental data and numerical calculations.     

Micrometre double-quantum ionization of Rydberg hydrogen using linearly and circularly polarized light

The European Physical Journal D - We investigate the two-color two-photon K-shell ionization of neutral atoms based on the relativistic second-order perturbation theory and independent particle...     

Stokes vector determination of polarized light propagation in turbid medium

We report a two dimensional Stokes vector imaging technique for transamination measurements of the polarization state of scattering medium. Measurement of the depth resolved Stokes parameters allows determination of the degree of polarization, birefringence, retardation, optical activity and characterization of the medium. The polarized light preserved and degree of polarization very with scatterer concentration. The transmitted intensity patterns by varying a polarization state of the incident laser light (λ = 632.8 nm) and changing analyzer configuration provides a useful information about concentration, orientation, and shape of the sample under investigation. The results are important for the understanding of polarization phenomenon in turbid media, like biological tissues.     

Diffusion of circularly polarized light in a disordered medium with large-scale inhomogeneities

It is shown that in the presence of diffusion of electromagnetic waves in random media containing large discrete scatterers, circular polarization can persist even after the radiation flux is isotropized. For scattering exactly in the backward direction, this effect is manifested as an increase in the interference contribution to the cross-polarized component of the intensity as the size of the scatterers increases. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 21–26 (10 July 1998)     

Amplification of a weak circularly polarized light signal in a multilevel atomic medium

Amplification of a weak circularly polarized light signal in a multilevel atomic medium excited by a strong linearly polarized resonant radiation is studied. A strong optical pumping may lead to the population inversion between magnetic sublevels of hyperfine components of the ground and excited states. Solution to the self-consistent system of density matrix equations and Maxwell equations for propagation of the weak signal allows analyzing optimum conditions of amplification.     

Imaging through turbid media by polarized light

A theoretical approach to the calculation of the image of an inhomogeneity in transillumination of tissue-like media using polarized radiation is developed in the approximation of the basic polarization modes. The method is based on the solution of the nonstationary transfer equation in the Fokker-Planck approximation. An expression for the edge spread function of the image of a totally absorbing half-plane is derived. A generalization to the image profile of inhomogeneity that represents a finite-width stripe is presented. The spatial resolution (sharpness) and contrast of image are analyzed at various time intervals of detection. The image parameters are compared for the polarization-difference technique and imaging in unpolarized light. It is demonstrated that the imaging using the difference of linear polarizations is similar to the imaging using the pulsed unpolarized radiation with a detection interval of about z/2c (z is the thickness of the sample and c is the velocity of light). The results are in agreement with the experimental data on the differential polarization transillumination of tissue-like media.     

Depolarization of linearly polarized light by a polycrystalline specimen

One of the methods in use for measuring the kinetics of isothermal crystallization of polymers is the recording of the depolarization of plane-polarized light by a microscope specimen. A relation between this depolarization and the volume fraction crystallized has been derived taking into account the polycrystalline nature of the specimen and assuming uniaxially birefringent crystallites. Uniform, all-directional increase in size of the crystallites would cause the depolarization to follow an Avrami-type equation with the Avrami exponent one unit “too high.” For spherulitic growth the correct Avrami exponent is found on the assumption that the birefringent entities in the spherulites do not increase in size but only in number. The depolarization by a polycrystalline specimen of uniform particle size is proportional to both specimen thickness and particle size. The derived relations are tested by random number calculation and their limits of reliability are indicated accordingly.     

光强连续可调的圆偏振光衰减器

利用1/4波片及偏光棱镜,设计了光强连续可调的圆偏振光衰减器,通过该器件可获得偏振度高且光强连续可调的圆偏振光.该衰减器解决了632.8 nm多膜氦氖激光器出射光为非完全自然光且偏振面随时间做随机变化导致实验误差的问题,为偏光信息测试系统提供了相对稳定的激光光源.     

Depth-resolved fluorescence measurement in a layered turbid medium by polarized fluorescence spectroscopy

We show that, when a turbid medium with a layered fluorophore distribution is excited by linearly polarized light, measurement of angle-resolved polarized fluorescence can provide depth-resolved fluorescence measurements.     

Photomagnetization of polydomain magnetic insulators by circularly polarized light

A new effect of magnetization of magnetic insulators due to light absorption by magnetic ions is proposed. The estimates for an yttrium ion garnet crystal are made.     

Dynamic Hall effect driven by circularly polarized light in a graphene layer

We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates--under oblique incidence--an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed E and B fields which are, however rotating with the light's frequency.     

Formation of linearly polarized light with axial symmetry by use of space-variant subwavelength gratings

We present a novel method for forming linearly polarized axially symmetric beams with various polarization orders that is based on computer-generated space-variant subwavelength gratings. We introduce and experimentally demonstrate that our space-variant polarization state manipulations are accompanied by a phase modification of a helical structure that results from the Pancharatnam-Berry phase. We have verified the polarization properties of our gratings for laser radiation at 10.6-microm wavelength.     

Coherent trapping of a three-level atom in a circularly polarized light

Using semiclassical theory, we study coherent trapping of a three-level atom, where the atom possesses a momentum of its center-of-mass motion and is irradiated only by a classical circularly polarized electromagnetic wave. We find that if the atom is initially in a coherent trapping state of it, under the zero- or first-order approximation, the atom is absolutely or nearly in the state hereafter.     

Transmission of polarized light through turbid media

The depolarization of light in multiple-scattering media with large (larger than the light wavelength) inhomogeneities is considered. The polarization state of the scattered light is described in the principal-mode approximation. Using the Fokker-Planck model, the polarization and intensity distribution of light are calculated in the vicinity of an inhomogeneity in the shape of an absorbing half-plane. The results of the calculations agree with the experimental data on transmission of light through turbid media.     

Random walk of polarized light in turbid media

We study the propagation of polarized light in turbid media as a random walk of vector photons. Both propagation and polarization directions of light are found to isotropize, following a power law of the number of scattering events. The characteristic length scale governing light isotropization and linear depolarization, the isotropization length , is derived using the exact Mie scattering for spherical particles. A simple relation is obtained for Rayleigh-Gans scatterers where is the transport mean free path and is the mean cosine of scattering angles.