Mechanism and symmetry properties of depolarization in weak scattering of light

The mechanism and some symmetry properties of depolarization upon weak scattering of light from a class of random media were studied theoretically. Departing from the angular distribution of the degree of polarization, our derivations showed the mechanism that induces the change of polarization can be split into two parts of different nature. One is the vectorial effect that redistributes the original light components, and the other is the interaction effect of the medium that modulates the correlation properties of the incident field. We also showed that there is dependence of the angular distribution on the incident polarization state; i.e., the angular pattern and its symmetry depend on both the orientation and ellipticity of the incident polarization. Random light was analyzed in the space-frequency domain.     

Localizable imaging study for the noninvasive diagnosis of lesions with the backscattering polarized light

The asymmetry appearing in the degree of polarization (DOP) distribution of the backscattering polarized light from tissues is investigated by using polarized Monte Carlo simulation. When the incident point is close to the boundary of the lesion inside the tissue, high asymmetry emerges regardless of the polarized direction of the incident light. A noninvasive method based upon the DOP asymmetry of the backscattering light is proposed to locate lesions hidden in live tissues by scanning a point light source. Imaging of the front projection on complicated lesion structures is demonstrated with this method. Its transverse resolution, which is affected by the wavelength of incident light and the size of the scattering particle, can reach the diameter of the lesion scattering particle theoretically while the best longitudinal detection depth can be achieved by choosing a suitable incident wavelength according to the scattering characters of the tissue.     

Retrieval of the scattering and microphysical properties of aerosols from ground-based optical measurements including polarization. I. Method

A method has been developed for retrieving the scattering and microphysical properties of atmospheric aerosol from measurements of solar transmission, aureole, and angular distribution of the scattered and polarized sky light in the solar principal plane. Numerical simulations of measurements have been used to investigate the feasibility of the method and to test the algorithm's performance. It is shown that the absorption and scattering properties of an aerosol, i.e., the single-scattering albedo, the phase function, and the polarization for single scattering of incident unpolarized light, can be obtained by use of radiative transfer calculations to correct the values of scattered radiance and polarized radiance for multiple scattering, Rayleigh scattering, and the influence of ground. The method requires only measurement of the aerosol's optical thickness and an estimate of the ground's reflectance and does not need any specific assumption about properties of the aerosol. The accuracy of the retrieved phase function and polarization of the aerosols is examined at near-infrared wavelengths (e.g., 0.870 mum). The aerosol's microphysical properties (size distribution and complex refractive index) are derived in a second step. The real part of the refractive index is a strong function of the polarization, whereas the imaginary part is strongly dependent on the sky's radiance and the retrieved single-scattering albedo. It is demonstrated that inclusion of polarization data yields the real part of the refractive index.     

Second harmonic generation from electrically polarized CuCl-microcrystallite-doped glass

Second harmonic generation has been observed in electrically polarized CuCl-microcrystallite-doped glass. The glass was prepared by means of melting–quenching method and poled at 200°C for 1 h under applied dc field of 4 kV. The incident angular variation of SHG intensity for the polarized glass was measured. To explain the effect, it is proposed that the second-order nonlinear polarization results from the dipolar orientation in the glass.     

The theoretical design and fabrication of a prism-coupled polarization conversion ferroelectric liquid crystal light modulator

It is possible to probe directly the optical dielectric tensor configuration within thin smectic layers (less than 6 μm thick) of ferroelectric liquid crystals (FLCs) by the propagation of optical prism-coupled leaky Fabry-Pérot modes. Incident polarized monochromatic light couples into the resonant modes of the system and may be coupled out of the cell in an orthogonal polarization. The observed reflectivity is a series of sharp peaks on a low background response at certain well-defined incident angles. These sharp resonant features make the prism-coupling technique a possible route for commercial fabrication of voltage-modulated devices. However, previous prism-coupled cells with sharp resonant guided mode features are not practical from a device point of view because they incorporate silver layers not used in conventional cell design. In this paper we demonstrate for the first time operating a leaky guided mode FLC cell with conventional surface layers, in a sp-mixed polarization mode of operation, allows sharp features to be observed which are modulated in intensity by an applied d.c. voltage. The prism-coupled cells used here are designed to be compatible with current FLC device technology.     

Theoretical analysis of polarization patterns from incoherent backscattering of light

Anisotropy in the polarization of the backscattered light from a polarized laser beam incident upon a scattering medium has been observed experimentally. When the beam is viewed through an oriented polarizer, characteristic patterns in the backscattered light are observed. We present here a simple explanation of these patterns, using the theory of incoherent scattering of light by spheres. It appears that the major contribution to the observed patterns comes from the double scattering of light.     

Optical Mode Characterization of the Configuration of a Thin Ferroelectric Liquid Crystal Cell Under an Applied Electric Field

Abstract

It is possible to probe directly the optic dielectric tensor configuration within a thin (about 3·5 μm) smectic layer of a ferroelectric liquid crystal (FLC) by the propagation of prism-coupled leaky Fabry-Perot optic modes. Incident monochromatic polarized light couples into the resonant modes of the system which are then observed as sharp dips in the reflected signal at specific incident angles. This work has shown that the FLC has an optic tensor profile which is consistent with the chevron formation of the smectic layers observed in X-ray scattering measurements. Also it is shown that under the application of a low d.c. applied field the optic dielectric tensor configuration distorts in a manner consistent with the retention of the chevron structure up to fields of about 10⁶ Vm^?1.     

Form-birefringent space-variant inhomogeneous medium element for shaping point-spread functions

We experimentally characterize the properties of an element that generates a doughnutlike point-spread function by converting the linearly polarized incident field to radially or azimuthally polarized light utilizing space-variant inhomogeneous medium (SVIM) form-birefringent subwavelength structures. To fabricate the high-aspect-ratio SVIM structures, we developed a chemically assisted ion-beam-etching process that permits control of the fabricated form-birefringent structure profile to optimize the effect of birefringence and the impedance mismatch on the substrate-air interface. Fabricated elements perform efficient polarization conversion for incident angles as large as 30 degrees, where the extinction ratio is found to be better than 4.5. The intensity distribution in the far field shows that our SVIM device generates a doughnut point-spread function that may prove useful for various applications.     

Detection of biological particles by the use of circular dichroism measurements improved by scattering theory

Light scattered from optically active spheres was theoretically analyzed for biodetection. The circularly polarized signal of near-forward scattering from circularly dichroic spheres was calculated. Both remote and point biodetection were considered. The analysis included the effect of a circular aperture and beam block at the detector. If the incident light is linearly polarized, a false signal would limit the sensitivity of the biodetector. If the incident light is randomly polarized, shot noise would limit the sensitivity. Suggested improvements to current techniques include a beam block, precise angular measurements, randomly polarized light, index-matching fluid, and larger apertures for large particles.     

Symmetrically coated pellicle beam splitters for dual quarter-wave retardation in reflection and transmission

A trilayer pellicle that consists of a high-index center layer that is symmetrically coated on both sides by a low-index film can be designed to produce differential reflection and transmission phase shifts of +/- 90 degrees at oblique incidence and equal throughput for the p and the s polarizations. Such a device splits a beam of incident linearly polarized light into two orthogonal circularly polarized components that travel in well-separated angular directions. Examples of infrared dual quarter-wave retarders that use a symmetrically coated Ge pellicle at 77 degrees angle of incidence are presented. A 50-50% splitter requires a symmetric pellicle with at least five layers. Error analysis shows that the thicknesses of the high-index layers must be tightly controlled. These circular polarization beam splitters are intended for operation with a well-collimated light source and can be used as the basis of a novel circular polarization Michelson interferometer.     

Polarization properties of inversely twisted nematic liquid-crystal gratings

Based on the Jones matrix representation of twisted nematic liquid crystals (LC's), we have carried out a theoretical analysis of the polarization properties of inversely twisted nematic (ITN) LC gratings. Some interesting polarization behaviors are expected in the ITN LC grating. When a linearly polarized light parallel or perpendicular to the grating direction is incident on the ITN LC grating, the diffracted light in the 0th order is linearly polarized with the same polarization direction of incident light, while the diffracted light in high orders is linearly polarized perpendicular to that of incident light. Using a multirubbing alignment technique, we have practically prepared an ITN LC grating with ?45 degrees inversely twisted structures. The experimental investigations of the optical characteristics of the ITN LC grating demonstrate agreement with theoretical expectations.     

Characterizing Mammalian cells and cell phantoms by polarized backscattering fiber-optic measurements

Fiber-optic, polarized elastic-scattering spectroscopy techniques are implemented and demonstrated as a method for determining both scatterer size and concentration in highly scattering media. Measurements of polystyrene spheres are presented to validate the technique. Measurements of biological cells provide an estimate of the average effective scatterer radius of 0.5-1.0 mum. This average effective scatterer size is significantly smaller than the nucleus. In addition, to facilitate use of polarization techniques on biological cells, polarized angular dependent scattering from cell suspensions was measured. The light scattering from cells has properties similar to those of small spheres.     

Polarization dependence of surface-enhanced Raman scattering in gold nanoparticle-nanowire systems

We study the polarization dependence of surface-enhanced Raman scattering (SERS) in coupled gold nanoparticle-nanowire systems. The coupling between the continuous nanowire plasmons and the localized nanoparticle plasmons results in significant field enhancements and SERS enhancements comparable to those found in nanoparticle dimer junctions. The SERS intensity is maximal when the incident light is polarized across the particle and the wire, and the enhancement is remarkably insensitive to the detailed geometrical structures of the nanoparticles.     

Depolarization of diffusely reflecting man-made objects

The polarization properties of light scattered or diffusely reflected from seven different man-made samples are studied. For each diffusely reflecting sample an in-plane Mueller matrix bidirectional reflectance distribution function is measured at a fixed bistatic angle using a Mueller matrix imaging polarimeter. The measured profile of depolarization index with changing scattering geometry for most samples is well approximated by an inverted Gaussian function. Depolarization is minimum for specular reflection and increases asymptotically in a Gaussian fashion as the angles of incidence and scatter increase. Parameters of the Gaussian profiles fitted to the depolarization data are used to compare samples. The dependence of depolarization on the incident polarization state is compared for each Stokes basis vector: horizontal, vertical, 45 degrees, 135 degrees, and right- and left-circular polarized light. Linear states exhibit similar depolarization profiles that typically differ in value by less than 0.06 (where 1.0 indicates complete depolarization). Circular polarization states are depolarized more than linear states for all samples tested, with the output degree of polarization reduced from that of linear states by as much as 0.15. The depolarization difference between linear and circular states varies significantly between samples.     

Polarized light scattering by aerosols in the marine atmospheric boundary layer

The intensity and polarization of light scattered from marine aerosols affect visibility and contrast in the marine atmospheric boundary layer (MABL). The polarization properties of scattered light in the MABL vary with size, refractive index, number distributions, and environmental conditions. Laboratory measurements were used to determine the characteristics and variability of the polarization of light scattered by aerosols similar to those in the MABL. Scattering from laboratory-generated sea-salt-containing (SSC) [NaCl, (NH(4))(2) SO(4), and seawater] components of marine aerosols was measured with a scanning polarization-modulated nephelometer. Mie theory with Gaussian and log normal size distributions of spheres was used to calculate the polarized light scattering from various aerosol composition models and from experimentally determined distributions of aerosols in the marine boundary layer. The modeling was verified by comparison with scattering from distilled water aerosols. The study suggests that polarimetric techniques can be used to enhance techniques for improving visibility and remote imaging for various aerosol types, Sun angles, and viewing conditions.     

Optical Transmission in Polycrystals

Optical transmission in a polycrystalline aggregate consisting of randomly oriented anisotropic transparent crystals has been worked using Mueller matrices. The interesting results are: (1) A completely polarized beam of light emerges as partially polarized light. (2) The completely polarized part of the emergent beam is in the same state of polarization as the incident light for optically inactive crystallites. (3) The intensity of the completely polarized part not only depends on the optical and the geometrical parameters of the crystallite but also on the polarization state of the incident light. (4) For optically active crystallites the medium behaves as an isotropic optically active solid with a rotatory power equal to the mean rotatory power of the single crystal. This helps one in extracting most of the gyration tensor components in enantiomorphic optically active crystals.     

Spectrogonio radiometer for the study of the bidirectional reflectance and polarization functions of planetary surfaces. 1. Design and tests

We have developed a spectrogonio radiometer to measure in the laboratory (-35 degrees C to +30 degrees C) the bidirectional reflectance and polarization distribution functions of various types of planetary material from the UV to the near-IR (310-4800 nm). The major, to our knowledge, novel feature of this instrument is that it is capable of measuring dark to translucent materials with a high degree of radiometric accuracy under most viewing geometries. The sample surface is illuminated with a large monochromatic and polarized parallel beam (incidence: 0 degrees-90 degrees ), and the total intensity and the two polarized components of the reflected light are measured (observation, 0 degrees-80 degrees; azimuth, 0 degrees-180 degrees). The scientific and technical constraints, the design, and the performances and limitations of the system are presented in this first paper.     

Geometrical-optics approximation of forward scattering by gradient-index spheres

By means of geometrical optics we present an approximation method for acceleration of the computation of the scattering intensity distribution within a forward angular range (0-60 degrees ) for gradient-index spheres illuminated by a plane wave. The incident angle of reflected light is determined by the scattering angle, thus improving the approximation accuracy. The scattering angle and the optical path length are numerically integrated by a general-purpose integrator. With some special index models, the scattering angle and the optical path length can be expressed by a unique function and the calculation is faster. This method is proved effective for transparent particles with size parameters greater than 50. It fails to give good approximation results at scattering angles whose refractive rays are in the backward direction. For different index models, the geometrical-optics approximation is effective only for forward angles, typically those less than 60 degrees or when the refractive-index difference of a particle is less than a certain value.     

Large-Angle in-Plane Light Scattering from Rough Surfaces: Comment

A recent paper by Karabacak et al. [Appl. Opt. 39, 4658 (2000)], which discussed the scattering from rough surfaces in directions out of the plane of incidence, exhibited an error in the derivation of a polarization factor. An asymmetry in the scattering function for directions out of the plane of incidence and for circularly polarized incident light is predicted by the correct derivation of this factor and can be observed in their data.     

Influence of particle size and concentration on the diffuse backscattering of polarized light from tissue phantoms and biological cell suspensions

We present experimental results that show the spatial variations of the diffuse-backscattered intensity when linearly polarized light is incident upon highly scattering media. Experiments on polystyrene-sphere and Intralipid suspensions demonstrate that the radial and azimuthal variations of the observed pattern depend on the concentration, size, and anisotropy factor g of the particles that constitute the scattering medium. Measurements performed on biological-cell suspensions show the potential of this method for cell characterization.