Complex ray tracing in uniaxial absorbing media

We theoretically analyze waves propagating between an isotropic nonabsorbing medium and a uniaxial absorbing medium in the general case where the incidence plane does not coincide with the principal section of the uniaxial crystal. Expressions for the reflection and transmission coefficients are derived by a combination of complex ray tracing and the 4 x 4 matrix method. The work presented here is useful for the design of optical systems that incorporate the use of birefringent absorbing components.     

Photon diffusion coefficient in scattering and absorbing media

We present a unified derivation of the photon diffusion coefficient for both steady-state and time-dependent transport in disordered absorbing media. The derivation is based on a modal analysis of the time-dependent radiative transfer equation. This approach confirms that the dynamic diffusion coefficient is given by the random-walk result D = cl(*)/3, where l(*) is the transport mean free path and c is the energy velocity, independent of the level of absorption. It also shows that the diffusion coefficient for steady-state transport, often used in biomedical optics, depends on absorption, in agreement with recent theoretical and experimental works. These two results resolve a recurrent controversy in light propagation and imaging in scattering media.     

The mechanism of liquid metal jet formation in the cathode spot of vacuum arc discharge

We have theoretically studied the dynamics of molten metal during crater formation in the cathode spot of vacuum arc discharge. At the initial stage, a liquid-metal ridge is formed around the crater. This process has been numerically simulated in the framework of the two-dimensional axisymmetric heat and mass transfer problem in the approximation of viscous incompressible liquid. At a more developed stage, the motion of liquid metal loses axial symmetry, which corresponds to a tendency toward jet formation. The development of azimuthal instabilities of the ridge is analyzed in terms of dispersion relations for surface waves. It is shown that maximum increments correspond to instability of the Rayleigh–Plateau type. Estimations of the time of formation of liquid metal jets and their probable number are obtained.

     

Propagation of Helmholtz-Gauss beams in absorbing and gain media

The propagation of Helmholtz-Gauss beams in media exhibiting loss or gain is studied. The general expressions for the field propagation, the time-averaged power on propagation, the trajectory of the beam centroid, the beam spreading, the nondiffracting distance, and the far field are derived and discussed. Explicit expressions of these parameters for Bessel-Gauss and cosine-Gauss beams are included. The general expressions can be applied straightforwardly to describe the propagation of Mathieu-Gauss and parabolic-Gauss beams in complex media as well.     

High-order dark solitons in nonlocal nonlinear media

Abstract

The high-order dark solitons in nonlocal nonlinear media are found up to fifth-order. The stability of high-order dark solitons is investigated numerically. The stability relation graphs of negative imaginary eigenvalues versus propagation constant are obtained. The propagation graphs of the input signal with noise added prove the result of the stability analysis to be correct.     

Definition of the diffusion coefficient in scattering and absorbing media

We revisit the definition of the diffusion coefficient for light transport in scattering and absorbing media. From an asymptotic analysis of the transport equation, we present a novel derivation of the diffusion coefficient, which is restricted neither to low absorption nor to a situation in which the specific intensity is quasi-isotropic. Our result agrees with previous expressions of the diffusion coefficient in the appropriate limit. Using numerical simulations, we discuss the implications of the proper choice of the diffusion coefficient for time-dependent transport.     

The dark side of galaxy formation

I discuss the discovery of a population of extremely luminous, but very dusty and very distant, galaxies in the submillimetre (submm) waveband. Almost all the light emitted by the stars in these galaxies is absorbed by interstellar dust (which is produced by the same stars) and re-radiated in the far-infrared. This leaves little to be detected at optical wavelengths and results in most of these galaxies being effectively invisible in even the deepest optical images obtainable with the Hubble space telescope. Yet this population contributes most of the light emitted by galaxies at wavelengths of lambda > or approximately equal 100 microm over the lifetime of the Universe. Together with other observations, this suggests that perhaps up to half of all the stars seen in galaxies today were formed in very dusty regions in the early Universe. Hence, studying the galaxies detected in the submm wavebands is critical for developing and testing models of galaxy formation and evolution. Individually, these luminous submm galaxies are forming stars a thousand times faster than our Galaxy is at the present-day, sufficiently fast to form all the stars in the most luminous galaxy in the local Universe within a short period, up to ca. 0.1-1 Gyr. Detailed study of a handful of examples of this population confirm these estimates and unequivocally identify the bulk of this submm-selected population with dusty, star-burst galaxies in the very distant Universe. The extreme faintness of this population in the optical and near-infrared wavebands, resulting from their obscuration by dust, means that our understanding of the detailed nature of these galaxies is only slowly growing. I give a brief summary of the properties of these highly obscured systems and describe the wide range of facilities currently being developed that will greatly aid in their study.     

Multiparameter measurement of absorbing liquid by time-resolved photoacoustics

Measuring constituent concentrations of processing liquids provides highly useful data for industrial process control. Techniques that allow online measurement will greatly save resources and energy, making them highly attractive for enterprises. In this paper, we develop a technique based on time-resolved photoacoustics for simultaneously measuring the optical absorption coefficient, acoustic speed, and thermal-acoustic transformation coefficient of an absorbing liquid, using an experimental setup that merely employs a nanosecond pulsed laser with millijoule energy and a single piezoelectric transducer with a wide frequency bandwidth. As investigated samples, we use potassium chromate, glucose, and their mixing solutions. Experimental results show that the value of each parameter measured in a mixed solution is approximately equal to the sum value of the same parameter in the constituent solutions. This means that a simultaneous measurement of these parameters enables us to calculate two or three constituent concentrations in a mixed liquid, if the constituent substances differ clearly from one another in terms of their optical absorption, acoustic speed, or thermal-acoustic transformation properties.     

Experimental determination of photon propagation in highly absorbing and scattering media

Optical imaging and tomography in tissues can facilitate the quantitative study of several important chromophores and fluorophores. Several theoretical models have been validated for diffuse photon propagation in highly scattering and low-absorbing media that describe the optical appearance of tissues in the near-infrared (NIR) region. However, these models are not generally applicable to quantitative optical investigations in the visible because of the significantly higher tissue absorption in this spectral region compared with that in the NIR. We performed photon measurements through highly scattering and absorbing media for ratios of the absorption coefficient to the reduced scattering coefficient ranging approximately from zero to one. We examined experimentally the performance of the absorption-dependent diffusion coefficient defined by Aronson and Corngold [J. Opt. Soc. Am. A 16, 1066 (1999)] for quantitative estimations of photon propagation in the low- and high-absorption regimes. Through steady-state measurements we verified that the transmitted intensity is well described by the diffusion equation by considering a modified diffusion coefficient with a nonlinear dependence on the absorption. This study confirms that simple analytical solutions based on the diffusion approximation are suitable even for high-absorption regimes and shows that diffusion-approximation-based models are valid for quantitative measurements and tomographic imaging of tissues in the visible.     

Physical essence of erosional plasma formation and the formation of plasma flows by the action of different types of high-energy sources on absorbing condensed media

A study is made of erosional plasma processes occurring with the exposure of metals to a laser beam, a plasma, and a combination of the two. It is found that the ionization of the initially heated vapors is of a thermal nature, whether an erosional plasma is formed by an impulsive electrical discharge or a near-surface optical discharge.Institute of Physics, Academy of Sciences of Belarus, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 62, No. 5, pp. 685–686, May, 1992.     

Estimation of a radiative property of scattering and absorbing media

This study proposes a simple estimation method for the albedo, which is one of the radiative properties of scattering and absorbing media. The method enables us to determine the albedo from the measured emittance of the medium without a complicated inverse analysis, using the relation that the emittance of the optically thick medium depends only on the albedo when the scattering occurring in the medium is assumed to be isotropic. This study also deals with a measurement method for the emittance of the scattering and absorbing medium, which is necessary to estimate the albedo, and demonstrates the validity and the usefulness of both the measurement method for the emittance and the estimation method for the albedo. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Yet another look at light scattering from particles in absorbing media

We examine the scattering properties of particles contained in absorbing media. Rather than consider energy fluxes through arbitrary integrating spheres, we examine the extinction from its fundamental definition: the energy removed from the plane wave, or incident beam. The resulting energy received by a detector contains two terms: one the result of the incident beam traversing through the medium that would have occurred if the particle were not present, and a correction term due to the presence of the particle. Both terms have the same dependence on the pathlength that the beam travels between two arbitrarily located parallel planes and are independent of where the particle is located within the medium. The result is that the definition of the extinction cross section is not dependent on a reference plane or the particle location within the medium.     

Ultrasound-modulated optical tomography of absorbing objects buried in dense tissue-simulating turbid media

Continuous-wave ultrasonic modulation of scattered laser light was used to image objects buried in tissue-simulating turbid media. The buried object had an absorption coefficient greater than the background turbid medium. The ultrasonic wave that was focused into the turbid media modulated the laser light that passed through the ultrasonic field. The modulated laser light that was collected by a photomultiplier tube reflected the local mechanical and optical properties in the zone of ultrasonic modulation. Objects buried in the middle plane of 5-cm-thick dense turbid media were imaged with millimeter resolution through the scanning and detecting alterations of the ultrasound-modulated optical signal. The optical properties of the dense turbid media included an absorption coefficient of 0.1 cm(-1) and a reduced scattering coefficient of 10 cm(-1) and were comparable with those of biological tissues in the visible and near-IR ranges. The dependence of the ultrasound-modulated optical signal on the off-axis distance of the detector from the optic axis and the area of the detector was studied as well.     

Unified analytical inversion of reflectometric and ellipsometric data of absorbing media

We present a unified two-step analytical inversion of reflectometric and ellipsometric data of absorbing media. Instead of a direct determination of the optical constants n, kappa from reflectometric or ellipsometric measurements, we first calculate the real and the imaginary part eta, gamma of the normal component of the wave vector in the absorbing medium. New and simple analytical formulas are obtained for eta and gamma in terms of rho(s), rho(p) or tan psi, delta, respectively, where rho(s), rho(p) and tan psi, delta are the measured reflectometric and ellipsometric parameters of the absorbing medium. The optical constants are then easily determined analytically again from eta, gamma. We use the new formulas to compare the sensitivity with experimental errors due to the inversion of reflectometric and ellipsometric data.     

Spectroscopy of one-dimensionally inhomogeneous linear absorbing media with arbitrary frequency dispersion

This study focuses on a one-dimensionally inhomogeneous absorbing plane-parallel plate with dielectric properties characterized by a diagonal tensor of dielectric permittivity. The research has found that based on the reflection and transmission coefficients of p- and s-polarized plane monochromatic waves with frequency ω known for a certain range of incidence angles the coordinate dependence of all the components of the dielectric permittivity tensor of the plate material at frequency ω is determined in a unique manner according to the proposed reconstitution algorithm.     

Stokes reciprocity equations and density of modes for absorbing stratified media

One considers the influence of absorption on Stokes reciprocity equations and on the density of modes (DOM) for stratified media. The reciprocity equations are extended to absorbing media by using the transfer matrix formalism and introducing a special complex conjugation. A method of calculation for the DOM is presented in the Wigner-time approach and applied to periodic and quasi-periodic absorbing stratified structures. Examples of the role of absorption are given in the soft-X-ray domain.