Influence of absorption coefficient fluctuations on the heating of a weakly absorbing medium by intense optical radiation

The problem of heating the opacity fluctuations in a transparent solid medium by powerful optical radiation is numerically solved. The dependence of the absorption coefficient of the medium on the thermoelastic stresses is taken into account.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 36, No. 2, pp. 327–330, February, 1979.     

Light scattering by a random distribution of particles embedded in absorbing media: diagrammatic expansion of the extinction coefficient

We address the problem of the modeling of the extinction coefficient into an absorbing medium, including a random distribution of identical scatterers of arbitrary size. We show that the extinction coefficient, including losses in the host medium, can be derived from a diagrammatic expansion arising from the rigorous multiple-scattering theory of electromagnetic waves in random media. While in previous approaches the contribution to the extinction coefficient due to the absorption in the host medium and due to the absorption and scattering by the particles were evaluated separately and heuristically, our approach is based on a derivation from first principles.     

Refractive index and extinction coefficient determination of an absorbing thin film by using the continuous wavelet transform method

We present the continuous wavelet transform (CWT) method for determining the dispersion curves of the refractive index and extinction coefficient of absorbing thin films by using the transmittance spectrum in the visible and near infrared regions at room temperature. The CWT method is performed on the transmittance spectrum of an a - Si(1-x)C(x):H film, and the refractive index and extinction coefficient of the film are continuously determined and compared with the results of the envelope and fringe counting methods. Also the noise filter property of the method is depicted on a theoretically generated noisy signal. Finally, the error analyses of the CWT, envelope, and fringe counting methods are performed.     

Light scattering by a random distribution of particles embedded in absorbing media: full-wave Monte Carlo solutions of the extinction coefficient

We present a numerical investigation of the light scattering in an absorbing medium with randomly distributed scatterers. The extinction coefficient is derived from an ensemble of numerical solutions of Maxwell's equations for many different realizations of the system. Results are in good agreement with the predictions given by the effective medium theory under the independent-scattering approximation. Beyond the independent-scattering approximation, we explore the domain of validity of an effective medium theory that takes into account correlations between pairs of scatterers. A good agreement is obtained with a filling ratio up to 30% for scatterers with a relative refractive index contrast lower than 20% and size parameters near unity.     

Photon diffusion coefficient in an absorbing medium

A number of investigators have recently claimed, based on both analysis from transport theory and transport-theory-based Monte Carlo calculations, that the diffusion coefficient for photon migration should be taken to be independent of absorption. We show that these analyses are flawed and that the correct way of extracting diffusion theory from transport theory gives an absorption-dependent diffusion coefficient. Experiments by two different sets of investigators give conflicting results concerning whether the diffusion coefficient depends on absorption. The discrepancy between theory and the earlier set of experiments poses an interesting challenge.     

Piezo-absorption coefficients of weakly and strongly absorbing crystals

The effect of stress on a crystal is to distort not only the index ellipsoid but also the absorption ellipsoid. The piezo-absorption tensor like the photoelastic tensor is a fourth rank polar tensor. The methods of obtaining the components of the piezo-absorption tensor in weakly absorbing crystals are given. It is shown that by measuring the coefficients of reflection at normal and oblique incidence in stressed and unstressed metals it is possible to obtain changes in refractive and absorption indices. From this the piezo-optic and the piezo-absorption coefficients of metals may be obtained.     

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.     

Recording the electromagnetic field excited in fracture of dielectrics and weakly conducting materials

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 23, No. 4, pp. 96–98, July–August, 1987.     

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.     

Sintering of stacked pellets for controlling the temperature coefficient of dielectrics

Bearing in mind the excellent dielectric properties at high frequency of the oxides Ba₅Nb₄O₁₅, BaNb₂O₆, ZnNb₂O₆ and Zn₃Nb₂O₈ (?_r ∼ 20-45, tan(δ) < 10 × 10⁻⁴, ρ_i > 10¹⁰ Ω cm), “glass composites” of these materials were investigated in order to control their permittivity temperature coefficients for various applications. In a first method, samples constituted of mixed powders with temperature coefficient of opposite sign have been conventionally sintered. The obtained properties are not the expected ones because of the high reactivity between the mixed phases. In a second method, samples constituted of two stacked pellets with temperature coefficient of opposite sign have been co-sintered by the sinter-forging process, i.e. while maintaining a pressure, the two stacked pellets were co-sintered, leading to a good mechanical behaviour. More importantly we show that this technique avoids the delamination at the interface and that the so obtained dielectric properties are globally in good agreement with the mixing rule. Clearly, it is demonstrated that the temperature coefficient of these dielectrics can be tuned using the sinter-forging process, opening a new route for the optimization of multilayer capacitors.     

Evaluation of the optical constants and thicknesses of weakly absorbing non-uniform thin films

Formulae are deduced and evaluation procedures, both graphical and by computer, are derived for the evaluation of the optical constant n(v and k(v), the mean thickness d and the relative thickness variation $\text{Δd}\text{d}\text{= γ}$ of transparent and weakly absorbing wedge-shaped thin films. The films may be free or supported on thick transparent substrates of known refractive index n_s(v). The evaluation procedure uses only transmission data, i.e. the spectral transmission curves and their envelope curves which together define sets of tangency points used in the calculations.     

Empirical relationships between extinction coefficient and visibility in fog

Relationships between visibility and an extinction coefficient that is due to fog in optical windows that are free from molecular absorption are derived. The extinction coefficients in the visible (0.55 microm), the near IR (1.2 microm), and the mid IR (3.7 microm) are comparable to and roughly twice as much as that in the far IR (10.6 microm) when visibility is less than a few hundred meters. The advantage of far-IR radiation compared with shorter wavelengths grows as visibility exceeds 500 m. Correspondingly, the relationship between extinction coefficient and visibility becomes more sensitive to variations in the particle-size distribution of fog.     

Plano-concave microcuvette for measuring the absorption coefficient of highly absorbing liquids

A powerful and simple method based on the use of a plano-concave microcuvette was investigated for measuring the absorption coefficient of highly absorbing liquids. A plano-convex lens put on a plane-parallel plate formed a microcuvette with small, continuously varying thicknesses. This microcuvette was filled with liquid and illuminated by a homogeneous beam. The parabolic variation of the liquid thickness generates a Gaussian spatial intensity distribution behind the cuvette. This Gaussian profile, detected by a CCD camera, was used to determine the absorption coefficient of the liquid. An absorption coefficient as high as 1.54 x 10(4) cm(-1) was measured by use of high-concentration malachite green dye solutions. A comparison of the results with data extrapolated from those of conventional methods showed good agreement.     

Influence of weakly absorbing solid aerosol particles in lowering the optical breakdown threshold of air

The influence of a subtance in the disperse state on the optical breakdown conditions of air is investigated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 4, pp. 582–585, April, 1988.     

Analytical method of determining optical constants of a weakly absorbing thin film

We propose an analytical method for determining the refractive index and the extinction coefficient of a weakly absorbing thin film. This method is based on measurements of the reflectance extreme and corresponding transmittance of the film at normal incidence. Simulations of the theoretical accuracy of the method are given. The effect of the errors of reflectance and transmittance measurements on determination of the optical constants is also analyzed. The method is successfully applied to calculate the optical constants of indium tin oxide films.     

Experimental study of the thermal conductivity of weakly absorbing liquids in layers transparent to infrared radiation

The thermal conductivities of a number of organic liquids are determined by a transient method under irregular thermal conditions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 6, pp. 972–978, June, 1976.     

Distortions of the extinction coefficient profile caused by systematic errors in lidar data

The influence of lidar data systematic errors on the retrieved particulate extinction coefficient profile in clear atmospheres is investigated. Particularly, two sources of the extinction coefficient profile distortions are analyzed: (1) a zero-line offset remaining after subtraction of an inaccurately determined signal background component and (2) a far-end incomplete overlap due to poor adjustment of the lidar system optics. Inversion results for simulated lidar signals, obtained with the near- and far-end solutions, are presented that show advantages of the near-end solution for clear atmospheres.     

Spurious surface finish improvement of the infrared extinction coefficient of alpha quartz

Infrared extinction coefficients obtained using quartz samples lapped with a 3 mum abrasive compound were found to be much smaller than anticipated. The improvement in extinction coefficient is traceable to the irregular surface finish. To demonstrate this improvement, polished quartz samples were evaluated and compared to evaluations after lapping the same sample with 3 mum and 12 mum abrasives. In one sample, a 3 mum finish resulted in an almost 30% improvement in the extinction coefficient evaluated at 3500 cm(-1). Lapping of the sample to a 12 mum finish resulted in an almost 90% improvement. These improvements in alpha correspond to improvements in Q(IR ) (3500) from 2.58 million (polished) to 3.03 million (3 mu) to 4.71 million (12 mum). The improvement is caused by wavelength dependent transmission losses due to surface irregularities. The surface transmission losses can be reduced by polishing the optical surfaces or by applying index matching fluids to 3 mum lapped surfaces.     

Modeling of the refractive index and extinction coefficient of binary composite films

Southwell's analysis of optical multilayers within the limits of very thin films has been extended to include absorption in the multilayer for predicting the effective values of the refractive index n(e) and extinction coefficient k(e) of mixed-composition binary homogeneous films over a wide spectral region, including the high-absorption (k > 10(-2)) region. It has been found that n(e) in general is a complicated function of the optical parameters (n(1), k(1), n(2), k(2)) and volume fractions (f(1), f(2)) of the component materials in a homogeneous layer, and the expression for n(e) becomes the same as that predicted by the Drude model in the spectral region where the layers are transparent. Moreover, according to the present analysis, the volume fractions of the product of the refractive index and the extinction coefficient of the component materials of a binary composite film are additive and the sum equals the product of the effective refractive index and extinction coefficient of the composite film.