Interaction of a high-power laser beam with low-density porous media

We have experimentally investigated the processes of laser light absorption and energy transfer in porous targets made of “agar-agar” (C₁₄H₁₈O₇) with an average density of 1–4 mg/cm³ illuminated by the focused beam of a neodymium laser with an intensity of 10¹⁴ W/cm² within a pulse of duration 2.5 ns. Many important scientific and technical problems, e.g., inertial-confinement thermonuclear fusion, the creation of lasers in the x-ray regime, and the modeling of astrophysical phenomena under laboratory conditions, can be successfully addressed by using low-density porous media as components of such targets. In our experiments with porous targets of variable density and thickness we used optical and x-ray diagnostic methods, which ensured that our measurements were made with high temporal and spatial resolution. We show that a region forms within the porous target consisting of a dense high-temperature plasma which effectively absorbs the laser radiation. Energy is transferred from the absorption region to the surrounding layer of porous material at up to 2×10⁷ cm/s. Experimental data are in good agreement with the predictions of our theoretical model, which takes into account the specific features of absorption of laser radiation in a porous material and is based on representing the energy transfer within the material as a hydrothermal wave. Zh. éksp. Teor. Fiz. 111, 903–918 (March 1997)     

Polarization visualization of scattering media with CW laser radiation

The method of polarization visualization of a multiply scattering medium containing macroinhomogeneities based on analysis of polarization spatial distribution of a scattered linearly polarized light is discussed. The treatment is based on statistical properties of the effective optical path distribution of scattered field components. The influence of media scattering properties and the geometry of the experiment on the inhomogeneity image contrast obtained with use of polarization degree and of normalized scattered intensity of radiation as visualization parameters are discussed, as well as spatial resolution achieved in these both cases. Using the results of theoretical analysis and of the experimental model, the relationship between the shapes of spatial distributions of polarization degree and the intensity of the scattered light is considered as a function of the position of the visualized object (an absorbing half-plane immersed in a plane layer of the scattering medium). The opportunities for enhancing the quality of the images formed in this way are also discussed.     

Dynamics of high-power ultrashort laser pulses in resonant media

Work supported by Russian Fund for Basic Research, Project 93-02-14271.     

Speckle diagnostics of multiple scattering media with the use of frequency-modulated laser radiation

A method for probing randomly inhomogeneous multiple scattering media with the use of frequency-modulated laser radiation is considered. The method is based on analysis of the dependence of the blinking index of time-averaged speckles formed upon scattering of the probing radiation in a medium on the frequency modulation depth of the probing radiation. In the case of a binary frequency modulation, the blinking index of the detected speckle-modulated radiation is determined by the cosine Fourier transform of the probability density of the optical path-length difference of partial components of the scattered field in the probed medium. The values of the probability density of the optical path-length difference reconstructed with the use of the proposed method from the measured blinking index of speckles for model scattering media (fluoroplastic layer and layer of TiO₂ particles on a glass substrate) are in a good agreement with the results of statistical simulation of the probing radiation transfer in multiple scattering media.     

Absorption of laser radiation in ultracold plasma

In this paper, we analyze the spectral behavior(optical thickness, shape and linewidth) of laser radiation absorption under the correlation heating of ions in an ultracold plasma. The Voigt formula is used to find the absorption coefficient.The spectral line width is shown to grow with time while the optical thickness reduces. Our modeling results are in good agreement with the experimental findings reported in the literature.     

Monte-Carlo simulation of light scattering in fractal porous media

Monte-Carlo simulation of light propagation in a porous medium with a mass fractal morphology was carried out. It was shown that the simulation results can be used to analyze experimental data on light scattering and to study the porous medium structure. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 193–196, March–April, 2008.     

Photophysical processes stimulated in nanoporous silicon by high-power laser radiation

Photoprocesses initiated on the surface of porous silicon irradiated with laser radiation with wavelengths (λ = 266, 337, and 532 nm) in a wide range of intensities (up to 2 × 10⁷W/cm²) were investigated. Laser-induced luminescence and laser mass-spectrometry were used as experimental procedures. X-ray reflection was used to determine the parameters of the porous silicon films. The photoluminescence spectra obtained at different wavelengths and low intensities were analyzed. This analysis showed that for an optically thin layer of porous silicon the luminescence spectrum does not depend on the wavelength of the exciting radiation. This indicates the existence of a separate system of levels in porous silicon that are responsible for the luminescence. The behavior of the photoluminescence spectra as a function of the intensity q of the exciting radiation was investigated. It was shown that the luminescence intensity is a nonlinear function of q. At high intensities of the exciting radiation, the luminescence intensity saturates and a short-wavelength shift of the spectra is observed; this is due to the high concentrations of photoexcited carriers. This increases the probability of the experimentally observed nonequilibrium photodesorption of H₂ and Si from the surface of porous silicon.     

Theory of laser action in scattering gain media

A laser model based on feedback produced by scattering has been developed to explain the narrow linewidth emission and input-output behavior observed in scattering gain media. The model is based on the transient two-level laser equations and includes the detailed spectral properties of the dye gain system. Monte Carlo methods were employed to calculate the threshold gain required for modeling the input-output and linewidth emission characteristics.     

Time-independent polarized radiation in radiators with scattering media

The necessary condition of stationarity in radiators with scattering media is deduced. It is obtained from the fact that all primary emitted radiation disappears with probability unity when radiation is time-independent. The disappearance due to the absorption by matter and to escape from radiator is taken into account. The condition of stationarity includes the characteristics of radiation polarization, the coefficients of medium absorption, and the elements of the Green's function matrix. The relations between intensity components of radiation are derived from the condition. The results may be used in investigations of various radiators with scattering media.     

Interaction of laser radiation with a low-density structured absorber

A theoretical model is proposed for computing simulations of laser radiation interaction with inhomogeneous foam materials doped with heavy elements and undoped materials. The model satisfactorily describes many experiments on the interaction of the first and third harmonics of a 200 J pulsed PALS iodine laser with low-density porous cellulose triacetate targets. The model can be used to analyze experimental data and estimate the reality of experimental results.     

Absorption of CO laser radiation by NO

Absorption of CO i.r. laser radiation by NO has been studied over the temperature range 300°–4000°K using a grating-tunable CO laser in conjunction with a room-temperature absorption cell and a shock tube. The CO laser line with strongest absorption at elevated temperatures was determined to be the V = 7 → 6, J = 12 → 13 line at 1935.4817 cm^-1, which is nearly coincident with the ²Π_{$\text{3}\text{2}$}V = 0 → 1, J = 37/2 → 39/2 transition in NO. The absorption cell measurements (300°K) were used to infer the position of the NO absorption line (a Λ-doublet at 1935.492 and 1935.497 cm^-1) as well as collision-broadening parameters in pure NO and NO dilute in foreign gases: 2γ° (collision-broadened full width at half maximum in cm^-1 atm^-1 at 300°K) = 0.110, NO-NO; 0.072, NO in Ar; 0.069, NO in Kr; 0.109, NO in N₂. Calculations of the NO absorption coefficient at 1935.4817 cm^-1 are presented for a range of conditions applicable to current studies in combustion and NO_x kinetics. Shock tube measurements (630°–4000°K) supporting these calculations are also reported.     

Reflectance of two-layer composite porous media with linear-anisotropic scattering

We report an analysis of the hemispherical reflectance of composite slabs made up of two porous layers. To allow realistic modeling of most porous materials, anisotropic scattering is considered. The reflectance is obtained by using the method of spherical harmonics to solve the equation of transfer. Results from the P-11 approximation are presented for a wide range of governing parameters, including the single-scattering albedos and scattering phase-function coefficients for both porous layers. The effects of anisotropic scattering are illustrated.     

Role of light-induced scattering in the optical limitation of laser radiation on the basis of fullerene-containing media

A theoretical and experimental study of the contribution of light-induced scattering to nonlinear optical limitation is made. It is shown that light-induced scattering makes a considerable contribution to the optical limitation of radiation by fullerene-containing solutions. Radiation is scattered by small-scale (1–10 μm) inhomogeneities of density, and the corresponding increments are rather large. Because of this, scattering can be initiated by small-scale inhomogeneities of the input beam. Numerical simulation of nonlinear scattering is made, and the angular distribution and limitation of radiation in optical limiters are calculated. The calculation results are compared with the experiment.     

Separation of viscothermal losses and scattering in ultrasonic characterization of porous media

This paper presents a method for separating viscothermal and scattering losses in ultrasonic characterization of porous media. This method is based on variations of the static pressure of the saturating fluid. Experimental results were already presented in previous papers and the losses separation was verified experimentally. The aim of this paper is to present an analytic justification of this losses separation in the case of this experimental method and to show that it is possible to estimate acoustic parameters without the knowledge of scattering characteristics. The standard scattering length is used to renormalize speed and transmission through the porous medium, described as an equivalent fluid. Under certain assumptions corresponding to a weak scattering regime, it is shown how viscothermal and scattering losses can be separated easily without knowing scattering characteristics. Application of this model is presented in the case of weak scattering in a polyurethane foam and in the limit case of stronger scattering in a glass beads sample.     

固体中多孔媒质球的声共振散射特性

利用Flax等提出的声共振散射模型,以铁中多孔石墨为例数值上研究了平面声纵波入射时固体中多孔媒质球体背向散射纵波和散射横波的频谱以及不同阶分波的共振散射谱,并由此研究了铁中多孔球形石墨不同模式的共振频率随石墨孔隙率的变化。结果表明,球形石墨的背向散射谱及各阶分波中的共振频率均随孔隙率的增加向低频移动,其形态也发生变化。因此,如果用实验测定球墨铸铁样品的超声背向散射谱或各阶分波中共振频率,有可能确定多孔石墨的孔隙率。