Vitalyi E. Gusev

Generation of deformation waves in the processes of photoexcitation and recombination of nonequilibrium carriers in silicon

The shape of acoustic pulses excited in crystalline silicon during absorption of Nd3+ YAG laser radiation was studied as a function of incident light intensity. It was revealed that the amplitude of the dilatation wave is saturated while the duration of the compression pulse is shortened. A theoretical model is suggested which explains the above experimental facts by a decrease in the time of Auger recombination for nonequilibrium carriers with an increase in their concentration for higher intensity of the optical excitation. The value of the Auger constant obtained from the experiment is ∼5×10−31 cm6s−1.

Resonant ultrasound spectroscopy using optical excitation and detection

We propose a new technique for Resonant Ultrasound Spectroscopy (RUS), using chopped optical excitation and interferometric detection with focused laser beams. Spheroidal vibrational modes of a sintered, polished silicon nitride sphere of diameter ∼7 mm were observed, and the elastic constants of the sample derived. Varying the excitation geometry affects the relative strengths of the resonance peaks.

Nonlinear regime of laser‐induced piezoexcitation of the Rayleigh acoustic wave in CdS1−xSex crystal

The Rayleigh‐type surface acoustic wave piezoexcitation induced by the interband absorption of laser pulses in CdS1−xSex crystal placed in a dc electric field was investigated. We detected the surface acoustic wave (SAW) pulses with a profile of the particles’ vibrational velocity consisting of three consequent phases and we observed nonlinear (logarithmic) increase of the SAW amplitude with the increase of laser intensity. Our experiments directly demonstrated that the efficiency of SAW laser‐induced piezoexcitation is more than 4 orders of magnitude higher than the efficiency of SAW laser‐induced thermoelastic generation.

Non-stationary heat conduction of a porous medium

The thermal diffusion process is examined for a porous sample with idealized arrangement and form of the pores, when its surface is illuminated by a modulated light. A formula for the frequency dependence of the average surface temperature is derived. It is shown that it depends on the porosity, the form of the pores, and the ratio between a characteristic pore size and the thermal wavelength. In the limiting cases of low frequency of modulation and low porosity the results agree well with those quoted in the literature. The frequency dependence of the surface temperature of a microporous rubber sample, glass filtering crucibles, and leather samples have been measured by a PA cell and compared with the analytical results. The influence of various processes on the heat diffusion in porous media is discussed.

Idealized limiting models for semiconductor energy gap depth-profiling with picosecond optoacoustics

The reconstruction of the spatial variation of the energy gap of a semiconductor from the profiles of laser-generated and laser-detected picosecond acoustic pulses is analyzed. The proposed method for depth-profiling is based on the fact that electron-hole (e-h) plasma, photogenerated in a spatially inhomogeneous energy potential, exhibits hydrodynamical drift in addition to the usual diffusion.

New photoacoustic mean for the determination of surface recombination velocity in piezocrystals

A theoretical analysis is given of the surface recombination effect on laser generation of longitudinal acoustic pulses in piezocrystals. The acoustic pulses are excited through the inverse piezoelectric effect as a result of the electric fields induced by the spatial separation of photogenerated electrons and holes. Two regimes of dynamics of nonequilibrium electron-hole plasma are considered: the regime of independent motion of opposite-sign charge carriers and one of their ambipolar diffusion. The possibility of generating acoustic pulses with a duration equal to the surface recombination time is predicted. It is shown that the profile of pulse of longitudinal strain is essentially transformed as the surface recombination rate increases. Being based on these facts, a new photoacoustic mean is suggested to determine the high surface recombination velocity (s approximately equals 106 cm/s) in piezocrystals.

Theory of thermal wave interference induced by laser action on a normally cut layered structure

The exact analytical solution for the thermal wave multiple reflections and interference in a normally cut layered structure is derived. The analysis predicts the existence of the new characteristic spatial scale in the considered phenomena—the length of thermal wave synchronism, which depends on the modulation frequency of the laser-induced head flux and the thermal diffusivities of the alternating layers. Both the amplitude and the phase of the photothermal response exhibit pronounced variation with modulation frequency when the length of the thermal wave synchronism is close to the spatial period of the structure.

Physical mechanisms of light-induced polarization effects "on reflection" in GaAs

Non-steady-state polarization response of photoexcited GaAs has been investigated with the specular micropolarimetry particularly in crystals with laser induced damages of near surface layer. Physical models of the polarization response in the excited GaAs due to electron-hole plasma photogeneration are discussed: the nonlocal electron-hole plasma model the optically induced piezogyrotropy the electrooptic effect in the Dember field. 1.

Possibility of ultrahigh shock-pulse generation by radiation pressure of superintense femtosecond light bursts

The conditions for shock-wave generation by UV light pressure are examined. It is shown that in the nonrelativistic regime of laser intensities 7 1018 W/cm2 < I < 7 1020 W/cm2 the radiation pressure can exceed thermokinetical pressure of hot electrons. In a time scale less than 10 fs, the formation of shock-pulse with Gbar pressure can be achieved.