Acoustooptic interaction

Acousto-optic interaction is reduced to the effects of optical refraction and diffraction only at low intensities of optical radiation. With increasing light intensity, nonlinear effects of light on the environment begin to play an increasing role. Due to electrostriction and the effects of heating the medium by optical radiation, alternating elastic stresses arise in it and sound waves are generated with frequencies from audible to hypersonic - so-called. optoacoustic or photoacoustic phenomena.

In the field of high-power optical radiation, as a result of the simultaneous occurrence of light diffraction by ultrasonic and the generation of ultrasonic waves as a result of electrostriction, ultrasonic light amplifies with light. In particular, with the propagation of intense laser radiation in the medium, t. Mandel'shtam-Brillouin stimulated scattering, in which thermal acoustic noise is amplified by laser radiation, accompanied by an increase in the intensity of scattered light. Optoacoustic effects also include the generation of acoustic oscillations by periodically repeated light pulses, which is caused by alternating mechanical stresses resulting from thermal expansion during periodic local heating of the medium with light.

The effects of acousto-optic interaction are used both in physical research and in technology. Diffraction of light by ultrasound makes it possible to measure the local characteristics of ultrasonic fields. The angular dependences of the diffracted light determine the radiation pattern and the spectral composition of the acoustic radiation. Analysis of the diffraction efficiency at various points in the sample allows us to reconstruct the picture of the spatial distribution of the sound intensity. In particular, the visualization of sound fields is carried out on the basis of acousto-optic effects. Using Bragg diffraction, it is possible to obtain information on the spectral, angular, and spatial distribution of acoustic phonons in the DV region of the phonon spectrum. This method is of value for the study of nonequilibrium acoustic phonons, for example, in the conditions of phonon (acoustoelectric) instability in semiconductors, due to the enhancement of ultrasonic supersonic drift of charge carriers.

Acousto-optic diffraction also makes it possible to measure many parameters of a substance: the speed and absorption coefficient of sound, elastic moduli of the 2nd, 3rd and higher orders, elastic-optical constants, and other quantities. So, from the Bragg condition for the known values ​​of the ultrasonic frequency f and the wavelength of light , and the measured angle of 20 B between the incident and diffracted light rays determine the speed of sound: (where 20 B is the Bragg angle). On the basis of the values ​​of C s thus obtained, the full matrix of elastic moduli Cij is calculated for various directions. Sound absorption coefficient can be found by comparing the intensities I1 and I2 of diffracted light, measured at two positions of the incident light beam, displaced relative to each other by a distance, and along the direction of propagation of the sound wave:

When high-intensity sound waves propagate in the medium, higher-order elastic modulus data is obtained by measuring with Bragg diffraction the amplitudes of harmonics arising in the wave, which are proportional to the nonlinear elastic moduli of the corresponding orders.

To study the dispersion of sound speed and its absorption coefficient at hypersonic frequencies, Mandel'shtam-Brillouin scattering is used. Passing a coherent optical radiation beam through the medium and fixing the scattering angle 0, it is possible from Bragg conditions to determine the speed of sound Cv at a given frequency f by the spectral shift f of the Mandelstam-Brillouin component. Based on half width measurements Mandelshtam-Brillouin component is determined by the absorption coefficient at this frequency:

On the basis of optoacoustic sound generation, a method of photoacoustic spectroscopy was created to obtain the optical absorption spectra of substances in various physical states. In this method, the light absorption coefficient is measured by the intensity of sound vibrations, excited by periodically interrupted light. For example, when a gas is periodically heated, sound vibrations arise in it with an amplitude proportional to the absorbed light energy. By changing the wavelength of the incident light, it is possible to obtain a photoacoustic spectrum of a substance — a complete analogue of the absorption spectrum, measured by conventional methods. The advantage of photoacoustic spectroscopy is in the high sensitivity of the method, which makes it possible to obtain optical absorption spectra in a wide range of light wavelengths, including both the strong absorption region and the transparency region; In addition, this method measures only that part of the energy of the incident radiation, which is actually absorbed by the substance, and the scattered radiation does not make any contribution. This allows us to study the absorption spectra of samples with poor surface quality: powders, loose, porous materials, biological objects.