This paper considers the investigation of photoacoustic transformation in naturally-gyrotropic and magnetoactive crystals, with internal stress under sound excitation in different modes by Bessel light beams (BLB). In the range of high modulation frequencies (Ω > 1 MHz), the dependence of the photoacoustic response amplitude on the radial coordinate ρ exhibits resonant phenomenon, which can be used to increase the resolution of photoacoustic spectroscopy for media with internal stresses.The expressions for amplitudes of photoacoustic signals in strained crystalline samples were obtained under different boundary conditions, taking into account the dependence of the thermoelastic coupling coefficient on the initial strain in the sample. It was showed that a resonant increase in the amplitude signal is related to the dependence on the geometric parameters of the sample-piezoelectric transducer system, the values of Murnagan constants, the mode composition of the Bessel light beam, and its amplitude modulation frequency.Keywords: photoacoustic spectroscopy, Bessel light beams, magnetoactive gyrotropic materials W pracy przedstawiono wyniki badań transformacji fotoakustycznej poprzez dźwięk generowany laserowymi wiązka-mi Bessela o różnych modach w kryształach naturalnie żyrotropowych i magnetoaktywnych, z wewnętrznymi naprężeniami. Stwierdzono, że w zakresie modulacji o wysokiej częstotliwości (Ω > 1 MHz), zależność amplitudy odpowiedzi fotoakustycznej od radialnej współrzędnej wykazuje efekt rezonansowy. Efekt ten może być wykorzystany do podwyższenia rozdzielczości spektroskopii fotoakustycznej w ośrodkach z wewnętrznymi naprężeniami.Otrzymano wyrażenia na amplitudy sygnałów fotoakustycznych w próbkach krystalicznych z wewnętrznymi naprężeniami, przy różnych warunkach brzegowych. Brano przy tym pod uwagę zależność współczynnika sprzężenia termoplastycznego od wewnętrznych naprężeń w próbce. Wykazano, że rezonansowy wzrost amplitudy sygnału jest zależny od geometrycznych parametrów układu próbka -przetwornik piezoelektryczny, wartości stałych Murnagana, modów wiązki Bessela i częstotliwości modulacji.
Photoacoustic transformation of the TE mode of a Bessel light beam (BLB) has been studied for piezoelectric detection in short-period superlattices formed by magnetoactive crystals of bismuth germanate (Bi 12 GeO 20 ) and bismuth silicate (Bi 12 SiO 20 ) types. It is shown that the resulting signal amplitude can be controlled using optical schemes of BLB formation with a tunable cone angle. A resonant increase in the signal amplitude has been found in the megahertz range of modulation frequencies and its dependences on the BLB modulation frequency, geometric sizes of the two-layer structure and piezoelectric transducer, radial coordinate of the polarization BLB mode, and dissipative superlattice parameters are analyzed.
535.13The mechanism for the formation of photodefl ection signals in magnetoactive superlattices irradiated by polarized modes of Bessel-Gaussian light beams is studied. The feasibility of controlling the spatial distribution of the temperature distribution in test samples with subsequent thermo-optical excitation of photodefl ection signals is established. A method is proposed for nondestructive monitoring of the geometrical parameters of magnetoactive superlattices by means of laser photodefl ection spectroscopy.Introduction. Laser photodefl ection spectroscopy is widely used in research on solids [1-3]. It is distinguished by its universality, high sensitivity, and relative ease of making measurements [4,5]. In this paper, a photodefl ection method is used to study short-period two-layer magnetoactive superlattices formed by cubic crystals of bismuth germanate (Bi 12 GeO 20 ) and bismuth silicate (Bi 12 SiO 20 ). Polarized TE-and TH-modes of Bessel-Gaussian light beams (BGLB) are used to excite thermoelastic oscillations in the samples [6,7].Laser photodefl ection spectroscopy is based on the conversion of the energy absorbed from a light beam in the volume of a sample into a thermal fi eld which produces a refractive index gradient in the sample and the surrounding medium. The defl ection from the horizontal of a low-power probe laser beam as it passes through the region with a nonuniform refractive index yields information on the optical, dissipative, thermal, and other characteristics of the sample.Determination of the BGLB Energy Loss Rate and of the Defl ection Angles. Let an amplitude modulated Bessel light beam, e.g., with TE-polarization, be incident normally on a magnetically active superlattice (Fig. 1) consisting of cubic crystals such as bismuth germanate or bismuth silicate. In the long-wavelength approximation [8,9] a two-layer superlattice can be represented as a single-layer crystal with its optical axis perpendicular to the boundary of the layers, since the beam is normally incident on the sample. A two-layer magnetoactive superlattice is characterized [10] by uniaxial complex dielectric ε ij and induced optical activity G ij tensors. The corresponding principal values of these effective tensors are given by
photcacoustic spectra or natural and magnetic circular ;iichrosm for a number of gyrotropic samples. Theoreti-0-8194-1070-5/92/$4.OO
The peculiarities of thermooptical excitation of elastic waves in nonlinear crystals, which appear due to modulated absorption of the energy from a Bessel light beam at the second harmonic, were theoretically examined. The amplitude-phase characteristics of the photoacoustic signal were found, and the conditions of the most effective thermooptical excitation of the sound were determined. The received results can be used in the design of new optoacoustic scanners for diagnostics of subsurface structures and new quantum electronics and integrated optics devices.
The results of finite-element modeling of controlled laser thermosplitting of crystalline silicon are presented. The case of treatment by two laser beam with wavelengths, namely 0.808 µm and 1.06 µm is studied. Calculations of the thermoelastic fields formed in a single-crystalline silicon wafer as a result of consecutive two-beam laser heating and action of coolant were performed for silicon crystalline orientations: (100), (110), (111). Modeling was performed for circular and U-shaped laser beams. The results received in the presented work, can be used for the process optimization concerning the precise separation of silicon wafers by laser cutting.Keywords: laser cutting, thermoelastic stresses, silicon wafers, crack W pracy przedstawiono wyniki modelowania metodą elementów skończonych termorozszczepiania krystalicznego krzemu przy pomocy wiązek laserowych. Analizowano przypadek działania dwóch wiązek laserowych o długości fali 0,808 µm i 1,06 µm. Obliczenia pól termosprężystych, formowanych w krystalicznym waflu krystalicznym, prowadzono jako efekt kolejnych działań dwuwiązkowego ogrzewania laserowego i chłodzenia dla orientacji (100), (110), (111) krystalicznego krzemu. Modelowanie prowadzone było dla wiązek o przekroju kolistym oraz w kształcie litery U. Otrzymane wyniki mogą być wykorzystane do optymalizacji precyzyjnego cięcia laserem wafli krzemowych.
The process of thermo-optical excitation of sound by opposing light waves is studied in the magnetically active medium. The influence of natural and induced optical activity on the resulting photoacoustic (PA) response is analyzed. A method of controlling the amplitude of PA signal is proposed. The method is based on the possibility to affect the interference energy dissipation by an external magnetic field.
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