Acousto-optic interaction with leaky surface acoustic waves in Y-cut LiTaO3 crystals

The acousto-optic interaction with leaky surface acoustic wave radiation into the bulk of YX-cut LiTaO₃ crystals has been investigated. The light incidence and diffraction angles corresponding to the strongest acousto-optic interaction were calculated and measured as functions of the acoustic wave frequency. The dependencies of the diffracted light intensity on the amplitude of radio-frequency voltage applied to the interdigital transducer (IDT) were studied. Our acousto-optic measurements revealed generation, by the IDTs, of slow shear bulk acoustic waves propagating at different angles depending on their frequency. A secondary acousto-optic interaction from the bulk waves radiated by the receiving IDT has been studied.     

基于布拉格声光衍射场的光强非均匀空间分布研究

为了保证合成孔径激光雷达测试中信号光的光束质量,从波动方程出发,推导出传统的衍射光源布拉格声光作用的耦合波方程。根据测试对光束质量的不同要求,借助有限差分方法分析衍射场光强的空间三维分布,发现声场振幅的不均匀分布会影响声光衍射场的光强分布,光强分布不均造成实验中信息光源的探测难度。通过光强空间分布研究增大有效通光孔径到3 mm,解决了声光调制器光束衍射质量问题,改进后的光束质量达到实验要求。该研究方法适用于改进声光器件工作参数。     

Light-induced acoustic effect in LiNbO3：Fe：Ce crystals

The phenomena of acoustic emission in LiNbO3：Fe：Ce crystals have been observed in the process of light-induced quasi-breakdown. It is found that the ultra-high frequency acoustic signal introduced into the crystal is modulated by the low frequency acoustic waves. Its frequency increases with the increase of the intensity of incident light and its jump period of breakdown is the same as that of the photovoltaic current Ic, the change of light-induced refractive index △n and the diffracted light intensity L. This phenomenon has been analysed in this paper, which is caused by the inverse piezoelectric strain effect of the jump of space charge field during the quasi-breakdown.     

Estimation methodology for amplitude of second-order harmonic in nonlinear surface acoustic wave measured by optical diffraction method

An estimation methodology for amplitude from the fundamental to fourth-order harmonic composed of nonlinear surface acoustic waves on a LiNbO₃ substrate with a 128° Y-X propagation axis was demonstrated. First, equations for estimation of amplitudes from diffracted light intensity were shown and then numerical simulations of main factors were carried out. Normalized diffraction light intensities due to components of nonlinear surface acoustic waves were measured and their amplitudes were obtained from measured values. Finally, the amplitude of the second harmonic propagating on a gold thin film of about 50 nm thickness was estimated. It was found that the amplitude of the second harmonic increased continuously from 0.8 to 2.0 Å with propagation distance in the thin film.     

基于激光偏振特性的水下声信号探测方法研究

水下航行体辐射的声波传播到水-空气分界面上时,会在水表面形成微扰,使水表面元产生微倾角.提出一种利用激光偏振特性对水下声信号进行探测的方法,不直接利用散射光强信息,而是利用激光在布儒斯特角附近入射水表面时,反射光偏振特性随入射角剧烈变化的特点对水表面微扰进行高精度探测.建立了该方法的理论模型,得到了入射波、出射波垂直分...     

Acoustic light diffraction, associated with the modulation of the polarization of the light, in an easy-plane antiferromagnet

The intensity of acoustic light diffraction by an easy-plane antiferromagnet in the Raman-Nath regime, due to a photoelastic interaction of antiferromagnetic origin, is calculated in the case when there is no modulation of the refractive index to first order in the acoustic deformations, and the entire effect is due to the linear modulation of the light polarization. Quantitative estimates are made for FeBO₃. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 317–321 (25 February 1997)     

Diffraction of light in intermediate regime of acoustooptical interaction at oblique incidence

The diffraction of light by ultrasound in an isotropic medium at arbitrarily small angles of incidence onto the acoustic layer is calculated. The intermediate regime of diffraction is considered, for which the conditions of observation of the Raman-Nath and Bragg diffraction are not satisfied in terms of the wave parameter. The effect of the acoustooptical interaction length and angle between the incident light beam and the acoustic wave on the light intensity distribution over the diffraction peaks is studied as a function of the acoustic power. Specific features of the transition from the Raman-Nath to the Bragg diffraction regime through the intermediate regime of diffraction are analyzed.     

Narrow band laser-generated surface acoustic waves using a formed source in the ablative regime

A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.     

Photorefractive surface waves in a thin Bi12TiO20 crystal

A theoretical model considering total internal reflections of fanned beams from the side crystal surfaces and their coupling with the pump beam is proposed. The interaction of these beams results in the light intensity redistribution and in the generation of the photorefractive surface wave. An experimental observation of the light intensity self-concentration near the surface of a thin photorefractive Bi₁₂TiO₂₀ crystal is reported. It results in an acceleration of the photorefractive energy exchange process.     

Influence of the sample-probe coupling on the resolution of transmitted near-field optical image

Numerical simulation of photon scanning tunneling microscopy is presented to study the near-field distribution in the vicinity a dielectric surface with one-dimensional sub-wavelength structures. Multiple scattering between the probe tip and the sample has been taken into account implicitly by matching electromagnetic boundary conditions at interfaces. The near-field intensity in transmission mode through two ridges on surface has been modeled in order to analyze the resolution of the system. The effects on the signal by the sample-tip coupling, the polarization of the incident light, and the angle of incidence are investigated. We find that the capability to recognize the feature will be improved when the tip–object interaction is strong.     

低频声表面波对细激光束的衍射效应

对于频率为几十赫兹的液体表面声波,实验上观察到反射光所形成的稳定、清晰的激光干涉图样.改变表面声波振幅,进一步发现了干涉图样的调制效应.调制效应明显地表现为干涉条纹的缺级,并且随表面声波振幅变化.依据波动光学原理,分析了调制作用下光束干涉所产生的光强解析分布及条纹特征,得到了干涉条纹角宽度与表面声波之间的解析关系,并据此对表面声波的特性参量波长和振幅及各级条纹相对光强分布规律进行了测定.     

Influence of UV light irradiation on film thickness distribution of tin oxide films by photochemical vapour deposition

Tin oxide (non-doped) films have been prepared by a photochemical vapour deposition (photo-CVD) from Tetramethyltin (TMT) (Sn(CH₃)₄) and O₂ (containing O₃). A low-pressure mercury lamp was used as the light source. The effect of the UV light irradiation on the film thickness distribution along 5 cm×5 cm area was examined. By piling Teflon films on the surface of the suprasil window, the light intensity of 184.9 nm UV wavelength of the low-pressure mercury lamp was controlled, while that of 253.7 nm wavelength through the Teflon hardly changed. As a result, the uniformity of the film thickness distribution was improved as the light intensity (184.9 nm) increased. The UV 184.9 nm light irradiation may have improved the uniformity of the reactive species distribution in the vapour phase, which may result in the formation of the uniform thickness distribution.     

Diffraction of pulsed laser radiation from an acoustic wave with frequency- and phase-shift keying

Bragg acoustooptic diffraction of nanosecond pulsed laser radiation from a frequency- and/or phase-keyed acoustic wave is studied experimentally and theoretically for exact synchronization of laser pulses and signal keying. It is shown that the diffraction field for short pulses is practically stationary and is determined by the positions of acoustic signal keying over signal aperture. The application of this type of signals for the formation of a multibeam diffraction pulsed radiation field is considered. A method is proposed for transforming the angular spectrum of laser radiation intensity from the initial Gaussian to a nearly rectangular spectrum. This may considerably increase the efficiency of high-power technological lasers used in material processing (laser cutting, welding, engraving, etc.), in which the action of radiation is of the thresh-old type in light intensity. The possibility of correcting the angular intensity distribution for a pulsed fiber laser, which increases the thermal efficiency of radiation from such a laser, is established experimentally.     

Improvement in dose threshold of α-Al2O3:C using thermally assisted OSL

α-Al₂O₃:C phosphor was studied for improvement of its dose threshold using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The dose threshold of phosphor depends on the standard deviation of the background signal which affects the signal-to-noise ratio of the instrument. In case of OSL measurement, the background signal is due to the scattering of the stimulation light intensity from the planchet. On reducing the stimulation light intensity, the scattering component and thus the standard deviation of the background signal reduces considerably. The reduction in stimulation light intensity increases the readout time due to the dependence of decay constant of OSL signal on the former. The decay constant depends on the photo-ionization cross-section of the OSL active traps in the phosphor participating in OSL phenomenon and thus on the readout temperature due to the temperature dependence of photo-ionization cross-section. In order to achieve the same decay constant for two sets of measurements for α-Al₂O₃:C i.e. to take the OSL measurement in the same time at lower stimulation light intensity as that taken for higher light intensity, the temperature of measurement for α-Al₂O₃:C for the former was raised. Moreover, the increase of readout temperature does not affect the standard deviation in the background signal of OSL readout. The optimized elevated temperature for α-Al₂O₃:C was found to be 85 °C as the main dosimetric peak starts giving signal due to depletion of its filled traps, at temperatures higher than 90 °C. As a result of lowering the stimulation intensity at higher temperature (85 °C), the standard deviation in the background signal and thus the overall dose threshold of α-Al₂O₃:C was found to improve by 1.8 times.     

Acoustic characteristics of a barrier-discharge XeCl excilamp

The generation of an acoustic signal by means of voltage pulses (f = 15 kHz) applied to the electrodes of a barrier-discharge excilamp based on a Xe/Cl₂ = (50?C500)/1 mixture kept at a pressure of 5?C500 Torr is studied. It is shown that, from the time variation of the acoustic signal intensity, one can judge the time instant the excilamp starts operating in a steady mode. Optimal (in power and efficiency) operating conditions of the excilamp are found (Xe/Cl² = 240/1, p = 98 Torr, ?? ?? 9.5%). It is experimentally demonstrated that the discharge energy at a low pressure is spent largely on heating the gas. This is indicative of the volume heat release and volume glow discharge (as the pressure grows, the efficiency of this source of energy consumption drops and more and more energy is spent on acoustic vibration excitation). Under higher pressures, the Fourier spectrum of the acoustic signal becomes richer, the intensity of the spectrum rises, and the dispersion of the signal grows. At very high pressures, the intensity of the acoustic signal drops to a level corresponding to the natural vibrations of the excilamp envelope without the discharge (when the discharge is quenched, the Fourier spectrum of the signal becomes depleted and contains only harmonics corresponding to the carrier frequency of voltage pulses from the power source).     

Impact fracture of ZnSe ceramics

Structurally different ZnSe ceramics prepared by various techniques were subjected to fallingweight impact fracture. Mechanoluminescence (ML) pulses generated during the motion and multiplication of dislocations were detected, as well as acoustic emission (AE) pulses produced predominantly during the growth of macroscopic (on the specimen scale) cracks. The luminescence began immediately at the moment of contact of a striker with the surface of the specimen, whereas the emission of sound occurred within 50–100 μs after the impact. The emission maxima in the ML and AE time series coincided with each other. The signal series were used to construct energy distributions upon the emission of light and the generation of sound. It was established that the ML amplitude (the number of emitted photons) is proportional to the energy released due to dislocation rearrangements, and the intensity (the square of the amplitude) of AE pulses is proportional to the energy released due to discontinuities of the material. It was found that the ML energy distribution follows a power law, which indicates the self-organization of an ensemble of dislocations during rapid plastic deformation. The AE energy distribution, on the contrary, was found to be random, i.e., typical of the growth of non-interacting cracks. It was shown that the efficiency of the interaction of dislocations depends, to a certain extent, on the technological prehistory of ZnSe ceramics.     

Interaction of light with acoustic waves excited by an inphase multielement transducer in the 1.0–2.5 GHz range

Interaction between a weakly divergent optical beam and an acoustic wave generated in the range 1.0–2.5 GHz by an inphase multielement electroacoustic piezoelectric transducer is analyzed. A piezoelectric (Y + 36°)-cut LiNbO₃ plate is fixed on the surface of an X-cut LiNbO₃ acoustic duct with the help of metallic sublayers (Cr, Cu, In, Cu, or Cr). The inphase structure of the transducer is formed by the upper electrodes inter-connected by short conductors. The signal is applied through a coaxial Chebyshev transformer. The efficiencies of electroacoustic conversion and acoustooptic interaction are calculated as functions of frequency. The experimental setup, method, and results are described.     

Generation and detection of electromagnetic field with charged material oscillation

We generated an electromagnetic field using the spatial oscillation of a charged material such as a polyimide film. The film was vibrated with acoustic waves at 1 Hz–1 kHz. For charged films, changes in electric field intensity with acoustic wave irradiation were detected using an antenna. The electric field intensity and phase were found to be related to the surface voltage and electrical polarity. The surface potential distribution matches the electric field distribution that was measured by scanning the local excitation. These results indicate that this phenomenon can be used to measure the electrical properties of charged materials.     

Signal enhancement in Rayleigh wave interactions using a laser-ultrasound/EMAT imaging system

Enhancement of signal amplitudes from Rayleigh wave interaction at solid surface features has been investigated when signals were detected by an in-plane electromagnetic acoustic transducer (EMAT). A laser-ultrasound system was used to inspect surface-breaking slots, serving as artificial defects. Nd:YAG laser pulses were delivered onto a metal surface via an optical fiber and focused to a line source by a cylindrical lens. An in-plane EMAT receiver detected transient surface acoustic waves. A-scan signals and B-scan images from surface defects revealed increased signal amplitude up to 2.8+/-0.3 depending on the distance of the transducer from a slot. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave. A supporting computer model was derived to show that experimental signal enhancements were consistent with numerical predictions.     

Diffraction of light by an acoustic wave with three frequency components

Collinear light diffraction by three-frequency sound is investigated theoretically. The amplitude distributions of transmitted and diffracted light waves along the cell are calculated for different amplitudes of sound signals. The dependence of the intensity of principal diffraction peaks on the frequency difference between acoustic signal components is studied for different amplitude ratios of these components. It is shown that the character of this dependence for a wave being in synchronism differs substantially from that for two other waves characterized by detuning. The dependence of the amplitudes of principal and parasitic diffraction peaks on the efficiency of acoustooptical interaction is investigated. It is demonstrated that parasitic sideband components in diffracted light can play a considerable role if the diffraction efficiency is sufficiently high and exceeds 80%.