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.     

Spatial dependence of the dynamic optical transfer function of a transparent medium perturbed by an acoustic signal

Based on a spatially dependent (SD) dynamic optical transfer function (DOTF) of a medium perturbed by an acoustic signal, a dynamic theory has been developed of the acousto-optical interaction (AOI) of light beams in the field of an arbitrary acoustic signal under conditions of the diffraction distortion of its amplitude profile (DDAP). A system of integro-differential equations is derived for angular and frequency DOTF spectra of coupled waves in the AOI region. An analytical DOTF model has been constructed for a weak acousto-optical coupling. The characteristics of transient processes (TPs) in the diffracted-beam field have been investigated for the spatially dependent DOTF. It is demonstrated that in most practical situations this dependence is manifested rather strongly; however, it does not always affect the TP. Thus, when the light and acoustic beam apertures exceed some characteristic values d^* and L^*, related to the acoustic frequency and speed and the AOI geometry, the effect of the SD DOTF on the transient process can be neglected. Otherwise, the DDAP leads to the asymmetry of the dynamic dependences of the field amplitude of the off-axis angular-spectrum components of the diffracted beam. The parameters d^* and L^* have been calculated for the AOI of several types in lithium niobate and paratellurite crystals. The TP characteristics are illustrated by plots calculated for the DDAP in the case of the AOI in paratellurite. Tomsk State University of Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–48, May, 1999.     

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.     

Research on detection method of multi-frequency weak signal based on stochastic resonance and chaos characteristics of Duffing system

Weak signal detection has been widely used in many fields such as military and national economy. Aiming at the problem that the traditional stochastic resonance (SR) method can’t obtain the signal amplitude when detecting weak signals, the frequency and amplitude of the weak signal are obtained by combining the SR and chaos characteristics of the two-dimensional Duffing system. Firstly, the effects of two-dimensional Duffing system parameters a, b, k, noise intensity D on the Kramers rate and signal-to-noise ratio (SNR) are analyzed under the Gaussian white noise environment. The results show that the damping ratio K can hinder the SR effect of the system to some extent. Secondly, to solve the misjudgment of the state method of the weak signal amplitude in the detection, the Lyapunov exponent is used to assure the threshold's range, and the threshold of the chaotic critical state is found. Finally, the paper gives the processes of frequency and amplitude detection of multiple high-frequency signals, which realizes the effective detection of the frequency and amplitude of multiple high-frequency signals in a Gaussian white noise environment, and successfully applies the method to the accurate detection of boundary voltage amplitude in electrical impedance tomography.     

Frequency tripling of convergent beam employing crystals tiling in large-aperture high-energy laser facilities

In inertial confinement fusion, ultraviolet laser damage of the fused silica lens is an important limiting factor for load capability of the laser driver. To solve this problem, a new configuration of frequency tripling is proposed in this paper. The frequency tripling crystal is placed on downstream of the focusing lens, thus sum frequency generation of fundamental frequency light and doubling frequency light occurs in the beam convergence path. The focusing lens is only irradiated by fundamental light and doubling frequency lights. Thus, its damage threshold will increase. LiB₃O₅ (LBO) crystals are employed as frequency tripling crystals for its larger acceptance angle and higher damage threshold than KDP/DKDP crystals’. With the limitation of acceptance angle and crystal growth size are taken into account, the tiling scheme of LBO crystals is proposed and designed optimally to adopt to the total convergence angle of 36.0 mrad. Theoretical results indicate that 3 LBO crystals titling with different cutting angles in θ direction can meet the phase matching condition. Compared with frequency tripling of parallel beam using one LBO crystal, 83.8% (93.1% with 5 LBO crystals tiling) of the frequency tripling conversion efficiency can be obtained employing this new configuration. The results of a principle experiment also support this scheme. By employing this new design, not only the load capacity of a laser driver will be significantly improved, but also the fused silica lens can be changed to K9 glass lens which has the mature technology and low cost.     

Ultrasonic enhanced Brillouin light scattering in water

Brillouin light spectra in water was experimentally investigated by a scanning Fabry-Perot interferometer. The optical properties of the scattering media were modulated by a plane ultrasound wave. An enhancement up to 10 times of the Brillouin signal by modulation with the ultrasound wave has been seen clearly. The dependence of the Brillouin signal amplitude on the ultrasound frequency, amplitude and the position of the ultrasound field based to the light beam was investigated experimentally. Very strong dependence of the enhancement with experimental conditions was obtained.     

Free carrier Voigt magneto-absorption in cubic semiconductors

A compensation bridge for measuring Voigt magneto-absorption in semiconductors has been setup for X-band. Room temperature measurements for phase and amplitude, as a function of magnetic field up to 9.8 kOe, have been carried on n-type single crystals of germanium, at 9.02 GHz. Theoretical calculations have been done for the amplitude and phase measurements as a function of magnetic field taking into account lattice scattering. There is a good agreement between the experimental and the theoretical results.     

Electrooptic modulation in a multimode exodiffused waveguide

The variation of the phase shifts obtained for different modes of an exodiffused electrooptic waveguide modulator has been calculated.The calculated results have been compared to experimental values obtained by using a scanning Fabry-Perot to determine the relative side-band intensities of the modulated light in different modes if a fixed amplitude (20 V) and frequency (300 MHz) electric signal is applied to the modulator. Good agreement has been found between the predictions and the experimental results.     

All-optical switch based on photonic crystal microcavity with multi-resonant modes

We present a design of all-optical switches based on one-dimensional photonic crystals (1D PhC) doped with nonlinear optical materials. The 1D PhC switch structure is composed of a PhC cavity sandwiched by two accessional PhC microcavities. The center PhC cavity has two resonant frequencies with nearly the same quality factors (Q), while the accessional PhC cavities have the same resonant frequency, which is equal to one of the resonant frequencies of the center cavity. The two accessional PhC cavities cause reduction of Q value in this resonant frequency and result in different Q values of two modes. We realize all-optical switch effect by selecting pump light wavelength at the low Q mode and probe light wavelength at the other mode. The theoretical simulations by using the finite difference time domain method show that the pump light intensity required to realize optical switch effect in the designed switch is 50 times smaller than that in one-dimensional photonic crystals cavity with only one resonant mode.     

Electro-Optic Coefficient Measurements for Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Te Single Crystals at 1550 nm Wavelength

Nonlinear optical materials of the type Zn _x Cd_1-x Te single crystals, where x = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, and 1.0, have been grown by the Bridgman method, using a vertical furnace. We have investigated the electro-optic (E-O) coefficient and refractive index of Zn _x Cd_1-x Te single crystals at optical communication wavelength (1550 nm). In the case of CdTe crystal, the E-O coefficient was 15.5 × 10^-12 m/V, which is the biggest among the E-O coefficients of Zn _x Cd_1-x Te crystals. The E-O modulation signal was very big in low frequency range (DC-100kHz), but the signal amplitude became smaller as we increased frequency above 100 kHz. We also found the acousto-optic modulation at CdTe single crystals.     

The orange luminescence band in MgO crystals

Nominally pure MgO crystals exhibit a bright orange luminescence when excited with u v or visible laser light This luminescence has been characterized using a variety of measurements with pulsed and cw laser sources Its amplitude increases with temperature to about 400 K. while the lifetime of emission decreases from 270 μs at 290 K to 9 μs at 433 K This emission appears to originate in hole recombination with Fe²⁺ ions Hole trapping at V^?_OH and V^?_Al centers may occur as an intermediate step Bleaching with 514 5-nm Ar⁺ laser light is reversible upon exposure to u v radiation Present data are compared with those obtained previously on the thermoluminescence of irradiated MgO crystals.     

X-ray determination of the mean Debye-Waller factors,amplitudes of vibration and Debye temperatures of six crystals with CsCl structure

From the integrated intensities of Bragg reflections measured at room temperature with an x-ray powder diffractometer, the mean Debye-Waller factorB, the average r.m.s. amplitude of vibration (ū ²)^1/2 and the Debye temperatureθ, have been determined for six crystals with the CsCl structure. The energy of formation of a Schottky pair in these crystals has been estimated from an empirical relation between the x-ray Debye temperature and the energy of formation.     

Self-pulsation and optical chaos in self-pumped photorefractive BaTiO3

Sinusoidal intensity oscillations of the phase conjugated wave reflected from a self-pumped photorefractive BaTiO₃ crystal have been observed. The oscillation amplitude is shown to be increased by providing optical feedback of the light scattered during grating formation. The oscillation frequency ? can be tuned from 0.024 Hz to 2 Hz by changing the light intensity I₀ of the object beam from 0.7 W/cm² to 100 W/cm². A power law ? ∝ I^β₀ ∝ σ with β = 0.89 was observed over this range of intensities (σ = photoconductivity). Transitions to a chaotically oscillating or a stable reflecting state have been observed at higher power levels or with increased optical feedback. An additional pump beam near the entrance face of the signal beam can decrease the build-up time for the self-pumping process by a factor of five or more.     

Magnetization dynamics in epitaxial films induced by femtosecond optical pulses near the absorption edge

Peculiarities of the magnetization dynamics induced in iron garnet films by laser pulses with a frequency detuning near the absorption edge have been studied experimentally. It has been found that the dependence of the observed signal amplitude on the pumping energy becomes nonmonotonic with an increase in the pumping frequency. At the same time, the pumping energy corresponding to the maximum amplitude of the signal and this maximum signal amplitude decrease. Moreover, the signal amplitude starts to decrease with an increase in the pumping energy at frequencies within the absorption band. The observed phenomena are possibly caused by generation of magnetostatic spin waves and the effect of ultrafast optically induced demagnetization.     

Detection of weak signals based on a new class of transformations of random series

For detecting weak signals (when their amplitude is comparable with the amplitude of the noise track) a new class of transformations of random series to a straight line has been suggested. It has been shown that these transformations are quasi-linear and can be defined as a signal-to-staircase transformation (SST). The height of a step defines an amplitude of the detected signal and the step length its duration. The SST can be applicable for a wide class of random series having different statistical nature. The verification of this new method based on the analysis of the real signal/noise tracks containing registration of different earthquakes with small amplitudes has been realized. It has also been shown that different situations which have been found from the real-data analysis demonstrate the high sensitivity and efficiency of the new method suggested.     

Hydrogen maser frequency shifts due to coherently excited Δm F =±1 transitions betweenF=1 levels of the atomic hydrogen ground state

Hydrogen maser frequency shifts, caused by the multiple quantum transition nonlinearities of a resonant multiple frequency excitation of the atomic hydrogen four level ground state system have been investigated. The oscillation characteristics of hydrogen maser operation with simultaneously excited, low frequencyΔm _F =±1 transitions between theF=1 states of the atomic hydrogen ground state have been analysed theoretically and explicit formulas for hydrogen maser frequency shifts and amplitude response have been derived for arbitrary maser oscillation amplitude and a small signal approximation for theΔm _F =±1 “Zeeman” transitions. The comparison with experimentally observed hydrogen maser frequency shifts was specialized to small magnetic fields, for which the difference between the resonance frequencies of the two low frequency,Δm _F =±1 Zeeman transitions is small compared to the linewidth. Special emphasis was placed on the evaluation of frequency pulling effects for a Zeeman transition excitation at off-resonance conditions. For this case the theoretical formulation of frequency pulling effects becomes insensitive against simplifying assumptions about the radiation damping phenomena and a particular good agreement between experiment and theory can therefore be expected. Experimental conditions have been specified, for which the uncertainty of hydrogen maser frequency due to Zeeman transition induced frequency shifts does not restrict the present frequency stability of a hydrogen maser frequency standard.     

Nucleation kinetics in a crystalline TCNQ salt

The allotropic transformation of a TCNQ salt has been studied. For this the nucleation frequency of ensembles of small crystals was measured as a function of time, temperature, and crystal size by means of light microscopy. It was found that the measured proportion of transformed crystals can be described by a function of the type α=f(k_T·k_s·t) with k_T depending on temperature according to an Arrhenius equation, and k_s being proportional to the edge length of the crystals. Furthermore the results cannot be described on the assumption of a single stationary nucleation frequency. The results are discussed with reference to other transformations, and it is concluded that there is a distribution of stationary nucleation frequencies.     

相位调制激光多普勒频移测量方法的改进

本文对现有相位调制激光多普勒频移测量方法进行了改进,通过定义新的鉴频参量来同时利用相位调制信号直流和交流分量中的有用信息进行多普勒频移测量.由于相位调制信号直流分量中包含着调制信号光的Fabry-Perot干涉仪光强透过率,所以这一改进本质上是将基于Fabry-Perot干涉仪的边缘技术激光多普勒频移测量方法的优势引入到相位调制测量方法中,以提高其自身的性能.理论上证明改进后的相位调制激光多普勒频移测量方法无需对信号光的光强进行测量,所以可以进一步简化探测系统的结构和较少噪声混入的通道.另外,通过对改进前后鉴频和测量灵敏度曲线进行对比,还证明了其具有更高的测量灵敏度和动态范围.实验上对硬目标反射的频移可控信号光进行测量,不但证明了理论的正确性,而且证明了改进后的相位调制激光多普勒频移测量方法,测量动态范围提高约1倍,测量标准偏差降低约35%.     

Raman scattering of light in N-benzylaniline crystals

Single crystals of N-benzylaniline (C₆H₅-CH₂-NH-C₆H₅) have been grown, X-ray structural analysis of these crystals has been performed, and their symmetry has been identified as corresponding to the space group C _2h ⁵ . Raman spectra of N-benzylaniline single crystals in polarized light for different geometries of the experiment have been analyzed. Frequencies and symmetry of lattice phonons have been determined.     

Optical bistability in nonlinear surface-plasmon polaritonic crystals

Nonlinear optical transmission through periodically nanostructured metal films (surface-plasmon polaritonic crystals) has been studied. The surface polaritonic crystals have been coated with a nonlinear polymer. The optical transmission of such nanostructures has been shown to depend on the control-light illumination conditions. The resonant transmission exhibits bistable behavior with the control-light intensity. The bistability is different at different resonant signal wavelengths and for different wavelengths of the control light. The effect is explained by the strong sensitivity of the surface-plasmon mode resonances at the signal wavelength to the surrounding dielectric environment and the electromagnetic field enhancement due to plasmonic excitations at the controlled light wavelengths.