Anisotropic light diffraction in a biaxial crystal of alpha-iodic acid

A program for calculating characteristics of acousto-optic interaction in biaxial crystals is developed and used to analyze light diffraction by ultrasound in an iodic acid crystal. Crystal cuts with maximum acousto-optic figures of merits are found. It is shown that in the region of the optical axes the frequency dependences of the Bragg angles under anisotropic diffraction are substantially different from those in uniaxial crystals.     

Spatial structure of acousto-optic phase matching in uniaxial crystals

The spatial structure of Bragg angles and the transfer functions of an acousto-optic cell are calculated for the cases of isotropic and anisotropic light diffraction in a uniaxial crystal. Their change with ultrasound frequency is traced. The possibility of image processing by acousto-optic filtration of the spatial spectrum of an image is considered. The results of experimental visualization of the transfer functions of a calcium molybdate crystal cell are presented.     

双轴晶体中二次谐波产生的最佳相位匹配条件

从二次谐波产生的理论出发，讨论了双轴晶体中二次谐波产生的相位匹配条件，推导了有效非线性光学系数的精确计算公式，以有机非线性光学晶体二苯甲酮为例，给出了对两种基波(1064μm和0808μm)二次谐波产生的相位匹配条件的数值模拟及相应的有效非线性光学系数的精确计算结果，并确定出相应的最佳相位匹配条件. 关键词： 二次谐波产生 最佳相位匹配 有效非线性光学系数 二苯甲酮晶体     

Calculation of nonlinear optical polarization and phase matching in biaxial crystals

We present a general, analytical formula for the calculation of the unit vectors of the direction of vibration of the electric field strength in biaxial crystals. By means of this formula it is possible to calculate the effective nonlinearity for second-order nonlinear optical frequency conversion processes in biaxial crystals. For two highly efficient nonlinear optical crystals we have calculated the effective nonlinear susceptibility and walk-off angles. The results have been compared with numerical as well as experimental data of previous papers.     

双轴晶衬底上双轴晶单晶薄膜的椭偏测量

本文给出了两种测量双轴晶衬底上双轴晶单晶膜的折射率和膜厚的方法及测量的公式.我们利用这种方法测量了所研制的KTP光波导薄膜层的折射率和膜厚.     

双轴晶体中光的波面的形状

利用数学软件Mathematica 5.2的三维参数作图法,用计算机画出了双轴晶体中光的波面的立体形象,并说明了它的一些特征.     

Rotational diffusion and orientation relaxation of rodlike molecules in a biaxial liquid crystal phase

The longitudinal relaxation time and the complex dielectric polarizability of rod-like molecules with dipole moment parallel to the long axis in a biaxial nematic liquid crystal are calculated using as model the rotational Brownian motion in a mean field potential so reducing the problem to a solution of a set of linear differential-recurrence relations for statistical moments (the appropriate equilibrium orientational correlation functions). The solution of this set is obtained by matrix continued fractions. Moreover, simple analytic equations (based on the exponential separation of the time scales of the intrawell and overbarrier (interwell) relaxation processes), allowing one to understand the qualitative behavior of the system and accurately predicting the longitudinal complex polarizability for wide range of the barrier height and anisotropy parameters, are proposed.     

Collinear acousto-optic interaction in a single tellurium crystal

The problem of designing acousto-optic imaging filters based on tellurium crystal is discussed. The measurement and calculation results of acousto-optic properties of tellurium are presented. The collinear acousto-optic interaction in tellurium crystal is experimentally implemented for the first time at the light wavelength of 10.6 μm.     

Analysis on dependence of phase matching angle on temperature in KDP crystal

The dependence of the phase matching (PM) angle on temperature in type-I second harmonic generation (SHG) and type-II third harmonic generation (THG) of KDP is experimentally investigated. The experimental results show that as the crystal temperatures vary by 1 °C, the external PM angles of the KDP doubler and tripler change 100 μrad and 220 μrad, respectively. The PM angle of KDP tripler is more sensitive to temperature variation than is that of the KDP doubler. Comparing the numerical calculations and the previous reports, a discrepancy has been found among the different reported results. Based on the analytical solution of the temperature dependence of the SHG process it is shown that a small error in the thermo-optic coefficients of the crystal is the main factor source of the discrepancy.     

Electric-field-induced polar biaxial order in a nontilted smectic phase of an asymmetric bent-core liquid crystal

A polarization reorientation process has been studied by means of optical second-harmonic generation in an optically uniaxial smectic phase of an asymmetric bent-core liquid crystal. A nontilted polar smectic order with a biaxial order is induced by applying an electric field to the uniaxial nonpolar smectic phase. This phenomenon was well simulated by the two-dimensional Langevin process, i.e., electric-field-induced continuous molecular reorientation against thermal agitation. The simulation suggests that about 200s of molecules form a polar domain and cooperatively respond to the applied field. The existence of the polar domains and their reorientation are consistent to the dielectric measurement; a high dielectric constant of about 60 at the kHz range is markedly suppressed by applying a bias field.     

Picosecond pulse of variable duration by phase matching frequency doubling crystal

Report on a semiconductor saturable absorber mirror (SESAM) passively mode-locked picosecond Nd:YVO₄ laser, with adjustable durations between 8.7 and 70 ps. The pulse duration was adjustable by varying the length of type II phase matching KTiOPO₄ (KTP) crystal. Type II phase matched second harmonic interaction in combination with the anisotropic emission characteristic of Nd:YVO₄ introduced additional bandwidth limiting through birefringent filtering.     

Space phase matching and self-frequency-doubling red laser in Nd:GdCOB crystal

By using the calculation theory of the phase-matching direction and effective nonlinear coefficient of frequency doubling in biaxial crystal, we calculate the effective nonlinear coefficients at different frequency-doubling phase-matching directions of the three main fluorescent lines (936, 1060, 1331 nm) of Nd:GdCOB crystal. The space distribution curves of effective nonlinear coefficient are drawn. Two crystal specimens are cut at the optimal type I phase matching angles: for specimen 1, θ=65°, Φ=34.6° (the first octant); for specimen 2, θ=66.5°, Φ=145.5° (the second octant). When pumped by Datachroom-5000 pulsed dye laser, the output self-frequency-doubling red laser in specimen 2 is 0.62 mJ, and the corresponding conversion efficiency is 2.5%. Under the same pumping condition, the output red laser of specimen 1 is much smaller than that of specimen 2, which is well in agreement with the theoretical calculation.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

Effects of structural irregularities on modulational instability phase matching in photonic crystal fibers

The effect of structural irregularities in photonic crystal fibers on scalar and vector modulational instability (MI) processes is studied by numerical simulations and experiments. For an anomalous-dispersion regime pump, variations in core ellipticity as small as 0.5% over length scales of the order of several meters are shown to have a negligible effect on scalar MI, yet they completely suppress vector MI. In contrast, for a normal-dispersion regime pump, vector MI is shown to be robust against such fluctuations.     

Photoelastic and acousto-optic properties of the KDP crystal with application to wide-angle acousto-optic tunable filters

Acousto-optic properties of the KDP crystal are analyzed with application to wide-angle acousto-optic tunable filters. The analysis is based on the measurements of the effective photoelastic constant in KDP crystals at the optical wavelengths of 405, 532.5, and 633 nm. The disagreement between the experimental and literature data is discussed.     

The elusive thermotropic biaxial nematic phase in rigid bent-core molecules

The biaxial nematic liquid crystalline phase was predicted several decades ago. Several vigorous attempts to find it in various systems resulted in mis-identifications. The results of X-ray diffraction and optical texture studies of the phases exhibited by rigid bent-core molecules derived from 2,5-bis-(p-hydroxyphenyl)-l,3,4-oxadiazole reveal that the biaxial nematic phase is formed by three compounds of this type. X-ray diffraction studies reveal that the nematic phase of these compounds has the achiral symmetry D_2h, in which the overall long axes of the molecules are oriented parallel to each other to define the major axis of the biaxial phase. The apex of the bent-cores defines the minor axis of this phase along which the planes containing the bent-cores of neighboring molecules are oriented parallel to each other.     

Vavilov-Cherenkov radiation in the biaxial triglycine sulphate crystal

Some properties of the Vavilov-Cherenkov radiation in the biaxial triglycine sulphate crystal have been analysed. The radiation properties have been studied for the cases of protons moving along the main three dielectric axes. The measurements were performed with the 660 MeV proton beam.A quantitative agreement of experiment with Muziká's theoretical calculations on the directional properties was observed. In all the cases a qualitative agreement concerning azimuthal distribution of the radiation intensity of the extraordinary waves of both polarities as well as concerning the polarization properties predicted by Obdrálek's formulae has been obtained.Dedicated to the Memory of Professor . Muziká.On leave from Department of Nuclear Physics, Comenius University, Bratislava, Czechoslovakia.The authors are grateful to Prof. V. P.Dzhelepov, Director of the Laboratory of Nuclear Problems, to Prof. L. I.Lapidus for support of the experiment as well as to Prof. L. M.Belyaev and I. V.Gavrilova (the Institute of Crystallography of the USSR Academy of Sciences) for supplying triglycine sulphate crystals, to J.Obdrálek (Charles University, Prague) for fruitful discussions and helpful advice, to Dr. R.Pose for assistance in obtaining negative colour film, to E.Janikova and R.Janik for great assistance in the experiment.     

Influence of disorder in crystal structure on ferroelectric phase transitions

A new model of ferroelectric phase transitions in disordered crystals is developed. The model takes into account the nonequivalence of the structural environment of identical ions, which alters the balance of forces governing ferroelectric structural instability. In contrast with its predecessors, the new model can be used for qualitative and, in many cases, quantitative predictions of the temperature range of the diffuse ferroelectric phase transition and the value of the disorder-induced shift of the average transition temperature as a function of the degree of disorder and chemical composition. This conclusion is confirmed by a comparison of the theoretical results with an abundance of known experimental facts. Zh. éksp. Teor. Fiz. 111, 1817–1832 (May 1997)     

Influence of dipole-dipole correlations on the stability of the biaxial nematic phase in the model bent-core liquid crystal

A molecular theory of biaxial nematic ordering in the system of bent-core molecules has been developed in the two-particle cluster approximation which enables one to take into account short-range polar correlations determined by both electrostatic dipole-dipole interaction and polar molecular shape. All orientational order parameters and short-range correlation functions are calculated numerically as functions of temperature in the uniaxial and in the biaxial nematic phases, and the results are compared with the ones obtained in the mean-field approximation and in the cluster approximation but without taking into consideration the dipole-dipole interaction. It is shown that short-range polar correlations and, in particular, the dipole-dipole correlations dramatically increase the temperature of the transition into the biaxial nematic phase and enhancing its stability range. The results are also very sensitive to the value of the opening angle of a model bent-core molecule.