Features of acousto-optic interaction in uniaxial gyrotropic crystals

The features of noncollinear acousto-optic interaction in gyrotropic crystals in the intermediate diffraction regime and the regime of Bragg diffraction were investigated. The dependence of the efficiency of diffraction in gyrotropic paratellurite, tellurium, and quartz crystals on the intensity of the ultrasonic wave, acousto-optic interaction length, and incident-light polarization was investigated. It is shown that the gyrotropy of the crystal is responsible for the appearance of a multiple-peak structure of the Bragg maximum. It has been established that in the case of propagation of incident and diffracted waves in the vicinity of the optical axis of the crystal, the diffraction efficiency is independent of the polarization state of the incident light. The results of theoretical calculations are in agreement with the experimental results obtained for uniaxial crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 105–110, January–February, 2000.     

The spatial structure of acousto-optic phase matching in biaxial crystal of alpha-iodic acid

Anisotropic acousto-optic diffraction in biaxial crystal of alpha-iodic acid has been studied in the case in which the wave vector of the ultrasound is almost orthogonal to one of the optical axes and the directions of incident and diffracted light beams are close to the optical axis. It is shown that, owing to the complex form of wave surfaces in the field of optical axes of a biaxial crystal, the considered variant of acousto-optic interaction is very sensitive to changes in the direction of the wave vector of the ultrasound that imposes special requirements to accuracy of manufacturing of acousto-optic devices.     

A simplified model for second-harmonic generation in uniaxial crystals: phase matching condition

The phase matching condition for second-harmonic generation can be satisfied by a simplified model making use of the conservation of energy and momentum. By using the crystal indicatrix together with our model, we are able to classify uniaxial crystals as Type I and II non-linear optical materials.     

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     

Quasi phase matching in two-dimensional nonlinear photonic crystals

We analyze quasi-phase-matched (QPM) conversion efficiency of the five possible types of periodic two-dimensional nonlinear structures: Hexagonal, square, rectangular, centered-rectangular, and oblique. The frequency conversion efficiency, as a function of the two-dimensional quasi-phase-matching order, is determined for the general case. Furthermore, it is demonstrated for two basic feasible motifs, a circular motif and a rectangular motif. This enables to determine the optimal motif dimensions for achieving the highest conversion efficiency. We find that a rectangular motif is more efficient than a circular motif for quasi-phase-matched processes that rely on a single reciprocal lattice vector (RLV), and that under optimal choice of motif dimensions, it converges into a one-dimensional periodic structure. In addition, in a few specific cases we found that higher order QPM can be significantly more efficient than lower order QPM.     

Broad angle phase matching in subdiffractive photonic crystals

We show that photonic crystals made of materials with normal dispersion allow broad angular range phase matching in nonlinear wave mixing processes if tuned to the subdiffractive (or equivalently self-collimated) beam propagation regimes for the frequencies of both interacting waves. This allows efficient parametric mixing of narrow beams. We demonstrate this idea by numerical simulation of the second harmonic generation in two-dimensional photonic crystal in particular nonlinear material (AlGaAs) in planar waveguide geometry.     

Noncollinear double quasi phase matching in one-dimensional poled crystals

We demonstrate simultaneous phase matching of two different nonlinear processes, using a noncollinear interaction in periodically poled crystal with single grating. The noncollinear scheme provides phase-matching solutions over continuous regions of the optical spectrum and can be used for multiple-harmonic generation as well as all-optical effects. We have demonstrated experimentally third-harmonic generation of a 3 microm pump wavelength in a noncollinear configuration using a periodically poled LiNbO3 crystal. We observed, in good agreement with theoretical calculation, very broad spectral and thermal acceptance bandwidths, as well as a relatively narrow angular bandwidth.     

Wide-aperture acousto-optic interaction in birefringent crystals

Regularities of the wide-aperture mode of light diffraction by acoustic waves in crystals with different values of birefringence are considered. The diffraction has been investigated theoretically and experimentally with the aim of applying it in acousto-optic filters used in processing uncollimated optical beams and images. It is proved that the main filtration parameters depend on the birefringence of crystals used in processing luminous fluxes. By the example of wide-aperture filters based on paratellurite, magnesium fluoride, and potassium dihydrogen phosphate crystals, it is shown that the choice of crystals with high birefringence may increase the maximum attainable angular aperture and throughput of devices. It is also proved that application of crystalline compounds of mercury and tellurium (characterized by high optical anisotropy) in filters increases spectral resolution and improves the quality of filtered images.     

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.     

Optical breathers in uniaxial crystals

Anisotropic crystals are shown to have three different mechanisms of the formation of breathers depending on the direction of the extraordinary wave propagation and on the symmetry of the medium. All uniaxial crystals with quadratic susceptibilities can be divided into three different groups, according to the crystal classes (CC). Each group is characterized by a universal structure of the breather zones (SBZ). The SBZ of the media with cubic susceptibilities does not depend neither on the crystal systems nor on the CC and coincides with the SBZ of the crystals with quadratic susceptibilities and the CC 3, 3m, 4, 4mm, 6, 6mm.     

Electrooptical parameters of uniaxial crystals

For the linear effect in classes 4, ¯4, 42, 4 · m, 6, 62, and 6 m, one of the optic axes always coincides with the high-order symmetry axis c when the electric field e lies in a plane perpendicular to that axis, while the other describes a circular cone around c as e rotates in that plane. The same rotation in classes 3, 32, 3 · m, 3 m, and m · 3 m causes both optic axes to deviate from c to opposite sides by equal angles, both describing circular cones. One of the optic axes always coincides with c for any direction of e in classes 42 and 62, while the other axis describes an elliptic cone around c as e rotates in a plane inclined to c. Both optic axes describe an elliptic cone in classes 3 m, 32, and m · 3 m.     

All-angle phase matching condition and backward second-harmonic localization in nonlinear photonic crystals

We demonstrate an isotropic phase matching in properly designed nonlinear two-dimensional photonic crystals. In addition, by combining left- and right-handed properties at the fundamental and second-harmonic frequencies, we obtain a backward second-harmonic generation. These two properties lead to an unusual second-harmonic localization effect in perfect lattice photonic crystals.     

Additional beams in optically uniaxial crystals

Reasons for occurrence of additional beams are established for rays reflected from a tilted prism face and transmitted through the interface between two crystal media. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–53, July, 2007.     

Effect of uniaxial strain on the structure of self-localized excitons in alkali halide crystals

We have used luminescence spectroscopy to establish the effect of low-temperature (100 K) elastic uniaxial strain on the configuration of a self-localized exciton (SLE) in alkali halide crystals (AHCs) at the instant of radiative relaxation. In face-centered (fc) alkali halide crystals, redistribution of the luminescence intensity occurs from the asymmetric SLE configuration to the symmetric SLE configuration (type III → II → I), while conversely in body-centered (bc) alkali halide crystals the redistribution occurs in favor of the asymmetric (polarized) SLE configuration (type I → II). External strain along the 〈100〉 direction leads to effective slip of the anions in the alkali halide crystals along the 〈110〉 direction, coinciding with the direction of compression of the SLE, which promotes creation of preferentially the symmetric SLE configuration, while strain along the 〈110〉 direction, acting perpendicularly to the length of the SLE, leads to elongation of the SLE, which promotes creation of the asymmetric SLE configuration with a higher degree of polarization. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 67–72, January–February, 2007.     

Generation of singular beams in uniaxial crystals

The process of generation and transformation of the structure of singular beams both coherent and incoherent, in a crystal-polarization compensator-polarizer system is studied. It is shown that this system can transform the topological charge of the initial beam. The value of the transformed charge depends both on the structure of the initial beam and on its polarization. The initial singular beams, transferring topological multipoles, are shown to acquire unique properties after passing through the system. This system, in particular, makes it possible to control the position of bound optical vortices and the magnitudes of their angular momenta, which may find practical application in devices for trapping, transportation, and mutual arrangement of microparticles.     

平顶高斯光束在单轴晶体中的传输

基于光束在各向异性单轴介质中的近轴矢量传输理论，对平顶高斯光束在单轴晶体中的传输 作了研究，得到了解析的传输公式. 利用该传输公式可用一种简单的方法研究平顶高斯光束 在单轴晶体中的传输. 研究结果表明，由于晶体的各向异性特性使平顶高斯光束初始的圆对 称性在传输过程中不能保持，并且其偏振状态也随传输而变化. 用数值计算例对晶体内源于 入射平顶高斯光束的寻常光和非常光的场分布以及x和y方向的偏振分量在空间中的演化都作 了详细说明. 关键词： 平顶高斯光束 单轴晶体 各向异性 近轴矢量理论 传输公式     

Optimal conditions of the acousto-optic interaction in crystals with gyrotropy

The conditions of the acousto-optic interaction in the crystals with gyrotropy are considered. The optimal length of the acousto-optic interaction and the diffraction efficiency in the gyrotropic direction are calculated. The diffraction efficiency is determined by the coupled wave method which allows calculation of the field interaction in the gyrotropic crystal [1]. It is theoretically shown and experimentally confirmed that the diffraction efficiency in the gyrotropic direction is independent of incident light polarization, when the polarization plane rotates by 45° along the interaction length.     

Optical phase shifting with acousto-optic devices

A novel optical phase-shifting method based on a well-known acousto-optic interaction is proposed. By using a pair of acousto-optic modulators (AOMs) and properly aligning them, we construct an optical phase shifter that can directly control the phase of a collimated beam. The proposed phase shifter is insensitive to the polarization of the incident beam when polarization-insensitive AOMs are used, and no calibration is necessary. The proposed approach is confirmed with experimental results.     

Optoacoustic interaction in uniaxial gyrotropic quartz crystals

The specific features are considered of the noncollinear optoacoustic interaction in uniaxial gyrotropic quartz crystals in an intermeadiate regime of diffraction close to the Bragg regime. A system of equations of coupled waves is presented which describes the optoacoustic interaction in uniaxial gyrotropic crystals in the Raman-Nath, intermedaite, and the Bragg regimes of diffraction. The dependences of the diffraction efficiency on the ultrasonic wave intensity, the optoacoustic interaction length, and on the incident light polarization are investigated. A multispike structure of the Bragg maximum is studied that appears in accounting for the crystal gyrotropy. A good agreement between the theoretical calculations and the known experimental results for a quartz crystal is shown. Mozyr State Pedagogical Institute, 28, Studencheskaya Str., Mozyr, 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 278–281, March–April, 1999.