Bragg diffraction of light by ultrasonic shear waves in a plane-parallel layer

We have studied anisotropic Bragg diffraction of light by ultrasonic shear waves in an optically isotropic plane-parallel layer. We have established the analytical dependences of the relative intensities and polarization azimuths of reflected and transmitted diffracted waves on the intensity of the ultrasound, the layer thickness, the angle of incidence, and the polarization azimuth of the incident light. We show that rotation of the plane of polarization of the diffracted wave is determined by the different Fresnel reflection of the s and p polarized components of the incident light in the plane-parallel layer. We have determined that in mismatched acousto-optic structures, deep amplitude modulation of transmitted and reflected light is possible which is an order of magnitude greater than the usual modulation in matched structures.     

Bragg diffraction of light by rectangular phase gratings

Bragg diffraction of light by rectangular phase gratings is considered, utilizing two-dimensional coupled-wave theory. Diffraction characteristics for two important cases of incident plane waves and Gaussian-profile beams are presented. It is shown that the diffraction properties depend on the following normalized parameters: the grating strength and a geometry parameter, the beam width and a displacement. Values of these parameters for the realization of both nearly transmitting and nearly reflecting regimes of diffraction are determined. The results obtained are used for the choice of the design parameters of integrated optic Bragg-type devices.     

磁光Bragg衍射中的相位失配分析

给出了任意倾斜偏置磁场作用下相位失配时微波静磁波与导波光的磁光耦合方程，分析了相位失配对导波光衍射效率及其衍射方向的影响.计算了YIG薄膜波导中静磁反向体波与导波光非共线作用的Bragg衍射效率, 传统磁化时计算结果与实验结果一致.计算表明，与传统磁化情形相比，适当的偏斜磁场可使导波光衍射效率提高6dB以上；当导波光入射角保持不变时，由磁场方向改变引起的相位失配对衍射效率的影响不大.因此，优化偏置磁场方向是改善磁光Bragg器件衍射性能、提高磁光带宽的有效方法. 关键词： 磁光效应 Bragg衍射 静磁波     

Extremely asymmetrical scattering of slab modes in periodic Bragg arrays

The steady-state extremely asymmetrical scattering of electromagnetic modes in a slab with a periodically corrugated boundary is analyzed theoretically. A new approach, based on allowance for the diffractional divergence of a scattered wave, is used with the approximation of slowly varying amplitudes and a Fourier analysis. The structure of the incident and scattered waves inside and outside the array is determined. The amplitudes of the scattered waves are found to be much larger than the amplitude of the incident wave. The typical time of relaxation to steady-state scattering is found to depend on the distance from the array boundary through which the incident wave enters the array. Conditions of applicability of the results obtained are also presented.     

Scattering properties of an individual metallic nano-spheroid by the incident polarized light wave

The scattering properties of a metallic nano-spheroid under the illumination of different polarized light waves are investigated using 3D boundary element method. The influences of different geometrical sizes of the nano-spheroid and incident directions of the illuminating light wave on the scattering spectrum are studied for different incident polarized light waves. The results show that the metallic nano-spheroid has two intrinsic resonant modes, corresponding to different polarization states and resonant wavelengths. The scattering enhancement, the resonant wavelength, and the location of the enhanced optical field are strongly dependent on the polarization properties of the illuminating light waves, and they can be modulated by appropriately choosing the polarization directions of the incident light wave.     

Quantum fluctuations in second-harmonic light generation

Second-harmonic light generation (SHLG) is analyzed from the viewpoint of the photon statistics of the fundamental and generated beams versus the path traversed by the two waves in the medium. The calculations lead to an anti-bunching effect for coherent incident light.     

入射角对Al0.5Ga0.5As-AlAs分布布拉格反射器反射光谱的影响

采用传输矩阵法对Al0.5Ga0.5As-AlAs材料的发光二极管分布布拉格反射器进行入射角的反射光谱研究,计算发现反射偏振光p和s随入射角的增大呈“V”形变化,在49.8°处有最小反射值。不同入射介质[以空气和限制层(Al0.7Ga0.3)0.5In0.5P材料]下的反射光谱受入射角的影响差异很大,其中入射角对空气入射介质的反射谱影响较小,由0°入射的反射率88.13%降至45°的84.94%,反射峰值波长蓝移仅10 nm;但入射角对(Al0.7Ga0.3)0.5In0.5P入射介质的反射谱影响很大,仅从0°到45°入射,反射率降幅就超过45%,反射峰值波长蓝移超过127 nm。为了减缓这种影响,提出了多波长布拉格反射器结构设计。计算表明多波长分布布拉格反射器在0ο~45°的入射角内比传统的分布布拉格反射器有更好的光谱特性,这对提高发光二极管的出光效率有现实意义。     

Simple and high efficient optical trapping using a cylindrical lens and a single plane wave of incidence

We suggest a simple and high efficient method for trapping particles in the evanescent field. In this method, a single plane wave is normally incident on the cylindrical surface of a cylindrical lens and then incident on the plane surface of the lens at an angle larger than the critical angle. Multiple reflections of light within the cylindrical lens create two evanescent waves with different directions in the transmitted field. Interference of two evanescent waves comes into being a standing wave which can stably trap particles close to the top of the cylindrical lens. Based on the Rayleigh approximation, we obtain analytical expressions of optical force acting on a Rayleigh particle placed in the vicinity of the lens. We find that the trap stiffness and trap depth is dependent on the radius of the cylindrical lens, wavelength and polarization of light, and incident angle at the lens–liquid interface.     

The theory of planar Bragg resonators

The reflecting properties of one-dimensional planar Bragg gratings are studied. A coupled resonator model for studying the diffraction of electromagnetic waves in an arbitrarily corrugated waveguide is suggested. It is based on exact relationships that follow from the two-dimensional boundary-value problem stated in terms of the Helmholtz equation. The specific relationships for the rectangular corrugation of the grating-forming plates are presented. The reflection coefficients of the Bragg gratings vs. corrugation length and incident radiation frequency are calculated. An analytical solution for the “narrow” corrugation is obtained.     

Narrow-band terahertz Bragg reflectors based on coupling of propagating and quasi-critical waves

We consider a planar system of narrow-band Bragg reflectors in which the transformation of an incident wave into a counterpropagating wave occurs via the excitation of a quasi-critical mode. The period of the structure with a new modification of Bragg mirrors is approximately twice as large as that in the traditional case, in which direct coupling between two counterpropagating waves takes place. Analysis of modified Bragg structures based on the method of coupled waves, in which high-frequency fields are quasi-optical beams, demonstrates the effectiveness of the proposed system of reflectors for the spacing between corrugated plates from 10 to 15 wavelengths λ. These conclusions were confirmed by direct numerical simulation. Such a superdimensionality is sufficient for using modified Bragg reflectors in high-power long-pulse free-electron lasers operating at short-wave frequency ranges up to the terahertz range.     

The quantum acousto-optic effect in Bose-Einstein condensate

We investigate the interaction between a single mode light field and an elongated cigar shaped Bose-Einstein condensate (BEC), subject to a temporal modulation of the trap frequency in the tight confinement direction. Under appropriate conditions, the longitudinal sound like waves (Faraday waves) in the direction of weak confinement acts as a dynamic diffraction grating for the incident light field analogous to the acousto-optic effect in classical optics. The change in the refractive index due to the periodic modulation of the BEC density is responsible for the acousto-optic effect. The dynamics is characterised by Bragg scattering of light from the matter wave Faraday grating and simultaneous Bragg scattering of the condensate atoms from the optical grating formed due to the interference between the incident light and the diffracted light fields. Varying the intensity of the incident laser beam we observe the transition from the acousto-optic effect regime to the atomic Bragg scattering regime, where Rabi oscillations between two momentum levels of the atoms are observed. We show that the acousto-optic effect is reduced as the atomic interaction is increased.     

Steady-state vectorial self-diffraction on a non-local photorefractive grating in a crystal of symmetry \overline{4}3m at symmetrical transmitting geometry

The steady-state two-wave interaction in a cubic crystal of the symmetry group 3m with the non-local photorefractive response in the absence of an external electric field is considered for the case of arbitrary interaction orientation with respect to the crystallographic coordinate system and for arbitrary intensities and polarization states of incident light waves. The self-diffraction problem is described on the basis of four coupled-wave equations in terms of the complex scalar amplitudes of components of the light waves with orthogonal linear polarization. The derived conservation laws are valid for the non-linear dependency of the photorefractive-grating amplitude on the modulation coefficient of the interference light pattern. It follows from these laws that the two non-unidirectional energy fluxes can form the total energy exchange between the two interacting light waves. A set of independent conservation laws allows us to decouple the coupled-wave equations and to obtain their analytical solution, at least, in the form of quadrature formulae. For example, such a solution is derived for the case of linearly polarized incident light waves and for the linearized dependency of the photorefractive-grating amplitude on the modulation coefficient. The explicit analytical expressions for the scalar amplitudes are obtained for the transversal electro-optic configuration of interaction. The possibility of polarization-state transformation of light waves without energy exchange between them is shown. Received: 30 July 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +7-3822/414321, E-mail: litvinov@ed.rk.tusur.ru     

Self-diffraction of light waves by a nonlocal photorefractive grating in a crystal with the \bar 43m symmetry

A frequency-degenerate steady-state two-wave interaction on a dynamic transmitting phase grating formed in a cubic crystal of the $\bar 43m$ symmetry group with a nonlocal photorefractive response is considered in the paraxial approximation. The conservation laws for the nonlinear system of equations of coupled waves, derived for an arbitrary orientation of interaction relative to the crystallographic axes and the polarization of incident light waves, indicate that the contribution to energy exchange between the interacting waves may come from fluxes in different directions. The possibility of nonunidirectional energy pumping from one wave to the other upon a change in their polarization state due to the interaction is demonstrated. For the transverse configuration of the interaction and linear polarization of incident waves, explicit analytic expressions for the scalar amplitudes of the orthogonal components of the light field are derived in the linear approximation in the coefficient of modulation of the interference pattern of light. The possibility of rotation of the polarization planes of light waves without a change in their intensity is demonstrated. For three particular configurations, the dependence of the efficiency of interaction of linearly polarized waves on the reduced length, orientation of the polarization vectors of the incident light waves, and the ratio of their intensities are analyzed.     

Self-modulation of light in piezoelectric crystals

Spectroscopic measurements were used to monitor the optical interference of ordinary (o-ray) waves with extraordinary (e-ray) waves by incidence of plane-polarized light and transmission through piezoelectriccrystals (alpha-quartz, LiNbO3, and LiTaO3). This observation confirms that the rotations of the vibration planes of the o- and e-ray waves originate from the dynamic gratings induced by the electric field of the incident light. This is a self-modulation process of the polarized incident light in piezoelectric crystals.     

均匀等离子体光栅的色散特性研究

最近研究表明,两束交叉激光脉冲在等离子体中传播时,可以诱导生成周期等离子体密度调制(或称等离子体布拉格光栅). 分别利用传输矩阵法和耦合模理论,推导了激光斜入射时等离子体光栅的色散关系. 两种方法均表明,均匀等离子体光栅存在着光子带隙结构,并且在带隙附近有强烈的色散. 当激光斜入射时,带隙结构会呈现不同的偏振特性：S偏振光的带隙宽度随着入射角的增大而逐渐变宽,而P偏振光的带隙宽度随着入射角的增大先迅速变窄,在布儒斯特角入射时带隙消失,然后又随着入射角的增大而迅速变宽. 超宽的光子带隙和超高的激光损伤阈值,使得等离子体光栅有望成为一种新型的操纵强激光脉冲的光子器件. 关键词： 等离子体光栅 传输矩阵法 耦合模理论 光子带隙     

倾斜光纤光栅谱特性

基于倾斜光纤光栅耦合模理论,采用数值分析法研究了前后向导模耦合时倾斜角度和调制深度对倾斜光纤光栅反射谱的影响,以及导模和辐射模耦合时针对不同的入射光偏振态在倾斜角度、调制深度、光栅长度等不同时倾斜光纤光栅透射谱(或反射谱)的特性.研究结果表明,在前后向导模耦合时,不同偏振态入射光的光谱几乎相同;而对于辐射模耦合,由于反射谱的包络受消光系数的调制,而不同偏振态入射光的消光系数不同,并且倾斜角度越大不同偏振态的入射光的消光系数差别越大,故在研究辐射模耦合时需要将偏振态予以考虑.     

倾斜光纤光栅谱特性

基于倾斜光纤光栅耦合模理论,采用数值分析法研究了前后向导模耦合时倾斜角度和调制深度对倾斜光纤光栅反射谱的影响,以及导模和辐射模耦合时针对不同的入射光偏振态在倾斜角度、调制深度、光栅长度等不同时倾斜光纤光栅透射谱(或反射谱)的特性.研究结果表明,在前后向导模耦合时,不同偏振态入射光的光谱几乎相同;而对于辐射模耦合,由于反射谱的包络受消光系数的调制,而不同偏振态入射光的消光系数不同,并且倾斜角度越大不同偏振态的入射光的消光系数差别越大,故在研究辐射模耦合时需要将偏振态予以考虑.     

Intermediate regime of light diffraction by surface ultrasonic waves in planar gyrotropic optical waveguides

The intermediate regime of optoacoustic interaction, similar to the Bragg one, with Rayleigh surface ultrasonic waves in gyrotropic planar waveguides is studied. A system of equations of bound waves is presented that describes planar optoacoustic interaction in the intermediate, Raman-Natoh, and Bragg regimes of light diffraction by surface ultrasonic waves. It is shown that the optical activity of a wavequide film favors the incident light energy pumping to diffracted light for a TE-polarization wave and decelerates this process for an incident TM-polarization wave. Mozyr' State Pedagogical Institute, 28, Studencheskaya St., Mozyr', 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 412–416, May–June, 1997.     

Generation of mutual coherence of eigenvibrations in <Emphasis Type="BoldItalic">α</Emphasis> -quartz at infrared frequencies by incidence of randomly polarized waves

Considering the interaction of the electric field of the incident light and piezoelectric lattice vibrations at infrared frequencies, we proposed the relationships to describe eigenvibrations in α-quartz. Orthogonal eigenvibrations are found to be the correlated oscillations with mutual coherence. Such arguments were confirmed by generation of polarized waves on reflection of randomly polarized incident light from α-quartz. By incidence of randomly polarized infrared light, the reflected waves are found to be polarized and vibrate in two directions of eigenpolarizations with mutual coherence, corresponding to the ordinary and extraordinary waves, respectively. This observation provides a novel technique to develop the new type of polarizer in the infrared frequency region. PACS 42.70.Ce; 77.22.Ej; 77.65.Ly; 78.20. Ci; 78.30.-j     

Photoresist Bragg gratings as beam splitters and beam reflectors for integrated optical systems

Based on the theory of coupled waves the diffraction efficiency (DE) of Bragg reflectors on planar glass waveguides is described for the case of obliquely incident waves. The coupling coefficients of the different types of mode conversion are determined both by a perturbational analysis and by fitting of the theoretical DE curves to the experimental data. Results on the fabrication and use of photoresist gratings on monomode glass waveguides as beam splitters and beam reflectors are given.