Photorefractive effect induced by polarization gratings in dye-doped liquid crystals

We report on the photorefractive effect induced by a polarization grating in the presence of dc voltage in a dye-doped liquid-crystal (DDLC) film. The writing beams are two orthogonally (left- and right-circularly) polarized laser beams that create a spatially polarization-modulated interference field with constant intensity. The photorefractivity is ascribed to the absorption anisotropy of the azo dye. The unique dichroism of a DDLC cell causes a spatial variation in the absorption of light in response to a polarization-modulated interference field. Such a variation establishes a space-charge field in the presence of dc voltage, generating photorefractivity. Two-beam couplings were also verified and measured dynamically during the formation of the photorefractive grating in this study.     

Nonlinear interaction of moving space-charge and photoconductivity gratings in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystal

A nonlinear interaction of moving space-charge and photoconductivity gratings is experimentally investigated. In the presence of a dc electric field, a crystal is irradiated with an oscillating interference pattern with a spatial frequency K and an oscillation frequency ω. An ac electric field with a frequency \gM is also applied to the sample. At certain frequencies ω and \gM, the crystal exhibits two types of interacting oscillations: the space-charge grating moving with velocity |ω−Ω|/K and the photoconductivity grating moving with velocity −ω/K. The effect is studied using the method of the nonstationary photoelectromotive force in a photorefractive Bi₁₂SiO₂₀ crystal.     

Rigorous three-dimensional theory of the parametric excitation of space-charge waves in semiconductors

We analyze the stability against parametric excitation of space-charge waves of the space-charge field induced in a semiconducting crystal by a traveling light grating. We show that when the grating velocity is low, an important element of the analysis is the allowance for higher Fourier harmonics of the field. By combining analytic and numerical methods we study the stability against an increase in the amplitude of small three-dimensional perturbations of a general type. We find that instability is possible only within a single range of light-pattern velocities and that it leads to selective excitation of one-dimensional perturbations. Finally, we use the results of our analysis to interpret experiments on the generation of spatial subharmonics in sillenite crystals. Zh. éksp. Teor. Fiz. 114, 1034–1046 (September 1998)     

Vector Approach to Calculating Space-Charge Fields in Crystals withntrast Light Interference Pattern

A new method for calculating space-charge fields in photorefractive crystals with shallow-lying traps illuminated by a light interference pattern of arbitrary contrast and shape is suggested. A space-charge field in a BSO:Cd crystal with shallow-lying traps illuminated by a sine light interference pattern with the high contrast m = 0.99 is calculated.     

Rectification of space-charge waves upon optical and electrical excitation

A comparative analysis is performed for three optical and electrical methods of exciting space-charge waves in photosemiconductors: (i) excitation by an external ac electric field combined with a static interference pattern, (ii) excitation by a moving interference grating, and (iii) excitation by an oscillating interference grating. It is shown that, in the case when space-charge waves are excited using a combination of all three methods, the dependence of the direct current passing through a sample on the excitation frequency exhibits two peaks that correspond to the resonant excitation of two modes of space-charge oscillations, namely, drift waves and trap recharging waves. It is noted that experimental observation of the peak attributed to the excitation of trap recharging waves should not pose any problems, whereas observation of the second peak associated with the excitation of drift waves is significantly complicated because of the small magnitude of the effect, especially for materials with a low electrical conductivity (or a long Maxwell relaxation time).     

TE0导模干涉刻写周期可调亚波长光栅理论研究

通过棱镜耦合激发非对称金属包覆介质波导结构中的TE₀导波模式, 利用两束TE₀模的干涉从理论上实现了周期可调的亚波长光栅刻写. 分析了TE₀模式的色散关系, 刻写亚波长光栅的周期与激发光源、棱镜折射率、光刻胶薄膜厚度及折射率之间的关系. 用有限元方法数值模拟了金属薄膜、光刻胶薄膜和空气多层结构中TE₀导模的干涉场分布. 研究发现, 激发光源波长越短, TE₀ 模干涉刻写的亚波长光栅周期越小; 光刻胶越厚, 刻写的亚波长光栅周期越小; 高折射率光刻胶有利于更小周期亚波长光栅的刻写. 相较于表面等离子体干涉光刻, 基于TE₀ 模的干涉可在厚光刻胶条件下通过改变激发光源、棱镜折射率、光刻胶材料折射率、特别是光刻胶薄膜的厚度等多种方式实现对亚波长光栅周期的有效调控.     

Holographic Reflection Diffraction Gratings in Absorbent Photopolymer Media

Record and readout of holographic reflection gratings in absorbent photopolymer materials with a dye-sensitizer are theoretically investigated. An analytical model describing the spatiotemporal transformation of holographic grating field during record is constructed with allowance for the constant optical absorption and diffusion processes. It is demonstrated that the optical beam attenuation during record results in the spatial inhomogeneity of the grating profile and its growth kinetics. A degree of influence of the optical attenuation and contrast of interference pattern on the integral diffraction characteristics is evaluated based on numerical modeling.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 62–70, March, 2005.     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Strong and fast phase modulation for quantitative analysis of photorefractive gratings

A new approach for studying photorefractive gratings in two-wave mixing experiments by a phase modulation technique is presented. The introduction of a large-amplitude, high-frequency sinusoidal phase modulation in one of the input beams blurs the interference pattern and provides powerful harmonic signals for accurate measurements of the grating diffraction efficiency η and the output phase shift ϕ between the transmitted and diffracted waves. The blurring of the light fringes can be used to suppress the higher spatial harmonics of the grating, allowing a space-charge field with sinusoidal profile to be recorded. Although the presence of such a strong phase modulation affects the beam coupling in a rather complicated way, it is shown that for the special case of equal intensity input beams, the effect of the phase modulation on η and ϕ is reduced to a weakening of the coupling strength. The potentialities of the technique are illustrated in a study of refractive-index waves excited by running interference patterns in a Bi₁₂TiO₂₀ crystal. Expressions for the diffraction efficiency and the output phase shift are derived and used to match numerically calculated curves to the experimental data. The theoretical model is supported by the very good data fitting and allows the computation of important material parameters. PACS 42.40.Ht; 42.40.Kw; 42.70.Nq     

The possibility of reconstruction of two-dimensional random inhomogeneities in a shallow sea by frequency shifts of the spatial interference structure of the sound field

Fluctuations of frequency shifts of the spatial interference structure of the sound field in the oceanic waveguide, caused by a two-dimensional field of a random perturbation, are described. Various schemes of observation point spacing are considered. The possibility is shown to reconstruct the spatial spectrum of waveguide perturbations by measuring the spatial spectrum of the frequency shift of the interference pattern. The results of the theoretical treatment are illustrated by the examples of background internal waves and bottom roughness. The sensitivity of monitoring based on measurements of frequency shifts of the interference structure of the sound field is estimated. For medium perturbations by background internal waves, the fluctuations of liquid layer vibrations, sound speed, and temperature, which are minimum detectable by frequency shifts of the interference pattern, were estimated.     

Tilted superstructure fiber grating used as a Fourier-transform spectrometer

We propose and demonstrate a new method for fabricating a compact all-fiber spectrometer by using a tilted superstructure fiber grating to couple light out of a fiber core and produce an interference pattern in the near field. The Fourier transform of the interference pattern serves as a direct measurement of the optical input spectrum, and the superstructure grating design offers several degrees of freedom that permit control over the resolution and bandwidth of the spectrometer. For single-wavelength operation, the proposed scheme offers the possibility of making wavelength measurements with picometer-level precision over a broad 80-nm wavelength range while simultaneously providing coarser precision over a 160-nm range.     

用光辐照法在SBN:Cr晶体中写入动态阵列平面光波导

利用扩展的准直He-Ne激光束通过菲涅耳双棱镜所形成的干涉光场辐照SBN:Cr晶体， 同时沿晶体光轴方向施加适当的直流电场，可在晶体中形成类似于体相位光栅结构的阵列平 面光波导. 采用马赫-曾德干涉仪光路实时测量了所写入阵列平面光波导的横向折射率分布 ，其峰值接近10-4. 初步的导光测试结果表明，利用周期结构光辐照并辅以适当的外加电场在SBN:Cr晶体中写入阵列平面光波导是可行的. 并且由于SBN:Cr晶体的快速响应特性，所写入的光波导是动态的，可随着写入光的撤除而快速消失，或通过改变双光束夹角 关键词： SBN:Cr晶体 结构光辐照 阵列平面光波导 折射率分布 外电场     

Fixing the photorefractive soliton

We report the formation of permanent two-dimensional 12-microm waveguides in a bulk strontium barium niobate crystal. The waveguides are made by formation of a photorefractive spatial soliton in which the space-charge field induces ferroelectric domains that are permanently polarized opposite to the crystal c axis. The fixed waveguide propagates light with 80% efficiency. These results make possible the permanent recording of intricate optical circuitry in the volume of a bulk crystal.     

Self-Bending of the Light Beam Trajectory in Photorefractive Crystals in an External Alternating-Sign Electric Field

The self-action effects of a light beam propagating in a cubic photorefractive crystal in a square-wave electric field are studied. It is demonstrated that the dark conductivity of the crystal or its illumination by noncoherent radiation distorts the beam profile with the initial Gaussian intensity distribution. The results of investigations into the influence of the nonlinearity of the space-charge field formation on self-bending of the beam trajectory are presented. It is demonstrated that the evolution of modulation instability in the light field distribution over the beam cross section depends on the initial beam profile, its asymmetry, and the direction of bending caused by the inhomogeneity of induced refractive-index changes.     

Effect of background internal waves on the interference structure of the sound field in a shallow sea

The results of a theoretical analysis of the effect produced by an anisotropic field of background internal waves on the localization of the interference pattern in a shallow sea are presented. The space-time variability of the interference invariant and the smearing of the observation direction of interference fringes are considered in a wide frequency range. The stability of the interference pattern formed by both the superposition of the fields of different mode groups and separate mode groups is analyzed in comparison with the unperturbed waveguide. Numerical calculations are performed for longitudinal and transverse orientations of the acoustic track relative to the propagation direction of internal waves.     

Grating Interferometer Using ± 1st Order Beams for Step-Profile Altitude Difference Measurement

We propose a grating interferometer for step-profile altitude difference measurement. There are two main characteristics in this interferometer. The first is that the intensity distribution of the interference pattern is independent of the wavelength of the laser-diode used. No change of the intensity distribution occurs when the wavelength fluctuates. The second is that the measuring range is much larger than the wavelength of the light source because the spatial period of the grating is much larger than the wavelength. Sinusoidal phase modulating interferometry is easily applied to detect the phase variation of the interference pattern by vibrating the grating sinusoidally. The thickness of a 3.5-inch disk is measured with an accuracy of less than 0.5 μm.     

Modelling of finite-size grating waveguide filters on the basis of a free-space diffraction approach

The field reflection coefficientR of a high strength and short length waveguide grating section is obtained by resorting to a free-space diffraction approach whereby the waveguide grating is excited by means of a virtual high index prism. The effective index of the guided mode, the coupling coefficient between contradirectionally propagating modes of the grating waveguide as well as the modal field are first obtained in the case of an infinite grating by interpreting the results given by an exact diffraction solution by means of a three-wave coupled mode formalism. These results are then introduced in the coupled wave analysis of the structure of finite extent to provide the field at the entrance of the waveguide grating section where the usual overlap integral with the incident field delivers the reflection coefficient. We neglect only the result of radiation mode propagation. This assumption can be made when the grating section is longer than the propagating length of all the leaky modes.     

Modelling of finite-size grating waveguide filters on the basis of a free-space diffraction approach

The field reflection coefficient R of a high strength and short length waveguide grating section is obtained by resorting to a free-space diffraction approach whereby the waveguide grating is excited by means of a virtual high index prism. The effective index of the guided mode, the coupling coefficient between contradirectionally propagating modes of the grating waveguide as well as the modal field are first obtained in the case of an infinite grating by interpreting the results given by an exact diffraction solution by means of a three-wave coupled mode formalism. These results are then introduced in the coupled wave analysis of the structure of finite extent to provide the field at the entrance of the waveguide grating section where the usual overlap integral with the incident field delivers the reflection coefficient. We neglect only the result of radiation mode propagation. This assumption can be made when the grating section is longer than the propagating length of all the leaky modes.     

Estimation of field in plane impedance waveguide with local geometrical inhomogeneity. I

The effect of an inhomogeneity in the form of a circular cylinder the properties of whose base are described by a certain impedance and whose lateral surface is ideally conducting on the field of a point source in a plane impedance waveguide is studied. In a scalar approximation, the problem is reduced to a two-dimensional integral equation over the surface of the inhomogeneity. A solution is constructed by the method of successive approximations; the solution of the problem for a regular impedance waveguide is used as an initial approximation. The effect of a local ionospheric inhomogeneity on the field of an electrical dipole in the earth-ionosphere waveguide is estimated.Leningrad State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 8, pp. 908–918, August, 1991.     

Imageless microscopy of surface patterns using optical waveguides

Recent experiments using a grating coupled low-index nanoporous silica supported monomode waveguide have demonstrated that living cells can cause heavy distortion of the grating coupler resonance lines, in some cases even leading to a separation into two resonance peaks. These findings stand in contrast to previously reported data, where simple peak broadening was observed during cell attachment and spreading using less sensitive waveguide designs. In order to explain these observations, we apply the local interference method to simulate the effects of inhomogeneity patterns on the surface of grating coupled planar optical waveguides and obtain the resonant peaks for the modes. It is shown that analyte inhomogeneities affect both the position and shape of the resonant peaks. Depending on the deposited cell or domain size, refractive index contrast and waveguide design, peak shift, peak deformations or peak splitting can be observed. On the basis of simulation data, characteristic parameters of the resonant peaks such as peak width at half maxima, overall width, central position and peak integral are connected for the first time to quantitative parameters of the inhomogeneity patterns; like analyte covered sensor area, surface averaged effective refractive index and domain size. Our results indicate that by careful investigations of the incoupling resonant peaks, quantitative information about sample inhomogeneities at the micrometer scale can be obtained, thus allowing for a new generation of simple, low cost, label free and imageless optical sensors, which are well suited for high throughput screening applications. PACS 42.82.Et