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Two-wave mixing and fanning effect in Bi12TiO20 under an alternating electric field
Abstract
Numerical calculation of the two-wave mixing gain factor for arbitrary orientation of the holographic grating vector using the coupled wave theory developed for optically active electro-optic crystal enabling pump-beam depletion is presented. Two-dimensional angular dependencies of the gain factor are measured in Bi12TiO20 photorefractive crystal under an alternating external electric field using the fanning effect. It is found experimentally that the angular distribution of fanning light is independent of the polarization state of the pump beam. This fact could not be explained by the existing theory.
Studying Bi12TiO20 photorefractive fiber-like crystals we experimentally proved that fiber geometry has many advantages compared to the usual bulk samples. Double phase- conjugate mirror with its conversion efficiency up to 8% has been recorded in a wide range of the pump's incidence angles at lambda equals 632.8 nm for 3 mW input light power. Anomalous behavior of the fanning effect time response has been observed in Bi12TiO20 fiber-like samples: the higher external electric field we apply to the fiber the faster response time we get.
The effect of nonunidirectional energy exchange on the intensity and polarization state of the signal wave is analyzed in the case of symmetric two-wave coupling on a transmission photorefractive grating produced by a diffusion mechanism in a gyrotropic cubic crystal belonging to the point group 23.
Bi12SiO20 and Bi12TiO20 crystals were grown using the melting technique. The crystals were characterized by DTA, X-ray diffraction, and chemical etching. The fanning effect is known as a self-scattering process in which a single laser light beam self-generates scattering in an asymmetrical way. The intensity distribution of fanning light in the far field measured under an applied electric field, normalized to the intensity measured without a field represents the two-wave mixing gain factor γ. For the experiments an argon laser with λ =515nm and the maximum power of 0.5 W was used and the scattered intensity was recorded. The factor γ(θ,φ) for a set of samples was calculated using the intensity distribution in the far field, recorded with a CCD camera. The values of γ(θ,φ) were correlated with the optical inhomogeneity of the crystals.
Optical suppression of a double phase conjugation has been demonstrated in a photorefractive Bi12TiO20 waveguide-like crystal. A dynamic phase conjugation mirror was recorded by two mutually coherent He-Ne laser beams coming from the one side of the samples and the beam of an independent laser from the opposite side. In the absence of any one of coherent input beams the other one generates the phase-conjugate of the third beam. The situation changes when both coherent beams are simultaneously present. The phase conjugated beam, which was produced by one of the coherent pumps, could either be enhanced or almost completely suppressed by introducing the second pump beam into a system. The final intensity of a phase conjugated beam depends on the polarization angle of the second beam compared with the first one. Almost total suppression is observed when the input beams were orthogonally polarized.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Propagation of laser radiation through a photorefractive crystal is accompanied by a gradual increase of unintentional photoinduced light scattering. The process emerges from an initial scattering of the incident beam (pump beam) at the air-crystal boundary or from inhomogeneities within the crystal causing a slight deviation from the light distribution of the pump beam. By the photorefractive effect small refractive-index changes are recorded which, subsequently, diffract the pump beam. If the originally scattered wave and the wave diffracted from the pump beam are in phase, scattering grows continuously by amplification. The photoinduced refractive-index inhomogeneities can be regarded as parasitic holograms, the process as holographic scattering (HS). Those spurious structures, which are recorded at the same time as the desired holograms, are among the main drawbacks limiting the applicability of thick holographic recording media in data storage and image processing applications. On the other hand, HS can advantageously be exploited in applications for optical communication systems, optical limiting, motion detection, and plays an important role in the generation of self-pumped phase conjugators and the realisation of double phase conjugate mirrors. Further, HS is a common phenomenon that reflects the properties of various material parameters, thus being a valuable tool for material investigations. Its major advantage as compared to two-wave mixing is that experiments are much simpler to perform (only one beam, no sensitivity to vibrations). This review outlines the origin of HS, the parameters influencing it, presents techniques how to avoid HS and applications in which HS plays an essential role. Moreover, we give a short overview on materials exhibiting that phenomenon, focussing on recent results for three model substances with differing mechanism of photorefractive response: the acentric crystals $rm Sr_xBa_1-xNb_2O_6$ (SBN) and $rm LiNbO_3$, as well as the centrosymmetric crystal $rm Na_2[Fe(CN)_5NO]cdot 2H_2O$ (sodium nitroprusside, NaNP).
We demonstrate that the fanning light emerging from an optically active fiberlike bismuth titanium oxide (Bi12TiO20) crystal is linearly polarized either parallel to the applied electric-field Vector or perpendicular to it, independently of the external ac electric field, the crystal length, and the pump beam polarization state. We show also that a photorefractive surface wave can be generated at the opposite crystal sides, depending on the pump beam polarization. (C) 1996 Optical Society of America
We present an analytical theory of vectorial wave coupling in photorefractive cubic crystals which are, in general, optically active. The theory is based on the systematic use of the spatial symmetry properties and the apparatus of the Pauli operators to deal with two-dimensional vectors and matrices. It allows one to give a unified description of a wide spectrum of photorefractive phenomena, including the efficiency and polarization properties of Bragg diffraction, polarization two-beam coupling enhanced by ac fields, the influence of the photoelastic effect, etc. Applications of the theory to crystals of the sillenite family and to particular photorefractive phenomena are given. A good qualitative agreement between the theoretical predictions and experimental data for Bi12TiO20 (BTO) crystals is shown.
We develop a theory of vectorial wave coupling in cubic photorefractive crystals placed in an alternating ac field to enhance the nonlinear response. It is proven in the general case that despite essential differences between the diffusion and the ac nonlocal responses, the latter keeps the light interference fringes straight during the interaction. This fundamental feature allows, under certain restrictions, to reduce the nonlinear problem of vectorial coupling to the known linear problem of vectorial Bragg diffraction from a spatially uniform grating, which admits an exact solution. As a result, the nonlinear vectorial problem can be effectively solved for a number of practically important cases. These cases include nontrivial polarization effects and also the high-contrast effects. The presence of conservation laws involving the polarization degrees of freedom is shown. A number of particular cases relevant to experiments with BTO crystals are considered.
The history of dynamic holography is very short, but nevertheless several specific periods of its development can be distinguished. The main purpose of this paper is to illustrate them using the experimental evidence obtained for photorefractive crystals and to give an idea of salient properties of photorefractive crystals related to their use in dynamic holography.
We study the polarization properties of volume phase gratings induced in photorefractive materials that are both optically active and linearly birefringent, such as Bi12SiO20, Bi12GeO20, and Bi12TiO20. Analytic expressions for the diffraction efficiency and for the orientation and the ellipticity of the diffracted light have been derived for the 〈110〉 and 〈001〉 configurations under the assumption of a nondepleted readout beam. The physical parameters that determine the polarization characteristics of the diffracted light are the orientation of the readout vector, the crystal thickness, the optical activity, and the electric-field-induced linear birefringence. Our results are valid for arbitrary values of these parameters. The particular cases of zero optical activity, zero linear birefringence, a thin crystal, or a specific readout-vector orientation can be deduced from the general formulas. The analytic expressions that we have derived permit an optimal choice of different parameters for the image-processing and beam-coupling applications of these materials.
A new kind of optical self-defocusing is described that in steady state is independent of optical beam power and is strongly asymmetric. The physical mechanism responsible is the photorefractive effect. We present a theory that explains the observed dependence of this self-defocusing on polarization, angle of incidence, beam size, and crystal orientation. Experimental results, using a single-domain crystal of BaTiO3, are presented that show excellent quantitative agreement with the theory. Possible device applications are discussed, including an optical diode and a low-power bistable device with permanent memory.
Photoinduced light scattering was observed in cerium-doped (SrxBa1–x)1–y(Nb2O6)y crystals having the
composition x = 0.61, y = 0.4993. It was found that this effect is due to holographic amplification of light
scattered by crystal defects. An analysis is made of static and dynamic characteristics of self-amplification of
scattered light in the crystals. A theoretical model of the process is constructed assuming that the hologram
recording process is of the diffusion type. Theoretical results are compared with the experiment.
It is shown that the two-wave amplification in an alternating field in photorefractive crystals of the sillenite family is strongly affected by the drift length of photoinduced charge carriers. Consequently in samples with short drift lengths the two-wave amplification in an ac field is much less effective than that in a dc field.
We present a theoretical description of the coupling of energy and polarization between two beams in an optically active photorefractive medium. We apply this analysis to the problem of cross-polarization two-beam coupling and determine the dependence of the cross-polarization coupling gain on the input polarization states of the interacting beams and the properties of the coupling medium. In particular, we show that by a proper choice of the input polarization states, the cross-polarization gain seen in typical optically active photorefractive materials may be increased by more than 2 orders of magnitude over that previously observed. As a test of our theoretical treatment, we also apply it to standard two-beam coupling in Bi12SiO20 and obtain close numerical agreement with earlier experimental results in this material.
The polarization properties of diffraction from volume phase gratings in photorefractive sillenite crystals such as bismuth silicon oxide (Bi12SiO20), bismuth germanium oxide (Bi12GeO20), and bismuth titanium oxide (Bi12TiO20) are strongly modified by the presence of concomitant natural optical activity and electric-field-induced linear birefringence. A set of coupled-wave equations that characterize the Bragg regime has been derived for the 〈110〉 and the 〈001〉 crystallographic orientations typically employed in volume holographic storage and multiwavemixing applications. The predicted anisotropic behavior of the grating diffraction is experimentally confirmed, and a significant efficiency improvement is shown to occur for proper choice of the operating mode and the probe beam polarization in a given configuration.
The energy transfer between two beams (signal and reference, respectively) writing a dynamic-volume hologram in photorefractive BSO crystals is applied to the image amplification of a diffuse object. The image intensity transmitted by the crystal is amplified 10 × in the presence of the pump reference beam. The crystal is used in the drift recording mode (applied electric field, E0 = 10 kV cm−1; fringe spacing, Λ = 3 μm), and beam coupling is induced by the nonlocal response of the crystal that is due to the fringe displacement at a constant speed. We have applied this two-wave mixing configuration to a real-time optical-processing operation; the related energy transfer and stationary image amplification permit the mode pattern visualization of a vibrating structure.
The basic physical mechanisms of hologram formation, as well as the major specific features of Bragg diffraction from anisotropic volume phase holograms recorded in cubic photorefractive crystals, are discussed. Special consideration is given to a new dynamic effect of hologram enhancement on recording an interference pattern running with a certain ‘resonance’ speed, the phenomenon underlying a new efficient mechanism of hologram recording, or the ‘long drift length‘ (LDL) mechanism. It is shown that weak light beams can be enhanced and an amplified complex conjugate light wave can be generated in cubic photorefractive crystals in the stationary regime. The maximum amplification is achieved for the shifted LDL hologram in four-wave mixing experiments with positive feedback when two oppositely travelling pump beams polarized orthogonally and at ± 45° to the plane of incidence result in the buildup of the hologram. The main phenomena discussed are illustrated by experimental results obtained in studies of cubic photorefractive BSO and BTO (Bi12 TiO20) crystals.