Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity

The first single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector waveguide laser with two thermally fixed photorefractive gratings as resonator mirrors is reported. The optically pumped (λ_p=1480 nm,120-mW incident power) laser emits up to 1.1 mW at λ_s=1561.1 nm. The threshold pump power is 70 mW. Received: 7 June 2001 / Published online: 30 October 2001     

Electrically switchable reflection holograms formed using two-photon photopolymerization

Two-photon holographic photopolymerization was used to form switchable Bragg gratings composed of layers of phase-separated liquid-crystal (LC) domains interspersed with cured, crosslinked polymer. These holographic polymer-dispersed liquid crystals form a periodic structure which diffracts red light due to nanostructured planes ∼250 nm in spacing. These structures were formed by interfering two 90-fs pulses coherently upon a reactive syrup consisting of acrylate monomer, liquid crystal, and a two-photon dye. The large two-photon cross-section allows excitation of the two-photon dye that results in electron transfer between this dye and the monomer. Diffraction efficiencies of approximately 10% were obtained, which can be modulated using an electric field applied across the film. Switching speeds below 1 ms were observed due in part to the small size of the LC domains. Received: 10 April 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

The role of carrier mobility in holographic recording in LiNbO3

We investigate the role of carrier mobility in holographic recording in LiNbO₃ crystals. Both normal holographic recording (single wavelength, single trap) and two-center recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude. Received: 22 February 2001 / Revised version: 5 March 2001 / Published online: 27 April 2001     

Associative recall in a volume holographic storage system based on phase-code multiplexing

We present two different techniques on how to realize a content-addressed holographic memory when using phase-code multiplexing, relying on simple intensity measurements rather than phase distributions. Theoretical and experimental results of associative recall in a phase-coded system designed for digital data storage will be presented and compared to the corresponding method when using angular multiplexing. Received: 2 August 2001 / Revised version: 20 September 2001 / Published online: 23 November 2001     

Material characterization by holographic methods

After an introduction to the theoretical fundamentals, different holographic techniques for material characterization are presented: Isotropic and anisotropic diffraction measurements, interferometric techniques, holographic microphotometry, isotropic and anisotropic holographic scattering and self-diffraction experiments, beam-coupling topography and neutron holography.     

Speed and diffraction efficiency in feedback-controlled running holograms for photorefractive crystal characterization

We report the measurement of the diffusion length, the Debye screening length and the quantum efficiency of photoelectron generation in strongly light absorbing photorefractive Bi₁₂TiO₂₀ crystals, using fringe-locked running hologram experiments. The effective applied electric field inside the sample is also computed and self-diffraction is considered. The novelty here, as compared to formerly reported experiments, is that the diffraction efficiency is now measured simultaneously with the hologram speed v. From these data the above referred to photorefractive and experimental parameters are obtained without the need for additional experiments. The method is used to analyze two photorefractive Bi₁₂TiO₂₀ crystal samples, in different experimental conditions, using the 514.5 nm wavelength. The computed parameters are in good agreement with the available information about these samples. Received: 23 November 2000 / Published online: 21 March 2001     

Compact and portable holographic camera using photorefractive crystals. Application in various metrological problems

We present a compact holographic interferometer that uses a photorefractive crystal of the sillenite family as a holographic recording medium. Its development is based on a previous prototype that showed lack of flexibility and portability. We briefly discuss the main improvements leading to a compact device. Applications of this instrument in various metrological problems are shown, among which are two that were not already considered using holography, namely measurement of a thermal expansion coefficient and detection of fingerprints. Received: 20 December 2000 / Revised version: 8 February 2001 / Published online: 20 April 2001     

Inscription of holographic gratings using circularly polarized light: influence of the optical set-up on the birefringence and surface relief grating properties

The polarization properties of the optical set-up used for holographic recording of diffraction gratings on azopolymer thin films are analyzed. The state of polarization of circularly polarized light is fully analyzed after reflection on a mirror at various incidences (Lloyd-mirror set-up). The Stokes analysis is performed using a photopolarimeter and the phase shift, the ellipticity and the azimuth orientation are compared with those calculated from Fresnel formulae. At large angles of incidence, an initially right circularly polarized (RCP) beam becomes elliptically polarized with an azimuth of nearly +45°. From these results, holographic diffraction gratings are recorded on an azobenzene-containing polymer thin film using (i) co- and contra-circularly polarized beams and (ii) a right circularly polarized beam interfering with a +45° linearly polarized light beam. Using Jones-matrix formalism, the polarization states of the diffracted orders from the birefringence (Δn) and the surface-relief (2Δd) gratings are derived and compared with experimental measurements. Finally, the induced local birefringences and surface-relief amplitudes are discussed in connection with atomic force microscopy measurements. The diffraction efficiencies obtained under the (+45°+RCP) and (LCP + RCP) (where LCP = left circularly polarized) configurations are thus compared and discussed. Received: 5 October 2001 / Revised version: 26 November 2001 / Published online: 17 January 2002     

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     

Influence of the optical activity on hologram formation in photorefractive crystals

It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ is predicted.     

Holographic interferometry for the structural diagnostics of UV laser ablation of polymer substrates

Holographic interferometry is examined for its potential as a diagnostic tool of the structural modifications effected in laser-processing applications. The interferometric ‘comparison’ of the holographic images of the sample recorded before and after irradiation enables the full-field spatially resolved detection of the induced structural modifications. The potential of the method is illustrated in the ablation of polymer (polymethylmethacrylate and polystyrene) films with nanosecond pulses at 193 and 248 nm. The detailed characterization and quantitative monitoring of the growth of the induced modifications as a function of laser-material parameters is attained. A most novel result is the observation of delocalised structural modifications at distances relatively far away (d≈2 cm) from the irradiated area. Received: 15 August 2001 / Accepted: 16 August 2001 / Published online: 2 October 2001     

Surface grating formation and erasing on a-Se films

Surface relief formation at holographic recording on amorphous selenium films was demonstrated and investigated. The presence of this optical phase modulation component is essential for ensuring significant, stable and erasable optical recording in a-Se films at 290–320 K temperatures, where conventional photodarkening was known as insignificant and unstable. Photocrystallization can only be observed in super-exposed a-Se films at the given experimental conditions of hologram recording. Erasing behavior of surface relief gratings under heat treatment was also investigated in order to reveal further details of the mechanism. Photoinduced structural transformations within the amorphous phase, connected to local ordering under the condition of light-induced fluidity, are proposed as an explanation for the relief formation and erasing. The observed reversible optical recording process may be useful for the various optoelectronic applications of photoconductive a-Se layers. Received: 12 June 2000 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Second-order grazing-angle scattering in uniform wide holographic gratings

Grazing-angle scattering (GAS) is a type of Bragg scattering in slanted wide periodic gratings. It occurs when the diffracted order satisfying the Bragg condition (scattered wave) propagates at a grazing angle to the grating boundaries. Previous research has been concerned only with first-order GAS, which has been shown to be a highly unusual type of scattering characterised by a strong resonant increase of amplitudes of the scattered and incident waves in the grating. In this paper, a rigorous numerical study of second-order GAS is presented for the case of bulk TE electromagnetic waves in planar holographic gratings. A highly unusual pattern of strong resonances in the grating, which is strongly different from that for first-order GAS, is predicted, described, and discussed. Physical interpretations of the predicted results are presented. In particular, a special new type of eigenmodes in a slanted wide periodic grating with large amplitude is predicted. These eigenmodes are shown to be guided by the grating alone without any conventional guiding effect in the structure. The typical field structure in such eigenmodes is investigated and discussed. Received: 16 September 2002 / Revised version: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +61-7/3864-9079, E-mail: d.pile@osa.org     

Calculation of shear angles in holographic gratings recorded in bleached silver-halide emulsions

Shearing phenomena in holographic gratings formed in bleached silver-halide emulsions due to chemical processing are evaluated. These effects are introduced because of the inability of emulsion-thickness changes to explain the anomalous detuning of the peak diffraction efficiency from the expected Bragg condition. Experimental results obtained with transmission and reflection gratings are presented and discussed considering shearing effects, and the values of the shear angles calculated from the Bragg-angle deviation measurements are also provided. In order to separate the shearing phenomenon and the shrinkage of the emulsion layer, a rehalogenating bleach without fixing is used because this bleach bath introduces a minimal change in the thickness of the emulsion. The influences of exposure, spatial frequency and slant angle on shear angles are briefly analyzed and discussed.     

Enhanced angular selectivity in volume holograms with speckle reference waves

A speckle-multiplexing scheme with enhanced angular selectivity for holographic storage is proposed. Angular selectivity in the proposed holographic storage system is theoretically and experimentally investigated. We find the effect of speckle reference wave on angular selectivity strongly depends on the techniques to perform multiplexing for holographic storage. Angular selectivity of a holographic storage system can be effectively enhanced as long as angular deviation of reading wave induces a lateral displacement of the speckle pattern on the hologram plane. When angular deviation of reading wave only induces a speckle wavefront tilt on the hologram plane, the speckle wave is not helpful to enhance the angular selectivity and the angular selectivity becomes to depend on material thickness (Bragg condition) only.     

Nonlinear diffraction due to the transient polarization in a thin film of atomic gases*

Nonlinear optical response of a thin layer of rarefied atomic vapor is examined by taking into account the atomic motion as well as collisions with the cell walls. Extraordinary pattern of self-diffraction spectrum due to the transient polarization is predicted. It is shown that the spectra are Doppler free and depend strongly upon the vapor thickness. A new possibility of signal enhancement is revealed and a simple device is suggested to enhance greatly the nonlinear reflection signal from a resonant vapor layer. Received: 3 July 1997/Accepted: 5 December 1997     

Diffraction efficiency and energy transfer in two-wave mixing experiments with Bi12SiO20 crystals

The diffraction efficiency and energy transfer are investigated in photorefractive Bi₁₂SiO₂₀ crystals (BSO). Dependence on fringe spacing, electric fields, light intensity and rise-time constants are reported. The optimum crystallographic orientation for each effect is determined for vertical polarization of the recording beams. It is shown that beam coupling is a very sensitive phenomenon in BSO crystals where charge transport lengths are equivalent to usual fringe spacings. Experimental results are interpreted on the basis of the nonlinear theory of self-diffraction developed by Kukhtarev et al. [10].     

Photorefractive properties of (1-10) and (111)-cut sillenite crystals when external electric field is applied along the direction of the optimum diffraction efficiency

We study the influence of the application of an external electric field on the grating formed in (1-10) and (111)-cut photorefractive sillenite crystals. Optimum conditions for application of the bias field are examined, taking into account the shift of the grating orientation for maximum diffraction efficiency along the crystal depth. The contribution of the piezoelectric effect is considered both in the electro-optic tensor and space charge field calculations. An analysis on the diffractive properties under these optimum conditions shows an increase of 30% in maximum diffraction efficiency in Bi₁₂GeO₂₀, and experimental verification for this is provided. Received: 5 November 2001 / Revised version: 18 April 2002 / Published online: 8 August 2002     

Theoretical and experimental aspects of optical storage of information via refreshing by inverse seeding in a photorefractive Ba0.77Ca0.23TiO3 crystal (BCT)

We present a new experimental setup for the optical storage of information via refreshing by inverse seeding (OSIRIS), which allows a sixfold increase of the storage time of holograms in a Ba_0.77Ca_0.23TiO₃ crystal (BCT). The setup consists of two four-wave mixing processes with common amplified signal waves and phase-conjugated (pc) waves. Temporal behaviours of the amplified and pc signal waves for the OSIRIS experiment as well as for the common four-wave mixing experiment are compared and discussed. The solutions of coupled equations under the depleted-pump approximation are obtained in order to estimate the pc reflectivities and coupling gains of gratings inside the crystal. Received: 13 December 2000 / Revised version: 26 January 2001 / Published online: 27 April 2001