Ronchi-grating-based measurement of photorefractive recording response time.

We report on a simple technique for the measurement of the recording response time in photorefractive materials. Three different material samples were successfully measured, and their response times, as well as their dependence upon the recording/measurement light irradiance, were also determined and compared with available data in the literature in order to assess the reliability of this technique.

Photoinduced Schottky barrier in photorefractive materials.

We report on the first experimental evidence of a Schottky barrier effect produced by the action of light in an otherwise purely Ohmic contact between a nominally undoped photorefractive titanosillenite Bi12TiO20 crystal and a transparent conductive SnO2 electrode. The photorefractive crystal is sandwiched between two transparent electrodes and a Schottky barrier is built up in the illuminated crystal-electrode interface under the action of light with photonic energy large enough to excite charge carriers from the Fermi level into the conduction band. The contact remains purely Ohmic under illumination with photonic energy below that of the Fermi gap and the photoinduced barrier almost disappears if the photonic energy is large enough to produce electron-hole pairs.

Self-stabilized recording of fixed gratings at high temperature in LiNbO3:Fe.

A high quality fixed holographic grating was recorded in a photorefractive LiNbO(3):Fe crystal at about 100 degrees C in a homemade temperature-controlled vacuum chamber. The recording was carried out using self-stabilization techniques with lambda=532 nm beams guided onto the crystal by polarization maintaining fibers. The diffraction efficiency of the fixed grating was eta=0.

Fixed holograms in iron-doped lithium niobate: simultaneous self-stabilized recording and compensation.

We analyze the mechanisms leading to a highly diffractive fixed hologram in photorefractive Fe-doped lithium niobate crystals by simultaneous self-stabilized holographic recording and compensation at moderately high temperatures. We show that a partially compensated running hologram is produced during recording under this condition and discuss the performance of the process in terms of the operating temperature, the degree of oxidation ([Fe(3+)]/[Fe(2+)] ratio) of the sample, and the effect of the absorption grating arising from the spatial modulation of the Fe(2+) concentration produced during photorefractive recording. We experimentally measure the evolution of the uncompensated remaining hologram during recording and the evolution of the diffraction efficiency of the fixed hologram during white-light development and show that the maximum fixed grating modulation to be achieved is roughly limited by Fe-dopant saturation.

Holographic phase-shift measurement during development of a fixed grating in lithium niobate crystals.

We report what is believed to be the first direct measurement of the grating phase-shift evolution during white-light illumination for the development of a fixed grating in an Fe-doped lithium niobate crystal. Stabilized holographic recording is shown to be essential for such measurements. Experimental data are in good agreement with theory and allow computation of the relevant material parameters for the sample under analysis.

Avoiding hologram bending in photorefractive crystals.

Dynamic coupled-wave theory predicts the bending of recorded hologram phase planes in most photorefractive crystals. Bent holograms occur in LiNbO(3) and other photovoltaic crystals that are particularly interesting as holographic storage media and result in a reduced overall diffraction eff iciency. We show that hologram bending in LiNbO(3) can be avoided or at least sensibly reduced by use of an actively stabilized recording technique.

Phase-compensated holographic recording based on anisotropic photorefractive diffraction.

We report a simple actively stabilized setup for holographic recording on sillenite photorefractive crystals that is based on the anisotropic diffraction properties of these materials. The method is much simpler than previously published ones, requires neither phase modulation nor synchronous phase-sensitive detection, and needs no external reference. We describe the successful operation of this stabilization procedure for a Bi(12)TiO(20) crystal in a practical holographic interferometry setup, using the 632.

Adjustable phase control in stabilized interferometry.

We report an optoelectronic feedback loop that permits the active stabilization of an interferometric setup for any chosen value of the phase between the interfering beams. This method is based on phase modulation and homodyne detection techniques. The phase can be stabilized with a precision of better than 1 deg for our experimental conditions.

Photorefractive response time measurement in GaAs crystals by phase modulation in two-wave mixing.

We describe a new method for photorefractive response time measurement in fast photorefractive crystals based on continuous phase modulation in two-wave mixing. We report experimental results for undoped semi-insulating GaAs that are in good agreement with theory. Values obtained for the response time and photoelectron generation quantum efficiency are consistent with previously published data.

Electrophotochromic gratings in photorefractive Bi(12)TiO(20) crystals.

We present the first experimental demonstration of quasi-permanent holographic recordings in Bi(12)TiO(20) crystals through what is to our knowledge a new electrophotochromic effect. This arises from spatial redistribution of some centers that are different from photorefractive centers. Light diffracted from this photochromic grating records a secondary photorefractive hologram.

Selective two-wave mixing in photorefractive crystals.

A new technique that permits the selective steady-state continuous detection of phase (photorefractive) or amplitude (photochromic) effects in two-wave mixing experiments is described. We show the capabilities of this method for the selective stabilized recording of any one of both types of grating that is chosen. Experimental results from holographic recording in Bi(12)TiO(20) are reported.

Adaptive fringe-locked running hologram in photorefractive crystals.

We have recorded running holograms in a photorefractive crystal with an applied electric field, using a 90 degrees phase-shift fringe-locked interference pattern of light. This method provides a simple way to obtain optimal conditions for nonstationary holographic recording. The experiment that we describe allows us to calculate the crystal diffusion length L(D) and provides direct evidence of the occurrence of running holograms in photorefractive crystals.

Thickness and refractive index dispersion measurement in a thin film using the Haidinger interferometer.

A Haidinger interferometer setup was adapted for accurate measurement of thickness and refractive index dispersion in transparent films using some spectral lines of a commercial argon-ion laser. Experimental results are reported and compared with those from other available methods.

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals.

We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation.

Fourier synthesis for fabricating blazed gratings using real-time recording effects in a positive photoresist.

A method is described which allows the relative phase-shift adjustment of two successively recorded holographic gratings. The residual real-time recording effect in a positive resist was used as a reference hologram for operating an actively stabilized holographic setup and for adjusting both the spatial frequency and the corresponding phase shift of the successively recorded Fourier components to produce a predetermined periodic profile structure in the film. Synthesis of a blazed holographic grating is reported here as an illustration of the possibilities of the process.

Self-diffraction for intrinsic optical modulation evolution measurement in photoresists.

Two-wave mixing at a holographic grating being recorded in a positive photoresist film is used for simultaneously and independently stabilizing the holographic setup itself and for measuring the real-time optical modulation evolution during recording. The method is more sensitive than directly measuring the diffraction efficiency with a probe beam, being particularly interesting for very low efficiency values. This technique is used here for studying the photoreaction kinetics in a Shipley AZ-1350J positive photoresist, and measurements acree with those made with a well-established spectrophotometric method.

Analysis of an active stabilization system for a holographic setup.

An active stabilization system for a holographic setup based on detection of phase shift between the interference pattern and a reference hologram is described. Its basic feature is the possibility of operating for 0, pi or +/-pi/2 at will, always in a null-detection mode. The reference hologram may be a previously recorded permanent hologram or a real-time (even reversible) one.

Stabilized holographic recording using the residual real-time effect in a positive photoresist.

The weak real-time effect in a positive-resist film was used for operating an actively stabilized holographic setup in order to carry out the entire recording process in a stabilized mode. Illustrative experimental results are reported. The successive recording in phase of spatially shifted holographic gratings is also shown.