Paulo Magno Garcia

Phase‐shift measurement in photorefractive holographic recording

A simple direct method for measuring the interference pattern of a light‐to‐recorded hologram phase shift in photorefractive crystals is reported. The method is used for studying the recording in Bi12SiO20 under an externally applied electric field. The results are then compared with theory and used for computing the density of photoelectron traps.

Transport length, quantum efficiency, and trap density measurement in Bi12SiO20

We report the measurement of diffusion transport length LD, quantum efficiency φ for photoelectron generation, and the Debye screening length and associated trap density NA in an undoped Bi12SiO20 (BSO) photorefractive crystal. The method is based on the optical erasing of holographically recorded gratings in the crystal. The measurement of the diffraction efficiency evolution is considerably facilitated profitting from polarization properties of anisotropic diffraction in BSO‐type crystals in a self‐diffraction experiment. Data processing is carried out using simple linear regression techniques for experimental conditions in two limiting situations: far from and well into trap saturation conditions. We prove that optical erasure closely verifies the decay law predicted by theory even if a highly contrasted pattern of light is used for recording. We measured NA=6.3×1016 cm−3, agreeing in order of magnitude, with already reported values, but found out LD=0.1 μm, which is too much lower than data in literat...

Phase modulation in two-wave mixing for dynamically recorded gratings in photorefractive materials

We derive an analytical expression describing the effect of phase modulation in two-wave mixing for dynamically recorded phase-shifted holograms in photorefractive materials. The temporal harmonic terms that arise from phase modulation are explicitly formulated for small modulation (as used in our experiments), allowing for pump depletion and an arbitrary value of the phase shift. The theory is experimentally verified for the particular case of an unshifted hologram in a photovoltaic LiNbO3 crystal and for the general case of an arbitrarily shifted hologram in a Bi12TiO20 crystal subject to an externally applied electric field, always for the 514.5-nm-wavelength laser light. In both cases the present analytical formulation permits material parameters to be computed that are self-consistent and in agreement with data in the literature.

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.

Phase-controlled photorefractive running holograms

Abstract We report on the use of phase modulation in two-wave mixing with negative feedback to record stabilized photorefractive running holograms with arbitrarily fixed phase shift between the transmitted and the diffracted beams behind the crystal. By adequately choosing this phase one can select the running hologram speed and associated properties like diffraction efficiency. This technique is an alternative to the classical frequency detuning one with the advantage that the recording is at the same time actively stabilized referring to the own hologram being recorded. We describe the setup, analyse its behavior and report experimental results for a Bi 12 TiO 20 crystal.

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.

Shape-asymmetry of the diffraction efficiency in Bi12TiO20 crystals: the simultaneous influence of absorption and higher harmonics

Abstract Studying moving holographic gratings in a Bi 12 TiO 20 crystal with an applied external field, we have observed the appearance of a “shape-asymmetry” in the diffraction efficiency as a function of frequency detuning. The effect is mainly absorption-induced; for large modulation depth it is further enhanced by the influence of higher harmonics.

Charge–carrier diffusion length in photorefractive crystals computed from the initial hologram phase shift

The phase shift between the pattern of light onto a photorefractive crystal and the resulting hologram at the very beginning of the recording process in two-wave mixing is analyzed and measured as a function of the applied electric field. These data allow one to compute the diffusion length of photoexcited charge carriers and to evaluate the actual electric field inside the crystal. A diffusion length of 0.14 μm is measured in a nominally undoped photorefractive Bi12TiO20 crystal using a 532 nm wavelength laser illumination, in agreement with results obtained from other methods.

Scattering reduction for highly diffractive holograms in LiNbO3 crystals

We show that self-stabilized recording in photo-voltaic LiNbO3 crystals produces highly diffractive holograms with a 3- to 4-fold light scattering reduction. Low scattering is probably due to the nonstationary mechanism involved in self-stabilized recording that may not allow parasitic scattering patterns in the crystal to be efficiently recorded.

Feedback-controlled running holograms in strongly absorbing photorefractive materials

We propose a mathematical model for the movement in absorbing materials of photorefractive holograms under feedback constraints. We use this model to analyze the speed of a fringe-locked running hologram in photorefractive sillenite crystals that usually exhibit a strong absorption effect. Fringe-locked experiments permit us to compute the quantum efficiency for the photogeneration of charge carriers in photorefractive crystals if the effect of bulk absorption and the effective value of the externally applied field are adequately taken into consideration. A Bi12TiO20 sample was measured with the 532-nm laser wavelength, and a quantum efficiency of Φ=0.37 was obtained. Disregarding absorption leads to large errors in Φ.