Photorefractive Thin Films Research Articles

Bipolar two-dimensional analysis of grating dynamics in photorefractive thin films

A two-dimensional analytical model is developed to describe transient grating formation for bipolar transport in photorefractive devices. This approach is suitable for structures in which the film thickness is comparable with or smaller than the grating vector. A crucial feature of the model is that it takes into account the edge effects by including both parallel and perpendicular components of the fields and the currents. The treatment is entirely general and valid for any geometry, in particular, for parallel (image amplifiers, etc) and for perpendicular (Pockels readout optical modulators, etc) photorefractive devices. The role of bulk and surface charges in each configuration is discussed. The results are illustrated in a few examples showing the effect the material and geometrical parameters have on the device resolution thus imposing more stringent limits upon the resolution than those found in previous studies.

Optical grating recording in lossy photorefractive thin films: space charge waves in 2D geometry

Dynamic grating formation in lossy PR thin films is investigated when the incident beams are detuned and the applied electric field is parallel with the grating vector. Starting with a two-dimensional formulation of the problem a differential equation, first order in time and fourth order in space, is derived for the parallel component of the electric field. An analytic solution of this equation subject to the boundary conditions (for the tangential component of the electric field and the normal components of the electric flux and current) is obtained under steady-state conditions. For the complete time domain the differential equation is solved numerically under the approximation that the Debye length is small relative to the film thickness. It is shown that under the effects of attenuation and detuning the perpendicular component of the space charge field may considerably increase. The space charge wave excited gives rise to a non-zero steady-state perpendicular component of the space charge field that may be comparable with the parallel component.

The effect of substrates on the transverse geometry photorefractive thin films

Although photorefractive thin films in the transverse geometry retain most of the basic features of the bulk photorefractive materials, the abrupt changes in the dielectric constant across the boundary regions always accompany polarized charge gratings. These gratings are generated by the perpendicular component of the space charge field and they are absent in the bulk transport equations. Our systematic theoretical calculation indicates that these boundary charge gratings will strongly influence device performance. Adjustment of the dielectric properties of media sandwiching the thin film can serve as a method by which control device performance.

Edge effects in photorefractive thin films

We have used a two-dimensional charge transport model to study edge effects in photorefractive thin films. Our result shows the presence of a large surface charge layer that causes saturation of trap-limited-field in the large fringe spacing limit. Quadratic electro-optic materials were used as an example to quantitatively study the contribution of the surface-charge-layer field to the total photorefractive grating. We clearly demonstrate that the device performance can be dominated by either surface charge or bulk trap charges.

Surface Charge Grating in the Transverse Geometry Holographic Recording Thin Film Devices

ABSTRACTWe have used a two-dimensional charge transport model to study edge effects in photorefractive thin films. Our result shows that in addition to the bulk space charges, the large surface charges near the edges make a significant contribution to the space charge field. Quadratic electro-optic material were used as an example to study the surface-charge-layer-field quantitatively. The conditions for the device performance be dominated by either surface charge or bulk trap charges are clearly demonstrated.

Holographic recording in photorefractive thin films: Edge effects

The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two-dimensional formulation (coordinates x and z parallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one-dimensional result, i.e., edge effects are remarkable. In particular, significant space-charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film.

Analysis of photorefractive stratified volume holographic optical elements

An analytical model is developed to study layered structures of photorefractive thin films, a promising application of stratified volume holographic optical elements. The theory gives a closed-form solution for the first-order diffraction efficiency of such elements. A new holographic multiplexing scheme that uses the photorefractive phase shift induced by an applied electric field is also analyzed with this theory.

Steady-state four-wave mixing in photorefractive quantum wells with femtosecond pulses

Photorefractive AlGaAs–GaAs multiple-quantum-well structures perform as ultrafast field autocorrelators during steady-state four-wave mixing by use of femtosecond pulses. The photorefractive thin films operate in the Raman–Nath diffraction regime with thin transmission gratings. The thin gratings permit degenerate and nondegenerate four-wave mixing without the need to satisfy phase-matching conditions. Nondegenerate mixing has several advantages over degenerate four-wave mixing. The autocorrelation is broadband and is viable for photon energies larger than the band gap of the photorefractive sample, and the diffraction efficiency is unaffected by the bandwidth of the femtosecond pulses. In addition, the readout laser can be a low-power low-cost cw diode laser.