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.

The effect of substrates on the transverse geometry photorefractive thin films

This contribution reviews the present knowledge on photorefractive thermal fixing, considering phenomena and their applications to long-lifetime holographic devices. Most attention is devoted to LiNbO3, where a coherent and clear picture has recently emerged and most applications are implemented. Discussions an the physical basis for thermal fixing as well as an the standard mathematical model are presented. Relevant experimental results an thermal fixing are described and the capability of the model to reliably interpret them is emphasized. In addition, either bulk or waveguide devices of potential interest in telecommunications are revised and discussed.

Long-Lifetime Photorefractive Holographic Devices via Thermal Fixing Methods

The distribution of potential and electric field are determined in an insulator-photorefractive-film-insulator sandwich due to the incidence of a single optical beam. The role of surface states and the distinction between surface charge and charge residing in the vicinity of the surface are discussed. The mathematical solution for the steady state is obtained, based on the well known transport equations of photorefractive theory. In particular, edge effects caused by an applied voltage, by surface states and by the attenuation of the optical beam are discussed through a number of examples. Approximate mathematical solutions are obtained and their accuracy is discussed.

Nonlinear and linearized solutions for edge effects in thin photorefractive materials

A differential equation and boundary conditions are derived for the amplitude of the first harmonic of the space-charge field in the case of a lossy thin photorefractive film with anisotropic conductivity that is illuminated by a moving interference pattern. It is shown that for certain values of the anisotropy parameter, detuning, and film thickness, a new resonance phenomenon may occur. This resonance is distinct from and additional to the one that relies on the agreement between the speed of the interference pattern and that of the space charge wave, and has been known for a long time.

Optical grating recording in anisotropic photorefractive thin film: Dimensional resonance

The distribution of free electrons and the formation of the electric field from the dark condition are analysed in an insulator-photorefractive (PR) film-insulator sandwich illuminated by a single optical beam in the absence of an external electric field. Boundary conditions are derived that take into account the dynamics of fast surface states. The mathematical solution for the transient process is obtained, based on the well known transport equations of photorefractive theory. The role of surface states and attenuation is discussed. In particular, it is shown that the character of transients in the boundary depletion layers depends on the ratio of the charged surface state lifetime to the dielectric relaxation time. It is also shown that the width of the depletion layers varies with time if the diffusion length differs from the Debye length.

https://iopscience.iop.org/article/10.1088/1464-4258/2/5/310/pdf

The influence of surface states and light attenuation on transient phenomena in photorefractive 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.

Apriel K. Hodari

Papers

Edge effects in photorefractive thin films