Frederick Vachss

Nonlinear photorefractive response at high modulation depths

The response of photorefractive materials to sinusoidal intensity patterns of high modulation depths is considered. It is shown that the induced refractive-index response in this regime is highly nonlinear. Concise analytic expressions for the various nonlinear harmonics of the photorefractive response field are developed and compared with exact numerical solutions of the underlying charge-transport equations as well as with the results of previous theoretical models over a broad range of physical parameters. We show that the nonlinear response characteristics are strongly dependent on both the magnitude of applied electric fields and the relative concentrations of charge donor and acceptor sites in the material. For drift dominated recording, in particular, we determine analytically that in the limit of a minimal acceptor/donor ratio, the amplitudes of higher spatial harmonics of the response reach a maximum and eventually decay as functions of increasing applied field, whereas these amplitudes reach a nonzero limit in the case of a near-unity acceptor/donor ratio. Finally, we generalize our results to account for diffusion effects in an appended derivation.

Higher-order analysis of the photorefractive effect for large modulation depths

In an extension of earlier work by Moharam et al. [ J. Appl. Phys.50, 5642 ( 1979)], we use a perturbation scheme to obtain a higher-order expression for the steady-state photorefractive effect at large modulation depths and moderate applied electric fields. The expression is applicable for paraelectric crystals for which there is no photovoltaic effect and under conditions of weak coupling. The dependence of the size of the correction term on modulation depth, concentration of donor–trap sites, and applied field is discussed. Comparison of both the first-order and the second-order theories to the Kukhtarev model, valid for small modulation depths, allows the range of validity to be quantified for certain regimes.

Measurement of the electrogyratory and electro-optic effects in BSO and BGO

Abstract The linear electrooptic and electrogyratory coefficients of BSO and BGO are measured in a transverse electrooptic configuration. Using the photorefractive properties of these materials we determine the true electric fields present in the samples during the measurements and thus obtain accurate values for these coefficients independent of contact losses.

Holographic beam coupling in anisotropic photorefractive media

The process of two-beam holographic coupling in optically anisotropic media is analyzed. We consider holographic media possessing birefringence, optical activity, and spatial modulation of these effects. Coupled equations are derived, and expressions for diffraction efficiency, angular and wavelength sensitivity, and polarization properties of the interacting beams are obtained. Results are compared with those of experiments with holograms formed in photorefractive Bi12SiO20 crystals.

Selective enhancement of spatial harmonics of a photorefractive grating

We demonstrate a technique by which the amplitudes of the various spatial harmonics of a holographic grating written in a photorefractive material may be increased with respect to one another. We show that for holographic recording with a moving intensity grating, the grating velocity may be chosen so as to amplify selectively a given spatial harmonic. This result is derived analytically and tested by measurements of the first three spatial harmonics of a grating written in a photorefractive Bi12SiO20 crystal.

Synthesis of a holographic image velocity filter using the nonlinear photorefractive effect

We demonstrate a technique by which components of an image moving with a given speed and direction may be selectively enhanced. By using the strong grating velocity sensitivity of the nonlinear photorefractive response, we show that a BSO crystal used in a schlieren imaging system may be used as a real-time velocity filter. The velocity selectivity, tunability and resolution constraints of this system are discussed in detail.