Muthu Jeganathan

Evolution and propagation of grating envelopes during erasure in bulk photorefractive media

We present a comprehensive analysis of the evolution of a grating envelope, in both transmission and reflection geometry, during the erasure process in various readout configurations in bulk photorefractive media. We derive a single partial differential equation that describes grating evolution. This equation shows that under certain conditions the grating envelope propagates and shares several features with either bright or dark solitons. Details of the grating envelopes directly relate to the dynamics of the observed diffraction efficiency. We derive analytical expressions for diffraction during nondestructive readout. Finally, we discuss the effects of nonzero absorption, finite dark conductivity, fringe bending, fanning, and nonlinear responses on the propagating envelope.

Theory of two-center transport in photorefractive media for low-intensity, continuous-wave illumination in the quasi-steady-state limit

A two-center photorefractive model permitting all center-to-band transitions and electron–hole charge transport unifies several models describing subsets of these transitions. Using the quasi-steady-state approximation for low continuous-wave illumination, the model derives the first-order Fourier harmonic of the steady-state space-charge distribution and fields and the corresponding response rates. The model describes electron–hole competition, complementary gratings, double-exponential time-dependent behavior, and nonlinear dependence of response rates on intensity. The model can be used to make new predictions of photorefractive behavior of crystals in which two species of impurity center are present; an example is presented for BaTiO3.

Diffraction from thermally fixed gratings in a photorefractive medium: steady-state and transient analysis

We investigate diffraction from a thermally fixed grating in a photorefractive medium with an ion concentration that is large compared with the acceptor density. A numerical solution to the standard band transport model with mobile ions and the coupled-wave equations are used to study steady-state solutions. For the first time to our knowledge, transient responses are investigated during recording at elevated temperatures and during readout at room temperature. We consider the effect of large applied fields, which reduces the electronic screening and reveals the fixed ionic grating, on diffraction efficiency. The combined effects of self-diffraction, beam coupling, electrical revelation, and in some materials increased sensitivity to the applied field can result in a large diffraction efficiency even from a weak fixed grating. Results showing how transient solutions are affected by absorption in the photorefractive medium and by the magnitude of the incident intensity are also provided.

EFFECT OF APPLIED FIELDS ON THE BRAGG CONDITION AND THE DIFFRACTION EFFICIENCY IN PHOTOREFRACTIVE CRYSTALS

We explore theoretically and experimentally how application of an electric field to a photorefractive crystal affects the recording and the readout of holograms. We consider, for the first time to our knowledge, the effects of fringe bending caused by nonlinear two-wave mixing on the change in Bragg condition and diffraction efficiency as a function of the applied electric field. Practical performance limitations for holographic data storage and image amplification are discussed.

Trapping the grating envelope in bulk photorefractive media

We introduce the concept of traveling grating envelopes to study the erasure dynamics in bulk photorefractive media. This new technique provides considerable physical insight into the observed photorefractive transients. We propose and demonstrate experimentally, in a Ce-doped SBN:60 crystal, two novel ways of extending the grating lifetime by trapping the grating envelope inside the medium. The incident beam ratio and the readout geometry have the largest effect on how long the grating can be read out.

Effect of self-diffraction on erasure dynamics during readout at different wavelengths and geometries in photorefractive materials

Certain memory applications based on photorefractive media require the ability to conveniently fix holograms. An attractive alternative is to sufficiently prolong the readout time. Low photoexcitation at long wavelength augmented by self-diffraction can increase readout time by a few orders of magnitude. The interaction of the writing and readout waves, both of the same wavelength, with the photorefractive space charge field has been investigated theoretically and experimentally. Dynamics of beam coupling and self diffraction, in certain geometries, can lead to an initial increase followed by non-exponential decay of the diffracted signal during readout. Here we study how the difference in gain, lifetime, photoexcitation and absorption at different wavelengths - which require changing incident angles to satisfy the Bragg condition - affect the decay dynamics of photorefractive gratings.<<ETX>>