HOLOGRAPHIC STORAGE MEDIA BASED ON OPTICALLY-ACTIVE BISTABLE DEFECTS

We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the ''DX'' cen ter. Crystals containing these defects have the characteristics of loc al photorefractive materials in that their refractive index is modifie d in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the DX deep trap states into the conduction band, is persistent at low temper atures due to a capture barrier, E-cap, which limits reformation of th e DX centers. The effect is reversed by heating above an annealing tem perature, which scales with E-cap and varies with the crystal host and active dopant. A number of DX materials have now been identified with long-term persistence temperatures ranging from 50 to 180 K. In this paper, we briefly review the physics of the DX center and present theo retical estimates of several important optical properties of these mat erials based on a simple model. We calculate spatial resolution, maxim um refractive index shift, and sensitivity, and compare our prediction s with measurements on one member of the DX family, AlGaAs:Te. In a 34 5 mu m thick sample of this material doped at 9 X 10(17) cm(-3), we fi nd a refractive index shift, Delta n, of 2 X 10(-3) and an exposure se nsitivity, S, of 0.012 cm(3)/J. Our expectation that the maximum refra ctive index change scales linearly with the doping density is consiste nt with our previous measurement of Delta n = 1.1 X 10(-2) obtained fo r a sample of AlGaAs:Si doped at 4 X 10(18) cm(-3). The measured value s of Delta n and S, are, respectively, two and three orders of magnitu de larger than corresponding values for the photorefractive material L iNbO3, and are shown here to be independent of exposing irradiance fro m 10(-3) to 10(8) W/cm(2). At the latter irradiance, the refractive in dex shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exh ibits high sensitivity, large refractive index change, and fast write time; all desirable properties of an optical holographic storage mediu m. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% f or grating periods from 0.13 to 15 mu m. No degradation of sensitivity is observed after large numbers of exposure-erasure cycles. Experimen ts with multiple-hologram exposures show that the DX materials require no exposure schedule: equal strength holograms are obtained using equ al exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system par ameters are estimated for a 1 Tbyte holographic storage device based o n angle multiplexing in a DX material. (C) 1998 American Institute of Physics. [S0021-8979(98)01002-0].