Holographic storage media based on optically active bistable defects

Abstract

We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the “$\mathrm{DX}$ ” center. Crystals containing these defects have the characteristics of local photorefractive materials in that their refractive index is modified in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the $\mathrm{DX}$ deep trap states into the conduction band, is persistent at low temperatures due to a capture barrier, $E_{\mathrm{cap}},$ which limits reformation of the $\mathrm{DX}$ centers. The effect is reversed by heating above an annealing temperature, which scales with $E_{\mathrm{cap}}$ and varies with the crystal host and active dopant. A number of $\mathrm{DX}$ 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 $\mathrm{DX}$ center and present theoretical estimates of several important optical properties of these materials based on a simple model. We calculate spatial resolution, maximum refractive index shift, and sensitivity, and compare our predictions with measurements on one member of the $\mathrm{DX}$ family, AlGaAs:Te. In a 345 μm thick sample of this material doped at ${\text{9×10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}},$ we find a refractive index shift, $\mathrm{\Delta n,}$ of ${\text{2×10}}^{\text{−3}}$ and an exposure sensitivity, $\mathrm{S,}$ of $\text{0.012\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{3}}\mathrm{/J}.$ Our expectation that the maximum refractive index change scales linearly with the doping density is consistent with our previous measurement of ${\text{Δn=1.1×10}}^{\text{−2}}$ obtained for a sample of AlGaAs:Si doped at ${\text{4×10}}^{18}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}.$ The measured values of $\mathrm{\Delta n}$ and $\mathrm{S,}$ are, respectively, two and three orders of magnitude larger than corresponding values for the photorefractive material ${\mathrm{LiNbO}}_{\mathrm{3}},$ and are shown here to be independent of exposing irradiance from ${10}^{\text{−3}}$ to ${10}^8\hspace{0.17em}{\mathrm{W/cm}}^{\mathrm{2}}.$ At the latter irradiance, the refractive index shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exhibits high sensitivity, large refractive index change, and fast write time, all desirable properties of an optical holographic storage medium. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% for grating periods from 0.13 to 15 μm. No degradation of sensitivity is observed after large numbers of exposure–erasure cycles. Experiments with multiple-hologram exposures show that the $\mathrm{DX}$ materials require no exposure schedule: equal strength holograms are obtained using equal exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system parameters are estimated for a 1 Tbyte holographic storage device based on angle multiplexing in a $\mathrm{DX}$ material.