The holographic recording characteristics of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) are investigated. The exposure sensitivity is characterized for single-hologram recording, and the M͞# is measured for samples as thick as 3 mm. Optically induced birefringence is observed in this material. © 1998 Optical Society of America OCIS codes: 090.2900, 090.4220, 210.2860.The characterization of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) 1,2 (PMMA) as a recording material for holographic memories is described in this Letter. This material consists of the polymer host matrix with added PQ molecules as photosensitive dopant. High-optical-quality samples of this material were made with variable thicknesses of up to 5 mm and in a variety of shapes. This material does not shrink after exposure and is lightweight, inexpensive, and durable, making it an attractive candidate for disk-based holographic memory systems.Sample preparation consists of dissolving PQ molecules in liquid methyl methacrylate together with a polymerization initiator. This solution is then poured into molds and allowed to polymerize in a pressure chamber at an elevated temperature. The molding process allows samples to be fabricated in a variety of geometries. Disks ranging from 2.5 to 10 cm in diameter with 1 -5-mm thickness were made. For a 1-mm-thick sample doped with a concentration of 0.7% of PQ molecules before exposure, the absorption reaches a maximum of 98.8% at 445 nm and is 58% for the 488-nm line of an argon laser, which was used in all experiments described in this Letter.A hologram was recorded by a pair of 488-nm beams, each incident upon the material at an outside angle of 21.5 ± . We monitored the growth of the hologram during recording by probing the sample with a Bragg-matched He-Ne laser beam. Figure 1 shows the diffraction eff iciency (diffracted power divided by the incident power) during recording in 1-mm-thick material. The diffraction eff iciency reached a maximum of 4.3% for an exposure energy of 2.5 J͞cm 2 . If exposure was allowed to continue, the diffraction eff iciency began to drop. After 20 J͞cm 2 of exposure with a single beam the hologram decayed to approximately 0.1%. At this point the material was completely exposed, and no more holograms could be recorded.We recorded permanent holograms that do not decay with subsequent illumination by stopping the exposure before saturation was reached and then baking the sample. Figure 2 shows the strength of a hologram as a function of baking time at a temperature of 55 ± C. The diffraction efficiency reached a maximum after 12 days and remained steady with continued baking. Figure 3 shows the selectivity curves for a weak and a strong hologram (2% and 35% diffraction eff iciency, respectively). The 2% hologram has a sinc-squared selectivity curve as expected for a 1-mm-thick hologram. The stronger hologram, on the other hand, has a selectivity curve that is distorted and shifted. For a holographic memory the diffraction eff iciency is relatively sma...

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The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range ͑M͞#͒ and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M͞# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits͞m 2 is experimentally demonstrated by use of a 1-mm-thick LiNbO 3 crystal as the recording medium.

A new multiplexing schedule is derived for multiplexing holograms in photorefractive polymers which do not exhibit mono-exponential recording behavior. An M-number (M/#) of 0.3 was measured experimentally by recording 20 holograms of roughly equal strength in a single location of 125-lm-thick material using peristrophic multiplexing. The eects of hologram dark-decay on the time-evolution of the M/# and the relative strengths of individual holograms is investigated. Ó 2000 Elsevier Science B.V. All rights reserved. Holographic data storage is a promising technology for the storage of large amounts of data. In order to be useful for holographic data storage applications, a material must be capable of achieving a high M/#, a property dependent on both the recording and erasure dynamics of the stored holograms. In this letter we report the recording of multiple holograms by peristrophic multiplexing in the novel material class of photorefractive (PR) polymers. A new recording schedule is devised to account for the variation of the erasure time constants as a function of exposure. The M/# measured from the recording of 20 equalized holograms was 0.3.The standard structures are sandwiches of the PR polymer between two glass slides coated with transparent electrodes made from indium tin oxide [1±6]. In particular, we used a composite derived from the ®rst high-performance PR polymer [7], consisting of (by weight) 42% poly-(N-vinylcarbazole) (PVK, polymer host), 7% N-ethylcarbazole (plasticizer), 25% each of the non-linear chromophores 2,5-dimethyl-4,4 H -nitrophenylazoanisole and 3-methoxy-4,4 H -nitrophenylazoanisole, and 1% 2,4,7-trinitro¯uorenone (sensitizer). The active layer thickness was d 125 lm. The holographic setup consisted of two plane-wave 633 nm recording beams at angles of 50°and 70°from the surface normal. They were both s-polarized and had equal intensities of approximately 500 lW/cm 2 at the sample surface. A weak (3.5 lW/cm 2 ) p-polarized readout beam counter-propagating to the 50°re-cording beam was used to perform a phase conjugate readout of the holograms. Under these conditions and at the externally applied ®eld used throughout these investigations (E 62 V/lm; necessary to induce bulk non-linearity), the logarithmically averaged holographic response time [8] of the material was 45 s and the internal diraction

We demonstrate holographic recording in a new photopolymer system. The recording material is created by copolymerization of an optically inert monomer, methyl methacrylate, and a second monomer that is optically sensitive. On exposure of the recording material to light, a portion of the optically sensitive component detaches from the polymer matrix and causes hologram amplif ication through diffusion of the free molecules. We measured postrecording grating amplif ications as high as 170% by this process. The recorded holograms are persistent at room temperature under continuous illumination at the recording wavelength. 