Available optical devices such as CDs and DVDs record information on the surface of the disc. To increase the capacity of optical storage media, Spanish researchers are adapting enhanced photopolymers and sol-gels to create holographic optical storage materials with potentially far more capacity than today's optical storage media.
"The theoretical information storage capacity of holographic memories is much greater than current techniques, and that is why we are developing thick photopolymers with good holographic properties," explains Manuel Ortuno of the Departamento Universitario de Optica at the University of Alicante, Spain.
Photopolymers based on polyvinyl alcohol-acrylamide (PVA) have many useful advantages for holographic recording: they have good energy sensitivities, their spectral properties adapt to the type of recording laser by changing the sensitizer dye, and they have high diffraction efficiencies together with acceptable resolution and signal-to-noise ratios. PVA is also relatively inexpensive and easy to manufacture, which makes the material practical for commercial holographic storage applications. The Alicante research team studied 1-mm PVA samples for use as a write-once, read-many holographic storage device.
The PVA solution was deposited in polystyrene molds under the force of gravity in a darkened room. After the material had sufficiently dried it was removed from the molds, cut into squares, and attached to glass substrates before placing it into the holographic system for exposure. A continuous wave argon laser emitting at 514 nm wrote diffraction gratings in the PVA material.
First, the laser was split into two beams of 5 mW/cm2 intensity and expanded to 1.5 cm. The object and reference beams were combined at an angle of 16.8° to produce a spatial frequency of 1125 lines/mm. The diffracted and transmitted intensities were monitored using a HeNe laser at 633 nma band of the spectrum where PVA has low sensitivity. Holographic efficiencies were measured as a function of the angle of reconstruction using a rotating stage.
The sum of the diffracted and transmitted intensities, which is indicative of the amount of light lost to absorption and scattering, was relatively constant between 78 and 84%. The maximum diffraction efficiency during exposure was approximately 70%. They also found that the material has a high angular sensitivity of 0.4°; outside this angle, the diffraction efficiency of the reconstructing beam drops off significantlya property not found in common holographic thin films and useful during holographic reconstruction. Finally, the team evaluated the thick polymers as they recorded nine distinct holographic gratings using angular multiplexing. The evaluation of the PVA material indicated high diffraction efficiency for each grating with sufficient separation between gratings to permit independent reconstruction.
Another material being investigated for holographic recording is photopolymerizable organic-inorganic hybrid materials prepared by the sol-gel method. This approach is designed to avoid the Bragg detuning and shrinkage that can occur in purely organic materials. The organic molecules are homogeneously imbedded within the porosity of the inorganic matrix, resulting in very low shrinkage, excellent optical quality, and ease of processing into different forms and thicknesses. "This is critical to holographic storage applications and a focus of our work," explains Gonzalo Ramos of the Laboratorio de Instrumentacion Espacial (Madrid Spain). They recently demonstrated shrinkage of only 0.4% from the initial material thickness using a mixture of organic monomers dispersed within silica matrices.