Real-Time Holographic Display Devices for Image Projection and Spatial Light Modulation

Available from UMI in association with The British Library. Liquid crystal and photorefractive materials and devices are investigated for application to holographic storage, real-time holographic image projection and display, and spatial light modulation. I present the characteristics of a technique for storing holograms in crystals of bismuth silicon oxide (BSO) that combines reversible photochromic effects with the more usual real-time photorefractive properties of the material. Photochromic effects have been observed before in these sillenite crystals but have generally been considered as a problem rather than as a possible mechanism for holographic storage. As the normal photorefractive behaviour is unaffected, simultaneous spatial multiplexing of both photorefractive (real-time) and photochromic (permanent) holograms is possible in the same crystal volume. Several applications are demonstrated ranging from image synthesis to holographic interferometry. These operations are based on the inherently separate nature of these two holographic gratings which can lead to relative phase shifts between the two simultaneously scattered fields by shifting the real-time grating with respect to the permanent one, using optical techniques. Permanent photochromic holograms recorded in BSO also show unexpected dynamic behaviour in which a fast increase in diffraction efficiency is observed upon illumination by a beam at the highly absorbed blue wavelengths. A novel scheme for spatial light modulation based on the spatial enhancement of the diffraction efficiency of photochromic holograms stored in BSO is demonstrated. The enhancement effect is dependent on a number of parameters, for example, crystallographic orientation, intensity and wavelength of the illuminating beam, angle of polarisation of the holographic readout beam, and externally applied high electric fields. These observations are consistent with the formation of a secondary, or parasitic, photorefractive grating upon the illumination of the permanent hologram by the additional blue beam. A theoretical model combining the effects of interference between contributions of diffraction from these two separate gratings (permanent and induced photorefractive) with different phase and polarisation properties is shown to be in good agreement with the experimental data. (Abstract shortened by UMI.).