Strain-biased ferroelectric-photoconductor image storage and display devices

A device suitable for image storage and display applications is described in which the image is stored as a spatially varying birefringence and can, by means of suitably polarized light, either be observed directly or projected onto a viewing screen. The basic device consists of a sandwiched structure of uniform thickness layers, comprising transparent electrodes, a photoconductive film, and a plate of fine-grained ferroelectric ceramic. In order to establish a preferred orientation of the polarization in the plane of the plate, a uniform strain is induced in the plate by placing it either in tension or compression ("strain biasing"). The plate then becomes birefringent, with the principal axes of the optical indicatrix along the strain axes. The magnitude of this birefringence can be controlled by an electric field applied in the thickness direction, and this forms the basis of the device operation. In operation, the image to be stored is projected or scanned onto the photoconductive film. A voltage applied to the transparent electrodes develops, in the ceramic, a transverse field having an intensity modulated by the photoconductive film. When the field is removed, the desired image is stored as a spatial modulation of the birefringence of the ceramic plate. To erase the image the entire structure is flooded with light in the presence of an electric field in the reverse direction, and the plate regains its initial state of uniform birefringence. A large number of experimental devices have been fabricated and tested in the laboratory. In addition to describing the basic device structure, experimental results are presented showing the level of performance obtained from present devices, and a physical interpretation of device operation in terms of domain switching processes is provided.