Patent GB-1334574-A (GB)
Title: Electro-optical memory
IPC: G02F1/05
Filed: 1971-05-03
Abstract: 1334574 Electro-optic storage arrangements; modulating light RCA CORPORATION 3 May 1971 [4 May 1970] 12668/71 Headings H3B and H4F [Also in Division H1] A surface charge on a material such as a ferroelectric is modified by incident light to record information, subsequent retrieval of the information being achieved by the effect of the material on a low power light beam, e.g. to cause polarization rotation or hologram reconstruction. Suitable materials are BaTiO 3 , SrTiO 3 , BaNaNbO 3 , or BaSrNbO 3  which may be doped or have lattice vacancies introduced to increase that conductivity. In the case of the first two materials vacancies are introduced by heating at 700 C. in hydrogen for several hours, followed by heating in oxygen at 900 C. to produce a high resistivity surface. A semiconductor material such as GaP or GaAs doped with Se, Te, S, Si or Sn may be used, a high resistivity surface being obtained by doping a narrow layer of the surface with Zn, Gd, Mn, or Mg. The BaTiO 3  and SrTiO 3  materials may be deposited as a thin photoconductive layer on a bulk conductor, Fig. 9 (not shown), or used in sheets of 1 mm. thickness. Storage.-A surface charge is established over the whole surface of the material, for example, by a corona discharge, Fig. 3, from an adjacent electrode 17; this may be aided by enclosing the material in an atmosphere of SF 6 , Fig. 6. Alternatively the charge may be set up by an electron beam, Fig. 7 (not shown), or by wiping the surface with an ionic salt dissolved in a volatile solvent. An information bearing beam of light is then incident on the surface, and provided the beam is over a given minimum intensity the surface charge is modified. In Fig. 3 a hologram of information bearing plate 26 is formed on a portion of the material 10 selected by deflection system 16 and mask 28 in the usual manner. The mask may be replaced by focusing optics. In Fig. 6 a two dimensional scanner 46 sweeps a modulated laser beam over the surface to store the modulating information. The information may be analogue or digital. Reading.-Reading in Fig. 3 is by reconstruction of the hologram stored, in a manner similar to other phase holograms. The image may be formed and detected by transmitted light at 40, or by reflected light at 26 coincident with the object. The intensity of the reading beam is below the minimum which will alter the stored charge. In Fig. 6 a light beam is passed through polarizer 52 and the storage material to analyser 54. It may be projected on to a screen 56, a film or television camera. The reading may be positive or negative depending on the orientation of the polarizer and analyser. In Fig. 6 a scanning system equivalent to components 42, 44 and 46 may also be used for reading one bit, or one word, at a time. The reading beam is preferably of low enough intensity for non-destructive readout. It is stated that light passing through the charged medium may undergo phase shifts of 180 degrees, which give hologram efficiencies of 30%; writing sensitivity is around 1 microjoule per cm.<SP>2</SP>. Erasure is by recharging the surface, or by flooding it with intense unmodulated light. The stored charge can exist for days or weeks, but for permanent storage in ferroelectrics the material is heated just above its Curie temperature during writing and then cooled.