Liquid crystal spatial light phase modulator

Abstract: An electro-optic material (2) has first (4) and second (6) regions formed therein. The optic axes of the first and second regions are antiparallel and define a domain inversion pattern. The difference in refractive index between the first and second regions is a function of an electric field applied thereto. The difference in refractive index enables spatial light phase modulators and refractive optical devices having electrical control to be formed.

Description: The present invention relates to a liquid crystal spatial light phase modulator. GB 2 173 605 and EP 0 486 769 both disclose optical devices which comprise a pair of elements having their optic axes arranged in different directions. GB 2 173 605 further describes providing a pair of electrodes in order to apply an electric field to the elements in order to control an optical condition of at least one of the first and second elements, for example, where electro-optic materials are used, to control the refractive indices thereof. The use of a single layer of liquid crystal material is also disclosed. In the GB 2 173 605 disclosure, in each embodiment in which electro-optic materials are used, the device comprises two separate elements, each of which is etched in a periodic pattern. US 3 832 033 discloses a ferroelectric-liquid crystal composite device for converting a thermal image to an optical image. The device comprises a regular ferroelectric single crystal substrate in contact with a liquid crystal film. A small electric field, which is less than the coercive field at room temperature, is applied across the device. A thermal image is projected onto the ferroelectric substrate, in which it causes domain inversions. The liquid crystal is aligned by the pattern of domain inversions corresponding to the thermal image. The image is made visible by directing linearly polarised light into the device and analysing the light emerging from the device with another polariser. The features of the preamble of claim 1 are known in combination from that document. According to the invention, there is provided electrically controllable liquid crystal spatial light phase modulator comprising a liquid crystal layer in contact with a first surface of an electro-optic material and means for applying an electric field across the liquid crystal layer and the electro-optic material, the electro-optic material comprises a plurality of first and second regions extending from the first surface, the direction of polarisation of the first regions being oriented in a first direction substantially perpendicular to the first surface and the direction of polarisation of the second regions being oriented in a second direction substantially opposte the first direction in the absence of an applied electric field, the field applying means being arranged to apply the electric field so as to switch off a pattern of phase shifts which exists in the absence of an applied field, wherein the electro-optic material and the means for applying an electric field across the liquid crystal layer and the electro-optic material are such that the applied electric field does not alter the directions of polarisation of the first and second regions. The liquid crystal layer may compromise a ferroelectric liquid crystal. The means for applying an electric field may comprise first and second transparent electrodes. It is thus possible to provide a device in which the effect of the domain inversion pattern is effectively amplified by the liquid crystal layer. By applying a sufficiently large electric field across the liquid crystal layer, the device can effectively be switched off. Furthermore, the properties of the device rely on the pattern of the first and second domains and not on having to form complex electrode patterns. Thus the need to provide patterned electrodes is avoided. The present invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a liquid crystal spatial light phase modulator constituting an embodiment of the present invention. A domain inversion pattern in an electro-optic material can be enhanced, thereby allowing thinner slabs of electro-optic material to be used, if the electro-optic material is used to define one wall of a liquid crystal cell, as shown in Figure 1. A ferroelectric liquid crystal cell 30 having one side formed using a conventional aligning film 32 and the other side formed of electro-optic material 34 having a fixed domain inversion pattern formed therein causes the liquid crystal 36 to align in one of two states at the surface of the electro-optic material in accordance with the domain inversion pattern. Application of an electric field across the cell, via transparent electrodes 38 and 40, aligns the liquid crystal 36 with the electric field, and thereby switches off the pattern of phase shifts. Cells of only a few microns thickness and having slabs of electro-optic material patterned to a depth of approximately 1 micron thickness or less are thus able to provide electrically controllable spatial light phase modulators. Many methods exist to create a spatially varying domain structure, i.e. a domain inversion pattern, within the electro-optic material 34. These include direct field reversal (polarising), electron beam written inversion, ion exchange reversal and optically written domains through internal fields induced by charge separation in photorefractive materials. Ion exchange in non-centrosymmetric crystals may be used to lower the Curie temperature for domain inversion along the optic axis in ion exchanged regions of the electro-optic material 34. Thus lithographically defined domain inversion may be performed to create the domain inversion pattern. Fabrication of a domain inversion pattern is described in a paper entitled "Quasi-phase-matched second harmonic generation in a MiTaO 3 waveguide" Kazuhisa Yamamoto, Kiminori Mizuuchi and Tetsuo Taniuchi, IEEE journal of quantum electronics, vol 28, no. 9, September 1992. Variations in refractive index formed between ion exchanged and non-ion exchanged regions may be reduced or removed by annealing. Direct writing of inverted domains using electron beam lithography has the advantage of avoiding the changes in refractive index asociated with ion exchange techniques and also allows relatively deep inverted regions to be formed. For example, lithium niobate substrates 1mm thick have been completely inverted by electron beam writing of one surface of the substrate. The pattern of the first and second domains may be arranged to form a phase hologram which is arranged to act as a lens, a beam steering device, Fourier plane filters, a Fresnel type zone lens, or which forms a binary optic device. It is thus possible to provide a very versatile electrically controllable optical device.

Claims: An electrically controllable liquid crystal spatial light phase modulator comprising a liquid crystal layer (36) in contact with a first surface of an electro-optic material (34) and means (38,40) for applying an electric field across the liquid crystal layer (36) and the electro-optic material (34), the electro-optic material (34) comprising a plurality of first and second regions extending from the first surface, the direction of polarisation of the first regions being oriented in a first direction substantially perpendicular to the first surface and the direction of polarisation of the second regions being oriented in a second direction substantially opposite the first direction in the absence of an applied electric field, the field applying means (38, 40) being arranged to apply the electric field so as to switch off a pattern of phase shifts which exists in the absence of an applied field, characterised in that the electro-optic materials and the means for applying an electric field across the liquid crystal layer and the electro-optic material are such that the applied electric field does not alter the directions of polarisation of the first and second regions. A modulator as claimed in claim 2, characterised in that the liquid crystal layer (36) comprises a ferroelectric liquid crystal. A modulator as claimed in claim 1 or 2, characterised in that the means for applying an electric field comprise first and second transparent electrodes (38, 40).

Inventors: DAVIS GILLIAN, MAY PAUL, DAVIS, GILLIAN, MAY, PAUL
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1995
Main classification: G02F1
IPC codes: G02F1