Diffractive liquid crystal spatial light modulators with optically integrated fine-pitch phase gratings

We describe opto-electronic LC spatial light modulators which allow the generation of alterable diffraction gratings down to a 1 mu m period. Optically defined binary and blaze phase gratings are used, in which the diffraction is altered by a parallel aligned nematic LC phase modulation layer. This enables the creation of arbitrary fine-pitch gratings. Different types of modulators have been investigated theoretically and experimentally. The theoretical results show that, in the case of small grating structures, the exact wave theory has to be applied to calculate the diffraction properties. The grating modulation then becomes dependent on the light polarisation and the grating period. Modulators with gratings of 1 to 10 mu m period have been fabricated. The modulator structure, the LC phase modulation layer, the problems of combining LCs with fine-pitch gratings, and the modulator properties are discussed. Good diffraction efficiencies and extinction ratios for the modulator diffraction orders have been obtained for monochromatic light, in addition to short switching times.