Nematic liquid crystal modes for Si wafer-based reflective spatial light modulators

Single-domain nematic liquid crystal (LC) devices based on either the polarization-rotation or the birefringent effect or both are investigated for reflective spatial light modulators (SLMs). We evaluate each LC mode in terms of its contrast ratio, optical efficiency, operating voltage and its tolerance in cell-gap. We study the hybrid-aligned, the 0-, 45- and 63.6°-twisted nematic LC modes which can be operated in either normally white (NW) or normally black (NB) mode. We also include the mixed twisted nematic and self-compensated twist nematic modes for NW and the tilted homeotropic mode for NB. Two-dimensional simulations have been carried out to compare with experimental observation for one of the NB modes implemented in active matrix reflective SLMs to elucidate the effect of fringe-fields. To improve the optical efficiency, we also study polarization-independent LC phase gratings using patterned alignment for reflective SLMs. The basic equations for the diffracted and non-diffracted intensities for both the reverse-twist and orthogonal-twist two-domain LC phase gratings have been derived. The device parameters, the operating voltage and the optical diffraction efficiency are given for various cases with a twist angle equal to or less than 90°.