Nematic LC modes and LC phase gratings for reflective spatial light modulators

Single-domain nematic liquid crystal (LC) devices based on the polarization-rotation effect, the birefringent effect, or both have been investigated for reflective spatial light modulators (SLMs) which use a polarizing beam splitter to separate the input light beam from its orthogonal output beam. We have evaluated each LC mode in terms of its contrast ratio, optical efficiency, operating voltage, and tolerance to cell-gap nonuniformity. We studied the hybrid- aligned and the 0°-, 45°-, and 63.6°-twisted nematic LC modes, which can be operated either normally white (NW) or normally black (NB). We have also investigated the mixed twisted nematic (MTN) and self-compensated twisted nematic (SCTN) modes in NW and the tilted homeotropic mode in NB. Two-dimensional simulations have also been carried out for both NW and NB modes implemented in active-matrix-driven reflective SLMs to elucidate the effect of fringe fields, which tend to generate disclination lines in high-field on-pixels adjacent to low-field off-pixels. Numerical examples are given to illustrate that, for the NB modes, the disclination lines occurring in the field-on bright state appear dark and reduce the optical efficiency. However, for the NW modes, the disclination lines occurring in the field-on dark states generate a light leakage which degrades the contrast ratio. To improve optical efficiency, we have also studied polarization-independent LC phase gratings using patterned alignment with opposite twist angles for reflective SLMs. The basic equations for the diffracted and nondiffracted intensities have been derived. The device parameters, operating voltage, and optical efficiency are given for various cases with a twist angle equal to or less than 90°.