<title>Multilevel nematic liquid crystal phase gratings</title>

Abstract

Planar-aligned nematic liquid crystals cells with an array of parallel electrodes can perform phase modulation with profiles going from binary to blazed gratings. A nematic liquid crystal with high birefringence (BL006) allows reducing the thickness of liquid crystal cells and thus reducing switching times. But the discrete electrodes and the small thickness have negative consequences on the shape of the phase profile. Liquid crystal phase gratings with 192 separately controlled electrodes were fabricated. The electrode distances and widths were 3 or 4 micrometers at a cell thickness of 6 micrometers . Perpendicular and parallel alignment of the liquid crystal with respect to the electrode grating were investigated. Far field diffraction measurements and phase measurements with a Mach-Zehnder interferometer were performed to characterize the gratings. The director distribution was modeled in 2 dimension and the resulting phase profiles were calculated with a Jones matrix method. It allows a comparison with the measurements. It was found that in the liquid crystal grating cell the in- plane electric field has a large influence on the optical properties. The in-plane electric field between the high- and low-voltage electrodes forms an unfavorable director deformation that limits the diffraction efficiency to approximately 65% for linear polarized light. Both, the measured and the simulated phase profiles, show a typical structure where valleys appear and worsen the optical performances. The optimization of such a grating is a compromise of large thickness, that smooths the valley structures, and small thickness, that reduces the in-plane switching effect and the switching time.