Cumulative Birefringence Effects of Nanosecond Laser Pulses in Dye-doped Planar Nematic Liquid Crystal Layers

New cumulative effects in laser-induced birefringence have been observed under 10Hz-pulse-repetition-rate, nanosecond-duration laser irradiation of azo-dye-doped planarnematic liquid crystal layers at incident intensities I ∼ 1–10 MW/cm2. An irradiation geometry with the incident polarization parallel to the nematic director was used. This geometry does not permit a first-order electric field induced reorientation of the nematic molecules, allowing us to exclude its contribution to the nonlinear response. New laserinduced birefringence effects with a buildup time of several seconds to minutes manifest themselves in: • the appearance of a polarization component perpendicular to the nematic director; • two different modes of spatial pattern formation with different patterns for parallel and perpendicular polarization: (1) At I ∼ 1–5 MW/cm2, the perpendicular polarization component forms a fourleaf-clover (a Maltese-like cross) spatial pattern in the far-field from the initial Gaussian spatial intensity distribution. The incident, parallel polarization component forms a round spot with a single ring spatial pattern. (2) At higher incident intensities (I ∼ 5–10 MW/cm2), a second regime of pattern formation is observed in the form of high definition patterns and only for the polarization component parallel to the nematic director.