Rewriteable Photoalignment of Liquid Crystals as a Route to High-Laser-Damage- Threshold Active Beam Shapers

The fabrication of rewriteable liquid crystal (LC) cells using high-damage-threshold photoswitchable azobenzene materials has been investigated as a route to all-optical active beam shapers. Beam shapers are used to modulate the intensity profile of OMEGA EP's petawatt-peak-power laser beams. All-optical photoswitchable beam shapers would allow intensity profile tailoring to occur at higher-fluence locations in OMEGA EP's beam lines than is currently possible using electro-optical LC beam shapers. Photoswitchable LC cells were assembled using one substrate coated with a non-switchable buffed polyimide alignment layer and a second substrate coated with an azobenzene-based photoswitchable alignment layer. The cells were filled with nematic E7 LC. Irradiating the cell with polarized UV light induced an azobenzene trans-to-cis isomerization, facilitating in-plane switching of LC alignment. The irradiation time was optimized to produce high-quality LC alignment. Different transmission functions were obtained after multiple alignment write cycles. During the fabrication of these devices, limitations in the azobenzene material's ability to continually rewrite were discovered, but high-quality rewritable behavior was obtained by exposing the photoswitchable layer to UV light through various masks.