Optically controlled bistable waveplates

We develop an optically controlled, rewritable half-wave plate based on a twisted nematic liquid crystal cell and present the relevant theory. The cell is bistable, with switching between states controlled by one-step illumination of a single PAAD azobenzene alignment layer with visible light. The photo-alignment properties of the layer enable reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. The resulting changes in transmission are observed for four different probe beam wavelengths in the visible and near infrared (NIR) spectral regime. We demonstrate a reproducible modulation of the transmitted polarization with an optical contrast of up to 90-100%. This work represents a first step towards creating LC modulators and phase shifters which can be controlled remotely using only external light sources.