Cholesteric diffraction devices with a field-controlled grating vector

Electrically-controlled diffraction gratings are developed on the basis of a cholesteric liquid crystal confined between two transparent electrodes. The electrodes are coated with unidirectionally treated alignment layers. The initial state (zero field) is planar, with the helix axis oriented normally to the electrodes. The applied field causes reorientation of molecules and creates structures modulated in the plane of. the cell. Surface alignment provides unidirectional uniformity of the modulation. The parameters of the modulated structures and light diffraction are controlled by the cholesteric pitch, cell thickness and applied voltages. Diffraction regimes of both Raman-Nath and Bragg types are demonstrated. In the Raman-Nath regime, the electric field allows one to control continuously the deflection angle by changing the periodicity of modulations. This effect can be used in various beam steering devices. The variation in the diffracted beam direction can be more than 20 degrees. Typical working voltages are less than 10V. We present both the experimental results and 3D computer simulations of modulated structures caused by the field.