Electrically-controlled two-dimensional gratings based on layers undulations in cholesteric liquid crystals

We developed electrically-switchable two-dimensional diffractive gratings with periodic refractive index modulation arising from layers undulations in cholesteric liquid crystal. Two-dimensional layer undulations occur above the threshold voltage when a planar cholesteric cell of thickness much larger than the cholesteric pitch is subjected to an electric field. The periodic structure of the layers undulations and corresponding spatial modulation of an average refractive index in the plane of a cell allows us to produce diffraction patterns with a square-type arrangement of diffraction maxima. The cholesteric cell can be switched by pulses of ac voltage between two states: one with flat layers of a planar cholesteric texture and another with square lattice of periodic director modulation associated with layer undulations that produces two-dimensional diffraction patterns. The periodicity of the developed two-dimensional phase gratings and intensities of the diffraction maxima can be tuned by changing the applied field magnitude. The diffractive properties of gratings are practically independent of the polarization state of the incident beam and can be optimized for different wavelengths by appropriately choosing the cholesteric pitch, cell thickness, and surface treatment. The potential applications include beam steering devices, optical waveguides, devices for splitting monochromatic beams and beam multiplexing.