A Low Voltage Liquid Crystal Phase Grating with Switchable Diffraction Angles

We demonstrate a simple yet high performance phase grating with switchable diffraction angles using a fringe field switching (FFS) liquid crystal (LC) cell. The LC rubbing angle is parallel to the FFS electrodes (i.e. alpha = 0 degrees), leading to symmetric LC director distribution in a voltage-on state. Such a grating exhibits three unique features: 1) Two grating periods can be formed by controlling the applied voltage, resulting in switchable diffraction angles. In our design, the 1st diffraction order occurs at 4.3 degrees, while the 2nd order appears at 8.6 degrees. 2) The required voltage to achieve peak diffraction efficiency (eta similar to 32%) for the 1st order is only 4.4 V at lambda=633 nm as compared to 70 V for a conventional FFS-based phase grating in which alpha approximate to 7 degrees, while the 2nd order (eta similar to 27%) is 15 V. 3). The measured rise and decay time for the 1st order is 7.62 ms and 6.75 ms, and for the 2nd order is 0.75 ms and 3.87 ms, respectively. To understand the physical mechanisms, we also perform device simulations. Good agreement between experiment and simulation is obtained.