LC-beam steering device based on sub-wavelength diffractive optical element structure

A theoretical analysis of liquid crystal (LC) beam steering structures based on Sub-Wavelength Diffractive Optical Elements (S-DOE) was performed. Rigorous Diffraction Analysis shows a significant diffraction efficiency gain of up to 40% in using Sub-Wavelength LC structures, compared to regular DOE structures. However, due to Fringe-Field Effects, the formation of a high Diffraction Efficiency Index Grating Structure inside a liquid crystal layer is possible, only if the grating pixel'S aspect ratio (height to width) is much smaller than unity (approx. 0.1). This in turn, requires the use of a small aspect ratio LC cell. However, Simulations of LC director behavior show, that such ultra-thin LC Cells will fall short of providing the 2π phase modulation necessary for an effective beam steering. One possible solution for this issue is the generation of a Blazed Phase Grating inside a Non-Symmetrical Reflective Fabry-Perot Resonator. Such configuration essentially allows an increased phase modulation magnitude at the expense of a very high sensitivity of such structure to liquid crystal thickness variations- as was verified by computer simulations. A second possible solution is the formation of a Cascaded Diffractive Gratings Stack based on ultra-thin LC layers. These gratings are identically reproduced in each of due to the Talbot Effect. We have studied a Cascaded system, consisting of four Ultra-thin liquid crystal layers separated by glass plates. The results show, that the thickness of each layer in the cascade decreases proportionally to number of layers, followed by an increased absorption. An Interesting feature of the cascaded structure is the discrete set of possible beam steering directions, which are determined by constraints of the Talbot Effect.