Switchable Zero-Order Diffraction Filters Using Fine-Pitch Phase Gratings Filled with Liquid Crystals

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Abstract
Zero-order transmission efficiency is controlled using electrically switchable liquid crystals confined in periodic fine grooves. Trapezoidal grating profiles are designed in order to improve the extinction ratio. Using the index matching and mismatching conditions of liquid crystals with the grating grooves, we achieve a transparent window for the entire visible range in the matching state, and a dark color filter in the mismatching state. Spectral transmittance is changed through the frequency-dependent anisotropic permittivity of the liquid crystal. Polarization-insensitive operation is performed using a two-stack device with crossed grating directions.