Fast tunable metamaterial liquid crystal achromatic waveplate

Abstract

Photonic metamaterials combined with liquid crystals (LCs) for tunability is a great niche for building miniature devices with high performance such as fast flat tunable lenses, tunable filters, and waveplates. Sub-wavelength or nano-grating surfaces are homogenized to uniaxial waveplates with negative birefringence of unique dispersion when the period is less than the wavelength by at least a few times. This uniaxial metasurface, combined with the LC layer, is shown to act as a tunable retardation achromatic waveplate with 8 μm thick LC layer operating over wide spectral and angular ranges, as compared to using two nematic liquid crystal (NLC) retarders of thicknesses on the order of 30-60 μm, when no metasurface is used. Hence the device becomes miniature and 50× faster due to the thinner liquid crystal layer. The silicon nano-grating of 351 nm pitch and 0.282 fill factor is designed and fabricated to operate in the short-wave infrared range (SWIR). Switching between three achromatic retardation levels: full-, half-, and quarter-waveplates is accomplished by changing the applied voltages on the NLC cell with a switching time of a few milliseconds. This device has applications in fast broadband shutters, low coherence phase shift interferometry, ellipso-polarimetry, dynamic control of light intensity, and smart windows.