Fast tunable metamaterial liquid crystal achromatic waveplate

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.