>270° phase shift in aluminum gate tunable conducting oxide metasurfaces for continuous optical beam steering at 1550 nm

In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components (prisms or lenses) as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces have typically been encoded at the time of fabrication, which fixes the achievable phase and amplitude for each elements in an array. However if actively controlled metasurface elements can be designed to dynamically control the phase shift and amplitude change imposed by each metasurface element, we could realize phased arrays to enable complex spatio-temporal wavefront engineering. We report here design and experimental demonstration of a tunable conducting oxide metasurface that achieves such active control by incorporating materials with voltage-tunable optical permivitties, such as indium tin oxide (ITO), into a metasurface [1]. We design a metasurface that consists of an aluminum back plane, HfO2 gate dielectric followed by a 14 nm thick ITO active layer, and a periodic array of aluminum patch antennas. We choose the dimensions of the Al antennas so that the antenna magnetic dipole resonance occurs at 1550 nm. By applying a gate bias between the Al antenna and ITO active layer, charge accumulation or depletion occurs at the ITO/HfO2 interface. This results in modulation of the ITO complex permittivity, thus altering the metasurface reflection phase and amplitude. The designed metasurface is capable of >270° phase shift. Our design enables independent control of each metasurface element enabling electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continious beam steering