Single-Gate Active Beam Steering Graphene Metasurface

Optical beam steering is a next-generation technology with applications in optical communications, laser patterning, three-dimensional laser point clouds and LiDAR systems. Conventional beam steering devices based on mirrors rotated by motors have a 360° horizontal field-of-view, but suffered from its bulky size and high heat generation. MEMS-based LiDAR, which combines an actuator and a micro-scanner, and flash LiDAR, based on diffractive elements, were adopted to address those issues. However, they still suffered from low durability and significant power consumption. Metasurface-based beam steering device offers several breakthroughs, including miniaturization, reduced power consumption, reduced heat generation and higher frame rates.<br/>Active metasurface beam steering device can be implemented utilizing liquid crystals, phase change materials, or carrier injection into Transition metal dichalcogenide monolayers or conductive oxides. However, previous studies have faced challenges such as a small field-of-view within a few degrees, poor diffraction efficiency or directivity. In conventional electrically tunable beam steering devices, they generally require a complex circuit driver inside to individually control each metaatom. Those system may also involve dielectric breakdown problems between adjacent metaatom. To scale up to practical beam steering technologies in the future, it is necessary to simplify the driving mechanism and improve performance.<br/>In this study we developed a beam steering technique using only single-gate voltage. The limitations in design freedom imposed by removing a complex circuit driver can be overcome by carefully determining the structural parameters of the metasurface. In conventional forward design, a local periodic approximation is commonly adopted to understand overall performance of metasurface in metaatom level. However, this approximation is not appropriate when there are significant interferences between neighboring metaatoms, resulting in unwanted sidelobes and poor directivity. Recently, inverse design, which determines structural parameters based on its performance, has been introduced to address these shortcomings. We used a genetic algorithm to efficiently deal with large design hyperspace and find a device structure with high directivity and field-of-view without a complex circuit configuration. The metasurface consists of a gold grating and a graphene ribbon array with different widths and gaps. The height, width, gap and operating wavelength of the metasurface were optimized as design parameters. The designed metasurface is placed on a gold back reflector and a 200 nm low-stress silicon nitride dielectric spacer.<br/>We measured modulation in diffraction efficiency and directivity at the 0th and -1st diffraction angles by varying a gate voltage applied to the device. When the gate voltage of 10 V is applied, the directivity at the 0th diffraction angle (45°) is approximately 0.675. This value decreases as the gate voltage is gradually decreased, reaching 0.325 at a gate voltage of -70 V. Meanwhile, the diffraction efficiency reaches 16%, gradually decreases to 8.5% when the gate voltage decreases. This confirms that the reflected wavefronts can be actively switched with a field-of-view of 68° with single-gate voltage.