GHz-Speed Wavefront Shaping Metasurface Modulators Enabled by Resonant Electro-Optic Nanoantennas.

Electrically tunable metasurfaces that control the amplitude and phase of light through biasing of nanoscale antennas present a route to compact modulator devices. However, most platforms face limitations in bandwidth, optical efficiency, and tuning response. Electro-optically tunable metasurfaces achieving both GHz amplitude modulation and transmissive wavefront shaping in the telecom range are presented. The resonant electro-optic nanoantenna design consists of a silicon nanobar atop thin-film lithium niobate, with gold electrodes. The nanobar is a periodically perturbed optical waveguide that supports high quality factor (Q > 1000) guided mode resonances excited with free-space light. Voltage biasing the lithium niobate tunes its refractive index, modulating the resonance of the nanobar through evanescent mode overlap. Absolute transmittance modulation of 7.1% with ±5 V applied voltage is demonstrated, and the modulation dependence on the resonance quality factor is shown. Additionally, the modulation bandwidth of these devices exceeds 800 MHz, and the electrode limitations on this bandwidth are studied. Finally, how this resonant antenna platform can enable wavefront shaping metasurfaces is shown. A beamsplitting metasurface device is demonstrated, whose diffraction efficiency can be modulated with a bandwidth of 1.03 GHz. The high-speed modulation and wavefront control capabilities of this platform provide a foundation for compact, high-bandwidth free-space communications and sensing devices.