  Free-space electro-optic (EO) modulators operating at gigahertz and beyond
are attractive for a wide range of emerging applications, including high-speed
imaging, free-space optical communication, microwave photonics, and diffractive
computing. Here we experimentally demonstrate a high-speed plasmonic
metasurface EO modulator operating at a near-infrared wavelength range with a
gigahertz modulation bandwidth. To achieve efficient intensity modulation of
reflected light from an ultrathin metasurface layer, we utilize the bimodal
plasmonic resonance inside a subwavelength metal-insulator-metal grating, which
is precisely tuned to satisfy the critical coupling condition. As a result,
perfect absorption of -27 dB (99.8%) and a high quality (Q) factor of 113 are
obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer
inside the grating, we achieve a modulation depth of up to 9.5 dB under an
applied voltage of $\pm$30 V. The 3-dB modulation bandwidth is confirmed to be
1.25 GHz, which is primarily limited by the undesired contact resistance. Owing
to the high electrical conductivity of metallic gratings and a compact device
structure with a minimal parasitic capacitance, the demonstrated device can
potentially operate at several tens of gigahertz, which opens up exciting
opportunities for ultrahigh-speed active metasurface devices in various
applications.
