  Metasurfaces composed of planar arrays of sub-wavelength artificial
structures show promise for extraordinary light manipulation; they have yielded
novel ultrathin optical components such as flat lenses, wave plates,
holographic surfaces and orbital angular momentum manipulation and detection
over a broad range of electromagnetic spectrum. However the optical properties
of metasurfaces developed to date do not allow for versatile tunability of
reflected or transmitted wave amplitude and phase after fabrication, thus
limiting their use in a wide range of applications. Here, we experimentally
demonstrate a gate-tunable metasurface that enables dynamic electrical control
of the phase and amplitude of the plane wave reflected from the metasurface.
Tunability arises from field-effect modulation of the complex refractive index
of conducting oxide layers incorporated into metasurface antenna elements which
are configured in a reflectarray geometry. We measure a phase shift of {\pi}
and ~ 30% change in the reflectance by applying 2.5 V gate bias. Additionally,
we demonstrate modulation at frequencies exceeding 10 MHz, and electrical
switching of +/-1 order diffracted beams by electrical control over subgroups
of metasurface elements, a basic requirement for electrically tunable
beam-steering phased array metasurfaces. The proposed tunable metasurface
design with high optical quality and high speed dynamic phase modulation
suggests applications in next generation ultrathin optical components for
imaging and sensing technologies, such as reconfigurable beam steering devices,
dynamic holograms, tunable ultrathin lens, nano-projectors, and nanoscale
spatial light modulators. Importantly, our design allows complete integration
with electronics and hence electrical addressability of individual metasurface
elements.
