Abstract

 Optical metasurfaces comprised of judiciously designed nanostructures facilitate the control of light in ultra-thin layers to perform similar functions as conventional bulky optical elements. The highly engineerable scattering properties of resonant optical antennas underpin the operation of such metasurface-based flat optics. Thus far, the choice of antenna has been limited to shaped metallic and high-index semiconductor nanostructures that support geometrical plasmonic or Mie resonances. Whereas these resonant elements offer strong light–matter interaction and excellent control over the scattering phase and amplitude, their electrical tunability has proven to be quite limited. Here, we demonstrate how excitonic resonances in atomically thin 2D semiconductors can be harnessed as a different, third type of resonance to create mutable, flat optics. These strong materials-based resonances are unmatched in their tunability with various external stimuli.

To illustrate the concept, we first we explore the fundamental efficiency limit of atomically-thin optical elements and its relation to the exciton decay dynamics. We then show how electrical gating can be used to effectively turn the exciton resonance on/off, and leverage this to demonstrate: (i) a tunable atomically-thin lens; (ii) dynamic beam steering in a 2D metasurface; and (ii) high-efficiency free-space optical modulation in a hybrid-2D metasurface.