Excitonic Beam Steering in Monolayer Molybdenum Diselenide Based Active Metasurfaces

We demonstrate dynamic beam steering in an active van der Waals metasurface by combining the large tunability of the complex refractive index near the excitonic resonance in monolayer molybdenum diselenide (MoSe₂) with an array of gold electrodes. While previous demonstrations of reconfigurable metasurfaces depend on the geometric parameters of dielectric or plasmonic Mie resonators, our design relies solely on excitonic resonances which are decoupled from geometric resonances. By varying the voltages between adjacent electrodes, we can control the phase profile of the reflected light from our MoSe₂metasurface. We generate a momentum space map of the reflected beam by imaging the intensity profile at the Fourier plane with a CCD camera. Our experiments show that at 6 K, the reflected light can be steered to angles between -30° to 30° at the A exciton wavelength of 754 nm. We attribute the dynamic control to the large refractive index modulation of over 200% through tuning the carrier density in MoSe₂ up to 7 x 10¹²/cm². Our results suggest the potential to utilize the tunability of excitonic resonances in two-dimensional transition metal dichalcogenides for wavefront shaping in emerging photonic applications.