Active Wavefront Shaping in Transmission from Tunable Excitonic Resonances

We demonstrate dynamic beam steering in transmission by utilizing the large tunability of the complex refractive index enabled by excitonic resonances. While reconfigurable metasurfaces have been explored using tunable materials such as transparent conducting oxides, liquid crystals, or phase change materials, these concepts rely on coupling to resonant antennas or optical cavities which are sensitive to geometric parameters. Rapid advances in understanding excitonic resonances intrinsic to materials have stimulated thinking about active metasurfaces where the excitonic material can act both as the resonant scatterer and the tunable material. However, demonstrations of active metasurfaces exploiting excitonic resonances for amplitude and phase modulation have been limited to operating in reflection. Here, we show how in excitonic materials where the quantum yield is over 70%, for example, in monolayer transition metal dichalcogenides, quantum dots, or perovskites, we can modulate the phase shift of the transmitted light by over 250°. By varying the voltages from ±5 V between adjacent electrodes, we can then dynamically steer the transmitted beam to ±39° near the exciton resonance. Our results suggest the potential of leveraging the tunability of excitonic resonances for arbitrary wavefront shaping in emerging photonic applications.