(Keynote) Van Der Waals Active Metasurfaces and Heterostructures for Phase Modulation and Polarization Conversion

Abstract

A grand challenge for nanophotonics is the realization of tunable metasurfaces enabling active control of the key constitutive properties of light – amplitude, phase, wavevector and polarization. Active metasurfaces that enable dynamic modulation of reflection amplitude, phase and polarization have been recently explored using several active materials and modulation phenomena, including carrier index in plasmonic ENZ structures, reorientation of liquid crystal molecules, electrooptic effects in quantum well heterostructures and index change in phase change materials. The rapid advances in understanding of exciton resonances in layered van der Waals materials has now stimulated thinking about active metasurfaces that exploit excitonic modulation phenomena to enable ‘van der Waals active metasurfaces’. As one example, I will describe recent advances in electrically reconfigurable polarization conversion across the telecommunication wavelength range in van der Waals layered materials, integrated in a Fabry-Pérot cavity. The large electrical tunability of the excitonic birefringence in tri-layer black phosphorus enables spectrally broadband polarization conversion over nearly half the Poincaré sphere. We observe both linear to circular and cross-polarization conversion with voltage, demonstrating dynamic access to polarization diversity. As a second example, we discuss the observed large gate tunability of the complex refractive index in monolayer MoSe2 by Fermi level modulation near the A and B excitonic resonances for temperatures between 4 K to 150 K. By tuning the charge density, we observe both temperature and carrier dependent epsilon-near-zero response in the permittivity and transition from metallic to dielectric near the A exciton energy, and directly observe active phase modulation in monolayer MoSe2 gated heterostructures, whose voltage dependence is consistent with our complex index measurements. These results have broad implications for the use of monolayer transition metal dichalcogenides in active metasurfaces, and I also will give a general outlook for the wide range of possibilities for active van der Waals metasurfaces.