Active Excitonic 2D Metasurfaces Using Exciton Polaritons in Monolayer TMDs

Monolayer transition metal dichalcogenides (TMDs) like WS₂ exhibit strong exciton resonances in the visible spectral range that govern their optical response. The excitonic light-matter interaction in these 2D quantum materials is inherently strong and highly tunable, which can be leveraged to realize mutable flat optical elements as well as novel spin-valley coupled information carriers. To unleash the full potential of exciton-enhanced wavefront shaping and active optical switching in atomically thin metasurfaces, it is essential to gain a detailed understanding of the exciton’s quantum mechanical properties and its coupling to hybrid light-matter quasiparticles known as exciton polaritons.<br/><br/>Recently, it was shown theoretically that exciton polaritons can exist as excitations from the continuum of three-dimensional electromagnetic modes even for atomically thin layers of materials. Coupling to such 2D exciton polaritons (2DEPs) was subsequently demonstrated experimentally in a free-standing monolayer membrane via photoluminescence excitation and outcoupling through a photonic crystal. Realizing such a strongly coupled light-matter state in on-chip photonic environments promises new opportunities for tunable wavefront shaping, photonic sensing, as well as probing of fundamental quantum-mechanical properties of light scattering by excitons.<br/>However, direct integration of coherently coupled 2DEP functionality into nanopatterned monolayers on substrates has so far remained elusive due to the stringent requirements on the dielectric environment as well as monolayer quality to support 2DEPs.<br/><br/>Here, we experimentally demonstrate active control of coherent coupling to 2DEPs in a monolayer of WS₂ on a quartz substrate, allowing for enhanced photonic functionality given directly by the geometry of the monolayer itself. We accomplish guiding along the monolayer by managing the refractive index contrast between the top and bottom side via a 230nm thick capping layer of PMMA. Using guided mode resonances in sub-wavelength gratings structured in mm-sized continuous WS₂ monolayers, we realize polarization-selective dynamic phase control of light scattered coherently off the hybrid light-matter state. The sub-wavelength nature of the grating together with the high quality and homogeneity of the initial monolayer leads to an electrically and/or thermally tunable phase modulation of the reflected light when exciting close to the 2DEP guided mode resonance.<br/><br/>Further utilization of photonic metasurface concepts allows for active amplitude switching of higher diffraction orders via binary blazed gratings, leading to expected modulation depths exceeding 80% stemming from an atomically thin optical element. This opens a path to full active control over the complex optical response in tailored atomically thin metasurfaces via exciton resonance tuning.