Exciton-Enhanced Light Scattering in Atomically-Thin Metasurfaces

Nanophotonic metasurfaces employ dense arrays of optically-resonant nanostructures to manipulate the properties of light in ultra-compact optical coatings. By harnessing plasmonic or Mie resonances in metallic or dielectric nanoparticles, the phase and amplitude of the scattered light can be controlled at the nanoscale. Based on rapid advances in metasurface design, metasurfaces are now widely applied in flat optical elements for beam steering, lensing, and holography. However, novel applications in dynamic holography and augmented reality require metasurfaces and metadevices with actively-tunable functionality. So far, the use of plasmonic and Mie-resonances in active metasurfaces is limited as their optical resonances are difficult to tune dynamically.<br/><br/>Monolayer transition metal dichalcogenides, such as WS₂ , exhibit strong exciton resonances in the visible spectral range that dominate their optical response. The excitonic light-matter interaction in these 2D quantum materials is inherently very strong and highly tunable, which can be leveraged to realize mutable flat optical elements. To unleash the full potential exciton-enhanced light scattering in atomically-thin metasurface elements, it is essential to first achieve detailed understanding of the role of the exciton’s quantum mechanical properties in passive nanophotonic wavefront shaping.<br/><br/>Here, we employ atomically-thin metasurface lenses carved out of a monolayer of WS₂ to directly study the influence of exciton decay and dephasing on the metasurface functionality and spectral line shape. We fabricate 500 μm diameter zone plate lenses using electron-beam lithography and reactive-ion etching. At resonance, excitonic light scattering strongly enhances the focal intensity. We systematically characterize the focal shape and focusing efficiency as a function of wavelength using confocal microscopy. To study the influence of exciton-phonon scattering and dephasing on the optical functionality of the lens, we then characterize the efficiency spectrum as a function of temperature.<br/><br/>At ambient conditions, the spectrum shows a strong asymmetric line shape revealing that the scattered light fields are directly governed by the monolayer susceptibility. This enables an almost background-free measurement of the optical properties of the monolayer, and thereby the excitonic light-matter interaction. Careful analysis of the line shape shows that the relative contribution from resonant excitonic light scattering is comparable to the non-resonant monolayer scattering. For decreasing temperatures on the other hand, the exciton energy shows a blue-shift, non-radiative decay and dephasing are suppressed, and the exciton becomes fully radiative. As a result, the asymmetric line shape not only narrows and increases in amplitude, but also transitions into a more symmetric line shape. This directly shows the increasing prevalence of the exciton resonance in the focusing efficiency. By comparing the results to numerical simulations and an analytical model, we show that the efficiency of the metasurface lens directly scales with the excitonic oscillator strength.<br/><br/>The results give direct insight in the role of exciton dynamics in optical wavefront shaping using atomically-thin metasurfaces. A full understanding of the role of exciton resonances in metasurfaces paves the way for dynamic components, combining tunable effects in quantum materials with classical metasurface optics.