Let’s Twist Again: High-Quality-Factor Metasurfaces to Enhance Spin In Molecules and Monolayer Materials

Many inversion-asymmetric materials, including chiral molecules and certain van der Waals materials, exhibit a differential absorption of left and right circularly polarized light that is nearly five orders of magnitude less than their absorption of unpolarized light. Such weak differential absorption challenges applications such as single molecule circular dichroism spectroscopy, all-optical enantio-specific synthesis, and efficient valleytronic data encoding for quantum information. Here, I describe approaches to enhance helicity-dependent optical absorption, emission, and carrier relaxation in molecules and monolayered materials. We rely on high-quality-factor (high-Q) metasurfaces, which, when placed in the near-field of a molecular or monolayer sample, precisely control the amplitude, phase, and polarization of light. Each metasurface enables substantial and uniform-sign enhancements of both the electric and magnetic fields of light, with local photon spin fields that can be enhanced by several orders of magnitude.<br/><br/>First, we show how high-Q metasurfaces enable circular dichroism from a molecular monolayer. We fabricate silicon metasurfaces and functionalize them with short, ~10-mer DNA oligonucleotides. We measure their circular dichroism in a home-built table-top polarization sensitive spectrometer, and also show how this technique is sensitive to changes in the CD handedness through double- stranded to single-stranded DNA denaturing. Next, we show metasurface designs that can enable high-yield enantioselective photochemistry at visible and ultraviolet wavelengths. By overlapping the metasurface optical resonances with the chiral molecular resonance, we project a 2000-fold improvement in the yield of a photoionization reaction. Finally, we extend this platform for valleytronic applications with two-dimensional transition metal dichalcogenides (TMDCs). We integrate TMDC monolayers with high-Q metasurfaces to improve and control valley-specific absorption and emission, to realize solid-state, optically addressable spin qubits. Through the coupling to the metasurface modes, the degree of polarization of exciton and trion emission from each valley can be enhanced, even up to 190K. Combining Si-compatible photonic design with molecular and 2D materials integration, our work makes an important step toward all-optical enantioselective sensing and separation as well as on-chip quantum optical information systems approaching room-temperature operation.