Light-Matter Coupling with Bound States in The Continuum in Van der Waals Metasurfaces

The extraordinary properties of layered van der Waals (vdW) materials, like hexagonal boron nitride (hBN) and Transition Metal Dichalcogenides (TMDCs), make them an appealing platform to investigate and engineer light-matter interactions at the nanoscale. When reduced to atomically thin monolayers, they display attractive characteristics, such as strongly bound excitons and optically addressable spin defects, while in their bulk form they exhibit significant optical anisotropy and possess high refractive index values greater than 4, surpassing common semiconductor materials. This makes vdW materials highly desirable for achieving low-loss optical resonances and for vertically stacking different materials for developing novel all-dielectric nanophotonic structures.<br/>Here, we exploit the concept of quasi-bound states in the continuum (qBIC) to create high-quality optical resonances in dielectric metasurfaces made of hBN and TMDCs. Our method is entirely monolithic, using only van der Waals materials, and achieves optical resonances with Q factors exceeding 10² through a two-step fabrication process. We demonstrate spectral tuning across the entire visible spectrum in hBN qBIC metasurfaces [1], and enhance light-matter coupling with intrinsic spin defects in hBN [2]. We observe a remarkable 25-fold increase in photoluminescence intensity and spectral narrowing of defect emissions, with a linewidth below 4 nm full width at half-maximum. Furthermore, our platform offers exciting possibilities for strong light-matter coupling, as seen in the distinct anti-crossing behavior between qBIC resonances and intrinsic excitons in monolithic TMDC WS₂ metasurfaces, achieving Rabi splitting values up to 116 meV under ambient conditions [3]. Our results illustrate how combining qBIC photonic metasurfaces with van der Waals materials paves the way for novel hybrid nanophotonic platforms and room temperature polaritonic devices.<br/><br/><br/>[1] L. Kühner, L. Sortino, <i>et al.</i> “High-Q Nanophotonics Over the Full Visible Spectrum Enabled by Hexagonal Boron Nitride Metasurfaces” <i>Advanced Materials</i> 35, 13 (2023)<br/>[2] L. Sortino, <i>et al.</i> “Optically addressable spin defects coupled to bound states in the continuum metasurfaces” arXiv:2306.05735 (2023)<br/>[3] T. Weber, L. Kühner, L. Sortino, <i>et al.</i> “Intrinsic strong light-matter coupling with self-hybridized bound states in the continuum in van der Waals metasurfaces” <i>Nature Materials</i> (2023)