Room Temperature Mie-Polaritons in a Monolayer WSe₂ Coupled to van der Waals Nanophotonic Structures

Dielectric nanostructures exhibit low optical losses and a wealth of distinct Mie resonances in contrast to their plasmonic counterparts(<i>1, 2</i>). Meanwhile, the strong light−matter coupling has attracted long-standing interest due to its fundamental importance in Bose-Einstein condensation, polariton lasing, and potential in quantum optics applications(<i>3</i>). However, strong coupling with relatively broad Mie resonances has been rarely observed. Here, we employ relatively narrow Mie resonances in high-refractive-index WS₂ nanoantennas placed on gold, and demonstrate strong exciton-photon coupling (Mie-polaritons) in monolayer WSe₂ exhibiting room temperature stable excitons with a large oscillator strength.<br/>To achieve the Mie-polaritons, we fabricate nanoantennas by etching thin film WS₂ (30-nm thickness) that is exfoliated on a gold substrate, and then stack a monolayer WSe₂ onto them by using a dry transfer method. We use nanoantennas with radii (<i>r</i>) from 83 to 155 nm, which allows gradual tuning of the Mie resonance through the WSe₂ exciton energy. Thanks to the high refractive index of WS₂ and the gold substrate, Mie resonances with a narrow bandwidth of <i>Γ_M</i>≈ 105 meV are achieved in WS₂ nanoantennas, suitable for observation of the strong coupling with WSe₂ excitons. The Mie resonances in the hybrid monolayer WSe₂/nanoantenna structures undergo significant changes compared with bare WS₂ structures. Typical anti-crossing behaviour is observed using dark-field scattering spectroscopy. The energy splitting between the upper (UBP) and low (LPB) polariton branches is fitted as <i>Γ</i>_p~86 meV, which is higher than the value reported in plasmonic systems (~50 meV)(<i>4</i>). Considering the linewidth of Mie resonance and neutral exciton (<i>Γ</i>₀ ≈ 38 meV), the strong coupling condition is thus satisfied at room temperature. Our results provide a promising platform for the realisation of the strong coupling in nanophotonic structures and controlled light emission in nanophotonics(<i>5-7</i>). Furthermore, one of the keys to our results is the use of hybrid dielectric/metal structures, achievable in a straightforward fashion with layered van der Waals materials.<br/><br/><b>Reference:</b><br/>1. A. I. Kuznetsov <i>et al</i>, <i>Science</i> <b>354</b>, aag2472 (2016).<br/>2. L. Sortino<i> et al.</i>, <i>Nature communications</i> <b>10</b>, 5119 (2019).<br/>3. S. Dufferwiel<i> et al.</i>, <i>Nature Photonics</i> <b>11</b>, 497-501 (2017).<br/>4. D. Zheng<i> et al.</i>, <i>Nano letters</i> <b>17</b>, 3809-3814 (2017).<br/>5. L. Sortino<i> et al.</i>, <i>Nature communications</i> <b>12</b>, 6063 (2021).<br/>6. H. Ling, R. Li, A. R. Davoyan, <i>ACS Photonics</i> <b>8</b>, 721-730 (2021).<br/>7. P. G. Zotev<i> et al.</i>, <i>ACS Nano</i> doi.org/10.1021/acsnano.2c00802, (2022).