Freestanding Polymer Metasurface Supporting Higher-Order Optical Resonances for Strong Field Enhancement in TMD...

Enhancing light-matter coupling in two-dimensional (2D) semiconductors, such as transition metal dichalcogenide monolayers, remains a central challenge in nanophotonics due to their atomic thickness, which limits their interaction volume with light. Here, we demonstrate that higher-order optical resonances, including photonic guided modes (GMs) and quasi-bound states in the continuum (quasi-BICs) supported by a freestanding metasurface, provide exceptionally strong surface field enhancement, enabling efficient coupling with a tungsten disulfide (WS₂) monolayer. Triangular-lattice polymer patterns on silicon nitride membranes are fabricated to realize these higher-order modes. Simulations reveal that second-order modes possess optimal surface electric-field distributions that strongly overlap with the overlying WS₂ monolayer, significantly outperforming their first-order counterparts. Photoluminescence (PL) measurements confirm a remarkable PL enhancement factor of 193 for the second-order GM, over an order of magnitude greater than that of the first-order modes. These results establish higher-order modes in freestanding metasurfaces as a promising route to engineer light-matter interactions in 2D semiconductors for advanced nanophotonic and quantum photonic applications.