Freestanding Polymer Metasurface Supporting Higher‐Order Optical Resonances for Strong Field Enhancement in TMD Monolayers

ABSTRACT 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 (WS ₂ ) 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 WS ₂ 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.