Ultrathin photonic–plasmonic membranes supporting high-Q hybrid bound states in the continuum

Plasmonic bound states in the continuum (BICs) offer a promising route to overcome the intrinsic trade-off between field confinement and radiation loss in nanophotonic systems. By exploiting the coupling of plasmonic and photonic systems, plasmonic BICs can achieve sharp spectral selectivity with strong near-field enhancement, making them attractive for compact and efficient optical devices. However, realizing high-Q resonances in plasmonic structures remains challenging due to Ohmic losses and the requirement of relatively thick metallic films. Here, we numerically demonstrate an ultrathin freestanding photonic – plasmonic membrane that supports hybrid guided and quasi-BIC resonances with record-high Q-factors in plasmonic architectures. By integrating a thin Ag layer (10-20 nm) with a dielectric SiN membrane, the hybrid coupling suppresses radiative leakage while preserving deep-subwavelength confinement, yielding Q-factors exceeding 2370. Remarkably, even when the total thickness of the photonic-plasmonic membrane is reduced to approximately 30 nm, the resonance maintains Q > 1300 with strong near-field intensity, a performance regime that cannot be achieved in metal-only plasmonic systems. The demonstrated strategy resolves the long-standing trade-off between near-field localization and radiative loss in plasmonics by leveraging hybrid BIC formation in an ultrathin membrane platform. Because the architecture is compatible with standard thin-film processing and operates with metal layers only a few tens of nanometers thick, it provides a practical route to compact, high-performance components for nanophotonic sensing, surface-enhanced spectroscopy, quantum emitter coupling, nonlinear optics, and subwavelength optoelectronics.