Ultrahigh-Q plasmonic-photonic resonances with subwavelength confinement enabled by bound states in the continuum

Plasmonics, featured with enhanced light-matter interaction and strong wave localization, have been intensively studied theoretically and experimentally in the past few decades. However, the intrinsic ohmic loss in plasmonic-based devices is a challenging issue that limits the performance of such configurations, hindering their application potential. Herein, we propose a simple and effective strategy to achieve optical resonances with ultrahigh quality (Q) factors (∼ 10⁴) and subwavelength confinement (∼ λ²/20) in hybrid plasmonic-photonic structures. The proposed design consists of a silicon superstrate and silver substrate which are separated by a thin silica spacer, forming a composite waveguide layer that supports hybrid plasmonic-photonic guided modes (GMs) with ultrasmall mode area. By patterning low-index polymer gratings onto the stacked films, the obtained GMs can be effectively turned into guided mode resonances with ultrahigh Q factors. Leveraging this strategy, ultrasmall mode area far beyond the diffraction limit can be realized without sacrificing Q factors. Such findings provide a new paradigm to design high-performance plasmonic elements, which may show superiority in nonlinear optics, lasers, and sensors.