Quasi-Trapped Guided Mode in a Metasurface Waveguide for Independent Control of Multiple Nonlocal Modes

Most of the nanophotonic devices such as metasurfaces control light by coupling external waves to the eigenmodes of the nanostructure. Notably, nonlocal modes have garnered considerable attention owing to their exceptional high-quality factor characteristics. Specifically, employing multiple nonlocal modes provides the unique benefit of broadening the detection range for a variety of sensing applications. However, the simultaneous alteration of these modes by altering the geometric parameters impedes the ability to execute distinct functions across each spectrum. Thus, the necessity of disentangling the interdependence of these modes and regulating them individually is underscored. In this work, we demonstrated the independent regulation of multiple nonlocal modes by introducing an eigenmode termed a quasi-trapped guided mode (QTGM). We explore the characteristics of the QTGM within an integrated structure of a metasurface manifesting a quasi-trapped mode (QTM) and a waveguide capable of exhibiting a leaky guided mode (LGM). QTGM inherits the characteristics of both QTM and LGM, rendering it especially responsive to variations in the in-plane symmetry of the metasurface notwithstanding its confinement within the waveguide slab. Conversely, LGM is confined within the waveguide slab and remains unresponsive to such perturbations, thereby enabling the exclusive alteration of QTGM. Additionally, we demonstrated that this methodology permits the tuning of the Rabi-splitting and quality factor of Friedrich–Wintgen bound states in the continuum. The proposed approach offers significant potential for a wide array of applications that leverage multiple nonlocal modes, including biomolecular sensing, multispectral filtering, and multiphoton nonlinear processes.