Phonon-coupled perfect absorption in photonic-plasmonic hybrid metasurfaces

The simultaneous spectral and spatial confinement of optical modes, governed by high-Q resonances and subwavelength field localization, is critical for optimizing light-matter interactions. However, the coupling mechanisms in non-local metasurfaces, particularly those involving quasi-bound states in the continuum (QBIC) and van der Waals (vdW) materials in vibrational strong coupling (VSC), remain unclear. To address this, we propose a photonic-plasmonic hybrid metasurface supporting photonic QBIC, localized surface plasmon resonances (LSPRs), and plasmonic QBIC mode. At a grating duty cycle of R = 0.3, the structure supports LSPRs under normal incidence, while oblique excitation simultaneously activates photonic QBIC and LSPR modes. Although the Q-factor of photonic QBICs decreases under tilted incidence, their electromagnetic fields redistribute toward the interface, emulating the spatial confinement of LSPRs. This interfacial localization, combined with the high Q-factor of photonic QBICs, enhances spectral and spatial overlap with hexagonal boron nitride (h-BN) phonon-polaritons, enabling robust VSC with coupling strengths surpassing those of LSPRs. Increasing the duty cycle to R = 0.7 transitions the system to plasmonic QBIC modes, which exhibit unconventional asymptotic degeneracy upon coupling with phonons, revealing distinct polaritonic interaction dynamics. Furthermore, angle-tunable QBIC modes achieve near-unity absorption under critical coupling conditions, facilitating dual applications as mid-infrared thermal radiation sources and surface-enhanced infrared spectroscopy platforms for biosensing.