Tale of Two Resonances: Waveguide–Plasmon Coupling and High Q-Factor Engineering on the Nanoscale

Localized surface plasmon (LSP) excitations provide an efficient strategy for advancing nanophotonic designs and applications where strong field enhancement and confinement are often required on the nanoscale. They represent an important plasmonic paradigm for achieving strong light–matter interactions in both linear and nonlinear regimes, enabling the development of high-performance chemical and biological sensing approaches and nonlinear optics with low light intensities. However, the LSP resonance line width, limited by both radiative and resistive losses of metallic nanostructures, is significantly larger than the line width of the waveguided modes supported by low-loss dielectric microcavities with a significantly lower field confinement. Hybrid microcavity–plasmonic systems are, therefore, often used to reduce the resonant line width which improves the detection spectral resolution while maintaining strong confinement. Employing the remarkable quality factors of whispering gallery mode (WGM) microresonators, the hybrid LSP-WGM systems demonstrate sensing capabilities down to the single-molecule level. In this Perspective, we review the recent advances in the hybridization of LSPs and WGMs, focusing on the fundamental understanding of the underlying coupling mechanisms and corresponding mode hybridization regimes. We further discuss opportunities for applying heterogeneous plasmonic–photonic integration to tailor the nanoscale light–matter interactions and realize novel waveguide–plasmon coupling based nontrivial responses and highlight their prospective applications in quantum optics, chiral spin-optics, nonlinear nanophotonics, and sensing.