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

Abstract

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.

Strong Coupling of Plasmonic Nanorods with a MoSe2 Monolayer in the Near-Infrared Shortwave Region

“…The COM treats the plasmonic and exciton modes as coupled damped harmonic oscillators driven by an external electric field. With this model, the mode coupling is described by ( true ω normalpl − i γ normalpl 2 g g ω normalex − i γ normalex 2 ) ( true α β ) = ω ± ( true α β ) where ω pl and ω ex are the frequencies of the uncoupled plasma and exciton resonances, respectively; γ pl and γ ex are their decay rates (i.e., damping rates, resulting in full width at half-maximums of γ pl and γ ex ); g is the coupling strength; α and β are the relative weights of the resonances of the plasma and exciton, respectively, in the hybrid mode, with |α| 2 + |β| 2 = 1. The diagonalization of the matrix above yields two eigenvalues: ω ± = ω pl + ω ex 2 − i 2 false( γ pl + γ ex false) ± g 2 + 1 4 false[ δ − i false( γ pl − γ ex false) ] 2 with δ = ω pl – ω ex representing the detuning frequency.…”

“…According to previous studies, the ratio of the quality factor Q to the mode volume V indicates the strength of the light–matter coupling . Therefore, the coupling constant g ∼ Q / V and quality factor ( Q = ω pl /γ pl ) describe how long a photon can be confined within the cavity, and they are calculated from the spectral width γ ex and resonant frequency of the cavity ω pl . The second parameter is the effective field localization V eff that characterizes the confinement of the cavity mode.…”

“…The COM treats the plasmonic and exciton modes as coupled damped harmonic oscillators driven by an external electric field. With this model, the mode coupling is described by 41 i k j j j j j j j j j j j j j j y { z z z z z z z z z z z z z z i k j j j j y { z z z z i k j j j j y…”

Modes Trimming and Clustering in a Weakly Perturbed High‐Q Whispering Gallery Microresonator

“…Experimentally, complete polygon and star modes sequences are observed within the single microresonator, showing excellent consistency with the theoretical results. Besides the use of the MNF as the weak perturbation, it is also possible to use periodically arranged nanopartibles [31,32] for the construction of the polygon and star mode. The weakly perturbed microcavity system demonstrated here may open new opportunities for the majority of research and application ranging from the WGM-based nonlinear photonics, [22,23] to the active selection of lasing modes.…”

Shaping the Plasmonic Response: Hollow Gold Elliptical Nanotubes

“…The most common mechanism to achieve required LSP resonances is assembling several basic nanoparticles together where the near-field coupling results in hybridized or emerging plasmonic modes together with dramatic light confinement. [20] For instance, artificial magnetisms at optical frequencies are observed in ring-shaped plasmonic metamolecules with broken symmetry. [21,22] The interference between a discrete dark state and a broad bright continuum produces plasmonic Fano resonances with a sharp asymmetric Fano line shape in the spectral domain.…”