Fano-resonant propagating plexcitons and Rabi-splitting local plexcitons of bilayer borophene in TERS

Abstract

Optical nanocavity provides an opportunity to deeply study the light–matter interaction with notable findings such as Rabi splitting in strong coupling and Fano resonance in weak coupling. Here, we theocratically explore the plexcitons of a bilayer (BL) borophene synthesized on an Ag (1 1 1) film in a tip-enhanced Raman scattering (TERS) system, where the BL borophene is located in the nanocavity between the tip and substrate, stimulated by recent experimental synthesis [Liu et al., Nat. Mater. 21, 35 (2022)]. In the strong-coupling region, the negative real part of the dielectric function of the BL borophene manifests; the BL borophene is of plasmonic properties resulting in Rabi splitting of plexcitons with 310 meV. In the weak-coupling region, the spectra show typical asymmetry with a sharp change between a dip and a peak (Fano resonance). A balanced gain and loss facilitates single-mode lasing in the parity-time symmetry-broken regime, where single-mode lasing with a very narrow half-width is of ultrahigh enhancement factor up to 108. Fano-resonant propagating plexcitons are observed in the dip of Fano resonance, which is extremely sensitive to the excitation wavelength. Our results not only deepen the physical understanding of the plasmon–exciton coupling interaction in the TERS system but also provide a way to manipulate the light–matter interaction in the TERS system.