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Plasmonic radial breathing mode (RBM), featured with radially oscillating charge density, arises from the surface plasmon waves confined in the flat nanoparticles. The zero net dipole moment endows the RBM with an extremely low radiation yet a remarkable intense local field. On the other hand, owing to the dark mode nature, the RBMs routinely escape from the optical measurements, severely preventing their applications in optoelectronics and nanophotonics. Here, we experimentally demonstrate the existence of RBM in a hexagonal Au nanoplate-on-mirror nanocavity using a far-field linear-polarized light source. The polarization-resolved scattering measurements cooperated with the full-wave simulations elucidate that the RBM originates from the standing plasmon waves residing in the Au nanoplate. Further numerical analysis shows the RBM possesses the remarkable capability of local field enhancement over the other dark modes in the same nanocavity. Moreover, the RBM is sensitive to the gap and nanoplate size of the nanocavity, providing a straightforward way to tailor the wavelength of RBM from the visible to near-infrared region. Our approach provides a facile optical path to access to the plasmonic RBMs and may open up a new route to explore the intriguing applications of RBM, including surface-enhanced Raman scattering, enhanced nonlinear effects, nanolasers, biological and chemical sensing.

Simultaneous control of plasmon–exciton and plasmon–trion couplings is fundamentally interesting for tailoring the strong light–matter interaction at the nanoscale and is intriguing for developing high-efficiency optoelectronic and nonlinear photonic devices. Here, we integrate the monolayer WS2 with the Au nanosphere to take full advantages of both the strong excitonic effect and local field enhancement effect to realize strong resonance couplings between the dipolar plasmon mode and the exciton, as well as the trion, at room temperature. Interestingly, from the dark-field scattering spectrum, a transition from the dominated plasmon–exciton coupling to the plasmon–exciton–trion coupling in the hybrid system by simply increasing the radius of the nanosphere is revealed. This evolution of the scattering spectrum is further analyzed using the coupled-oscillator model to extract Rabi splittings of 89 and 48 meV for plasmon–exciton and plasmon–trion couplings, implying that the hybrid system enters the moderate coupling region. The moderate coupling imparts the hybrid system with a remarkable light-emitting capacity, rendering 1265- and 680-fold photoluminescence (PL) enhancement for the exciton and trion emissions, respectively. Our findings provide a facile way for the manipulation of excitonic quasiparticles in semiconductors at room temperature. [ABSTRACT FROM AUTHOR]