White-light driven photoluminescence from a few nanoplatelets in a plasmonic nanogap

Abstract Quantum emitters (QE) weakly coupled to plasmonic cavities are known to exhibit scattering spectra that are largely indistinguishable from those in the absence of the emitters themselves. Here, we report a strong fluorescence peak visible in dark-field spectra of gap plasmons. This emission arises under incoherent white-light excitation from a few emitters in a monolayer of nanoplatelets (NPLs) within nanoparticle-on-mirror (NPoM) cavities. Single-NPoM spectra reveal an intense 650 nm peak from a few NPLs, matching their fluorescence and confirmed as emission by quenching and detuning controls. Simulations resolve the dipole mismatch of face-down NPLs: the bright cavity mode feeds dark modes whose in-plane corner fields drive the excitons. The 650 nm peak grows linearly with excitation power, as photoluminescence does, and at nearly twice the rate of scattering. The NPoM chip appears red to the naked eye under focused sunlight, at irradiances about seven orders of magnitude lower than conventional focused-laser excitation. Dark-field spectra of emitter-loaded nanocavities carry both scattering and emission, enabling laser-free few-emitter spectroscopy and sunlight-driven nanophotonics.