Extraordinary Fluorescence from Few Nanoplatelets in Gap Plasmons

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 the presence of strong fluorescence peaks visible in the scattering spectra of gap plasmons. This extraordinary fluorescence (EF) arises under incoherent white-light excitation from a few emitters in a monolayer of 2D semiconductor nanoplatelets (NPLs) within nanoparticle-on-mirror (NPoM) cavities. Broadband white light excites the bright plasmonic mode of the cavity, near field coupling generates hybrid in plane fields that match the NPL exciton dipoles thus yielding strongly enhanced emission within a deep sub wavelength volume-despite the incoherent illumination, a regime where fluorescence is typically observed only in photoluminescence measurements with coherent excitation. Strikingly, dark-field scattering from single NPoMs reveals an extraordinary, intense, and narrow peak at 650 nm, spectrally aligned with the fluorescence of the NPLs. Simulations suggest that this EF arises from the in-plane field enhancement at the Ag nanocube (Ag NC) corners, aligning with the dipole moment of the NPLs to enhance spontaneous emission. This 650 nm emission exhibits a nonlinear increase with incident power of white light, a clear departure from linear plasmonic scattering, further supporting the fluorescent nature of this scattering peak. This EF is sufficiently strong that the NPL-coated NPoM arrays are visibly red to the naked eye under focused sunlight excitation. These findings unveil a pathway to harness few-emitter fluorescence with incoherent excitation, opening avenues for surface coatings with unique visual effects, and solar-driven advanced chip-scale photonic devices, including single-photon sources, light emitters, lasers and sensors.