Spectral focusing of broadband silver electroluminescence in nanoscopic FRET-LEDs
Few inventions have shaped the world like the incandescent bulb. While Edison used thermal radiation from ohmically heated conductors, some noble metals exhibit "cold" electroluminescence (EL) in percolation films, tunnel diodes, electromigrated nanoparticle aggregates, optical antennae, or scanning-tunnelling microscopy (STM). The origin of this radiation, which is spectrally broad and depends on applied bias, is controversial given the low radiative yields of electronic transitions. Nanoparticle EL is particularly intriguing since it involves localized surface-plasmon resonances with large dipole moments. Such plasmons enable very efficient non-radiative fluorescence resonance energy transfer (FRET) coupling to proximal resonant dipole transitions. We demonstrate nanoscopic FRET-LEDs which exploit the opposite process, energy transfer from silver nanoparticles to exfoliated monolayers of transition-metal dichalcogenides (TMDCs). In diffraction-limited hotspots showing pronounced photon bunching, broadband silver EL is focused into the narrow excitonic resonance of the atomically thin overlayer. Such devices may offer alternatives to conventional nano-LEDs in on-chip optical interconnects.