Photoluminescence enhancement of monolayer MoSe2 in a plasmonic nanocavity with Fano resonance and mode matching

Two-dimensional transition metal dichalcogenides (TMDs) exhibit remarkable optical properties. However, their applications in electronics and photonics are severely limited by the intrinsically low absorption and emission rates. Due to the strong local-field enhancement effect, plasmonic nanostructures are regarded as the ideal platform to enhance the photoluminescence (PL) of TMDs. To obtain a giant PL intensity, it is indispensable to apply plasmonic nanocavity with multiple resonances to simultaneously enhance the absorption at the excitation wavelength and boost the radiative rate at the emission wavelength. However, few works take the advantages of the multiple resonances in the nanocavity to augment both the PL absorption and emission processes. Here, we propose a silver (Ag) nanowire-on-mirror (NWoM) nanocavity and demonstrate the PL enhancement of monolayer MoSe2 using the multiple resonances of the nanocavity. By carefully designing the NWoM structure, we observe the Fano resonance resulted from the coherent interaction between the discrete exciton state of monolayer MoSe2 and the broadband plasmon mode. The Fano resonance, as a characteristic of the moderated coupling between plasmon and exciton, shows a remarkable ability of boosting the emission rate of the hybrid system utmost. Meanwhile, we align another resonance of NWoM nanocavity at the excitation wavelength to enhance the absorption of monolayer MoSe2. This good spectral overlap is accompanied with an excellent spatial overlap between the distributions of excitation and emission enhancement within the nanocavity that allows to observe an over 1800-fold enhancement of the PL intensity.