Nanocavity clock spectroscopy: resolving competing exciton dynamics in WSe₂/MoSe₂ heterobilayers

Long lived interlayer excitons in transition metal dichalcogenide heterobilayers hold promise for applications from high temperature exciton condensates to nano-lasers with extended spatial coherence and other 2D optoelectronic devices. However, their exciton dynamics are difficult to disentangle due to multiple competing processes with timescales varying over many orders of magnitude. Using a configurable nano-optical cavity based on a plasmonic scanning probe tip, we can control the radiative and nonradiative relaxation of intra- and interlayer excitons. Tuning their relative rates in a WSe₂/MoSe₂ heterobilayer over six orders of magnitude in tip-enhanced photoluminescence spectroscopy (TEPL) reveals cavity-induced crossover from nonradiative quenching to Purcell-enhanced radiation. Rate equation modeling with the interlayer charge transfer time as a reference clock allows for comprehensive determination from the long interlayer exciton (IX) radiative lifetime t_IX^rad = (94+/-27) ns to the five orders of magnitude faster competing nonradiative lifetime t_IX^nrad=(0.6+/-0.2) ps. This approach of nanocavity clock spectroscopy is generally applicable to a wide range of excitonic systems with competing decay pathways.