Spectrally and Spatially Selective Brightening of Interlayer Excitons in TMDC vdW Heterostructures with Large-quality-factor, Small-mode-volume Magnetic Plasmon Resonance

Interlayer excitons in van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) have received significant attention because of their great potential in quantum photonics, such as moiré potential trapped single-photon emitters and high spatial-coherence exciton lasers. However, radiative recombination of such peculiar excitons is fundamentally limited by their spatially separated electron–hole pairs in two different layers. A recent study shows that this physical constraint can be partially lifted by coupling a vdW heterostructure to a gold nanotip-on-mirror cavity, leading to Purcell-enhanced nonresonant emission of interlayer excitons. Yet, this broadband enhancement approach is bottlenecked by three unfavorable characteristics: (1) simultaneous radiation enhancement of both intralayer and interlayer excitons, depleting the formation of interlayer excitons; (2) the nonradiative decay of interlayer excitons is 5 orders of magnitude faster than their radiative decay, implying a quite low quantum yield; and (3) relying on extreme control of the nanometer-scale tip-mirror distance.To address these issues, the present proposal aims to theoretically and experimentally investigate selective brightening of interlayer excitons with an ultra-compact metal nanocubeon- mirror (NCoM) cavity sandwiching a TMDC vdW heterostructure. Compared to the broadband yet limited Purcell enhancement of the above-mentioned nanotip-on-mirror cavity, the high-Q (~25) magnetic dipole (MD) mode of NCoM allows for spectrally-selective, onresonance Purcell enhancement of interlayer excitons, being off-resonant with intralayer excitons; the small mode volume of MD (~104 nm3) enables spatially selective interrogation of photo-excited interlayer excitons within the cavity, avoiding unwanted contribution from the surrounding heterostructure region. Furthermore, tuning the MD resonance can study the scaling law of plasmonic enhancement with the TMDC-mirror/cube distance and explore cavity-induced crossover from non-radiative quenching to Purcell radiation enhancement, better understanding the formation and radiation mechanisms of interlayer excitons. In addition, combining coupled rate equation modeling and temperature-dependent, timeresolved photoluminescence spectroscopy of single NCoM-heterostructure cavities can disclose the correlation of intralayer and interlayer excitons under thermodynamic equilibrium. Finally, a full quantum model will provide microscopic understanding of cavityexciton interaction and in turn reveal the cavity properties.Overall, this project will develop a new plasmonic paradigm for manipulating radiative and non-radiative relaxation of interlayer excitons, including competing decay pathways and relaxation dynamics. Results of the project could pave the way for further investigating the elusive dynamics of multi-excitonic systems within the framework of plasmonic nanocavity quantum electrodynamics and also for exploiting TMDC vdW heterostructures to build highperformance ultra-compact nanophotonic and optoelectronic devices such as excitonic lasers and light-emitting diodes.