  Control of excitons in transition metal dichalcogenides (TMDCs) and their
heterostructures is fundamentally interesting for tailoring light-matter
interactions and exploring their potential applications in high-efficiency
optoelectronic and nonlinear photonic devices. While both intra- and interlayer
excitons in TMDCs have been heavily studied, their behavior in the quantum
tunneling regime, in which the TMDC or its heterostructure is optically excited
and concurrently serves as a tunnel junction barrier, remains unexplored. Here,
using the degree of freedom of a metallic probe in an atomic force microscope,
we investigated both intralayer and interlayer excitons dynamics in TMDC
heterobilayers via locally controlled junction current in a finely tuned
sub-nanometer tip-sample cavity. Our tip-enhanced photoluminescence
measurements reveal a significantly different exciton-quantum plasmon coupling
for intralayer and interlayer excitons due to different orientation of the
dipoles of the respective e-h pairs. Using a steady-state rate equation fit, we
extracted field gradients, radiative and nonradiative relaxation rates for
excitons in the quantum tunneling regime with and without junction current. Our
results show that tip-induced radiative (nonradiative) relaxation of intralayer
(interlayer) excitons becomes dominant in the quantum tunneling regime due to
the Purcell effect. These findings have important implications for near-field
probing of excitonic materials in the strong-coupling regime.
