Energy transfer driven brightening of MoS2 neutral exciton by ultrafast polariton relaxation in microcavity MoS2/hBN/WS2 heterostructures

Abstract Energy transfer is a ubiquitous phenomenon that delivers energy from a blue-shifted emitter to a red-shifted absorber, which has enabled plentiful photonic applications of light-emitting diodes (LEDs), lasers, solar cells, and display devices1–5. The fast-emerging two-dimensional (2D) semiconductors offer unique opportunities for exploring new energy transfer mechanisms in the atomic-scale limit enabled by confined geometry and van der Waals architectures, which transcend the conventional Förster and Dexter types. Herein, we have successfully designed and constructed a planar optical microcavity-confined MoS2/hBN/WS2 heterojunction, which realizes the strong coupling among donor exciton, acceptor exciton, and cavity photon mode for the first time. Such a configuration demonstrates the unconventional energy transfer via ultrafast polariton relaxation, leading to the brightening of MoS2 neutral exciton with a record-high enhancement factor of ~ 440, i.e., two-order-of-magnitude higher than the data reported to date. A short characteristic time of ~ 1.3 ps is extracted by setting up a high-resolution k-space transient-reflectivity spectroscopy. This ultrafast polariton relaxation is attributed to the significantly enhanced intra- and inter-branch exciton-exciton scattering to overcome the hot phonon bottleneck effect, as revealed by theoretical calculation with coupled rate equations. Our study not only opens a new direction of microcavity 2D semiconductor heterojunctions for high-brightness ultrafast polaritonic light sources, but also provides a new paradigm to study the ultrafast polariton carrier dynamics.