Origin of interlayer exciton-phonon coupling in 2D heterostructures

Excitons, bound electron-hole pairs, feature prominently in the optical spectra of two-dimensional (2D) materials and play a key role in optoelectronic applications. They can couple with phonons, as evidenced by their imprint on optical scattering experiments. Recent experiments on 2D heterostructures [1] have demonstrated that excitons in one layer can couple with phonons of the neighboring layer. This remarkable coupling has been used to study Raman-inactive phonons of neighboring layers and phonon polaritons of large bandgap substrates. Despite its experimental confirmation in numerous 2D heterostructures, the mechanism responsible for this phenomenon remains a mystery. In my talk, using a monolayer WSe2@hBN heterostructure as a prototype system and resonant Raman spectroscopy as a tool, I will discuss the microscopic mechanism that leads to the coupling of excitons and phonons across the layers. I will show the fundamental role of symmetries in interlayer exciton-phonon coupling, which leads to anomalous resonant Raman intensities of the hBN phonon modes in the WSe2@hBN heterostructure: the normally silent out-of-plane optical phonon mode of hBN is three orders of magnitude more intense than the in-plane Raman-active mode of hBN when in resonance with the WSe2 excitonic states. Our findings reveal that the deformation potential induced by the hBN phonons scatters the hole of the WSe2 excitons at the hBN interface, thereby giving rise to interlayer exciton-phonon coupling