Multiple vibrational resonances and Franck-Condon palisades in the low temperature photocurrent spectrum of interlayer excitons

Engineering the interactions between atomic motion and excitons is challenging since the interaction strength is fixed by atomic-scale configuration and electronic structure. Yet, current advances in stack-engineering of atomically thin vdw crystals allow careful tuning of both atomic and electronic structure. Here we report the emergence of strong exciton-phonon coupling in an atomically thin heterostructure composed of WSe₂ and MoSe₂. Strong coupling manifests as numerous photocurrent sidebands that form a palisade of multiple vibrational excited state resonances above the lowest excited state absorption feature. This can be understood through strong coupling between vibrations of the atomic lattice and interlayer excitons, giving rise to Frank-Condon transitions to multiple phonon states. Discretely spaced peaks with a period of 30 meV in the photocurrent spectra match the energy of the vibrational modes and are observed only when the device is finely tuned to charge neutrality, the condition at which electrons and holes across the interface are fully compensated. Strong vibrational coupling and multiple sidebands in the e-h pair generation process are hallmarks of localized excitons and may indicate self-trapping of excitons through strong interactions with the phonon field.