Multi-harmonic phonon assisted upconversion in van der Waals heterostructure p-n junction photodidodes

In atomically thin van der Waals heterostructures, the interaction between electrons and phonons is highly restricted compared to bulk materials, suggesting that precise stacking of atomic layers could be used to manipulate vibrational coupling to photoexcited electron-hole pairs. Here, we report on phonon assisted anti-stokes absorption near the interlayer exciton edge of vdW semiconductor heterostructures composed of 2L-WSe₂and MoSe₂. By carefully tuning the chemical potential of the heterojunction, we find highly rectifying I-Vcharacteristics reminiscent of conventional diode behavior. Using advanced photocurrent spectroscopy measurements, we observe a strong photocurrent peak at near-infrared photon energies with several low energy echoes spaced by 30 meV below this feature. These unusual features occur only when device is precisely tuned to the charge neutrality point. We attribute this behavior to multi-harmonic phonon assisted upconversion, a process by which multiple phonons contribute kinetic energy to the electronic ground state to produce an excited state electron-hole pair. It marks the first observation of strong resonant coupling in a semiconductor system and may herald a new generation of precision optical experiments on laser cooling of vdW heterostructures.