Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal-insulator-semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂ ). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS₂ , which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS₂ , followed by formation and radiative recombination of excitons in WS₂ . The photocarrier transfer rate can be described by Fowler-Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.