Optoelectronics of Graphene-based Van der Waals Heterostructures

The photoresponse of materials is governed by energy relaxation pathways of photo-excited electron-hole pairs. In graphene, due to frequent electron-electron collision and weak electron-lattice coupling, a novel transport regime is reached in which the photo-generated carrier population can remain hot while the lattice stays cool. In this talk, I will show that light is converted to electrical currents through a hot-carrier assisted thermoelectric effect in intrinsic graphene. The thermal energy slowly leaks to the lattice via two distinct processes of electron-phonon coupling that can be tuned by temperature and charge density. We also implemented a scheme to control the early behavior of photo-excited carriers before they collide with each other and ambient carriers to form a hot Fermi-Dirac distribution, which is realized in a graphene-boron nitride-graphene heterostructure. The weak electron-phonon coupling and frequent electron-electron scattering revealed above strongly alter the nature of particle and energy transport, leading to a collision dominant fluid behavior for electrons and unconventional thermal transport at the charge neutrality. In the last part of the talk, I will discuss our observation of highly-ordered photocurrent patterns at the charge neutral point of graphene, which may indicate a new regime of charge and thermal dynamics.