Hot carrier optoelectronic thermometry in van der Waals heterostructures.

Rapid cooling of hot carriers in high-mobility graphene heterostructures may expose emergent cooling regimes, yet probing the electronic temperature of pristine, charge-neutral graphene remains challenging. Here, we describe interlayer optoelectronic thermometry, which we developed to directly probe the Dirac excited state in the uniform two-dimensional plane of charge neutral, high-mobility graphene. This excitation scheme allows us to generate a photocurrent without the need for a finite charge density or an in-plane p-n junction, which would influence the energy relaxation pathways. After photoexcitation by short optical pulses, electrons and holes form a thermal distribution through graphene's rapid thermalization processes, prior to coming to equilibrium with the crystal lattice. By introducing an energy barrier for out-of-plane charge transport - imparted by a graphene/hBN interface - hot charge carriers at the top of the thermal distribution are filtered out, giving rise to strong interlayer photo-thermionic current. This interlayer photocurrent, which is exponentially sensitive to the number of charge carriers in the high-energy tail of the distribution, provides a high-resolution probe of the excited state temperature within the spatially uniform graphene layer. Our novel method opens new avenues for ultrafast control of hot carriers in pristine 2D layers, and easily extends to correlated electronic phases in van der Waals heterostructures.