Direct observation and control of near-field radiative energy transfer in a natural hyperbolic material

Heat control is a key issue in nano-electronics, where new efficient energy transfer mechanisms are highly sought after. In this respect, there is indirect evidence that high-mobility hexagonal boron nitride (hBN)-encapsulated graphene exhibits hyperbolic out-of-plane radiative energy transfer when driven out-of-equilibrium. Here we directly observe radiative energy transfer due to the hyperbolic phonon polaritons modes of the hBN encapsulant in intrinsic graphene devices under large bias, using mid-infrared spectroscopy and pyrometry. By using different hBN crystals of varied crystalline quality, we engineer the energy transfer efficiency, a key asset for compact thermal management of electronic circuits.Comment: 21 pages including Supplementary Material (Main text: 10 pages, 4 figures