  Van der Waals heterostructures have emerged as promising building blocks that
offer access to new physics, novel device functionalities, and superior
electrical and optoelectronic properties. Applications such as thermal
management, photodetection, light emission, data communication, high-speed
electronics and light harvesting require a thorough understanding of
(nanoscale) heat flow. Here, using time-resolved photocurrent measurements we
identify an efficient out-of-plane energy transfer channel, where charge
carriers in graphene couple to hyperbolic phonon polaritons in the
encapsulating layered material. This hyperbolic cooling is particularly
efficient, giving picosecond cooling times, for hexagonal BN, where the
high-momentum hyperbolic phonon polaritons enable efficient near-field energy
transfer. We study this heat transfer mechanism through distinct control knobs
to vary carrier density and lattice temperature, and find excellent agreement
with theory without any adjustable parameters. These insights may lead to the
ability to control heat flow in van der Waals heterostructures.
