  Fast carrier cooling is important for high power graphene based devices.
Strongly Coupled Optical Phonons (SCOPs) play a major role in the relaxation of
photoexcited carriers in graphene. Heterostructures of graphene and hexagonal
boron nitride (hBN) have shown exceptional mobility and high saturation
current, which makes them ideal for applications, but the effect of the hBN
substrate on carrier cooling mechanisms is not understood. We track the cooling
of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast
pump-probe spectroscopy. We find that the carriers cool down four times faster
in the case of graphene on hBN than on a silicon oxide substrate thus
overcoming the hot phonon (HP) bottleneck that plagues cooling in graphene
devices.
