# Out-of-plane heat transfer in van der Waals stacks: electron-hyperbolic phonon coupling

## TL;DR

This paper uncovers an efficient out-of-plane heat transfer mechanism in van der Waals heterostructures, where charge carriers couple to hyperbolic phonon polaritons, enabling ultrafast cooling and potential heat flow control.

## Contribution

It demonstrates a novel hyperbolic phonon polariton-mediated heat transfer process in van der Waals stacks, validated by theory and experiments with no adjustable parameters.

## Key findings

- Hyperbolic phonon polaritons enable ultrafast picosecond cooling.
- Heat transfer efficiency depends on carrier density and temperature.
- Excellent agreement between experimental results and theoretical predictions.

## Abstract

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.