Nearly every designer is concerned with the problems caused by excess heat and the need to manage that heat. Dissipating it is often a major design challenge, with the goal of getting it away from the source and associated components. (Keep in mind that “away” refers to that magical location in which the heat is no longer a system issue or, at the very least, no longer your issue but rather the concern of someone else.) Many techniques and mechanisms are used individually or in combination to achieve this, based on thermal conduction and convection (radiation-based cooling is far less common). Heat sinks, cold plates, forced air (fan) and unforced convection, heat pipes, and liquid cooling are just a few examples. Using advanced thermal physics principles that go beyond macro-basics like thermal conductivity and impedance, a group at the University of Virginia (UVA) has now developed what they claim is a far superior approach. Electronic heat typically dissipates outward and loses energy along the way, spreading like ripples in a pond. The team’s method, on the other hand, channels heat into waves that efficiently travel long distances. By using a special kind of crystal called hexagonal boron nitride (hBN), they found a way to move heat like a beam of light, sidestepping the usual bottlenecks that make electronics overheat.
Patrick Hopkins, Whitney Stone professor of engineering and professor of mechanical and aerospace engineering at UVA, stated, “We’re rethinking how we handle heat.” “We are directing it, not letting it slowly disappear,” The team used hyperbolic phonon-polaritons (HPhPs), which are special waves that are capable of carrying heat at extraordinary speeds, rather than the slow-moving heat vibrations known as phonons. The researchers achieved this by heating a tiny gold pad sitting on hBN. Instead of heat just spreading sluggishly, it excited the hBN’s unique properties, turning the energy into fast-moving “evanescent” polaritonic waves that very quickly carried the heat across and away from the interface between the gold and hBN (Figure 1).

First, the ultrafast visible pump is absorbed by the gold pad, which increases its temperature causing it to radiate. Consistent with launching of phonon polaritons via thermal radiation, the local incoherent dipole moment of the thermal radiation provides the energy matching and momentum matching to directly launch HPhPs within the hBN flake. As a result, a wide range of HPhP and transverse optical phonon modes are stimulated. Once launched, owing in part to the optical phonon and in part to the light nature of the HPhP quasiparticles, these modes can carry the thermal energy away from the heat source owing to the high heat capacity of the former and the high group velocity (with respect to acoustic phonons) of the latter. Their work extends earlier findings related to thermal conductivity in surface-confined PhPs in silicon carbide to volumetric hyperbolic modes and into the ultrafast regime.

This new mechanism has two significant features: speed and controllability. Once a polariton carries the thermal energy, it travels at ultrasonic speeds. This indicates that this mechanism can speed up the removal of heat from high-power or high-frequency electronic devices that accumulate heat through joule heating. This new mechanism also serves to employ thermal energy to launch polaritons, meaning that photonic circuits may be able to have a useful nanoscale source for photons.

In a nutshell, they demonstrated experimentally the capability of ultrafast thermal transport across solid–solid interfaces by transferring thermal energy from a gold pad that had been briefly heated into HPhPs that were supported by hBN. By overcoming the traditional limitations of phonon-dictated thermal boundary conductance, this mechanism sheds light on the potential contribution that polaritonic modes can make to interfacial heat transfer. They demonstrated that polaritonic coupling can facilitate the optical modes to move heat across and away from an Au–hBN interface over an order of magnitude faster than acoustic phonon conduction in the same system.

Will these developments radically affect techniques for directed removal of heat? There are many practical issues, of course, and the future is impossible to predict. The full paper, titled “Ultrafast evanescent heat transfer across solid interfaces via hyperbolic phonon–polariton modes in hexagonal boron nitride,” as well as a comprehensive Supplementary Information file, which provides additional details on the preparation and experimental arrangement, can be found online at Nature.