Opening a new nanoscale lane on the heat transfer highway

Thomas Beechem, an associate professor of mechanical engineering at Purdue University, is enthusiastic about heat transfer. He explained: “We have several ways of describing energy. When we talk about light, we describe it in terms of particles called ‘photons.’ Heat also carries energy in predictable ways, and we describe those waves of energy as ‘phonons.’ But sometimes, depending on the material, photons and phonons will come together and create something new called a ‘polariton.’ It carries energy in its own unique way, distinct from both photons or phonons.”

Polaritons, like photons and phonons, are not tangible particles but conceptual models for energy exchange.

Beechem offers an analogy: “Phonons are like internal combustion vehicles, and photons are like electric vehicles. Polaritons are akin to a Toyota Prius. They are a hybrid of light and heat, retaining some properties of both, yet they are distinct.”

While polaritons have been utilised in optical applications, their role in heat transfer has been largely overlooked, mainly because their effects become noticeable only at very small scales. Jacob Minyard, a Ph.D. student in Beechem’s lab, noted: “We know that phonons do most of the work in transferring heat. The effect of polaritons is only observable at the nanoscale. But now, with the advancement of semiconductors, there's a need to address heat transfer at this level.

“Semiconductors have become extremely small and complex. Those who design and build these chips find that phonons are not efficient in dispersing heat at these small scales. Our paper shows that at these dimensions, polaritons can play a larger role in thermal conductivity.”

Their work on polaritons has been recognised as a Featured Article in the Journal of Applied Physics. Beechem commented: “In the heat transfer community, we’ve previously been very material-specific in discussing the impact of polaritons. Jacob’s paper has shown that this isn’t an isolated phenomenon. Polaritons start to dominate heat transfer on any surface thinner than 10 nanometres, which is about twice the size of transistors on an iPhone 15.”

Beechem is optimistic about the implications of their research: “We’ve essentially added a new dimension to the heat transfer mechanism. As semiconductors continue to shrink, it becomes increasingly important to design them to utilise both phonons and polaritons effectively.”

Minyard’s paper begins to explore how this concept can be applied in practice. He said: “There are many materials in chipmaking, from silicon to dielectrics and metals. Our next step is to understand how these materials can be optimised for heat conduction, taking into account that polaritons offer a new method for energy transfer.”

Beechem and Minyard aim to demonstrate to chip manufacturers how to integrate these polariton-based nanoscale heat transfer principles into the physical design of chips, considering the materials used and the architecture of the layers.

While their current work is theoretical, Beechem and Minyard are eager to advance to physical experimentation, and Purdue University provides an ideal environment for this. Beechem praised the robust heat transfer community at Purdue: “We can consult with Xianfan Xu, a pioneer in this field, and Xiulin Ruan, known for his work in phonon scattering. At the Birck Nanotechnology Centre, we have the facilities to construct nanoscale experiments and unique tools for validating our theories. It’s an excellent place for a researcher.”