Directing Interfacial Heat Flow via Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride

Saman Zare
University of Virginia, Charlottesville, Virginia, USA _______________________________________

With the continued miniaturization of electronic devices to the nanoscale, heat generation approaches quantum limits, leading to significant increases in scattering and subsequent reductions in conventional thermal conduction. In parallel, ongoing advancements in nanoscale thermal engineering have identified radiative mechanisms as potential pathways to precisely tune both the magnitude and direction of heat flow at interfaces. Hexagonal boron nitride (hBN) has become one of the focal points of many efforts in this field due to its highly anisotropic crystal structure and wide optical bandgap, which together support hyperbolic phonon polaritons (HPhPs). These collective modes enable strong electromagnetic confinement at subwavelength scales, significantly boosting radiative heat transfer.

In this work, we present a combined theoretical and experimental investigation into radiative heat transport across metal/hBN interfaces under extreme non-equilibrium conditions. Such conditions frequently arise in high-power and high-speed electronic systems, necessitating advanced thermal management strategies to maintain device performance and reliability. Theoretically, we adopt a fluctuational electrodynamics (FED) framework to calculate the radiative heat flux from a metal film with high electron temperatures (that can exceed several thousand Kelvins) into hBN. Our simulations reveal that, at such elevated electronic temperatures, hot electrons rapidly scatter at the interface and can directly excite the HPhPs in hBN, allowing radiation-driven heat transfer to rival its conduction-driven counterpart. The ability of hBN to sustain hyperbolic modes across a broad frequency range is key to this enhanced radiative coupling.

To validate these predictions, we perform ultrafast pump-probe spectroscopy on a gold pad deposited atop hBN. By tuning the mid-infrared probe to the vibrational resonances of hBN, we capture sub-picosecond thermal dynamics that confirm the involvement of HPhPs in the rapid transfer of energy away from non-equilibrium electrons. The measured interfacial heat transfer rates closely align with FED-based theoretical models, underscoring the importance of polaritonic excitations in governing heat flow at the nanoscale. This study highlights the potential for hBN-based polaritonic architectures to serve as critical platforms for radiative thermal management, paving the way toward more efficient nanoscale heat dissipation strategies in next-generation electronic devices.

Email: saman.zare@virginia.edu