Enhanced heat transfer across h-BN interfaces via coupling between hot electrons and phonon-polaritons
Patrick E. Hopkins
University of Virginia – Charlottesville, Virginia, USA
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Thermal transport across solid-solid interfaces is vital for advanced electronic and photonic applications, yet conventional conduction pathways often restrict performance. In polar crystals, hybridized vibrational modes called phonon-polaritons offer a promising avenue to overcome the limitations of intrinsic phonon heat conduction. Our work demonstrates that volume-confined hyperbolic phonon-polariton (HPhP) modes can transfer energy across solid-solid interfaces at rates far exceeding phonon-phonon conduction. Using pump-probe thermoreflectance with a mid-infrared, tunable probe pulse with sub-picosecond resolution, we remotely and selectively observe HPhP modes in hexagonal boron nitride (hBN) via broadband radiative heating from a gold source. Our measurements ascertain that hot electrons impingent at the interface radiate directly into the HPhPs of hBN in the near field bypassing the phonon-phonon interfacial transport pathway. Such polaritonic coupling enables thermal transport speeds in solids orders of magnitude faster than possible through diffusive phonon processes. We thereby showcase a pronounced thermal transport enhancement across the gold-hBN interface via phonon-polariton coupling, advancing the limits of interfacial heat transfer. We support our experimental findings with predictions of this interfacial transport mechanism via fluctuational electrodynamics during conditions of strong electron-phonon nonequilibrium at metal/h-BN interfaces.
Our work demonstrates a new avenue for interfacial heat transfer based on broadband radiative coupling from a hot spot in a gold film to hBN HPhPs, independent of the broad spectral mismatch between the pump (visible) and probe (mid-IR) pulses employed. This methodology can be used to bypass the intrinsically limiting phonon-phonon conductive pathway, thus providing an alternative means of heat transfer across interfaces. Thus, the first major finding of our work is the ability to both spectrally and temporally resolve the thermally excited HPhP modes in hBN, providing clear evidence of the more efficient thermal transport that these hot HPhP carriers can provide. Secondly, our results illustrate that HPhP thermal coupling can occur across an all-solid heterogenous interface, and thus, for the first time, demonstrate that interfacial heat transfer can be enhanced via this PhP coupling. Further, our work demonstrates that HPhP modes do not need to be coherently excited, but rather can be optically stimulated by a graybody radiating in the near field. Our work unlocks the possibility of utilizing this NFRHT enhancement at solid-solid interfaces in intimate atomistic contact, by showing that this broadband radiative cooling effect is driven by PhP coupling that can delocalize the thermal energy from a radiating spot into an adjacent hyperbolic medium. This finding will redefine cooling of hot spots in materials and systems limited or dictated by interfacial resistances, such as high power or high frequency electronics, photonic circuits, memory, and thermophotovoltaics.
Email: peh4v@virginia.edu