Interfacial Thermal Radiation Driven by Hot Electrons in Non-Equilibrium

With the continued miniaturization of electronic devices to the nanoscale, heat generated by circuits enters the quantum regime, leading to significant increases in the scattering of heat and thus, a subsequent reduction in thermal conductivity. Recent advancements in nanoscale thermal engineering have introduced thermal radiation as a promising pathway to control the magnitude and direction of heat transfer across interfaces.
In this study, we investigate radiative transport in various metal films under extreme non-equilibrium conditions, with electron temperatures reaching up to 5000 K. Under these conditions, the high scattering rates can be leveraged to transfer heat radiatively across interfaces, bypassing the lattice. Using the formalism of fluctuational electrodynamics (FED), we simulate the radiative heat flux between a non-equilibrium metal film and a non-metal substrate. The trends observed in the calculated radiative conductance indicate that hot electrons scattering at the interface can release their energy by exciting the polaritonic and photonic modes within the absorber. Our simulated results show that radiative heat transfer can rival its conductive counterpart at high electron temperatures when phonon-polaritons are allowed in the substrate.
To further validate this thermal transport mechanism, we perform experimental studies on thermal interactions between a gold radiating pad in non-equilibrium and hyperbolic phonon polaritons (HPhPs) in hexagonal Boron Nitride (hBN). For this purpose, we employ an ultrafast pump-probe spectroscopy technique with a wavelength-tunable mid-infrared probe, which provides sub-picosecond spectral resolution of vibrational modes and polaritonic heat carriers. The ultrafast thermal transport rates we measure at Au/hBN interface due to the excitation HPhPs align closely with FED-based predictions, demonstrating the potential of radiative heat transfer via polaritonic excitations as a solution for nanoscale heat management in high-temperature electronic devices.