Quantifying the Effect of Nanofilms on Near-Field Radiative Heat Transfer

Recent measurements of near-field radiative heat transfer(NFRHT)between objects separated by nanometer-sized vacuum gaps have revealedthat thermal radiation at the nanoscale is remarkably distinct fromfar-field thermal radiation and can exceed the blackbody radiationlimit by orders of magnitude. Given the technological relevance ofthin films, there remains a significant need to experimentally explorehow such films influence NFRHT. Here, we report direct measurementsof the thickness-dependence of NFRHT between planar nanofilms of magnesiumfluoride (thickness ranging from 20 to 500 nm) performed using microfabricateddevices and a custom-developed nanopositioner. These results directlydemonstrate for the first time that nanofilms can enhance thermalradiation up to 800-fold above the blackbody limit and are as effectiveas bulk materials when nanoscale gaps have dimensions smaller thanthe film thickness. Finally, calculations based on fluctuational electrodynamicsshow good agreement with the measured gap-size dependence of the heattransfer coefficient for films of all thicknesses and provide physicalinsight into the observed dependence. The experimental techniquesand insights reported here pave the way for systematically exploringnovel thin films for near-field thermal and energy systems.