Rohith Mittapally1,Ju Won Lim1,Lang Zhang2,Owen Miller2,Pramod Sangi Reddy1,Edgar Meyhofer1
University of Michigan1,Yale University2
Rohith Mittapally1,Ju Won Lim1,Lang Zhang2,Owen Miller2,Pramod Sangi Reddy1,Edgar Meyhofer1
University of Michigan1,Yale University2
Radiative heat transfer (RHT) between objects separated by nanoscale gaps, referred to as being in the near-field (NF) of each other, is known to impact applications ranging from energy conversion to thermal management of electronics. Recently, several experiments validated early predictions of orders of magnitude enhancement in NF-RHT, aided by surface phonon polaritons (SPhPs) supported in polar dielectric materials. While these experiments using SiO<sub>2 </sub>validated the longstanding predictions of enhanced RHT, there remains a few unanswered questions: what is the fundamental limit to this NF-RHT and what material properties can enable this? Recent theoretical analysis [1] based on causal arguments suggests that the SPhPs in SiO<sub>2</sub> occur at a frequency far higher than the optimal. In fact, our optimization [2] for maximum NF-RHT suggests that materials supporting SPhPs close to an optimal frequency of 67 meV at room temperature can enhance this NF-RHT by 5-fold (a global bound) when compared to that of SiO<sub>2</sub>. In this talk, I will first discuss our theoretical analysis and report on how we utilized a custom-built nanopositioner to measure NF-RHT between Al<sub>2</sub>O<sub>3 </sub>- Al<sub>2</sub>O<sub>3 </sub>and MgF<sub>2</sub> - MgF<sub>2</sub> material systems. In these experiments, we observed a clear enhancement of NF-RHT beyond blackbody limit and even beyond that of SiO<sub>2</sub> at all gap sizes ranging from a few tens of nanometers to a few micrometers. Specifically, I will show that NF-RHT between MgF<sub>2</sub> at a gap size of 50 nm is 2.5-fold larger than that between SiO<sub>2</sub> and thus, approaches 50% of the global SPhP bound. The insights gained from these experiments and theoretical analysis will benefit exploration of materials further enhancing NF-RHT.<br/><br/>(1) Zhang, L.; Miller, O. D. Optimal Materials for Maximum Large-Area Near-Field Radiative Heat Transfer. <i>ACS Photonics </i><b>2020</b>, <i>7</i> (11), 3116-3129.<br/>(2) Mittapally, R.; Lim, J. W.; Zhang, L.; Miller, O. D.; Reddy, P.; Meyhofer, E. Probing the Limits to Near-Field Heat Transfer Enhancements in Phonon-Polaritonic Materials. <i>Nano Lett. </i><b>2023</b>, <i>23</i> (6), 2187-2194.