Greatly Enhanced Radiative Transfer Enabled by Hyperbolic Phonon Polaritons in α-MoO3

Orthorhombic molybdenum trioxide (alpha-MoO3) is a highly anisotropic hyperbolic material in nature. Within its wide Reststrahlen bands, alpha-MoO3 has hyperboloidal dispersion that supports bulk propagation of high-k phonon polariton modes. These modes can serve as energy transport channels to greatly enhance radiative heat transfer inside the material. In this work, large radiative transfer enabled by phonon polaritons in alpha-MoO3 is demonstrated. The study first determines the temperature-dependent permittivity of alpha-MoO3 from polarized Fourier-Transform Infrared (FTIR) spectroscopy measurements and then uses a many-body radiative heat transfer model to predict the equivalent radiative thermal conductivity of hyperbolic phonon polariton. Contribution of radiative transfer to the total thermal transport is experimentally determined from the Time-Domain Thermoreflectance (TDTR) measurements in a temperature range from -100 to 300 degrees C. It is found that radiative transfer can account for approximate to 60% of the total thermal transport at a temperature of 300 degrees C. That is, conductive thermal transport is enhanced by >100% by radiative transfer, or radiation inside alpha-MoO3 is greater than that of conduction. These additional energy pathways will have important implications in thermal management in new materials and devices.