Thomas Folland
University of Iowa – Iowa City, Iowa, USA
_______________________________________
Isotopically enriched hBN has been shown to have record-breaking propagation lengths for hyperbolic phonon polaritons (HPhPs), owing to the reduced scattering in isotopically pure materials. The hyperbolic properties of hBN have been pegged to have be transformative for applications in sensing, radiative heat transfer, and infrared detection. Studying the temperature dependence of the polariton propagation is critical for understanding both the physical mechanisms associated with polariton behavior and potential applications. Both h10BN and h11BN have been shown to have extended polariton lifetime at low temperatures; however, the full dielectric function is not available, hindering further efforts to utilize this material.
In our work, we study the temperature dependent optical properties of 10B and 11B enriched hexagonal boron nitride. First, we examine samples prepared on an Au mirror, which has strong background reflectance from the substrate that aids in referencing. We observe Fabry-Perot (FP) type absorption resonances in the mid-IR, which is well suited to determining the temperature dependence of TO phonons in the infrared. We also use sapphire substrates, which are better suited for completing a full dielectric function analysis close to both TO and LO phonon energies. Our dielectric function analysis extends previous methods implementing a method for backside reflection correction and addressing systematic errors. We find that optical losses in isotopically pure hBN at room temperature were previously underestimated by a factor of 2. This parameter is critical for realistic simulations of polariton propagation in hBN materials and heterostructures. The results are consistent with past measurements on Raman modes, and low temperautre hBN polaritons for which no model previously existed. Our results for the dielectric function show comparable shifts in the TO phonon energy as the FP resonances, suggesting that tracking the properties of FP modes is a suitable mechanism for estimating the temperature dependence of phonon properties. Finally, we calculate the quality factors of polaritons as a function of temperature and show that our temperature-dependent dielectric function closely matches prior experiments by Ni. This suggests our dielectric function improves on those previously published, even at room temperature, and can be used for predictive modeling for polaritonic devices. Our approach represents a transferable framework for different isotopes, polytypes, and dopants of van der Waals materials, which could be applied to other materials and systems.
Email: thomas-folland@uiowa.edu