Low-Dimensional Phonon-Polariton Materials for Higher-Order Hyperbolic Dispersion and Efficient Thermal Transport

Phonon polaritons are stimulated by coupling infrared photons with the polar lattice vibrations. Such quasi-particles offer low-loss, highly confined electromagnetic energy propagation at subwavelength scales. Here, we discuss the launching and manipulation of higher-order hyperbolic phonon polaritons (HPhPs) in low-dimensional materials.¹ Additionally, we discuss how non-equilibrium phonon polaritons in these materials can further enhance thermal transport as additional energy carriers.²<br/><br/>HPhPs in low-symmetry polar crystals offer ray-like light propagation with out-of-plane or in-plane hyperbolic wavefronts at deep-subwavelength scales. While hyperbolic dispersion in HPhPs implies multiple propagating modes at a given frequency, experimentally launching and probing the higher-order modes, especially for in-plane HPhPs, remains challenging. We report the experimental observation of higher-order in-plane HPhP modes, stimulated on the 3C-SiC nanowire (NW)/α-MoO₃ heterostructures. This demonstrates the advantage of leveraging both the low-dimensionality and low-loss nature of polar NWs to launch higher-order HPhP modes within two-dimensional α-MoO₃ crystals.<br/><br/>One-dimensional polar NWs not only bridge the wavevector mismatch between free-space light and the higher-order, high-momenta HPhPs but could also significantly impact thermal transport. While optic phonons typically contribute minimally to thermal conductivity, their hybridization with photons as phonon polaritons could facilitate conductive heat transfer in nanostructures. Although some signs of progress in phonon-polariton-mediated heat conduction have been made, experimental efforts so far suggest only very limited contribution from such modes. Here, by combining nanoscale real-space mapping of phonon polaritons within 3C-SiC NWs with direct thermal transport measurements of these NWs (coated with gold as an efficient polariton launcher), we unambiguously demonstrate phonon-polariton-mediated heat conduction, resulting in orders of magnitude enhancement compared to equilibrium predictions.<br/><br/>1. Lu, G.; Pan, Z.; Gubbin, C. R.; Kowalski, R. A.; De Liberato, S.; Li, D.; Caldwell, J. D., Launching and Manipulation of Higher-Order In-Plane Hyperbolic Phonon Polaritons in Low-Dimensional Heterostructures. <i>Adv Mater </i><b>2023,</b> <i>35</i> (22), e2300301.<br/>2. Pan, Z.; Lu, G.; Li, X.; McBride, J. R.; Juneja, R.; Long, M.; Lindsay, L.; Caldwell, J. D.; Li, D., Remarkable heat conduction mediated by non-equilibrium phonon polaritons. <i>Nature </i><b>2023,</b> <i>623</i> (7986), 307-312.