Low-temperature hyperbolic phonon polaritonics with suppressed phononic scattering.

Hyperbolic phonon polaritons (PhPs) enable unparalleled light manipulation due to their subwavelength confinement and anisotropic dispersions but suffer from substantial phononic scattering losses at room temperature. Recent cryogenic nanoimaging has demonstrated extended hyperbolic PhP propagation at low temperatures; however, the fundamental dissipation mechanisms remain elusive. Here, we unveil the intrinsic loss channels and scattering rates of hyperbolic PhPs in van der Waals alpha-molybdenum trioxide (α-MoO3) and bulk yttrium orthovanadate (YVO4) through integrated structural, vibrational, and polaritonic characterizations at cryogenic temperatures, complemented by anharmonic lattice dynamics analysis. We reveal that three-phonon interactions dominate dissipation within the entire hyperbolic frequency range; four-phonon processes become significantly inhibited at low temperature. Notably, suppressed phonon decay extends the PhP lifetime, leading to enhanced quality factors by ~97.8% in α-MoO3-based polaritonic crystals. This phonon suppression-induced prolongation of polariton lifetime also applies to YVO4, and temperature-dependent analysis of various PhP materials further confirms its universality. These insights elucidate the dissipation physics of hyperbolic PhPs and provide a foundation for designing high-performance polaritonic devices for low-temperature applications.