Direct Observation of Suppressed Optical-Acoustic Phonon Energy Coupling in Supported SWCNT at Cryogenic Temperatures.

Nonequilibrium among phonon branches critically influences nanoscale heat transport yet remains largely unexplored in one-dimensional (1D) systems, particularly at cryogenic temperatures. This work reports the first experimental quantification of optical-acoustic phonon coupling factor (G) in single-walled carbon nanotubes using the frequency-domain energy transport state-resolved Raman technique at cryogenic and room temperatures. Remarkably, a strong suppression of G is observed at low temperatures that exceeds the suppression of the coupling of interfacial phonon modes. As temperature increases, G is found to increase monotonically, consistent with enhanced anharmonic decay processes of optical phonons. At 93 K, the optical-acoustic phonon temperature difference exceeds 75% of the acoustic phonon temperature rise, which is reduced to about 33% at room temperature. The critical role of laser heating size on phonon nonequilibrium is elucidated, where it gets amplified for a more confined heating size. By utilizing the recently developed equivalent interfacial medium model, the intrinsic temperature-dependent interfacial thermal conductance based on acoustic phonon temperature is obtained. The results show that neglecting the nonequilibrium among phonon branches overestimates the interfacial conductance by ≈30% at room temperature. This research provides fundamental insights into phonon nonequilibrium in 1D nanoscale materials that strongly impact next-generation nanoelectronics and solid-state energy converters.