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

Abstract 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 _OA ) 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 _OA is observed at low temperatures that exceeds the suppression of the coupling of interfacial phonon modes. As temperature increases, G _OA 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.