Abnormal length-dependent thermal transport in silicon nitride nanoribbons by surface phonon polaritons

Surface phonon polaritons (SPhPs) are hybrid excitations arising from the coupling of mid-infrared photons with optical phonons, generating evanescent waves that propagate along the surfaces of polar dielectric materials. Owing to their long wavelengths and extended propagation lengths, SPhPs are particularly promising for enhancing the heat dissipation in micro- and nano-electronic devices. In this study, we fabricated silicon nitride (SiNx) nanoribbons and transferred them onto suspended micro-platforms to systematically investigate their thermal transport property. The thermal conductivity measurements were conducted in a temperature range of 300–480 K for SiNx nanoribbons with varying lengths. A distinct length-dependent enhancement in thermal conductivity is observed, indicative of the quasi-ballistic transport behavior. We attribute this enhancement to the contribution of propagating SPhPs along the surfaces of the polar SiNx nanoribbons. Further quantitative analysis reveals that the SPhP-mediated component constitutes a substantial fraction of the total in-plane thermal conductance for the 70 μm-long nanoribbon at 480 K. Our findings provide direct evidence that SPhPs act as effective heat carriers in low-dimensional polar dielectric systems, providing valuable insights for optimizing the heat dissipation in next-generation microelectronics and silicon photonics.