Surface electromagnetic waves double thermal conductivity of thin SiN membranes.

In recent times, the unique collective transport physics of phonon hydrodynamics motivates theoreticians and experimentalists to explore it in micro- and nanoscale and at elevated temperatures. Graphitic materials have been predicted to facilitate hydrodynamic heat transport with their intrinsically strong normal scattering. However, owing to the experimental difficulties and vague theoretical understanding, the observation of phonon Poiseuille flow in graphitic systems remains challenging. In this study, based on a microscale experimental platform and the pertinent occurrence criterion in anisotropic solids, we demonstrate the existence of the phonon Poiseuille flow in a 5.5 μm-wide, suspended and isotopically purified graphite ribbon up to a temperature of 90 K. Our observation is well supported by our theoretical model based on a kinetic theory with fully first-principles inputs. Thus, this study paves the way for deeper insight into phonon hydrodynamics and cutting-edge heat manipulating applications.

Efficient heat dissipation in micro/nano electronics requires long-distance propagation of heat carriers operated above room temperature. However, thermal phonons—the primary heat carriers in dielectric nanomaterials—dissipate the thermal energy after just a few hundred nanometers. Theory predicts that the mean free path of surface phonon-polaritons (SPhPs) can be hundreds of micrometers, which may improve the overall dissipation of heat in nanomaterials. In this work, we experimentally demonstrate such long-distance heat transport by SPhPs. Using the 3$\mathit{\omega }$ technique, we measure the in-plane thermal conductivity of SiN nanomembranes for different heater-sensor distances, membrane thicknesses, and temperatures. We find that thin nanomembranes support heat transport by SPhPs, as evidenced by an increase in the thermal conductivity with temperature. Remarkably, the thermal conductivity measured 200 μm away from the heater is consistently higher than that measured 100 μm closer. This result suggests that heat conduction by SPhPs is quasi-ballistically over at least hundreds of micrometers. Our findings pave the way for coherent heat manipulations above room temperature over macroscopic distances, which impacts the applications in thermal management and polaritonics.

Ballistic thermal transport is a remarkable nanoscale phenomenon with possible applications in microelectronics. In the past decade, research on ballistic thermal transport focused on the measurements of length-dependent thermal conductivity in semiconductor nanowires. In this Perspective article, we review the experimental demonstrations of this phenomenon in nanowires of various materials and sizes and at different temperatures. Our literature analysis reveals a controversy between works reporting two distinct pictures of ballistic conduction: perfectly ballistic conduction even at room temperature and weak quasi-ballistic conduction mainly below room temperature. Simulations seem to support the weaker version of the effect. Thus, future experiments are expected to resolve this controversy.

Surface phonon-polaritons (SPhPs) are evanescent electromagnetic waves that can propagate distances orders of magnitude longer than the typical mean free paths of phonons and electrons. Therefore, they are expected to be powerful heat carriers capable of significantly enhancing the in-plane thermal conductance of polar nanostructures. In this work, we show that a SiO₂/Si (10 $\mathsf{\mu }$m thick)/SiO₂ layered structure efficiently enhances the SPhP heat transport, such that its in-plane thermal conductance is ten times higher than the corresponding one of a single SiO₂ film, due to the coupling of SPhPs propagating along both of its polar SiO₂${}_{\mathrm{}}$ nanolayers. The obtained results thus show that the proposed three-layer structure can outperform the in-plane thermal performance of a single suspended film while improving significantly its mechanical stability.

Nanoscale heat conduction is limited by surface scattering of phonons but can be enhanced by surface phonon-polaritons (SPhPs), which are the hybridization of photons and optical phonons in polar materials. Here, we analyze the dispersion of SPhPs in a multilayer system consisting of a silicon (Si) layer sandwiched between two silicon dioxide (SiO₂) nanolayers. We find that SPhPs generated in SiO₂ nanolayers couple with guided resonant modes and propagate mainly in the nonabsorbent Si layer for microscale Si thicknesses. This coupling yields an enhancement in thermal conductivity with Si thickness. In contrast, for nanoscale Si thicknesses, evanescent components of SPhPs couple inside the Si layer, resulting in a higher thermal conductivity for thinner Si layers. The transition between these two different coupling phenomena provides the minimum of the in-plane SPhP thermal conductivity at a Si thickness of approximately 1 μm. Our finding brings deeper insight into thermal management in electronics and semiconductors.