Yunhui Wu

• PhD
• PostDoc Position at The University of Tokyo

We demonstrate that the time-averaged far-field thermal radiation between macroscopic bodies can be different from the Planck limit when their temperature difference is periodically modulated. This difference arises from the nonlinear dependence of the radiative heat flux on temperature and persists even for temperature-independent emissivities. By...

The decisive experimental evidence of enhanced heat conduction driven by surface phonon polaritons (SPhPs) has been recently demonstrated along polar nanofilms. However, a proper quantitative interpretation remains to be fully established. In this work, we provide a consistent theoretical explanation of the measured thermal conductivities of polar...

The development of emerging technologies, such as quantum computing and semiconductor electronics, emphasizes the growing significance of thermal management at cryogenic temperatures. Herein, by designing isotope interfaces based on the Golomb ruler, we achieved effective suppression of the phonon thermal transport of cryogenic graphene. The pronou...

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (SiO2) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twic...

Silicon carbide (SiC) is an important industrial material that enables the thermal stability of power electronics. However, the nanoscale phenomenon of ballistic thermal conduction, which may further improve the thermal performance, remains unexplored in SiC. Here, we reveal the length and temperature scales at which SiC exhibits quasi-ballistic th...

We derive and analytically integrate the models for the heater and sensor 3ω signals of the temperature field of anisotropic bulk materials and thin films. This integration is done by using the Fourier transform and expressing the frequency dependence of temperature in terms of the modified Bessel and Struve functions, which are well-implemented in...

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 e...

The performance of silicon-based thermoelectric energy generators is limited by the high thermal conductivity of silicon. Theoretical works have long proposed reducing the thermal conductivity by resonant phonon modes in nanopillars placed on the surface of silicon films. However, these predictions have never been confirmed due to the difficulty in...

Classical Planck's theory of thermal radiation predicts an upper limit of the heat transfer between two bodies separated by a distance longer than the dominant radiation wavelength (far-field regime). This limit can be overcome when the dimensions of the absorbent bodies are smaller than the dominant wavelength due to hybrid electromagnetic waves,...

Silicon thin films have great potential as chip-integrated Peltier micro-coolers and thermoelectric power generators due to their industry compatibility and cost effectiveness. Improving the thermoelectric figure of merit, zT, and therefore the device efficiency can be achieved by increasing the power factor while decreasing the thermal conductivit...

Nanoscale heat conduction is limited by the 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 (...

Efficient heat dissipation in micro/nanoelectronics 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...

The heat transport of surface phonon-polaritons propagating along a polar uniaxial anisotropic nanofilm is studied for different orientations of its optical axis, film thicknesses, and temperatures. For a hBN nanofilm, it is shown that: i) the propagation of polaritons can be described in terms of even and odd modes that generalize the transverse m...

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 e...

Silicon carbide (SiC) aims to be the number one material for power microelectronics due to its remarkable thermal properties. Recent progress in SiC technology finally enabled the fabrication of crystalline SiC nanostructures. Yet, the thermal properties of SiC at the nanoscale remain overlooked. Here, we systematically study heat conduction in SiC...

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 p...

Long-distance propagation of heat carriers is essential for efficient heat dissipation in microelectronics. However, in dielectric nanomaterials, the primary heat carriers - phonons - can propagate ballistically only for hundreds of nanometres, which limits their heat conduction efficiency. Theory predicts that surface phonon-polaritons (SPhPs) can...

Improving heat dissipation in increasingly miniature microelectronic devices is a serious challenge, as the thermal conduction in nanostructures is markedly reduced by increasingly frequent scattering of phonons on the surface. However, the surface could become an additional heat dissipation channel if phonons couple with photons forming hybrid sur...

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,...

Surface phonon-polaritons can carry energy on the surface of dielectric films and thus expected to contribute to heat conduction. However, the contribution of surface phonon-polaritons (SPhPs) to thermal transport has not been experimentally demonstrated yet. In this work, we experimentally measure the effective in-plane thermal conductivity of amo...

Thermal conduction becomes less efficient as structures scale down into submicron sizes since phonon-boundary scattering becomes predominant and impede phonons more efficiently than Umklapp scattering. Recent studies indicated that the surface phonon polaritons (SPhPs), which are the evanescent electromagnetic waves generated by the hybridation of...

Composites of Si and transition metal silicides have been studied extensively for waste heat re-covery applications because of their non-toxic, low cost and environmentally friendly nature. Composite samples made up of Si particles embedded in an Al doped β-FeSi2 matrix are synthesized by eutectoid decomposition. Presence and uniform spatial distri...

Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous thin free-standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed therm...