Saeko Tachikawa

A thermal diode, which, by analogy to its electrical counterpart, rectifies heat current, is the building block for thermal circuits. To realize a thermal diode, we demonstrate thermal rectification in a GaAs telescopic nanowire system using the thermal bridge method. We measured a preferred direction of heat flux, achieving rectification values ranging from 2 to 8% as a function of applied thermal bias. We demonstrate that the thermal boundary resistance between the thin part with the wurtzite crystal phase and the thick part with the zinc-blende crystal phase of the telescopic nanowire plays a crucial role in determining the amount and direction of heat flux rectification. This effect is confirmed by numerical solutions of the one-dimensional heat equation based on ab initio data. Additionally, we accounted for the effect of the thermal contact resistance. This work is the first experimental indication of rectification using a telescopic nanowire where we reveal the importance and role of the thermal boundary resistance in determining thermal rectification.

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, known as surface phonon-polaritons (SPhPs). Here, we experimentally demonstrate that the far-field radiative heat transfer between two non-absorbent bodies can also overcome Planck's limit, by coating them with an absorbent material to form a polaritonic waveguide. This super-Planckian far-field thermal radiation is confirmed by measuring the radiative thermal conductance between two silicon plates coated with silicon dioxide nanolayers. The observed conductance is twice higher than Planck's limit and agrees with the predictions of our model for the SPhP waveguide modes. Our findings could be applied to thermal management in microelectronics and silicon photonics.

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.