Vanadium dioxide (VO₂) exhibits a phase-change behavior from the insulating state to the metallic state around 340 K. By using this effect, we experimentally demonstrate a radiative thermal rectifier in the far-field regime with a thin film VO₂ deposited on the silicon wafer. A rectification contrast ratio as large as two is accurately obtained by utilizing a one-dimensional steady-state heat flux measurement system. We develop a theoretical model of the thermal rectifier with optical responses of the materials retrieved from the measured mid-infrared reflection spectra, which is cross-checked with experimentally measured heat flux. Furthermore, we tune the operating temperatures by doping the VO₂ film with tungsten (W). These results open up prospects in the fields of thermal management and thermal information processing.

We show that near-field electromagnetic heat transfer between multilayer thermal bodies can be significantly enhanced by the contributions of surface states at multiple surfaces. As a demonstration, we show that when one of the materials forming the multilayer structure is described by the Drude model, and the other one is a vacuum, at the same gap spacing the resulting heat transfer can be up to 40 times higher as compared to that between two semi-infinite materials described by the same Drude model. Moreover, this system can exhibit a nonmonotonic dependency in its heat transfer coefficient as a function of the middle gap spacing. The enhancement effect in the system persists for realistic materials.

Topological metamaterials offer new routes for control of waves, which are widely realized by Bragg scattering and/or local resonance. Understanding of topological transition by the interaction between these mechanisms is strongly desired to extend the design degrees of freedom for intriguing wave phenomena. Here, we demonstrate a phononic metamaterial consisting of C-shaped elements that enables us to investigate interaction between Bragg scattering and local resonance. We show that by adding resonance scattering a topological band gap is opened from a Bragg scattering based Dirac cone, and its band gap is controlled by the resonance frequency of the cavities relative to the Dirac cone frequency. In addition, we show that topological band-gap opening induced by the Bragg scattering can be reversed into an ordinary state or vice versa by judicious inclusion of the local resonance. Scattering cross-section analysis elaborates the combined effect of the two mechanisms on topological states. By employing lossy resonant elements, we further demonstrate a lossy topological insulator capable of one-way sound propagation immune to sharp corners, potentially leading to an energy-harvesting topological insulator. Our results provide a critical understanding of topological phenomena involving coupled scattering mechanisms.

Here, we show analytically that the thermal rectification via evanescent waves is obtained in the parallel semi-infinite bodies of the dielectric-coated silicon carbide and uncoated silicon carbide. The permittivity and the thickness of the dielectric coating are derived for maximizing the thermal rectification. In the nonequilibrium situation holding temperatures of 500 K for one body and 300 K for the other, either a coating with a high permittivity of 14 and a thickness of 1 nm or a coating with a low permittivity of 2 and a thickness exceeding 10 nm, results in rectifying coefficients of 0.4 to 0.44.

Near-field radiative heat transfer has been a subject of great interest due to the applicability to thermal management and energy conversion. In this letter, a submicron gap between a pair of diced fused quartz substrates is formed by using micromachined low-density pillars to obtain both the parallelism and small parasitic heat conduction. The gap uniformity is validated by the optical interferometry at four corners of the substrates. The heat flux across the gap is measured in a steady-state and is no greater than twice of theoretically predicted radiative heat flux, which indicates that the parasitic heat conduction is suppressed to the level of the radiative heat transfer or less. The heat conduction through the pillars is modeled, and it is found to be limited by the thermal contact resistance between the pillar top and the opposing substrate surface. The methodology to form and evaluate the gap promotes the near-field radiative heat transfer to various applications such as thermal rectification, thermal modulation, and thermophotovoltaics.