Makoto Shimizu

A thermal radiator with smart VO2-based metamaterials is investigated that passively adjusts its radiative properties as a function of temperature. At high temperatures, the proposed device exhibits low absorption at the vertical angles but high emission at high angles, facilitating effective heat dissipation even under solar irradiance, while at low temperatures it acts as a low-emitting material. It can be applied to a radiator used in spacecraft. The device property is experimentally demonstrated by measuring the reflectance at different temperatures and angles. The results demonstrate that the temperature change caused by the difference in solar illumination is reduced from 210.6°C for blackbody radiators to 120.8°C, even while maintaining a similar maximum temperature level when exposed to full sunlight.

A thermal radiator with smart VO2-based metamaterials is investigated that passively adjusts its radiative properties as a function of temperature. At high temperatures, the proposed device exhibits low absorption at the vertical angles but high emission at high angles, facilitating effective heat dissipation even under solar irradiance, while at low temperatures it acts as a low-emitting material. It can be applied to a radiator used in spacecraft. The device property is experimentally demonstrated by measuring the reflectance at different temperatures and angles. The results demonstrate that the temperature change caused by the difference in solar illumination is reduced from 210.6°C for blackbody radiators to 120.8°C, even while maintaining a similar maximum temperature level when exposed to full sunlight.

Thermal radiation management remains a challenge because of the incoherent and isotropic nature of electromagnetic waves. In this study, a multiband and angular-selective infrared emitter, consisting of a simple one-dimensional (1D) metal-dielectric-metal metamaterial, is demonstrated. Although this structure has been well-known as spectrally selective emitters, we analytically reveal that when the dielectric layer thickness is much smaller than the wavelength of interest (80°) can be obtained at multiple resonant wavelengths. Even though relatively broadband (8–14 μm) emission and angular shaping has been achieved through gradient ENZ structures in the far-IR range, the spectral control has been difficult. Recently, we have developed the directive emission with tunable spectral bands, as the emission peaks can be entirely characterized by geometrical parameters. This angular selective technology offers flexible control of thermal radiation and can be adjusted to specific applications.

Metal–insulator–metal (MIM) diodes facilitate rectification at high frequencies. However, the rectification ratio for light from visible to infrared spectra is insufficient. In this study, we developed a MIM diode with a strongly enhanced electric field achieved using the self-assembly of Pt nanoparticles (NPs) via atomic layer deposition. By shaping the tunneling barrier, current density and asymmetry were simultaneously enhanced by several orders of magnitude compared with the symmetric MIM diode. The diode efficiency of the proposed MIM diodes was experimentally demonstrated to be 231 times greater than that of the MIM diode without NPs. This strategy indicates significant potential for high-frequency rectification applicable in optical rectenna.

A possible way to realize high-efficiency solar-thermophotovoltaics (TPV) systems is to use TPV cells with high out-of-band reflectance to take advantage of the high-efficiency photon recycling effect. However, to take full advantage of this effect, a system that confine the thermal radiation from the emitter and effectively transports it to the TPV cell is required at the same time. In this study, the effect of non-ideal confinement performance is analytically demonstrated. The results suggest that a 15% energy loss in the confinement system may result in a system efficiency loss of about 10%, especially in the low input power density region. The superiority of the confined cuboid absorber/emitter system compared to the open type of the planar system is also analytically demonstrated.

This study proposes an optical rectenna that combines a hollow resonator with a metal–insulator–metal (MIM) tunnel diode that is capable of photoelectric conversion (at various visible and infrared wavelengths). It enables the conversion of thermal radiation with different peak wavelengths, such as sunlight and thermal radiation (from heat sources in various temperature ranges), into electric power. The MIM tunnel diode was placed on the wall of a hollow resonator. It rectified the induced current generated by the resonance of the magnetic field. The photoelectric conversion capability of the proposed device applied to visible light is experimentally demonstrated in this study.