In recent years, society has become more concerned about global warming, and there is a strong demand for power generation that does not emit, or emits less CO2 and for technological development (green innovation) that contributes to reducing energy consumption. In the field of plasmonics and metamaterials, on the other hand, the main subject is the improvement of the performance and the realization of new functions in optical devices by artificially modifying both the electromagnetic field itself and its interaction with matter. These possibilities of radiation field control are expected to help to achieve the developments of technologies that can contribute to the green innovation.

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This paper comprehensively overviews the principles of thermal radiation control by nano/micro-structures. First, we review the history of thermal radiation control, starting from the spontaneous emission of atoms placed in a microcavity. Next we discuss the effects of a microcavity and surface waves on thermal radiation. Finally, the narrow and wideband perfect absorbers based on metamaterials/metasurfaces are discussed.

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Thermophotovoltaic (TPV) systems, which convert heat into electricity by irradiating PV cells with thermal emission from heated objects, feature their high output power density and potentially high conversion efficiency. To increase the output power density and conversion efficiency of TPV systems, it is important to enhance the thermal emission above the bandgap energy of the PV cell while suppressing the emission below it. Here, we show our recent experimental demonstrations of far-field and near-field TPV systems based on photonic nanostructures. In the far-field experiment, we develop silicon rod-type photonic-crystal thermal emitters which exhibit near-infrared frequency-selective thermal emission with suppressed background emission, and demonstrate a heat-to-electrical conversion efficiency of 11.2% at 1338 K. In the near-field experiment, we develop a one-chip near-field TPV device integrating a thinfilm Si emitter and InGaAs PV cell with a deep sub-wavelength gap (＜150 nm), achieving photocurrent generation overcoming the far-field blackbody limit at 1192 K.

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Daytime and nighttime radiative cooling is possible if a surface has high solar reflectivity and high mid-infrared emissivity. While the main focus of radiative cooling is to achieve cooler surfaces, radiative cooling can be used to generate temperature difference against ambient temperature. In this review, we review our recent work where a combination of radiative cooler and a thermoelectric module can be an energy harvester. The unique feature of the device is that it can generate voltage whole day continuously which is quite challenging with other techniques. By further improving the output power, it is anticipated that the device can be used to self-power off-grid sensors.

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We propose metamaterial thermoelectric conversion that can produce electricity even in a uniform thermal radiation environment. A metamaterial absorber attached on the one end of a thermoelectric device absorbs thermal radiation emitted from the surrounding media, creating a thermal gradient across a thermoelectric element. We experimentally determined that a thermal gradient of 0.14 K on the device was created in a uniform thermal radiation environment at 364 K.

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This paper reports the development of an energy-harvesting (EH) device based on passive radiative cooling (RC). We first explain RC’s principle and then describe the development of the materials for a daytime RC. The non-radiative cooling materials of the silver nanowire matrix are also described. Finally, an EH device combined with a solar cell is presented, which our group developed. EH devices that can generate electricity even at night are expected to be used in places where grid power fails to not reach.

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Plasmonics enable significant improvements in the blue emission efficiencies of InGaN/GaN-based semiconductors. However, it remains difficult to achieve high efficiency using plasmonics and other methods in the green wavelength range of ~550 nm, which is the peak of human visual sensitivity. Semiconductor LEDs suffer from a systematic drop in efficiency in the green/yellow region, known as the “green gap,” and this gap must resolved. In this review, the author, who has been working on this issue since the early 2000s, explains a new method using Ag nanoparticles and dielectric nanofilm structures to solve this problem and obtains highly efficient green light emission.

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