分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Spectral/angular emissivity $e$ and absorptivity ${\alpha}$ of an object are widely believed to be identical by Kirchhoff's law of thermal radiation in reciprocal systems, but this introduces an intrinsic and inevitable energy loss for energy conversion and harvesting devices. So far, experimental evidences of breaking this well-known balance are still absent, and previous theoretical proposals are restricted to narrow single-band nonreciprocal radiation. Here we observe for the first time, to our knowledge, the violation of Kirchhoff's law using ultrathin ($0.6$ has been experimentally demonstrated under a moderate external magnetic field. Moreover, based on magnetized ENZ building blocks supporting asymmetrically radiative Berreman and surface ENZ modes, we show versatile shaping of nonreciprocal thermal radiation: single-band, dual-band, and broadband nonreciprocal emission spectra at different wavebands. Our findings of breaking Kirchhoff's law will advance the conventional understanding of emission and absorption processes of natural objects, and lay a solid foundation for more comprehensive studies in designing various nonreciprocal thermal emitters. The reported recipe of diversely shaping nonreciprocal emission will also breed new possibilities in renovating next-generation nonreciprocal energy devices in the areas of solar cells, thermophotovoltaic, radiative cooling, etc.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Daytime radiative cooling is a promising passive cooling technology for combating global warming. Existing daytime radiative coolers usually show whitish colors due to their broadband high solar reflectivity, which severely impedes applications in real-life situations with aesthetic demands and effective display. However, there is a trade-off between vivid colors and high cooling performance because colors are often produced by absorption of visible light, decreasing net cooling power. To break this trade-off, we design multilayered structures with coupled nanocavities and produce structural colors with high cooling performance. Using this design, we can obtain colorful radiative coolers which show a larger color gamut (occupying 17.7% sRGB area) than reported ones. We further fabricate colorful multilayered radiative coolers (CMRCs) and demonstrate they have temperature drops of 3.4 - 4.4 degrees on average based on outdoor experiments. These CMRCs are promising in thermal management of electronic/optoelectronic devices and outdoor facilities.
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