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Spectral/angular emissivity $e$ and absorptivity ${\alpha}$ of an object arewidely believed to be identical by Kirchhoff's law of thermal radiation inreciprocal systems, but this introduces an intrinsic and inevitable energy lossfor energy conversion and harvesting devices. So far, experimental evidences ofbreaking this well-known balance are still absent, and previous theoreticalproposals are restricted to narrow single-band nonreciprocal radiation. Here weobserve for the first time, to our knowledge, the violation of Kirchhoff's lawusing ultrathin ($<{\lambda}/40$, ${\lambda}$ is the working wavelength)magnetized InAs semiconductor films at epsilon-near-zero (ENZ) frequencies.Large difference of $|{\alpha}-e|>0.6$ has been experimentally demonstratedunder a moderate external magnetic field. Moreover, based on magnetized ENZbuilding blocks supporting asymmetrically radiative Berreman and surface ENZmodes, we show versatile shaping of nonreciprocal thermal radiation:single-band, dual-band, and broadband nonreciprocal emission spectra atdifferent wavebands. Our findings of breaking Kirchhoff's law will advance theconventional understanding of emission and absorption processes of naturalobjects, and lay a solid foundation for more comprehensive studies in designingvarious nonreciprocal thermal emitters. The reported recipe of diverselyshaping nonreciprocal emission will also breed new possibilities in renovatingnext-generation nonreciprocal energy devices in the areas of solar cells,thermophotovoltaic, radiative cooling, etc.

Nonreciprocal thermal radiation in ultrathin magnetized epsilon-near-zero semiconductors