  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 ($<{\lambda}/40$, ${\lambda}$ is the working wavelength)
magnetized InAs semiconductor films at epsilon-near-zero (ENZ) frequencies.
Large difference of $|{\alpha}-e|>0.6$ has been experimentally 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.

Optics;Materials Science;Applied Physics