Photoemission established tantalum phosphide as a Weyl semimetal, which hosts exotic Weyl fermion quasiparticles and Fermi arcs.

The recent observation of high-harmonic generation from solids 1-8 creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures 9,10 . All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field 11-20 , making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its

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The recent discovery of the first Weyl semimetal in TaAs provides the first observation of a Weyl fermion in nature and demonstrates a novel type of anomalous surface state, the Fermi arc. Like topological insulators, the bulk topological invariants of a Weyl semimetal are uniquely fixed by the surface states of a bulk sample. Here, we present a set of distinct conditions, accessible by angle-resolved photoemission spectroscopy (ARPES), each of which demonstrates topological Fermi arcs in a surface state band structure, with minimal reliance on calculation. We apply these results to TaAs and NbP. For the first time, we rigorously demonstrate a non-zero Chern number in TaAs by counting chiral edge modes on a closed loop. We further show that it is unreasonable to directly observe Fermi arcs in NbP by ARPES within available experimental resolution and spectral linewidth. Our results are general and apply to any new material to demonstrate a Weyl semimetal.

Objects around us constantly emit and absorb thermal radiation [1][2][3][4][5][6]. The emission and absorption processes are governed by two fundamental radiative properties: emissivity and absorptivity. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation [1,2,7]. This restriction limits the degree of freedom to control thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems such as solar cells [8]. Existing approaches to violate Kirchhoff's law typically utilize conventional magnetooptical effects in the presence of an external magnetic field [9,10]. However, these approaches require either a strong magnetic field (∼3T) [9], or narrow-band resonances under a moderate magnetic field (∼0.3T) [10], because the non-reciprocity in conventional magneto-optical effects is usually weak in the thermal wavelength range. Here, we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters that near completely violate Kirchhoffs law over broad angular and frequency ranges, without requiring any external magnetic field. The non-reciprocity moreover is strongly temperature tunable, open new possibilities for active non-reciprocal devices in controlling thermal radiation.[1] G. Kirchhoff, Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht, Annalen der Physik 185, 275 (1860).