Weyl fermions get wild under terahertz light

Weyl fermions were predicted to be massless elementary particles long ago, but have not yet been discovered in high-energy physics experiments. About a decade ago, Weyl fermions are realized in the so-called Weyl semimetals – a family of solid-state materials, which is a hallmark breakthrough in condensed matter physics.
As an effective description of low-energy excitations, the Weyl fermions have been very extensively investigated by a variety of methods in the linear response regime. Although theoretical studies have predicted strong nonlinear responses of Weyl fermions, an experimental work on the low-energy nonlinear properties of Weyl fermions is still missing. By carrying out time-resolved ultrafast terahertz (THz) high-harmonic generation spectroscopic experiment, we demonstrated how Weyl fermions respond to strong THz electromagnetic fields on the picosecond timescale. Their findings were obtained from the measurements on ultrathin films of tantalum phosphide (TaP) – a prototypical Weyl semimetal. By driving the material with intense THz electromagnetic pulses, the team observed emission of third-harmonic signals, that is an electromagnetic radiation at three times the frequency of the incoming THz lightwave.
The observed third-order harmonic radiation is highly characteristic: The radiation exhibits a cubic power law dependence on the driving pulse intensity. The third harmonic yield is sensitively dependent on the THz drive polarization, with a maximum for linear polarization and a minimum for circular polarization. Since these properties are found at room temperature and in an ultrafast picosecond timescale, this work demonstrates that Weyl semimetals are relevant for potential applications in next-generation imaging, sensing, and ultrafast electronics.
