Chiral terahertz wave emission from the Weyl semimetal TaAs

Y Gao,D E Kharzeev,Y Qin,X Chen,Y P Liu,Z Li,E J Philip,W L Zhang,J Qi,S Kaushik,H Weng,Y L Su,M K Liu

doi:10.1038/s41467-020-14463-1

Abstract

Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. Here, we discover strong coherent terahertz emission from Weyl semimetal TaAs, which is demonstrated as a unique broadband source of the chiral terahertz wave. The polarization control of the THz emission is achieved by tuning photoexcitation of ultrafast photocurrents via the photogalvanic effect. In the near-infrared regime, the photon-energy dependent nonthermal current due to the predominant circular photogalvanic effect can be attributed to the radical change of the band velocities when the chiral Weyl fermions are excited during selective optical transitions between the tilted anisotropic Weyl cones and the massive bulk bands. Our findings provide a design concept for creating chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics using Weyl physics.

Highlights

Weyl semimetals host chiral fermions with distinct chiralities and spin textures
Refs. 20,21,25 show the existence of a dominant helicity-dependent DC photocurrent owing to the circular photogalvanic effect (CPGE) in the Weyl semimetals (WSMs) TaAs
Similar THz emission was observed in the ferroelectric semiconductors[30,31], where the linear photogalvanic effect (LPGE) across the bandgap in a wide photon-energy range has been studied

Summary

Introduction

Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. 20,21,25 show the existence of a dominant helicity-dependent DC photocurrent owing to the circular photogalvanic effect (CPGE) in the WSM TaAs. In contrast, with linearly polarized light, a giant linear photogalvanic effect (LPGE) (or shift current) was observed in ref.

Results

Conclusion