# Chiral terahertz wave emission from the Weyl semimetal TaAs

## TL;DR

This paper reports the discovery of strong, coherent chiral terahertz emission from the Weyl semimetal TaAs, demonstrating ultrafast polarization control via colossal photocurrents driven by Weyl fermion excitations, opening new avenues in ultrafast opto-electronics.

## Contribution

It reveals a novel chiral terahertz emission mechanism in Weyl semimetals and demonstrates ultrafast polarization manipulation through large photocurrents induced by Weyl fermions.

## Key findings

- Strong chiral terahertz emission observed from TaAs.
- Ultrafast polarization control achieved via colossal photocurrents.
- Photocurrent maximized near 1.5 eV photon energy.

## Abstract

As a fascinating topological phase of matter, 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 chiral terahertz emission from the Weyl semimetal TaAs and demonstrate unprecedented manipulation over its polarization on a femtosecond timescale. Such polarization control is achieved via the colossal ultrafast photocurrents in TaAs arising from the circular or linear photogalvanic effect. We unravel that the chiral ultrafast photocurrents are attributed to the large band velocity changes when the Weyl fermions are excited from the Weyl bands to the high-lying bands. The photocurrent generation is maximized at near-IR frequency range close to 1.5 eV. Our findings provide an entirely new design concept for creating chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics using Weyl physics.