Harmonic generation provides an efficient tool for the study of ultrafast nonlinear dynamics. We report on time-resolved optical-pump terahertz-harmonic-generation spectroscopic investigation of ultrafast nonlinearity in a prototypical three-dimensional Dirac semimetal Cd3As2. A transient population inversion characterized by excessive nonthermal Dirac electrons and holes is found to be very sensitive and responsive to a periodic terahertz drive, leading to very efficient terahertz third-harmonic generation. Based on the Boltzmann transport theory, we analyze the terahertz field-driven kinetics of the transient Dirac fermions that is responsible for the observed strong terahertz nonlinearity.
@article{arxiv.2401.08257,
title = {Population inversion and ultrafast terahertz nonlinearity of transient Dirac fermions in Cd$_3$As$_2$},
author = {Changqing Zhu and Patrick Pilch and Renato M. A. Dantas and Anneke Reinold and Yunkun Yang and Faxian Xiu and Amilcar Bedoya-Pinto and Stuart S. P. Parkin and Roderich Moessner and Zhe Wang},
journal= {arXiv preprint arXiv:2401.08257},
year = {2024}
}