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Population inversion and ultrafast terahertz nonlinearity of transient Dirac fermions in Cd3As2

Dr. Changqing Zhu1, Mr. Patrick Pilch1, Dr. Renato M.A. Dantas2,3,4, Ms. Anneke Reinold1, Mr. Yunkun Yang5,6,7, Prof. Faxian Xiu5,6, Dr. Amilcar Bedoya-Pinto8,9, Prof. Stuart S.P. Parkin8, Prof. Roderich Moessner2, Prof. Zhe Wang1
1Department of Physics, TU Dortmund University, Dortmund, Germany. 2Max Planck Institute for the Physics of Complex Systems, Dresden, Germany. 3Department of Physics, University of Basel, Basel, Switzerland. 4Center of Physics, University of Minho, Braga, Portugal. 5State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai, China. 6Shanghai Research Center for Quantum Sciences, Shanghai, China. 7Beijing Academy of Quantum Information Sciences, Beijing, China. 8Max Planck Institute for Microstructure Physics, Halle (Saale), Germany. 9Institute of Molecular Science, University of Valencia, Paterna, Spain

Abstract

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.