Population Inversion and Dirac Fermion Cooling in 3D Dirac Semimetal Cd<sub>3</sub>As<sub>2</sub>
Changhua Bao, Qian Li, Sheng Xu, Shaohua Zhou, Shaohua Zhou, Xiangyu Zeng, Haoyuan Zhong, Qixuan Gao, Laipeng Luo, Dong Sun, Tian‐Long Xia, Shuyun Zhou, Shuyun Zhou
Abstract
Revealing the ultrafast dynamics of three-dimensional (3D) Dirac fermions is critical for both fundamental science and device applications. So far, how the cooling of 3D Dirac fermions differs from that of two-dimensional (2D) and whether there is population inversion are fundamental questions to be answered. Here we reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd3As2 by time- and angle-resolved photoemission spectroscopy with a tunable probe photon energy. The energy- and momentum-resolved relaxation rate shows a linear dependence on the energy, suggesting Dirac fermion cooling through intraband relaxation. Moreover, a population inversion is reported based on the observation of accumulated photoexcited carriers in the conduction band with a lifetime of 3.0 ps. Our work provides direct experimental evidence for a long-lived population inversion in a 3D Dirac semimetal, which is in contrast to 2D graphene with a much shorter lifetime.