Revealing the ultrafast dynamics of three-dimensional (3D) Dirac fermions upon photoexcitation 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) Dirac fermions and whether there is population inversion are fundamental questions that remain to be answered. Here we reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd3As2 by ultrafast time- and angle-resolved photoemission spectroscopy (TrARPES) with a tunable probe photon energy from 5.3 - 6.9 eV. 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 τn = 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 where the interband relaxation occurs on a much faster timescale.
@article{arxiv.2112.09371,
title = {Population inversion and Dirac fermion cooling in 3D Dirac semimetal Cd$_3$As$_2$},
author = {Changhua Bao and Qian Li and Sheng Xu and Shaohua Zhou and Xiang-Yu Zeng and Haoyuan Zhong and Qixuan Gao and Laipeng Luo and Dong Sun and Tian-Long Xia and Shuyun Zhou},
journal= {arXiv preprint arXiv:2112.09371},
year = {2022}
}