Population inversion and Dirac fermion cooling in 3D Dirac semimetal Cd${}_3$As${}_2$

Abstract

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 Cd${}_3$As${}_2$ 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 ${\tau }_n$ = 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.

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• Time- and Angle-Resolved Photoemission Studies of Quantum Materials
• Angle-resolved photoemission spectroscopy
• Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission
• Advancing time- and angle-resolved photoemission spectroscopy: The role of ultrafast laser development
• Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn${}_4$
• A newly-designed femtosecond KBe${}_2$BO${}_3$F${}_2$ device with pulse duration down to 55 fs for time- and angle-resolved photoemission spectroscopy
• Hidden charge density wave induced shadow bands and ultrafast dynamics of CuTe investigated using time-resolved ARPES
• Population inversion and ultrafast terahertz nonlinearity of transient Dirac fermions in Cd${}_3$As${}_2$
• Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd${}_3$As${}_2$
• Research on topological materials using ultrafast spectroscopy