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

TL;DR

This study investigates the ultrafast dynamics of 3D Dirac fermions in Cd$_3$As$_2$, revealing their cooling mechanisms and a long-lived population inversion, contrasting with 2D graphene behaviors.

Contribution

It provides the first direct experimental evidence of population inversion and detailed ultrafast relaxation dynamics in a 3D Dirac semimetal using TrARPES.

Findings

Linear energy dependence of relaxation rate indicates intraband cooling.

Observation of 3.0 ps population inversion in conduction band.

Contrasts with faster interband relaxation in 2D graphene.

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.