Transient reflectance of photoexcited Cd$_3$As$_2$

TL;DR

This study investigates ultrafast charge-carrier dynamics in the 3D Dirac semimetal Cd3As2 using transient reflectance measurements, revealing two relaxation processes linked to electron cooling via phonon emission.

Contribution

First ultrafast transient-grating measurements of Cd3As2 at different temperatures, identifying distinct electron relaxation processes and their relation to phonon interactions.

Findings

Charge carriers relax via two processes: 500 fs and 3.1 ps.

Faster process linked to filling of phase space near photon energy.

Slower process associated with cooling through phonon emission.

Abstract

We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd3As2, at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of duration 500 fs and the other of duration 3.1 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to a decrease in absorption, and the slower signal to a decrease in the light's phase velocity, at the probe energy. We attribute these signals to electrons' filling of phase space, first near the photon energy and later at lower energy. We attribute their decay to cooling by rapid emission of optical phonons, then slower emission of acoustic phonons. We also present evidence that both the electrons and the lattice are strongly heated.