  The topological three-dimensional Dirac semimetal Cd3As2 has drawn great
attention for the novel physics and promising applications in optoelectronic
devices operating in the infrared and THz regimes. Among the extensive studies
in the past decades, one intriguing debate is the underlined mechanism that
governing the nonequilibrium carrier dynamics following photoexcitation. In
this study, the temperature dependent photocarrier dynamics in Cd3As2 film has
been investigated with time-resolved terahertz spectroscopy. The experimental
results demonstrate that photoexcitation results in abrupt increase in THz
photoconductivity, and the subsequent relaxation shows a single exponential
relaxation for various temperatures and pump fluences. The relaxation time
increase from 4.7 ps at 5 K to 7.5 ps at 220 K, while the lifetime remains
almost constant of ~7.5 ps with temperature above 220 K. A Rothwarf-Taylor
model was employed to fit the temperature dependent relaxation time, and a
narrow energy gap of ~35 meV is obtained, which occurs around the Dirac node.
Our THz spectroscopy results demonstrate that the photocarrier relaxation in
Cd3As2 shows a semiconductor-like behavior, rather than hot carrier scatterings
in graphene and most of metals.
