Three dimensional (3D) Dirac semimetal exhibiting high mobility due to strongly suppressed backscattering of massless Dirac Fermions, has recently attracted enormous research interests as 3D analogues of graphene. From the prospects of probable application toward electronic/optoelectronic devices with extreme performance taking advantages of ultrahigh mobility of Dirac semimetals, it is crucial to understand the relaxation dynamics of photo-excited carriers and their coupling with lattice. In this work, we report ultrafast transient reflection measurements of photo-excited carrier dynamics in cadmium arsenide (Cd3As2), which is among the most stable Dirac semimetals confirmed experimentally. With low energy probe photon of 0.3 eV, photo-excited Dirac Fermions dynamics closing to Dirac point are probed. Through transient reflection measurements on bulk and nanoplate samples that have different doping intensities, and systematic probe wavelength, pump power and lattice temperature dependent measurements, the dynamical evolution of carrier distributions can be retrieved qualitatively using a two-temperature model. The photo-excited carrier dynamics of Cd3As2 is found to be very similar to that of graphene. However, in contrast to graphene, as optical phonon energies are relatively low in Cd3As2, the optical phonon couplings are slower but last over large electron temperature range.
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