Mid-infrared transient reflectance study of the Dirac semimetal Cd3As2 under strong optical pumping

The three-dimensional Dirac semimetal Cd3As2 has attracted intensive attention recently for its exotic properties. Understanding the hot carrier excitation and its subsequent relaxation is a prerequisite for Cd3As2-based optoelectronics functional in the infrared (IR) and terahertz (THz) frequency range. In this work, the photoexcited hot carrier dynamics of a bulk single crystal Cd3As2 in the mid-IR has been explored by using the time-resolved pump-probe reflectance (ΔR/R) measurement at room temperature under a strong optical pumping. By combining the experimental ΔR/R results and theoretical modeling, we analyzed the transient hot carrier redistribution upon a strong photoexcitation and the subsequent interband transitions. We show that the ΔR/R response of Cd3As2 has a complex behavior, due to the interplay of transitions between Dirac bands and transitions involving both the Dirac and non-Dirac bands. Throughout the mid-IR region measured, we find that ΔR/R is contributed primarily by changes in the refractive index, rather than by changes in the extinction coefficient. Our findings can help understanding the transient hot electron excitation and relaxation in Cd3As2 under an intense optical pumping, and provide a valuable reference for the Cd3As2-based ultrafast optoelectronics application.