Fig. 1. EDS mapping of Zn element distribution in (a) lightly doped (x=0.14) and (b) heavily doped (x=0.31) (Cd1−xZnx)3As2 films. (c) Normalized XRD patterns of (Cd1−xZnx)3As2 films with different doping concentration. (d) Raman spectra of the three samples. (e) Schematic diagram of our OPTP experimental setup and sample structure. (f) Tetragonal unit cell of Cd3As2.

Fig. 2. Pump-fluence-dependent differential THz transmission, ΔE/E0, as a function of delay time in (a) intrinsic, (b) lightly doped, and (c) heavily doped sample; the solid lines are the curves derived from a convoluted mono-exponential fitting. (d) The maximal ΔE/E0 versus pump fluence for three samples. The circles denote experimental data, and the solid lines represent the linear fitting curves. (e) Fitted decay time constants of three samples using a single exponential decay function at various pump fluences; the solid lines serve as guide to the eye. (f) The decay rate 1/τ is plotted with respect to pump fluence for the heavily doped sample, with the solid line representing a linear fit to the data.

Fig. 3. (a) Temperature-dependent transient THz transmission response of the heavily doped alloy, excited at a fixed pump fluence of 18.75  μJ/cm2. (b) Temperature dependence of the fitted decay time constants, where the solid lines are guide to the eye.

Fig. 4. Schematics for relaxation dynamics of photoexcited carriers near the Fermi level in (a) Dirac semimetal phase and (b) trivial semiconductor phase. It should be noted that the relaxation of holes in (b) is similar to that of electrons.

Fig. 5. Normalized THz differential transmission responses at various temperatures under a fixed pump fluence of 18.75  μJ/cm2 in (a) intrinsic and (b) lightly doped samples.

Fig. 6. Calculated electronic band structure of 0.31-Zn-doped (Cd1−xZnx)3As2 based on VASP.

Fig. 7. Transmitted THz time-domain signals of the heavily doped (Cd1−xZnx)3As2 thin film under different pump fluences and delay times. Inset shows the zoom-in around the THz peak.

Fig. 8. Complex THz photoconductivity dispersion and Drude-Smith model fitting under (a) various pump fluences at a delay time of 1 ps and (b) various delay times at 18.75  μJ/cm2. Blue square and red circle denote real and imaginary parts, respectively, and solid lines are fitting curves with the Drude-Smith model. The fitted plasma frequency ωp versus (c) pump fluence and (d) delay time.