Fig. 1. Experimental setup for static and transient THz-TDS. For static spectroscopy, the pump pulse is blocked. The THz beam splitter (BS) together with mirror M are used in reflection mode.

Fig. 2. THz time domain traces transmitting through a 54-μm-thick sample of As30Se30Te40 and dry air. The inset shows the corresponding Fourier transformed spectra.

Fig. 3. (a) Refractive index and absorption coefficient and (b) the real and imaginary parts of the complex permittivity of As30Se30Te40. The error bars indicate the standard deviation based on three individual measurements.

Fig. 4. (a) Differential reflectivity of the THz peak electric field as a function of pump–probe delay time for pump fluences of 0.32–0.82  mJ/cm2. The symbols are experimental results, and the solid lines represent the decay kinetics model shown in the inset. (b) Normalized decay curves, which show that the kinetics slows down at increasing pump fluence.

Fig. 5. 2D map of (a) the real part and (b) the imaginary part of the photoinduced ultrafast conductivity of As30Se30Te40 at a pump fluence of 0.7  mJ/cm2.

Fig. 6. 2D map of photoinduced refractive index change of As30Se30Te40 in reflection measurement at pump fluence of 0.7  mJ/cm2.

Fig. 7. Complex photoinduced conductivity of As30Se30Te40 at pump fluence of 0.7  mJ/cm2. The solid lines are the fits based on the Drude–Smith model.

Fig. 8. Temporal development of the carrier dynamics parameters for As30Se30Te40 extracted from the Drude–Smith model fits at different pump–probe delays. (a) Carrier scattering time τ, (b) backscattering parameter c, and (c) carrier density. The shaded areas indicate the time at which the probe pulse arrives before the pump pulse, with poorly defined fit parameters.