Fig. 1. Characterization of the WS₂ films with different layers used in this study. (a) Optical image of sample with size of 10 mm × 10 mm. The WS₂ film was plated on a 0.3 mm sapphire substrate by the CVD method. (b) Raman spectra of the three-layer WS₂ at different points. (c)–(e) Raman spectra of WS₂ films with 1–5 layers. (f)–(h) The surface roughness of the 1L, 3L, and 5L samples characterized by AFM.

Fig. 2. (a) Schematic diagram of the pump-probe-based optical configuration. (b) Schematic diagram of the marked area in (a). The probe light spot size is smaller than the pump light spot and falls in the center of the pump light spot on the sample. NPBS, non-polarized beam splitter; P, polarizer; F, neutral filter wheel; M, mirror; S, mechanical shutter; L, lens.

Fig. 3. (a) Static spectrum I₀ (black line) and dynamic spectrum I₁ (blue line). The time of pump excitation is marked. (b) Spectrum after differential processing. The open circles are the measured data, and the solid line represents the corresponding fitting result using a biexponential function. The spectra shown in this figure are the results of the three-layer WS₂ sample.

Fig. 4. Temporal evolution of the transmitted probe light intensity ΔI/I₀ of the WS₂ films with different layers (1L, 3L, and 5L). The open circles are extracted from the measured spectrum, and the solid lines are the results of fitting using a biexponential function.

Fig. 5. (a) Characteristic time of the carrier excitation process as a function of the material layers. (b) The layer dependence of the carrier fast decay characteristic time and (c) the slow decay characteristic time in the process of carrier decay. The corresponding solid lines in (b) and (c) are the fitting results using an exponential function.

Table 1. The Characteristic Times τ₂ and τ₃ of Different Layers