Fig. 1. Synthesis and structure characterization of α-In₂Se₃ nanoflakes. (a) Side view of the crystal structure of α-In₂Se₃ in R3m space group. The quintuple layers (QLs) are stacked with weak vdW interactions. (b) Raman spectrum of α-In₂Se₃. Labeled peaks represent A(LO + TO) and A(LO) modes, respectively. (c) AFM image of the α-In₂Se₃ nanoflakes with the thickness of 1.28 nm.

Fig. 2. THz generation from thin film and bulk α-In₂Se₃. (a) indicates the detection geometry pumped by 800 nm pulse. The incident pulse polarization angle θ and sample azimuthal angle ϕ within the ab plane are two degrees of freedom in studying the emitted THz properties. (b) Characterization of THz pulses from 40 nm thin film (blue) and bulk (gray) as a reference. (c) Polarization dependency of the THz pulse peak intensity and fitting by Eq. (1). (d) Azimuthal angle dependence of the THz amplitude with two types of fittings according to Eq. (2).

Fig. 3. Carrier dynamics investigated by reflective optical pump-probe method. (a) Schematic diagram of the measuring geometry. The UV pump beam (400 nm) and the IR probe beam (800 nm) are focused and overlapped on the sample surface with a spot size of 141 and 114 µm, respectively. The reflected signal is monitored by a silicon-balanced photodetector. (b), (c) Time-resolved reflectivity change ΔR/R of bulk and thin film α-In₂Se₃ as a function of pump-probe time delay at different pump fluence; (d)–(f) signal of reflectivity from thin films with decreased thickness of 10, 5.5, and 4.5 nm.

Fig. 4. Parameters of carrier dynamics retrieved from reflectivity decaying data. Left (a), (c), (e) are obtained from reference bulk sample. Right (b), (d), (f) are derived from thin-film sample. Normalized reflectivity data from (a) bulk and (b) 40 nm thin film. Dashed curves in (c) and (d) describe the amplitudes of fast (blue) and slow (red) decay varying along with pump fluence intensity. The relative amplitudes are shown in the insets. Dashed curves in (e) and (f) represent time constant change from fast (blue) and slow (red) decay at series of pump fluence.