Fig. 1. Schematic diagram of the open-aperture micro P-scan system. A 1/2 wavelength slide and a Glan prism are used to modulate the intensity of the laser. The chopper modulates the laser from 1 kHz to 500 Hz. An IDS camera and a Mitutoyo near-infrared objective lens (20×, NA=0.40) are used to build up the video microscope system (VMS). Two lock-in amplifiers are used to measure the power of the laser.

Fig. 2. Characterization of the continuous layered Bi2Se3 films. (a) Streaks of Bi2Se3 on Al2O3 substrate. The lattice constant values of the epitaxy Bi2Se3 film is 4.194 Å. (b) AFM profile of layered Bi2Se3 film. (c) Raman spectra of these five different quintuple layer (QL) films. (d) LAS of different QL films. The peak absorption wavelengths of different QLs are 506, 572, 616, 626, and 650 nm.

Fig. 3. Transmission spectra induced by the 600, 700, and 800 nm lasers. (a), (c), (e) Summary curves of different Bi2Se3 films (2 QL, 4 QL, 8 QL, 10 QL, 16 QL) excited by 600, 700, and 800 nm lasers; (b), (d), (f) fitting curves of Bi2Se3 (16 QL) fitted by different models excited by 600, 700, and 800 nm lasers.

Fig. 4. (a) Diagrammatic sketch of carrier transition in BisSe3. Different from linear excitation, nonlinear excitation will pump the electrons from the valence band to the conduction band rather than from the Fermi level. (b), (c), (d) Saturation intensity/intrinsic nonlinear absorption coefficient/FCA cross section of Bi2Se3 with different thickness excited by 600 to 800 nm lasers, where αNL0 is the intrinsic nonlinear absorption coefficient and Is is the saturation intensity of αNL0.

Fig. 5. Relaxation process of carriers in 16-layer Bi2Se3 films excited by 600–660 nm lasers. When the photon energy descends from ∼2.07  eV (600 nm) to ∼1.88  eV (660 nm), the fast relaxation time (τ1) of carriers corresponding to the band increases from 0.64 to 0.83 ps.

Table 1. FCA Cross Section σFCA of Bi2Se3 Films under Different Excitation Wavelengths (cm2, ×10−22)

Table 2. Saturation Intensity Is of Bi2Se3 Films under Different Excitation Wavelengths (GW/cm2)

Table 3. Nonlinear Absorption Coefficient αNL0 of Bi2Se3 Films under Different Excitation Wavelengths (cm/GW, ×104)

Table 4. Modulation Depth of Bi2Se3 (Unit: dB)