Figure 2: THz nonlinear behaviours of the Bi₂Se₃ topological insulator.

Integrated transmittance of Bi₂Se₃ 120-QL (a), Bi₂Se₃ 60-QL (b) and (Bi_0.9In_0.1)₂Se₃ 60-QL (c) films, respectively, as a function of the incident THz electric field amplitude. Experimental data are represented by red dots. The error bars on the order of 0.5% on both the integrated transmittance and electric field amplitude correspond to the statistical fluctuations of the measured signals averaged over 100 shots of the SPARC THz source. The dashed dotted blue line corresponds to a fit with a saturable absorption model that is described in the main text. Insets: spectrally resolved transmittance curves measured (solid lines) at 1 MV cm⁻¹ (red curve), 0.4 MV cm⁻¹ (green curve) and 0.1 V cm⁻¹ (blue curve) for Bi₂Se₃ 120 QL (a) and Bi₂Se₃ 60 QL (b). The spectrally resolved transmittance of (Bi_0.9In_0.1)₂Se₃ 60 QL at 0.1 V cm⁻¹ (blue curve) and 1 MV cm⁻¹ (red curve), which are superimposed in the limit of our sensitivity, is shown in the inset of c. The slow modulation in the spectrally resolved transmittances are related to a non-perfect compensation of water absorption in the THz range.