Fig. 1. Experimental setup for HHG and the crystal structure of Bi₂Se₃. (a) Experimental setup for driving laser polarization tuning and high harmonic polarization state measurement; (b) crystal structure of Bi₂Se₃ and schematic diagram of mid-infrared laser incidence, with the driving light making a 90° angle with the horizontal direction; (c) schematic diagram of the measurement of major axis direction and ellipticity angle; (d) schematic diagram of the three-dimensional crystal structure of Bi₂Se₃

Fig. 2. Polarization characteristics of the even harmonics H6 and H8 vary with the azimuth angle. (a) Polarization orientation angle α; (b) ellipticity ε

Fig. 3. Polarization characteristics of the odd harmonics H5 and H7 vary with the azimuth angle. (a) Polarization orientation angle α; (b) ellipticity ε

Fig. 4. Simulation results of the polarization characteristics of even harmonics H6 and H8 as a function of the azimuth angle. (a) Polarization orientation angle α; (b) ellipticity ε

Fig. 5. Band structure of Bi₂Se₃

Fig. 6. Orthogonal components of dipole moment phase in polar coordinate system. (a) αcv,k component of the transition dipole moment phase; (b) αcv,θ component of the transition dipole moment phase