Fig. 1. Experimental setup. The fundamental and SHG are linearly polarized in s-polarization direction. The mirror plane of Bi₂Se₃ is rotated to be parallel to the polarization of the pump field.

Fig. 2. HHG spectrum with and without the SHG, respectively; the intensity of the fundamental is 77 GW/cm²; gray line, the SHG intensity is 0; green line, the SHG intensity is 0.025 GW/cm²; ϕ = π. The small peak at 1.48 eV is the second-order diffraction of H9 from the grating-based spectrometer.

Fig. 3. Harmonic yield versus the phase delay of two-color laser fields. (a) Modulation of H4 for the SHG intensity of 0.025 GW/cm² (blue square and line) and 1.2 GW/cm² (orange square and line); (b), (c) Modulation of H5 (b) and H6 (c) with the same experimental condition as (a).

Fig. 4. Calculated high-harmonic spectra. (a) HHG from the surface states with and without the SHG; blue line, harmonic spectrum without the SHG; orange line, harmonic spectrum for the SHG intensity equal to 40 MW/cm² and ϕ = 0.6π. (b) HHG from the bulk states with and without the SHG; blue line, harmonic spectrum without the SHG; orange line, harmonic spectrum for the SHG intensity equal to 40 MW/cm² and ϕ = 0.6π.

Fig. 5. High-harmonic yield modulated with the phase delay of two-color laser fields. (a), (b) Modulation of H4 (a) and H5 (b) from the surface states, for the SHG intensity of 4 MW/cm² (red line) and 40 MW/cm² (blue line), respectively; (c), (d) modulation of H4 (c) and H5 (d) from the bulk states, for the SHG intensity of 4 MW/cm² (red line), 40 MW/cm² (blue line), and 400 MW/cm² (purple line), respectively.