Fig. 1. Lattice structure and the first Brillouin zone of single-layer MoS₂. (a) Lattice structure of single-layer MoS₂, where a₁ and a₂ are the two basis vectors of the lattice; (b) schematic of the first Brillouin zone of single-layer MoS₂

Fig. 2. Energy-band structure of single-layer MoS₂

Fig. 3. High-order harmonic spectra of the fundamental and second harmonics along Γ—K direction with different intensity ratios of the second harmonic and fundamental field. High-order harmonic spectra in (a) parallel and (b) perpendicular directions when intensity ratio is 0.001; high-order harmonic spectra in (c) parallel and (d) perpendicular directions when intensity ratio is 0.01

Fig. 4. Principle explanation diagram of high-order harmonic oscillation period

Fig. 5. Amplitude and phase of even-order harmonics oscillating in the parallel and perpendicular directions under different intensity ratios of the second harmonic and fundamental field. Amplitude ratio of even-order harmonics in (a) parallel direction and (b) perpendicular direction; phase of even-order harmonics in (c) parallel direction and (d) perpendicular direction

Fig. 6. High-order harmonic spectra in the parallel direction under different intensity ratios of the second harmonic and fundamental field. (a) Intensity ratio is 0.0001; (b) intensity ratio is 0.001; (c) intensity ratio is 0.01; (d) intensity ratio is 0.05

Fig. 7. Contribution of the second harmonic to the harmonic yield under different intensity ratios. High-order harmonic spectra with an intensity ratio of (a) 0.001 and (b) 0.01 compared to the high-order harmonic spectra with only the second harmonic field

Fig. 8. Amplitude ratio and phase oscillations of even-order harmonics in the parallel direction under different intensity ratios of the second harmonic and fundamental field. (a) Amplitude ratio; (b) phase