The enhancement of even-order harmonics near the cut-off of high-order harmonic spectra from monolayer MoS2 has been experimentally observed recently by several groups. Here we demonstrate that this enhancement can be interpreted as a result of spectral interference between half-cycles with opposite polarity by adopting a fully quantum mechanical calculation. We found that, due to the energy modulation induced by Berry connections, only half-cycles with the same polarity can generate high-order harmonics near the cut-off frequency, thus the lack of destructive interference leads to the enhanced intensity of the corresponding even-order harmonics. The explanation is supported by the frequency shift of the measured harmonic peaks. Our finding revealed the role of inter-half-cycle interference in high-harmonic generation (HHG) from non-centrosymmetric materials.

High harmonic generation (HHG) driven by counter-rotating bicircular (CRB) pulses excitation has been observed from several solid targets, where circularly polarized harmonics are emitted. We study this process using time-dependent density functional theory (TDDFT) to calculate the crystal orientation dependence of the circularly polarized high harmonics from a monolayer h-BN. The resulted can be interpreted by the real space electron dynamics of electrons in polar chemical bonds. The yield of circularly polarized high harmonics (CHHs) can be optimized by controlling the direction of valence electron dynamics. Our findings pave the way for exploring the binding potential from spectrum and all-optically processing information.