Orientation-dependent high harmonic generation from h-BN

We investigate high harmonic generation from monolayer h-BN by solving the semiconductor Bloch equations. The parallel-polarized odd-order harmonics at the cut-off region show anomalous angular dependence that the angular distribution of odd orders over 7.31eV exhibit a 30◦ shift with respect to others. We find that the harmonic energy is correlated to dynamics at the k dependent bandgap at the M point in the reciprocal space. Time frequency analysis indicates that the harmonics in the cut-off region along the zigzag direction are only generated in a specific polarity of the laser half-cycle, contributing to the observed angular shift. The phenomenon has significant implications for utilizing high harmonic spectroscopy in reconstructing the electronic bands.