Orientation dependence of temporal and spectral properties of high-order harmonics in solids

We investigate the connection between crystal symmetry and temporal and spectral properties of high-order harmonics in solids. We calculate the orientation-dependent harmonic spectrum driven by an intense, linearly polarized infrared laser field, using a momentum-space description of the generation process in terms of strong-field-driven electron dynamics on the band structure. We show that the orientation dependence of both the spectral yield and the subcycle time profile of the harmonic radiation can be understood in terms of the coupling strengths and relative curvatures of the valence band and the low-lying conduction bands. In particular, we show that in some systems this gives rise to a rapid shift of a quarter optical cycle in the timing of harmonics in the secondary plateau as the crystal is rotated relative to the laser polarization. We address recent experimental results in MgO [Y. S. You, Nat. Phys. 13, 345 (2017).1745-247310.1038/nphys3955] and show that the observed change in orientation dependence for the highest harmonics can be interpreted in the momentum space picture in terms of the contributions of several different conduction bands