Theory of solid-state harmonic generation governed by crystal symmetry

The solid-state harmonic generation (SSHG) derives from photocurrent coherence. The crystal symmetry, including point-group symmetry and time-reversal symmetry, constrains the amplitude and phase of the photocurrent, thus manipulates the coherent processes in SSHG. We revisit the expression of photocurrent under the electric dipole approximation and give an unambiguous picture of nonequilibrium dynamics of photocarriers on laser-dressed effective bands. In addition to the dynamical phase, we reveal the indispensable roles of the phase difference of transition dipole moments and the phase induced by shift vector in the photocurrent coherence. The microscopic mechanism of the selection rule, orientation dependence, polarization characteristics, time-frequency feature, and ellipticity dependence of harmonics governed by symmetries is uniformly clarified in our theoretical framework. Finally, we propose a nonlinear optical method for detecting the time-reversal symmetry breaking of crystals by using elliptical dichroism of SSHG. This work integrates nonequilibrium electronic dynamics of condensed matter in strong laser fields, and paves a way to explore more nonlinear optical phenomena governed by crystal symmetry.