Polarization anomaly in high harmonics in the crossover region between perturbative and extreme nonlinearity in GaAs

We investigate the characteristics of high harmonics (HHs) unique to nonperturbative nonlinear regime. show that polarization state HHs generated from GaAs changes drastically across crossover weak-field perturbative regime strong-field extreme regime, while linearly polarized infrared excitation field $({E}_{\mathrm{exc}})$ is fixed a particular crystal direction. The dependence on ${E}_{\mathrm{exc}}$-field strength reveals multiple emission processes with different orders and temporal phases contribute each order HH, interference among them plays pivotal role. This manifests itself as phenomenon: large HH ellipticity emerges in course crossover, despite fact hosts no magnetization or linear birefringence. These results demonstrate not only material's symmetry but also ultrafast dynamics largely affects polarization, hence, its provide useful experimental tool probe coherent light-driven solid-state materials.