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

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• High-order harmonic generation
• Nonlinear optics
• Polarization of light
• Ultrafast optics
• III-V semiconductors
• High-harmonic generation
• Infrared spectroscopy
• Optical absorption spectroscopy
• Optical techniques
• Photoluminescence