Enhancing high harmonic generation in GaAs by elliptically polarized light excitation

Reflecting crystal and electronic structures, high harmonic generation (HHG) in solids show intriguing behaviors that are absent in isotropic gases, and are expected to be novel light sources and spectroscopic tools. Recent studies reported that the efficiency of HHG in some materials can be enhanced by using elliptically polarized excitations, which we call "elliptical enhancement", while the mechanism underlying the phenomenon remains unknown. Here, we unambiguously demonstrate that the elliptical enhancement occurs in an archetypal direct-gap semiconductor GaAs and is accompanied by the emergence of nonlinear optical activity, despite that GaAs hosts no magnetization or linear birefringence. Remarkably, the elliptical enhancement becomes pronounced only at particular intermediate excitation strengths. The excitation-strength dependence and polarization state of the harmonics indicate that interference among multiple nonlinear emission processes plays a key role. Such an interference of nonlinear processes can be controlled by changing the ellipticity of excitation field when the HHG occurs in the region of anisotropic band structure, and hence, elliptical enhancement occurs effectively in higher-order HHG. Further, microscopic calculations imply that such interfering nonlinear processes are rooted in the interband transitions among multiple bands of a solid (in the case of GaAs, transitions between the conduction and light-, heavy-, split-off hole bands). Our study reveals that, besides the material properties, an optimal-strength excitation that can be used to fine tune the nonlinear dynamics is especially important for controlling HHG properties, providing an alternative perspective for ultrafast light-control of solid-state materials.