Elliptically Polarized High-order Harmonic Generation from Solids Driven by Linearly Polarized Mid-Infrared Pulse Combined with Terahertz Field

Perturbing and/or controlling high-order harmonic generation (HHG) from gases by (quasi-)static electric fields have been proposed since long ago, but never been realized in experiments, due to the difficulty of introducing strong enough (quasi-)static fields. For example, in Phys. Rev. Lett. 85, 732 (2000), theoretical investigation predicts that in the presence of a static field, elliptically polarized high-order harmonics can be generated by linearly polarized driving laser field. HHG from solids offers an alternative platform to test those theories due to the lower prerequisite on driving laser intensity. In this study, we experimentally demonstrate for the first time the generation of elliptically polarized even-order high harmonics from solid (100 nm, polycrystalline ZnO thin film) driven by a linearly polarized mid-infrared (70 fs, 3.6 μm) pulse dressed by a weak linearly polarized single-cycle terahertz (2 ps, 600 μm) pulse. We show that both the ellipticity and the orientation of the polarization ellipse depend on the angle (θ) between the mid-infrared and THz polarizations and vary across harmonic orders. For example, at θ=80°, the ellipticity varies from 0 to 0.4 and the major axis of the polarization ellipse varies by ~25 degrees, across 6th to 18th harmonic orders. Our study provides a novel way of generating elliptically polarized high harmonic lights. Exploring the origin of harmonic ellipticity would help us better understand the HHG process in solids.