Observing broken inversion symmetry in solids using two-color high-order harmonic spectroscopy

Abstract

We experimentally investigate the manifestations of broken inversion symmetry in solids in two-color interferometric measurements. Polarization-resolved, high-order harmonic spectra of ${\mathrm{SiO}}_{2}$ with different levels of crystallinity are obtained. In amorphous and polycrystalline ${\mathrm{SiO}}_{2}$, both even and odd harmonics of the two-color interferogram beat four times per fundamental period, which is similar to what was observed in atomic gases or nonoriented molecules. In contrast, when a crystalline ${\mathrm{SiO}}_{2}$ sample is utilized, the competition between the contributions of broken inversion symmetries of the medium and the incident electric field results in a distinctive feature: Both odd and even harmonics beat two times per fundamental period [as predicted in Phys. Rev. B 94, 115164 (2016)]. Our observations are explained by an intuitive model. Moreover, they are fully supported by numerical simulations based on semiclassical transport theory. Our experiments highlight the considerable potential of polarimetry in two-color interferometric measurements, which allow pure and unambiguous characterization of the complex polarization response and thus consequent studies of electron dynamics in solids.