Strong-field polarization-state control of higher harmonics generated in crystalline solids

Abstract

We demonstrate that the polarization states of higher harmonics emitted from crystalline solids (here silicon, quartz) are determined by both crystal symmetry and nonperturbative dynamics, opening the door to strong-field control of the harmonics' polarization states. Extending attoscience from atoms and molecules to condensed matter and nanosystems is currently one of the most fascinating frontiers of ultrafast physics. Adapting attosecond metrology techniques to observe and control electronic dynamics on sub-optical-cycle time scales opens up unprecedented opportunities for PHz electronic signal processing.Since its first observation by Ghimire et al.[1], the physics underlying high-harmonic generation (HHG) in solids has extensively been investigated (for a comprehensive review, see [2]). Recent studies have, for example, demonstrated isolated attosecond XUV pulses emitted from thin SiO 2 films [3], HHG from amorphous fused silica [4], graphene (enhanced by driving ellipticity) [5], or 2D transition metal dichalcogenides [5,6].A striking observation is the asymmetric driver-ellipticity dependence of the HHG yield for certain crystal orientations in MgO [7]. The asymmetric, non-Gaussian-shaped ellipticity profiles are in strong contrast to the ellipticity dependence in gas-phase HHG. Later works reported the generation of circularly polarized harmonics from single-color driver pulses [8,9]. To understand this peculiar behavior, we recently introduced an ab-initio time-dependent densityfunctional theory (TDDFT) framework that allows us to investigate the complex interplay between the coupled intraand interband dynamics giving rise to HHG without making a-priori assumptions [10], and we theoretically investigated the ellipticity dependence of the HHG yield in Si and MgO [11]. Here, we show that the polarization states of higher harmonics emitted from crystalline Si and quartz samples are determined by both crystal symmetry [8,9,[11][12][13] and nonperturbative dynamics [11], opening the door to strong-field control of the harmonics' polarization states.We irradiated free-standing, 2-µm-thin, (100)-cut crystalline silicon samples with 120-fs, 2.1-µm pulses from a Ti:sapphire-pumped OPA with a maximum peak intensity of 0.7 TW cm −2 (in vacuum). The driver pulse ellipticity ε was varied from ε = 0 (linear) to |ε| = 1 (circular) using a combination of quarter-wave plate (QWP) and half-wave plate (HWP) to keep the major axis of the polarization ellipse constant. Fig. 1(a)-(c) shows the harmonic ellipticities