Effect of crystal anisotropy on light-field-driven currents in dielectric crystal

Recent advances in the availability of intense, carrier-envelope phase-stabilized few-cycle lasers have led to the active study of the control of electrons using a light field in atoms, molecules, and solids. The field-driven ultrafast current in solids by strong fields is of far-reaching importance in view of ultrafast devices. Recent ab initio time-dependent density functional theory calculations [Wachter et al., Phys. Rev. Lett. 113, 087401 (2014)] predict that the crystal anisotropy manifests itself as the phase shift between induced currents along different crystal axes. The present work observes such a phase shift, clearly demonstrating that the electric current induced by a strong light field in an anisotropic crystal is sensitive to the orientation. A series of experiments has been carried out with few-cycle laser fields polarized parallel to the axes of quartz, (c) over cap and (a) over cap, respectively. Owing to the anisotropic atomic composition in the crystalline lattice, the transition to the tunneling regime takes place at lower intensity along the (a) over cap axis than along the (c) over cap axis. This implies that at a given tailored intensity, the tunneling transition occurs along the (a) over cap but not along the (c) over cap axis (still in the multiphoton regime). Hence, the currents induced by the two different mechanisms lead to an unequal accumulative phase, thus the nonzero phase shift. This work promotes an understanding of the strong field response of solids at the atomic level and in the subcycle time scale. (C) 2020 Optical Society of America11Nsciescopu