Abstract
We study the tunneling mechanism of nonlinear optical processes in solids induced by strong coherent laser fields. The theory is based on an extension of the Landau-Zener model with nonadiabatic geometric effects. In addition to the rectification effect known previously, we find two effects, namely perfect tunneling and counterdiabaticity at fast sweep speed. We apply this theory to the twisted Schwinger effect, i.e., nonadiabatic pair production of particles by rotating electric fields, and find a nonperturbative generation mechanism of the opto-valley polarization and photo-current in Dirac and Weyl fermions.
Highlights
A Detailed calculations for the 2D Dirac fermions A.1 Low frequency region A.2 High frequency region
Nonadiabatic geometric effects kicks in when we study pair production induced by rotating electric fields Ex +i Ey = EeiΩt [26,27], which we coin as the “twisted Schwinger effect”
We show that the three nonadiabatic geometric effects, i.e., rectification, perfect tunneling and counterdiabaticity, play an important role in understanding the nonperturbative versions of the opto-valley polarization and photo-currents in 2D and 3D Dirac/Weyl materials which are microscopically caused by the twisted Schwinger effect
Summary
Geometric effects [1] in electron dynamics have become a central research topic in condensed matter [2]. The Schwinger effect is fermionantifermion pair production in strong electric fields [13,14,15] and is known to originate from nonadiabatic tunneling in the momentum space [16,17,18,19,20]. Nonadiabatic geometric effects kicks in when we study pair production induced by rotating electric fields (or circularly polarized laser fields) Ex +i Ey = EeiΩt [26,27], which we coin as the “twisted Schwinger effect”. We show that the three nonadiabatic geometric effects, i.e., rectification, perfect tunneling and counterdiabaticity, play an important role in understanding the nonperturbative versions of the opto-valley polarization and photo-currents in 2D and 3D Dirac/Weyl materials which are microscopically caused by the twisted Schwinger effect. If these symmetries are broken, it is possible to realize finite U(1) photocurrent in a similar way as in the optical absorption mechanism proposed in [38, 40, 41]
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