Revealing the nonadiabatic tunneling dynamics in solid-state high harmonic generation

Abstract

Field-induced tunneling is one of the most fundamental quantum phenomena. In atoms this process has been successfully explored by attosecond interferometry based on high harmonic generation. Adapting this method to solids, the reconstruction of the subcycle tunneling dynamics calls for rigorous theoretical models that are able to properly map the experimental observables to the character of the tunneling process. Unlike in atomic gases, for crystalline solid-state systems the validity and applicability of the semiclassical trajectory-based model are still highly debated. Here we present a saddle-point analysis for solid-state systems which includes the quantum dynamics during tunneling. This allows us to quantify the initial conditions of electrons and holes when they emerge after the tunneling process in the classically allowed region. Our quantum trajectory simulations clarify the crucial role of the tunneling dynamics for the subsequent evolution and the harmonic emission from solids. Besides a nonzero initial electron-hole separation, a nonzero initial velocity of electrons/holes at the tunneling exits is revealed which arises from nonadiabatic tunneling. We find that depending on the ionization time, both inward and outward movements of the electron and its associated hole at the tunneling exit can occur. Our results provide intuitive insight into the nonadiabatic tunneling dynamics in solids and have direct implications for revealing fundamental quantum mechanical phenomena in solid-state systems with attosecond spectroscopic techniques.