Scientists reveal that nonadiabatic tunneling is crucial for understanding high harmonic generation from semiconductors

When matter is exposed to highly intense electromagnetic radiation, nonlinearities of the material may lead to the emission of light that contains very high multiples of the incident frequency. This process is called high harmonic generation (HHG) and can be used for the generation of extremely short light pulses with durations in the range of attoseconds (in 1 attosecond, i.e., 10^-18 s, light travels just 0.3 nanometer which is comparable to the diameter of atoms!). Such pulses form the basis of the rapidly developing field of attosecond science and can be used to resolve and explore ultrafast processes inside matter. One important vision of such investigations is to control the electronic dynamics directly by the rapidly oscillating electric field of light. This regime of light wave electronics has the potential to tremendously speed-up optoelectronic devices which would strongly improve if not revolutionize numerous applications.

Tunneling through regions of high potential which are classically not accessible is one of the most fundamental quantum phenomena. This process is also highly relevant when matter is excited by intense electromagnetic fields and high harmonics are generated. Although tunneling is a quantum mechanical process, up to now typically simplified so-called semiclassical methods have been used to determine electron trajectories in semiconductors. However, such approaches rely on assumptions which are not well justified in the regime of extreme light-matter interactions.

"Our novel quantum trajectory simulations clarify the crucial role of the tunneling dynamics for the evolution and the harmonic emission from semiconductors. Unlike in semiclassical models, our approach includes and demonstrates that electrons and holes may have a finite separation and finite velocities directly after tunneling" explains Ruixin Zuo, a PhD student in Prof. Torsten Meier's group at Paderborn University. "Our findings clearly highlight that tunneling in strong electromagnetic fields is not an adiabatic process but is non-adiabatic which means that electrons and holes gain energy from the electromagnetic field during tunnelling" adds Prof. Weifeng Yang from the Hainan University.

The findings of the Sino-German team 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. The novel approach and the obtained results are described in an article entitled "Revealing the nonadiabatic tunneling dynamics in solid-state high harmonic generation" which was recently published as a Letter in the open-access journal Physical Review Research.

Read the article: doi.org/10.1103/PhysRevResearch.5.L022040