Strong-field-approximation model for coherent extreme-ultraviolet emission generated through frustrated tunneling ionization

Abstract

An atom can be excited through tunneling when it is exposed to a strong laser field. This excitation process is known as frustrated tunneling ionization (FTI). Recently it has been reported that coherent extreme-ultraviolet emission can be achieved through FTI. In the present work we extend the strong-field-approximation (SFA) model developed by Lewenstein et al. to derive a quantum mechanical description of the FTI emission. An analytic expression is derived to describe FTI emission as four steps: tunneling of an electron from the ground state of an atom, acceleration of the electron in the continuum, recombination into the exited state of the parent atom, and free induction decay. The coherence property of FTI emission is studied using the attosecond lighthouse method, in which a spatially chirped laser beam generates FTI emissions in different directions. The spatial distribution of the FTI emission in the far field is analyzed using both the extended SFA model and the time-dependent Schr\odinger equation model. The results obtained from the two models agree well, thereby confirming the validity of our extended SFA model.