Nonadiabatic effect in high order harmonic generation revealed by a fully analytical method

TL;DR

This paper introduces a fully analytical method based on the strong-field approximation and perturbation expansion to distinguish nonadiabatic effects in high-order harmonic generation, revealing wavelength-dependent enhancements crucial for attosecond pulse production.

Contribution

The paper presents a novel analytical approach that explicitly separates nonadiabatic effects in HHG using a perturbation expansion on the Keldysh parameter, advancing understanding of wavelength influences.

Findings

Nonadiabatic effects depend on laser wavelength.

Short wavelengths significantly enhance HHG yields.

Analytical descriptions of electron exit velocity and position are provided.

Abstract

We propose a fully analytical method for describing high-order harmonic generation (HHG). This method is based on the strong-field approximation (SFA) and electron-trajectory theory, but utilizes the perturbation expansion on the Keldysh parameter $\gamma$. This expansion allows us to clearly differentiate the nonadiabatic and adiabatic effects on HHG. We show that the nonadiabatic effect relating to high-order expansion depends on the laser wavelength and remarkably enhances the HHG yields for cases of short wavelengths, providing deeper insights into wavelength-dependent HHG yields which are important in producing attosecond pulses. Especially, our method provides the analytical and accurate descriptions of nonadiabatic exit velocity and position of the tunneling electron at the tunnel exit. These descriptions are meaningful for constructing a fully analytical and quantitative Coulomb-included HHG model, which is crucial in HHG-based attosecond measurement.