Phase properties of the cut-off high-order harmonics

TL;DR

This paper investigates the phase properties of high-order harmonic generation in the cut-off regime, revealing key characteristics like equal dephasing, phase dependence on intensity, and implications for attosecond pulse production.

Contribution

It provides a combined numerical and analytical analysis of the cut-off regime, highlighting its significance and unique phase behavior compared to the plateau regime.

Findings

Equal dephasing between successive harmonics in the cut-off regime

Linear dependence of harmonic phase on fundamental intensity

Range of laser intensities for cut-off regime with enhanced atomic response

Abstract

The cut-off regime of high-order harmonic generation (HHG) by atoms in an intense laser field is studied numerically and analytically. We find that the cut-off regime is characterized by equal dephasing between the successive harmonics. The change of the harmonic phase-locking when HHG evolves from the cut-off to the plateau regime determines the optimal bandwidth of the spectral region which should be used for attosecond pulse generation via amplitude gating technique. The cut-off regime is also characterized by a linear dependence of the harmonic phase on the fundamental intensity. The proportionality coefficient grows as the cube of the fundamental wavelength, thus this dependence becomes very important for the HHG by mid-infrared fields. Moreover, for every high harmonic there is a {\it range} of laser intensities providing the generation in the cut-off regime and the atomic response magnitude in this regime can be greater than that in the plateau regime. Thus the cut-off regime substantially contributes to the harmonic energy emitted under typical experimental conditions where the laser intensity varies in time and space.