Spatial properties of odd and even low order harmonics generated in gas

Abstract

High harmonic generation in gases is developing rapidly as a soft X-ray femtosecond light-source for applications. This requires control over all the harmonics characteristics and in particular, spatial properties have to be kept very good. In previous literature, measurements have always included several harmonics contrary to applications, especially spectroscopic applications, which usually require a single harmonic. To fill this gap, we present here for the first time a detailed study of completely isolated harmonics. The contribution of the surrounding harmonics has been totally suppressed using interferential filtering which is available for low harmonic orders. In addition, this allows to clearly identify behaviors of standard odd orders from even orders obtained by frequency-mixing of a fundamental laser and of its second harmonic. Comparisons of the spatial intensity profiles, of the spatial coherence and of the wavefront aberration level of 5ω at 160 nm and 6ω at 135 nm have then been performed. We have established that the fundamental laser beam aberrations can cause the appearance of a non-homogenous donut-shape in the 6ω spatial intensity distribution. This undesirable effect can be easily controlled. We finally conclude that the spatial quality of an even harmonic can be as excellent as in standard generation.

Recent progress of below-threshold harmonic generation

“…In addition, one can generate a very short driving field with a wide range of frequency with a well controllable CEP stabilized for a long time and one can detect the phase of the emitted harmonics [32,33]. Along with these studies, some other properties of these low-order harmonics have been experimentally explored [34,35,67,68].…”

“…The generation of cavity enhanced harmonic generation not only leads to the frequency comb in the VUV range, but also draws much attention about the generation process itself, i.e., low-order harmonic generation, whose theoretical description is still unclear. The experimental investigation revealed the coherence [29,30], the CEP dependence [32,33], and some other properties of these harmonics [34,35,67,68]. In this subsection, we will review these progresses and mainly focus on some of the representative experiments.…”

“…The selection of a single harmonic order is necessary in some applications such as the seeding of the free electron laser. In 2015, the selection of isolated harmonics were investigated in the BTH regime [67] through experiments. In this study, the contribution of the surrounding harmonics was totally suppressed using the interferential filtering.…”

Restraint of spatial distribution in high-order harmonic generation from a model of hydrogen molecular ion

“…The effect of HHG from spatial properties of electron is a hot topic. [9][10][11][12] Lambert et al [13] discussed the spatial properties of harmonic generation in low order and Zhang et al gave the spatial distribution in HHG from a H + 2 and from a HeH 2+ . [14,15] However, there is no discussion on how to restrain the spatial distribution as far as we know.…”

“…In addition, only a molecular ion is considered to interact with the driving pulse in our simulation, and the harmonic yields are emitted from many molecules in actual experiment. [13,28,29] The harmonic is measured after propagating for a distance and has a divergence. Our simulation just aimed to give a theoretical expectation and the resolution of HHG from which nucleus may be measured by future tech-nique.…”