Focusing Properties of High-Order Harmonics

Maria Hoflund,Jasper Peschel,Chen Guo,Marius Plach,Per Eng-Johnsson,Eric Constant,Anne L'Huillier,Peter Smorenburg,Hampus Wikmark,Cord Arnold,Kévin Veyrinas,Hugo Dacasa,Sylvain Maclot

doi:10.34133/2021/9797453

Abstract

Many applications of the extreme ultraviolet (XUV) radiation obtained by high-order harmonic generation (HHG) in gases require a small focus area in order to enable attosecond pulses to reach a high intensity. Here, high-order harmonics generated in Ar with a multiterawatt laser system in a loose focusing geometry are focused to a few micrometers using two toroidal mirrors in a Wolter configuration with a high demagnification factor. Using a knife-edge measurement technique, we determine the position and size of the XUV foci as a function of harmonic order. We show that the focus properties vary with harmonic order and the generation conditions. Simulations, based on a classical description of the harmonic dipole phase and assuming that the individual harmonics can be described as Gaussian beams, reproduce the experimental behavior. We discuss how the generation geometry affects the intensity and duration of the focused attosecond pulses.

Highlights

High-order harmonic generation (HHG) is a highly nonlinear process that converts intense low-frequency light into attosecond pulse trains in the extreme ultraviolet (XUV) spectral region
The 19th harmonic is removed from the dataset because its position on the detector coincides with a damaged area on the microchannel plate of the spectrometer, making its detection unreliable
The different harmonics are found to be focused at different positions, with a separation of up to 1.5 mm, which is significant since the harmonic Rayleigh lengths are typically around 1 mm

Summary

Introduction

High-order harmonic generation (HHG) is a highly nonlinear process that converts intense low-frequency light into attosecond pulse trains in the extreme ultraviolet (XUV) spectral region. While XUV and X-ray radiation from free-electron lasers provide the required intensities for these types of studies [16, 17], they have only recently started to demonstrate the generation of attosecond pulses [18] Using these sources for experiments is still very challenging, whereas HHG-based sources already provide a mature technology for nonlinear pump-probe experiments in the attosecond range. To obtain both high intensity and attosecond pulse duration in the interaction with a low-density gas target, it is important to focus the broadband radiation on a small spot. Focus sizes of the order of 0:3 × 0:4 μm 2 have been measured for harmonics between 10 and 20 nm (i.e., 60-120 eV) focused with an ellipsoidal mirror [31]

Methods

Results

Discussion

Conclusion