Single-shot fluctuations in waveguided high-harmonic generation

TL;DR

This paper presents the first single-shot measurements of waveguided high-harmonic generation, characterizing fluctuations in pulse energy, divergence, and pointing, and identifying ionization-induced mode mixing as a key fluctuation source.

Contribution

It introduces the first characterization of shot-to-shot fluctuations in waveguided high-harmonic generation using a capillary waveguide filled with Argon gas.

Findings

Fluctuations depend on drive laser pulse energy and gas pressure.

Drive laser pointing does not induce high-harmonic fluctuations.

Ionization-induced nonlinear mode mixing is the main fluctuation source.

Abstract

For exploring the application potential of coherent soft x-ray (SXR) and extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is important to characterize the central output parameters. Of specific importance are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output energy, fluctuations of the direction of the emission (pointing instabilities), and fluctuations of the beam divergence and shape that reduce the spatial coherence. We present the first single-shot measurements of waveguided high-harmonic generation in a waveguided (capillary-based) geometry. Using a capillary waveguide filled with Argon gas as the nonlinear medium, we provide the first characterization of shot-to-shot fluctuations of the pulse energy, of the divergence and of the beam pointing. We record the strength of these fluctuations vs. two basic input parameters, which are the drive laser pulse energy and the gas pressure in the capillary waveguide. In correlation measurements between single-shot drive laser beam profiles and single-shot high-harmonic beam profiles we prove the absence of drive laser beam-pointing-induced fluctuations in the high-harmonic output. We attribute the main source of high-harmonic fluctuations to ionization-induced nonlinear mode mixing during propagation of the drive laser pulse inside the capillary waveguide.