Direct refractive index retrieval from interferometry measurements.

We present an alternative numerical method to the Abel inversion technique, which can be applied to complex non-symmetrical systems. A comparison with the Abel inversion algorithm was conducted. For benchmarking, the method was applied to a synthetic trace representing a plasma waveguide characterized by a constant parabolic density profile. Furthermore, the temperature and refractive index of a plume of hot air surrounding a non-cylindrical soldering iron were retrieved. Temperatures between 50 °C and 200 °C were successfully retrieved within the instrument precision. The proposed method allows robust and fast data retrieval while maintaining the accuracy and resolution of well-known methods, as Abel inversion.

High-harmonic generation wave front dependence on a driving infrared wave front.

With high-harmonic generation (HHG), spatially and temporally coherent XUV to soft x-ray (100 nm to 10 nm) table-top sources can be realized by focusing a driving infrared (IR) laser on a gas target. For applications such as coherent diffraction imaging, holography, plasma diagnostics, or pump-probe experiments, it is desirable to have control over the wave front (WF) of the HHs to maximize the number of XUV photons on target or to tailor the WF. Here, we demonstrate control of the XUV WF by tailoring the driving IR WF with a deformable mirror. The WFs of both IR and XUV beams are monitored with WF sensors. We present a systematic study of the dependence of the aberrations of the HHs on the aberrations of the driving IR laser and explain the observations with propagation simulations. We show that we can control the astigmatism of the HHs by changing the astigmatism of the driving IR laser without compromising the HH generation efficiency with a WF quality from λ/8 to λ/13.3. This allows us to shape the XUV beam without changing any XUV optical element.

Chirped pulse amplification in an extreme-ultraviolet free-electron laser.

Chirped pulse amplification in optical lasers is a revolutionary technique, which allows the generation of extremely powerful femtosecond pulses in the infrared and visible spectral ranges. Such pulses are nowadays an indispensable tool for a myriad of applications, both in fundamental and applied research. In recent years, a strong need emerged for light sources producing ultra-short and intense laser-like X-ray pulses, to be used for experiments in a variety of disciplines, ranging from physics and chemistry to biology and material sciences. This demand was satisfied by the advent of short-wavelength free-electron lasers. However, for any given free-electron laser setup, a limit presently exists in the generation of ultra-short pulses carrying substantial energy. Here we present the experimental implementation of chirped pulse amplification on a seeded free-electron laser in the extreme-ultraviolet, paving the way to the generation of fully coherent sub-femtosecond gigawatt pulses in the water window (2.3-4.4 nm).