Laser-driven high-flux source of coherent quasi-monochromatic extreme ultraviolet radiation for coincidence spectroscopy

TL;DR

This work develops and characterizes a laboratory-scale, high-flux, coherent XUV source based on high-harmonic generation in argon driven by frequency-doubled 515 nm pulses at 100 kHz. By combining meticulous beamline diagnostics, differential pumping, and a reaction microscope (COLTRIMS), the authors achieve quasi-monochromatic XUV light around $26.5$ eV with a flux of approximately $5\times10^{13}$ photons/s and demonstrate two-color pump-probe capabilities and two-photon ionization of argon. A key contribution is the detailed optimization of HHG via nozzle positioning and beam-iris control, coupled with phase-matching analysis that links observed yield to short-trajectory contributions before the focus. The work paves the way for lab-based XUV-pump/XUV-probe and coincidence experiments with high signal rates, and outlines concrete steps to further increase flux and extend applications to nuclear and electronic dynamics.

Abstract

We present a source of coherent extreme ultraviolet (XUV) radiation with a flux of 10$^{13}$ photons per second at 26.5 eV. The source is based on high-harmonic generation (HHG) in argon and pumped by a frequency-doubled 100 kHz repetition rate fiber laser providing 30 fs pulses centered at 515 nm. We report on the characterization of the source and the generated XUV radiation using optical imaging and photoelectron spectroscopy. The generated radiation is quasi-monochromatized using a suitably coated XUV mirror and used for coincidence spectroscopy of ions and electrons generated from a cold gas target. The high intensity of the focused XUV pulses is confirmed by the observation of two-photon double ionization in argon. Moreover, we demonstrate the capability to perform pump-probe experiments using XUV and visible laser pulses.

Figures (10)

• Figure 1: Overview of the experimental setup consisting of (right to left) HHG chamber, XUV photo electron time-of-flight spectrometer (XPETS), differential pumping stages and the reaction microscope (COLTRIMS). Our laser system consists of a Yb-fiber laser, a multi pass cell pulse compressor, a BBO for frequency doubling and chirped mirrors for visible light pulses. In the HHG chamber, the laser is focused into the argon gas target using a thin lens (150mm or 200mm focal length). The plasma generated in the laser focus is imaged onto a camera outside the chamber. A hole mirror followed by a thin aluminum filter are used to subtract the laser from the generated XUV beam. Further, the reflected laser is used for beam diagnostics. The XPETS is used for live measurements of the XUV spectrum and photon number estimation. In order to reduce the pressure from e-3mbar to $\leq e-10mbar$ in the COLTRIMS, the XUV beam propagates through three differential pumping stages (DPS). In addition, the second DPS is used to recombine the XUV beam with the 515nm beam for two-color experiments. The overlap of the two beams can be checked by moving in a mirror in the third DPS. A scintillation crystal is used to image the XUV beam. Finally, a mirror inside the COLTRIMS with Si-Sc coating focuses ($f = 75mm$) the XUV beam on the cold gas jet target. The inset shows a typical FROG (Frequency Resolved Optical Gating) trace and the retrieved pulse duration of 28fs, with additional dispersion up to the focus taken into account.
• Figure 2: Plasma image and evaluation. In (a) an image taken with the plasma imaging setup shows the plasma in the argon gas jet. Both, the gas nozzle and extractor are visible. The beam propagates along the z-axis and the gas jet along the x-axis. In (b) the dimensions (full width at half maximum, FWHM) and position of the plasma relative to laser focus (z-axis) and gas nozzle (x-axis) are determined.
• Figure 3: XUV divergence determined by scintillation imaging. In (a), the vertical and horizontal half angle divergence ($1/e^2$) of the XUV beam in dependence on the XUV generation position (z-axis) are shown. The images of the scintillation crystal hit by XUV radiation generated before (b) and after the focus (c) are presented as an example for the increasing divergence. In the latter case, the beam is cut due to apertures in the beam path. The round feature around the beam is caused by the crystal support structure. The maximum brightness divided by exposure time of (b) is 4.5 times higher compared for (c). Panel (d) shows the horizontal and vertical lineouts for (b) and (c) with normalized brightness for comparison.
• Figure 4: XPETS spectrum. In (a), the time-of-flight photoelectron spectrum captured with the XPETS is presented. It is dominated by the prominent harmonic peaks on top of a broad background due to scattered electrons. The post-processed photon energy spectrum is shown in (b). An estimation of the photon number per second allows determining the average power of the harmonic lines available in the COLTRIMS (i.e, behind the aluminum filter) with a precision of $\approx75%$. The measurement was carried out using a focusing lens of $f = 200mm$, position relative to focus $z = -0.8+-0.1mm$, laser power $P_L = 5.3\pm 0.1W$ after iris, and argon backing pressure of $p = 1.15+-0.05bar$.
• Figure 5: Scan of the HHG position. The nozzle position relative to the laser focus is scanned from -3.5mm to 1.8mm. The laser power was $P_L$ = 7.5W with the iris opened, at an argon pressure of p = 2bar.