Technical design report of a complete and compact broadband high-harmonics femtosecond beamline based on a modular hollow waveguide for photons generation centered on the upper region of the extreme ultraviolet spectral range

Abstract

We have successfully developed and implemented an entire and compact table-top high-order harmonics generation (HHG) setup from monochromatic and intense femtosecond ($10^{-15}$ s) laser pulses launched in a target composed of a high-purity monoatomic noble gas specie, which can be Argon or Helium, distinctively. Its frequency arrangement is distributed both in the full eXtreme UltraViolet (XUV, $22-124$ eV) spectral region and in the bottom part of the Soft-X Ray range (SXR, $124-132$ eV), at once. Specifically, the core of this coherent secondary light source is based solely on a homemade, modular, affordable, though sturdy, design. We take advantage of this opportunity to present our design guidance of the XUV generation from a hollow capillary waveguide apparatus, and our simple recipe regarding the alignment process of the latter, which is easily carried out thanks to our adjustable design. Then, a comprehensive description of our entire XUV beamline is described, and participate in adding essential contents to the existing literature. Concurrently, we conducted theoretical studies, in order to anticipate or explain our experimental results. Overall, we found very good consistency between the experimental and cost-effective time-consuming numerical results. Finally, our setup provides very good vacuum performance under high gas load pressures, to a few atmospheres. All of these attributes fulfill the requirements regarding ultrafast time-resolved pump-probe configuration in table-top element-sensitive spectroscopy of complex and integrated optoelectronic devices made of magnetic materials.

Figures (57)

• Figure 1: Workflow chart showing the progression of the project. Web diagrams are proposed for each task.
• Figure 2: Scrutinizing the HHG process. Left: illustration of the microscopic phenomenon of HHG for one emitter (one atom of noble gas). Right: coherent macroscopic building of HHG from the many atoms in presence, for an exploitable XUV signal.
• Figure 3: Predicted high cut-off energy for argon and helium vs femtosecond laser pulse duration $t_p$, for $\lambda_0 = 800$ nm. Green shadowed region is the pulse duration range where our cut-off is expected, considering pulse propagation in the plasma, and pulse temporal broadening in the entrance window via self-phase modulation and group delay dispersion (GDD). To illustrate the latter, insert shows the theoretical temporal broadening of femtosecond pulses before (Pulse in) and after (Pulse out) propagation through a L$=8$ mm BK$7$ thickness, for a Gaussian unchirped pulse with $t_0$ pulse duration (Fourier transform limited) at FWHM, in the form $t_p = t_0\sqrt{1+(4\ln2 (\mathrm{GDD}_{BK7}/t_0^2))^2}$ with GDD$_{BK7}$ = $43.96$ fs$^2 /$mm.
• Figure 4: The critical intensity (log-log scale) as a function of the laser pulse duration, for $\eta_{\textrm{cr}}=4\%$ and $\eta_{\textrm{cr}}=0.5\%$, for both Argon and Helium, respectively. Shaded areas (black $+$ for Helium, red for Argon) bounded in their upper part by the corresponding curve, with an intensity estimation $\pm 2 \%$ for both gas species, and in the lower part by the $x$-axis, show the region of interest in our experimental conditions, where we can apply a laser intensity. Vertical bounds indicate the range in which could possibly varied our pulse duration, giving a range of critical intensities we can reach.
• Figure 5: Phase matching pressure $P_{\textrm{PhMa}}$, for a medium centered at the laser focus ($z=0$), as a function of ionization degree $\eta$ in Ar and in He for four different harmonics ($\#q = 85, 41, 35, 19$ or $9.42$, $19.5$, $22.9$, $42.1$ nm, or $131.6$, $63.6$, $54.2$, $29.45$ eV, respectively), reading curves from left to right. Data for $n_{\textrm{XUV}}$ are taken from CXRO. The Argon under-curve for $\#q=19$ is shaded, showing the region where pressure must be applied for an, a minima, on-axis phase-matching. Other curves are not shaded for visual convenience, but their interpretation remains the same. a) is a close-up of Helium part. b) is the index of refraction explaining why the sequence of Helium curves are not monotonic in energy.