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

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.

💡 Research Summary

The manuscript presents a complete design, construction, and characterization of a compact, table‑top high‑order harmonic generation (HHG) beamline that operates in the extreme‑ultraviolet (XUV) and soft‑X‑ray (SXR) spectral ranges (22–132 eV). The core of the source is a modular hollow‑core waveguide (HCW) filled with high‑purity noble gases (argon or helium) at pressures up to a few atmospheres. By guiding an 800 nm, 30 fs, 3.5 mJ, 1 kHz femtosecond laser through the HCW, the authors achieve phase‑matched, long‑interaction‑length HHG, producing a continuous comb of odd harmonics from the 15th (≈22 eV) up to the 85th (≈132 eV).

The paper is organized into three main parts. First, a concise theoretical background is given, covering microscopic strong‑field ionization (ADK model, dipole phase) and macroscopic phase‑matching conditions, including re‑absorption and plasma defocusing. Analytical formulas are derived to relate waveguide diameter, length, and gas pressure to the optimal phase‑matching pressure. These formulas are validated with MATLAB‑based 1‑D wave‑propagation and time‑dependent Schrödinger equation (TDSE) simulations that incorporate the full laser intensity profile, ionization rates, and XUV propagation losses.

Second, the experimental setup is described in detail. The laser system, gas delivery network, vacuum architecture, and the modular HCW assembly are presented with schematics and part numbers. The HCW consists of interchangeable sections (entrance, main body, exit) joined by precision flanges, allowing rapid gas switching and waveguide replacement without breaking the main vacuum. Alignment is performed using a five‑step protocol that employs beam profiling cameras and piezo‑driven mirrors to achieve sub‑0.1 mrad pointing stability. Downstream, an aluminum filter chamber, a toroidal mirror for 90° beam steering, a recombination chamber, and a sample interaction chamber are integrated to enable pump‑probe experiments such as time‑resolved transverse magneto‑optical Kerr effect (T‑MOKE) measurements on magnetic thin films. The detection branch combines a micro‑channel plate (MCP) with a CCD spectrometer, providing simultaneous spectral and flux information.

Third, the results are reported. With argon, the harmonic cutoff is at ≈108 eV (73rd order); with helium, the cutoff extends to ≈132 eV (85th order). Measured photon fluxes reach 5 × 10⁸ ph s⁻¹ (Ar) and 1 × 10⁹ ph s⁻¹ (He), representing a 30 % improvement over comparable literature values. The experimental spectra agree with simulations within 95 % confidence, confirming the validity of the design equations. Vacuum performance under high gas load is demonstrated: even at 5 bar backing pressure, chamber pressures remain below 10⁻⁶ mbar, thanks to differential pumping stages and strategically placed throttling valves. A risk assessment addresses laser safety, high‑pressure gas handling, and vacuum failure, with mitigations such as interlocks, pressure sensors, and automatic shutdown.

The authors conclude that the modular HCW beamline provides a reliable, cost‑effective platform for ultrafast XUV science, especially for element‑specific pump‑probe spectroscopy of complex magnetic and optoelectronic devices. Future work will focus on further pulse compression (<10 fs), carrier‑envelope phase (CEP) stabilization, and extension to isolated attosecond pulse generation, thereby broadening the applicability of the system to attosecond spectroscopy and coherent control experiments.