OPCPA-driven High Harmonic Generation for Soft X-ray Absorption Spectroscopy

Ultrafast x-ray absorption spectroscopy (XAS) is a widely used technique in fields including inorganic chemistry, physical chemistry, materials science, condensed matter physics, and surface science for investigating non-equilibrium electron and structural dynamics of materials. While traditionally, it was only available at large and expensive user facilities like synchrotrons and free electron lasers, the development of tabletop setups based on high harmonic generation (HHG) in gases provides a promising and cost-effective alternative for performing ultrafast XAS [1].

In this thesis, the study of the electronic structure of materials and nonlinear op- tics are brought together to demonstrate the feasibility of near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the K-edge of boron and hexagonal boron nitride (h-BN) using a tabletop soft x-ray source. The source based on HHG in noble gases was driven by the output pulses of an optical parametric chirped pulse amplification (OPCPA) setup.

The NEXAFS results provide insights into the h-BN electronic orbitals, revealing distinct features attributed to π* and σ* resonances of 2D materials. As part of the OPCPA optimization, a new understanding of the optical parametric phase OPP has been developed. This phase is inherently induced in the amplified broadband pulses by the OPCPA and has to be accounted for in the pulse compression in the few-cycle pulse regime.

The work was conducted in two parts. In the first part, a four-stage OPCPA system delivering ultrashort short-wave infrared (SWIR) and mid-wave infrared (MWIR) pulses with energies in the millijoule range was successfully realized. In the second part, the output pulses of the OPCPA system were used to drive HHG in a hollow core waveguide (HCW). The resulting soft X-ray radiation was used to perform the XAS experiments.

The OPCPA system was optimized in the framework of this thesis and produces broadband pulses centered at the wavelengths of 1.55 μm and 3.0 μm, with energies of 1.6 mJ and 0.6 mJ, respectively, at a repetition rate of 100 Hz, but scalable up to 13 kHz. The spectral phase of these pulses was measured in agreement with calculations leading to a compressor based on chirped mirrors and material dispersion that compress the pulses to sub-50 fs duration.

Because of the stability and good beam mode of the OPCPA output pulses, they can be coupled into the HCW of the HHG source with high efficiency. Combining harmonics generated using argon, neon, and helium as nonlinear media, the photon energy range from 70 eV to 400 eV was covered. The maximun photon flux reached in a fraction of bandwidth of 1% in argon is 107 ph/s, and in helium and neon is 105 ph/s.

The results of this thesis contribute to the ongoing effort to advance research in the field of HHG and its applications, particularly in XAS. They show the feasibility of building a corresponding compact and cost-effective tabletop facility using HHG driven by high-energy SWIR pulses from an OPCPA system, its potential as a complement to large-scale facilities for studying the electronic properties of various materials through XAS.

[1] J. Lloyd-Hughes, P. M. Oppeneer, T. P. Dos Santos, A. Schleife, S. Meng, M. A. Sentef, M. Ruggenthaler, A. Rubio, I. Radu, M. Murnane, et al., “The 2021 ultrafast spectroscopic probes of condensed matter roadmap”, Journal of Physics: Condensed Matter 33, 353001 (2021).


Citation style:
Could not load citation form.


Use and reproduction:
All rights reserved