Efficient Process Delivers More Power to Soft X-ray Pulses

Research in lab

A research team that includes Paolo Carpeggiani (right) and Edgar Kaksis of Technische Universit├Ąt Wien (TU Wien), Austria, has demonstrated a novel scheme for generating pulses of soft X-rays. [Image: TU Wien]

An international team of scientists has devised a simpler and more efficient scheme for generating ultrashort X-ray pulses in a spectral region crucial for probing biological processes (ACS Photonics, doi: 10.1021/acsphotonics.2c01021). The researchers, led by Paolo Carpeggiani at Technische Universit├Ąt Wien (TU Wien), Austria, have demonstrated a scalable solution that could produce brighter pulses of soft X-rays in the 280–300 eV regime—which is widely targeted for spectroscopic studies of organic molecules.

Difficulties in creating pulsed X-rays

Ultrashort X-rays are generally produced using high-harmonic generation (HHG), in which intense light pulses from a near-infrared laser interact with a noble gas to create light at much higher frequencies. The laser must deliver high peak powers and high repetition rates to drive these nonlinear interactions, which in practice limits the choice to titanium sapphire (Ti:Sa) or ytterbium laser sources.

However, the wavelength of light emitted by both of these driver lasers is too short for HHG to create pulsed X-rays at energies greater than 220 eV. An extra step is needed to convert the initial light emission to a longer wavelength, and the most common approach has been to combine a Ti:Sa laser with optical parametric amplification (OPA) using a nonlinear crystal. The resulting wavelengths extend from 1300 nm to around 2000 nm, which, combined with HHG, enable generation of X-ray pulses with energies all the way up to 530 eV.

Stimulated Raman scattering to the rescue

The problem is that the conversion efficiency of the OPA process struggles to reach 25%, which limits the brightness of the X-ray pulses that can be produced. In contrast, Carpeggiani and colleagues have combined an ytterbium laser with a relatively new technique for frequency conversion called stimulated Raman scattering (SRS). By coupling the light from the laser into a hollow-core fiber filled with molecular gases, they show that more than 70% of the original pulse energy can be red-shifted to wavelengths of 1200–1300 nm—which is the sweet spot for efficient HHG to the sought-after 280–300 eV spectral range.

Experiments show that the scheme can produce soft X-ray pulses at energies up to 290 eV, above the absorption edge of carbon at 284 eV, and short pulse durations of around 20 fs. The method is also simpler and cheaper to implement than using Ti:Sa lasers. The biggest advantage, however, is that combining a more powerful ytterbium laser with more efficient frequency-conversion processes should yield brighter X-ray pulses at the carbon absorption edge. This is a critical requirement for future spectroscopic applications, since it would allow the analysis of samples in aqueous solutions as well as time-resolved studies of ultrafast processes in organic molecules.  

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