The relevance for radiology appli­cations is probably the most known advantage of X-ray beams with respect to visible radiation and can be traced back to their superior pene­tration depth. On a more funda­mental ground, however, the relevance of this photon energy range relies on the capability of probing inner shell electrons  and mapping molecular structures on the atomic-scale. Building on such capabilities, large efforts have been devoted by the scientific community to develop X-ray sub-pico­second sources able to access matter properties with a time resolution sufficient to access elemental molecular motions. Free electron lasers (FEL), nowadays available at several large-scale faci­lities around the world, represent a prime candi­date to generate femtosecond X-ray pulses with high brilliance. One of the main challenges to exploit the enormous potential of FEL sources is developing methods for tuning the spectral and temporal beam properties, a task which is customarily achieved at visible wave­lengths resorting to non-linear optics.

Now, a team of scientists from Italian Institute of Technology, University of L'Aquila, FERMI Trieste and Sapienza Univer­sity of Rome have shown the first evidence of self-phase modu­lation (SPM) in the soft X-ray regime. The experiment, performed in the facility FERMI­@elettra of Trieste, consists in the observation of spectral modulation after the interaction of focused FEL beams with a very thin metallic foil. “Our experiment demonstrates a new control knob for spectral shaping of FEL pulses. Blue to red shift accom­panied by bandwidth increase can be obtained by moving the input wavelength across the material's absorption edge”, Tullio Scopigno explains.

The atomic absorp­tion edges in the X-ray region feature sharp discon­tinuities: an optical transparent material can absorb light modifying the photon energy by less than 1 %, correspondingly generating specific core electron excitations. “This first observation of SPM effects in the soft X-Ray regime allows to unveil specific atomic properties on the subpico­second time scale. In parti­cular, the interplay with a light-induced out-of-equilibrium electron plasma generated on the femtosecond timescale in thin metallic foils”, concludes Carino Ferrante.

Below the absorption edge, the observed SPM is induced by Kerr effect, i.e. by a modi­fication of the non-linear refractive index mimicking the pulse intensity profile, which ultimately results into spectral broadening, accompanied by a redshift due to valence electrons heating. In striking difference, above edge, the highly excited core photo­electrons generated by the pulse leading edge form a transient hot dense ionized plasma, responsible for a sharp decrease of the refractive index. Consequently, the pulse trailing edge is accelerated giving rise to an asymmetric temporal compression which, in turn, results in a blueshift. The results provide a proof of concept for spectral shaping of soft X-ray pulses, a key milestone towards the development of new protocols for femto­second core electrons spectroscopies. (Source: CAS)

Reference: C. Ferrante et al.: Non-linear self-driven spectral tuning of Extreme Ultraviolet Femtosecond Pulses in monoatomic materials, Light Sci Appl. 10, 92 (2021); DOI: 10.1038/s41377-021-00531-8

Link: Graphene Labs, Istituto Italiano di Tecnologia, Genova, Italy