First evidence of spectral tuning in extreme ultraviolet light pulses after interaction with opaque materials
Since their discovery in 1895, X-rays played a key role to probe inner shell electrons (in view of their similar energies) and to map molecular structures on the atomic-scale (as typical interatomic spacings are comparable to X-ray wavelengths). Building on such capabilities, large efforts have been devoted to developing pulsed X-ray to enable accessing photophysical events with a time resolution sufficient to dissect elemental molecular motions. Within such a context, the recent advent of free electron lasers, high brilliance femtosecond X-ray facilities, offers unprecedented opportunities for capturing ultrafast dynamics. A necessary pre-requisite, however, it is the ability to manipulate spectral properties in a controlled manner: an extremely challenging task close to atomic-scale wavelengths, which is customarily achieved in the visible resorting to non-linear optics. In a new paper published in Light: Science & Application, a collaboration led by the Femtoscopy group of “La Sapienza” University involving FERMI, the Italian Institute of Technology and the University of L’Aquila, reported the first evidence of self-phase modulation (SPM) in the soft X-ray regime. The experiment, carried out at the EIS-TIMEX beamline of FERMI, consists in the achievement of spectral modulation after the controlled interaction of FEL ultrashort pulses with very thin metallic foils (100-300 nm). The experiment demonstrates an ideal “control knob” for spectral shaping of FEL pulses. By selecting an input wavelength across the material’s absorption edge, blue to red shift accompanied by bandwidth increase can be obtained as function of the fluence (Fig. 1). The atomic absorption edges in the X-ray region feature sharp discontinuities: an optical transparent material can turn into an absorber upon 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 unveils atomic processes on the sub-picosecond time scale. In particular, the interplay between propagating photon bunches and the out-of-equilibrium electron plasma consequently photo-generated in thin metallic foils, occurring within a few femtoseconds. Below the absorption edge, the observed SPM is induced by the 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 red-shift 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.
Figure 1. A Mg thin sample foil (thickness: 140 nm). Averages of single shot spectral differences between upstream PRESTO (Iu) and downstream WEST (Id) EUV spectrometers ( ΔI =〈Id − Iu〉) are reported for a FEL photon energy (~42.1 eV) below the Mg L2,3-edge (a) and for two photon energies (~51.5 and ~56.2 eV) above it (b, c). The red and blue curves correspond to ΔI at the highest and lowest fluence, respectively. Averaged FEL upstream spectra〈Iu(Eph)〉, scaled to improve readability, are shown by solid black lines. The blue-shift in (b) and (c) can also be appreciated comparing〈Iu(Eph)〉and〈Id(Eph)〉at the largest fluence (dashed black lines).
This research was conducted by the following research team:
Carino Ferrante1,2,3, Emiliano Principi4, Andrea Marini5, Giovanni Batignani3, Giuseppe Fumero3, Alessandra Virga2, Laura Foglia4, Riccardo Mincigrucci4, Alberto Simoncig4, Carlo Spezzani4, Claudio Masciovecchio4, Tullio Scopigno1,2,3
1 Graphene Labs, Istituto Italiano di Tecnologia, Genova, Italy
2 Center for Life Nano Science @Sapienza, Istituto Italiano di Tecnologia, Roma, Italy
3 Dipartimento di Fisica, Università di Roma “La Sapienza”, Roma, Italy
4 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
5 Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’Aquila, L’Aquila, Italy
Contact persons:
Carino Ferrante, email:
Tullio Scopigno, email:
