Generation of FEL pulses in the water window and beyond (up to ~800eV)


Although the shortest wavelength currently delivered to user experiments by FERMI operating in High Gain Harmonic Generation mode is about 4 nm, it has been recently demonstrated that pulses with a high degree of longitudinal coherence (first and second order) covering the water window and extending up to 790 eV can be generated, by taking advantage of the so-called nonlinear harmonic regime, (i.e. harmonics of the resonant FEL wavelength).  
Strong scientific cases exist for developing sources that deliver fully coherent and variable polarization ultra-short light pulses over the spectral region that covers the water window (up to the O K-edge at 530 eV) and the L-edges of the 3d transition metals up to 800 eV. The availability of such sources would make it possible: 

  • To address the ultrafast dynamics of organic molecules in solution; 
  • To investigate the electronic excitations in strongly correlated materials via pump-probe experiments at different core resonances, with both site and chemical sensitivity;
  • To extend all techniques based on coherent control to the water window and beyond.

High Harmonics in water window

Series of 400 consecutive single-shot spectra at 700 eV (a); statistics of the central wavelength stability (b), spectral intensity (c) and FWHM spectral bandwidth (d). A series of selected spectra are plotted in (e).


The scheme adopted at FERMI to overcome the Carbon K-edge (~300 eV) has been implemented in the FEL-2 line, setting the radiator of the second stage in linear horizontal polarization and generating odd higher harmonics on-axis. A detailed characterization of the FEL output at ~700 eV (third harmonic of 233 eV) in terms of intensity and spectral properties is reported in Fig.1. The energy per pulse at the source in a spectral range 500-800 eV was estimated to be about 150 -200 nJ, in good agreement with the expected ratio with the fundamental radiation of ~1%. Taking into account the focusing capability of the photon transport system, one obtains a fluence in the order of 50 mJ/cm2. The latter is well suited for those experiments (e.g., the studies of ultrafast spin dynamics or time resolved x-ray absorption spectroscopy) that do not require high brightness pulses and are difficult to perform using the standard slicing sources that can be implemented at Synchrotron facilities.


The success of the proof-of-principle experiments (as for example the measurement of the absorption spectrum of room-temperature water across the oxygen K-edge, and the magnetization dynamics of thin films at the Co and Fe L-edge) paves the way for extending the class of experiments based on the simultaneous control and manipulation of both phase and wavelength in the soft-x-ray spectral range so far inaccessible for seeded FELs. 
Moreover, the high shot-by-shot stability of the pulse intensity and spectral properties exhibited by the nonlinear harmonics of FERMI, associated with the remarkably low time jitter relative to the available user laser (a few femtoseconds), can be employed in a wide class of single-shot pump-probe soft-x-ray spectroscopic measurements. Furthermore, the FERMI FEL-2 scheme could lead to EUV (fundamental) pump–soft-x-ray (harmonic) probe studies entering in the regime of linear or nonlinear multidimensional spectroscopy.  Indeed, combining coherence with the possibility to generate synchronized pulses of different wavelengths enables a whole range of novel pump-probe experiments to investigate structural, electronic, and magnetization dynamics in the fields of condensed matter as well as atomic and molecular physics.


FEL Parameters 



Branch: FEL 2
Wavelength Range: 800 nm, 400 nm, 1.5-33 nm (i.e. 500-800 eV).
Pulse Duration: standard FEL-2

Others: Linear Polarization only available

G. Penco et al. Phys. Rev A 105, 053524 (2022)

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Last Updated on Monday, 05 December 2022 16:21