Spatially Separated crossed polarized FEL pulses


The FERMI team have recently demonstrated the possibility to generate spatially separated light pulses with parallel and crossed linear polarizations (SSPPU and SSXPU) in the extreme ultraviolet range. 
The configurations are obtained by tailoring the trajectory of electron bunches along the undulator line, demonstrating the possibility to obtain a doublet of pulses with independent polarization, pointing in different directions.
The need for such schemes has been suggested by EIS-TIMER scientists to introduce a new ensemble of experiments and explore additional features of the pump-probe technique.
It is well established that the interference of two pulses with linear parallel polarizations will result in a modulation of the light intensity, translated into a density grating at the sample position.
On the contrary, when the polarizations are crossed, a sinusoidal oscillation of the light chirality is obtained and, consequently, a polarization grating is induced in the sample.
This technique allows to:

  • detect circular dichroism and chiral properties of matter
  • characterize spin waves and magnons

Although the principle of this technique has been already demonstrated in the optical regime, FEL radiation can provide the proper wavelength range to investigate the chiral features of a sample down to the nanometer scale.

Spatially separated crossed polarised FEL

The undulator line is broken into two sections: the first one is lasing along a tilted direction with a specific polarization; in the second one the electron bunch follows a different trajectory, and the pulse is prepared with a polarization orthogonal to the first.
When the two sub-pulses impinge on the sample a polarization grating is induced and it can be exploited to explore magnetic and non -magnetic features, including dichroism and chirality. In the inset, a comparison between the response of a CoGd sample is shown. The black curve shows the intensity grating obtained with the FEL in the standard configuration; the purple curve shows the same behavior, obtained using the spatially separated parallel polarized pulses; the blue curve is obtained with the spatially separated crossed polarized pulses and shows the same behavior, obtained using the spatially separated parallel polarized pulses; the blue curve is obtained with the spatially separated crossed polarized pulses and shows the variation in the temporal domain of the excitations lifetime and presence.



Both SSU scheme were realized at FERMI FEL, on the FEL-1 branch, in 2020 and 2021.
From the machine point of view, the people involved demonstrated the possibility of generating and separating transversally two pulses along the undulator line, reaching a separation of ~5 mm in the horizontal plane at a wavelength of 31.25 nm, measured ~ 40 m away from the end of the undulator section. The global pulse energy was around 30 mJ, equally distributed between the two pulses.
Exploiting the experimental setup used to achieve ultraviolet transient gratings from four wave mixing experiments @ EIS-TIMER, a double pulse at 20.8 nm has been delivered to produce, on a sample of CoGd, a polarization grating (please refer to B. Wehinger, F. Bencivenga et al, to be published).
Besides, the direct demonstration of its feasibility in this wavelength range, we were also able to study the grating dynamics and compare it with that of  the density transient grating.



Our plans are focused on three main goals:

  • to extend the configuration to lower and higher harmonic orders, confirming and consolidating the control of this scheme in a wider range.
  • to automatize the trajectory setup to speed up the preparation and optimization of the FEL.
  • to characterize the optimal seed laser configuration and its correlation to the tilted directions of emission, demonstrating the micro-bunch rotation associated with it.


FEL Parameters 



Branch: FEL 1
Beamline : EIS-TIMER
Wavelength Range: 800 nm, 400 nm, standard FEL-1
Pulse Duration: standard FEL-1



Back to the main page 

Last Updated on Monday, 05 December 2022 16:22