Generation and use of ultra-short laser and FEL light pulses with orbital angular momentum


Optical vortices are helically phased light beams with a field dependence of exp(ilφ), where l is the topological charge and φ the azimuthal coordinate in the plane perpendicular to the beam propagation. Such beams carry an orbital angular momentum (OAM) of lℏ per photon. When generated in the extreme-ultraviolet range, optical vortices may be exploited to:
  • Gain insight into the local symmetry and chirality of a material;
  • Steer the magnetic properties of nanoparticles; 
  • Increase the resolution in microscopy and ptychography; 

Far-field Intensity

Measured far-field intensity (high left ) phase (high right), and intensity distribution in the focal plane (low) using zone plates with topological charge l=2


The FERMI team and collaborators implemented two different techniques for generating intense, femtosecond, coherent optical vortices in the extreme ultraviolet range [1]. The first method takes advantage of harmonic generation in a helical undulator, producing vortex beams at the second harmonic without the need for additional optical elements, while the second method relies on the use of a spiral zone plate to generate a focused, micron-size optical vortex with a peak intensity approaching 1014 W/cm2. These results pave the way to nonlinear optical experiments with vortex beams at short wavelengths. 
The interplay between IR light with OAM and XUV FEL light with circular polarization has been used, at the LDM beam line, to show the possibility of inducing an OAM-dependent dichroic photo-electric effect [2] and to demonstrate a laser-based scheme for generating atomic-scale charge current loops within femtoseconds [3].
FEL light with OAM produced by the spiral zone plates has been used, at the DIPROI beam line, to demonstrate the possibility of performing magnetic helicoidal dichroism by measuring the magnetic scattering from a permalloy dot [4]. New studies are foreseen to continue this activity. 


Future experiments will focus on the careful characterization of the wave front of OAM light generated in helical undulators. This radiation will then be used for exploring the interplay between spin and orbital angular momenta in magnetic helicoidal dichroism.   
Using the setup already available at DIPROI, ptychographic measurements for imaging the electron dynamics in metallic nanostructures will be also carried out. 


FEL Parameters 



Branch: FEL 1
Beamline : DIPROI, LDM
Wavelength Range: 800 nm, 400 nm, standard FEL-1 
Pulse Duration: standard FEL-1

[1] P.R. Ribič et al., Physical Review X 7 (3), 031036 (2017)
[2] G. De Ninno et al., Nature Photonics 14 (9), 554-558 (2020)
[3] J. Wätzel et al., Phys. Rev. Lett. 128 (15), 157205

Back to the main page

Last Updated on Monday, 05 December 2022 16:20