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Extreme-ultraviolet vortices from a free-electron laser

Extreme-ultraviolet vortices may be exploited to steer the magnetic properties of nanoparticles, increase the resolution in microscopy, and gain insight into local symmetry and chirality of a material; they might even be used to increase the bandwidth in long-distance space communications. However, in contrast to the generation of vortex beams in the infrared and visible spectral regions, production of intense, extreme-ultraviolet (XUV) and x-ray optical vortices still remains a challenge. Here, we present an in-situ and an ex-situ technique for generating intense, femtosecond, coherent optical vortices with tunable topological charge at a free-electron laser (FEL) in the XUV.
The first method takes advantage of nonlinear harmonic generation in a helical undulator and exploits the fact that such harmonics carry a topological charge of l = n-1, where n is the harmonic number. The experiment was performed at the FERMI FEL. An ultraviolet (250-nm) seed laser was used to energy modulate the electron beam (e-beam) in the first undulator (modulator), as shown in the top panel of Figure 1. The e-beam was then sent through a dispersive section (a four-dipole-magnet chicane), where the energy modulation was transformed into a current-density modulation (bunching) with Fourier components spanning many harmonics of the seed laser frequency. Such a bunched e-beam entered the helical radiator tuned to a fundamental wavelength of 31.2 nm (i.e., the 8th harmonic of the seed), producing coherent light in the XUV. The FEL was operated in the high-gain regime, close to the saturation point. Under these conditions, the interaction between the radiation at the fundamental FEL wavelength and the e-beam induced bunching at the second harmonic (15.6 nm), resulting in emission of coherent XUV vortices carrying unit topological charge (l = 1) at intensities on the order of 10−3 of the fundamental FEL emission; see bottom panel in Figure 1.

Figure 1 Top: The scheme to generate optical vortices at harmonics (in the present case at the 2nd harmonic) of the fundamental FEL wavelength. The optical vortex is separated from the fundamental FEL emission using a Zr filter. Bottom: Intensity profile of the generated optical vortex with a topological charge of l =1 (left), and interference with a Gaussian beam revealing the twisted nature of the vortex (right).


The second method relies on the use of a spiral zone plate (SZP), which is placed directly into the optical path of the fundamental (transversely Gaussian) FEL beam. The SZP simultaneously imprints a helical phase onto the FEL beam and focuses it, just like an ordinary Fresnel zone plate, generating a focused optical vortex. The scheme is shown in the top panel of Figure 2. The experiment was carried out at the TIMEX beamline at FERMI. An FEL beam at a wavelength of 26 nm was sent onto SZPs with different topological charges. The bottom panels show the resulting optical vortex with a topological charge of l = 3. The described method allows generating micron-size optical vortices with tunable topological charge and peak intensities approaching 1014 W/cm2, paving the way to nonlinear optical experiments with vortex beams at short wavelengths.
The methods developed here will allow carrying out some of the recently proposed experiments involving vortex beams in different areas of fundamental and applied physics ranging from optics to magnetic switching and from material characterization to super-resolution laser machining.

Figure 2 Top: The scheme to generate optical vortices with tunable topological charge using spiral zone plates. Bottom: Far-field intensity (left) and phase (middle) profiles of the optical vortex with l =3, and intensity in the focus (right).



This research was conducted by the following research team:

Primoz Rebernik Ribic1, Benedikt Rösner2, David Gauthier1, Enrico Allaria1, Florian Döring2, Laura Foglia1, Luca Giannessi1,4, Nicola Mahne1, Michele Manfredda1, Claudio Masciovecchio1, Riccardo Mincigrucci1, Najmeh Mirian1, Emiliano Principi1, Eléonore Roussel1, Alberto Simoncig1, Simone Spampinati1, Christian David2, and Giovanni De Ninno1,3

Elettra-Sincrotrone Trieste, Trieste, Italy
Paul Scherrer Institut, Villigen, Switzerland
Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
ENEA, C.R. Frascati, Frascati (Rome), Italy

Contact persons:

Emiliano Principi, email: emiliano.principi@elettra.eu


P. R. Ribič, B. Rösner, D. Gauthier, E. Allaria, F. Döring, L. Foglia, L. Giannessi, N. Mahne, M. Manfredda, C. Masciovecchio, R. Mincigrucci, N. Mirian, E. Principi, E. Roussel, A. Simoncig, S. Spampinati, C. David, and G. De Ninno, "Extreme-Ultraviolet Vortices from a Free-Electron Laser", Phys. Rev. X 7, 031036 (2017), DOI: 10.1103/PhysRevX.7.031036

Last Updated on Monday, 05 February 2018 18:26