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.
>Read more on the FERMI website
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).