Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wavefront. The distinctive way in which the photon spin dictates the electron motion upon light–matter interaction is the basis for numerous well-established spectroscopies. By contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable.
We carried out an experiment at the LDM beam line at the FERMI free-electron laser, with the aim of inducing an OAM-dependent dichroic photoelectric effect on photo-electrons emitted by a sample of He atoms. The experiment involved a large international collaboration and surprisingly confirmed that the spatial distribution of an optical field with vortex phase profile can be imprinted coherently on a photoelectron wave packet that recedes from an atom. Our results explore new aspects of light–matter interaction and point to qualitatively novel analytical tools, which can be used to study, for example, the electronic structure of intrinsic chiral organic molecules. The results have been published in Nature Photonics.
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Image: A VUV free-electron laser (violet) is used to ionize a sample of He atoms, and an infrared beam (red) to imprint orbital angular momentum on photo-emitted electrons. Credit: J. Wätzel (Halle university)