The next undulator source to look at orbital angular moment

It is known that a helical undulator does not generate any higher-order harmonics on the central radiation axis. As such, off-axis radiation in higher-order harmonics has been considered useless, but so far this problem has not been discussed further. Professor S. Sasaki and his colleagues (Argonne National Lab, USA) have recently published an interesting paper about this problem. They found that all the harmonics except the fundamental from a variable polarizing undulator, such as an Advanced Planar Polarized Light Emitter (APPLE) device, are expressed by Laguerre-Gaussian modes carrying orbital angular momentum, when it is phased to deliver circularly polarized radiation. As the advent of polarized X-ray sources has dramatically expanded the understanding of magnetism, the availability of intense X-ray beams carrying orbital in addition to spin angular momentum could open the door to new condensed matter research via X-ray scattering and spectroscopy methods. For more information, see the paper, "Proposal for Generating Brilliant X-Ray Beams Carrying Orbital Angular Momentum", S. Sasaki et al., Phys. Rev. Lett., 100, 124801 (2008).