An X-ray Vortex on the Horizon?
MAY 5, 2008
It is well known that a helical undulator does not generate any harmonics (except the fundamental) on the central radiation axis. Consequently, the off-axis radiation in the higher harmonics has been considered useless, and its properties have not been investigated closely. In a new paper published in Physical Review Letters, Shigemi Sasaki of the Accelerator Science Division and Ian McNulty of the X-ray Science Division, at the Argonne Advanced Photon Source (APS), propose that the electromagnetic Circularly Polarizing Undulator installed at X-ray Operations and Research beamline 4-ID-C at the APS—and similar permanent-magnet devices at the Advanced Light Source in Berkeley and elsewhere—are ideal sources for direct production of x-ray vortices. Indeed, evidence for vortex states has recently been observed from free-electron laser (FEL) sources operating in the infrared region of the electromagnetic spectrum.
In the x-ray region, these previously unappreciated properties of undulator radiation may also prove useful for spiral phase imaging of biological samples and to solve the structures of chiral proteins and pharmaceutical macromolecules by diffraction methods. The intensity and purity of these x-ray vortex beams will be vastly greater using ultra-bright FEL and energy recovery linac sources, potentially leading to novel femtosecond and quantum optics experiments with x-rays.
Sasaki and McNulty note that, just as the advent of polarized x-ray sources has dramatically expanded our 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.