At the BESSY II storage ring, a joint team of accelerator physicists, undulator experts and experimenters has shown how the helicity of circularly polarized synchrotron radiation can be switched faster – up to a million times faster than before.
They used an elliptical double-undulator developed at HZB and operated the storage ring in the so-called two-orbit mode. This is a special mode of operation that was only recently developed at BESSY II and provides the basis for fast switching. The ultra-fast change of light helicity is particularly interesting to observe processes in magnetic materials and has long been expected by a large user community.
In synchrotron radiation sources such as BESSY II, electron bunches orbit the storage ring at almost the speed of light. They are forced to emit extremely bright light pulses with special properties by periodic magnetic structures (undulators).
Experiments with polarized light pulses
Elliptical undulators can be used to generate also circularly polarized light pulses, which display a feature called helicity: the polarisation goes either clockwise or counterclockwise. Magnetic structures in materials react differently to circularly polarized light: Depending on the helicity of the X-ray pulses, they more or less absorb this radiation.
Image: This picture shows an X-ray image of the electron beam in TRIB-mode where two orbits co-exist: the regular orbit and the second one winding around it closing only after three revolutions.
Credit: F. Armborst/K. Holldack