Ultrafast Switching of Helicity

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. (Source: F. Armborst & K. Holldack)

At the BESSY II storage ring in Berlin, 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 inhouse and operated the storage ring in the 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 – the undu­lators. Ellip­tical undulators can be used to generate also circularly polarized light pulses, which display a feature called helicity: the polari­zation 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. Since the 1980s, this has been exploited in XMCD (X-ray Circular Dichroism) experiments to inves­tigate static and dynamic changes in magnetic materials or to image magnetic nano­structures on surfaces.

Especially for such imaging techniques, the user community at synchro­tron radiation sources has long wished for the possi­bility to quickly switch the helicity of the light, mainly because this directly results in a magnetic image contrast that makes bits in magnetic data storage devices visible and quanti­fiable. In the elliptical undu­lators typical for BESSY II (APPLE II), developed by the group around Johannes Bahrdt, the helicity of light is switched by a mechanical displace­ment of meter-long arrange­ments of strong permanent magnets, a process that sometimes takes up to minutes. The new method, however, is based on the combination of such undu­lators with a special orbit of the electron beam in the storage ring – generated by the TRIBs (transverse resonance island buckets). TRIBs have been experi­mentally explored by the acce­lerator expert Paul Goslawski at BESSY II. While the path of the electrons in the storage ring normally closes after one orbit, in the TRIBs mode the electrons run on different orbits during successive orbits and can thus emit X-ray pulses from different magnetic field confi­gurations, suggested Karsten Holldack and Johannes Bahrdt.

They were recently able to show that their idea actually works with the help of the existing double undu­lator UE56-2 at BESSY II in a pilot experiment: When passing through a specially prepared magnet arrange­ment of this double undulator, the electron bunches from different orbits in TRIBs mode emitted X-ray photons with the same wavelength but opposite circular polari­zation. Thus, in principle, XMCD signals from magnetic samples can now be studied at intervals of only 1 micro­second with right- and then left-circu­larly polarized light pulses. In the pilot experiment the XMCD signals from a magnetic sample (nickel in permalloy) were detected from revolution to revo­lution and the fast helicity change could be clearly demons­trated. With new undulators tailored for this purpose, special beamlines with ultrafast helicity change could be offered at BESSY II in TRIBs mode. Ulti­mately switching times could shrink to nano­seconds. (Source: HZB)

Reference: K. Holldack et al.: Flipping the helicity of X-rays from an undulator at unprecedented speed, Commun. Phys. 3, 61 (2020); DOI: 10.1038/s42005-020-0331-5

Link: Undulators at BESSY II, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany

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