Flipping the helicity of X-rays from an undulator at unprecedented speed

X-ray circular dichroism (XMCD), one of the main tools to study magnetism, benefits enormously from the capability of a fast alterable helicity of circularly polarized X-ray photons. Here we present a method for boosting the alternating frequency between right- and left-handed photons to the MHz regime, more than three orders of magnitude faster than state-of-the-art technologies. The method is based on a twin elliptical undulator installed in an electron storage ring being operated in a novel mode where the electron optics is tuned close to a resonance with electrons captured in transverse resonance island buckets. Propagating through the twin undulator, electrons from different islands emit photons of the same wavelength but of opposite helicity. These two helicity components can be alternated as fast as 2 ns. In a proof-of-principle experiment at BESSY II, we demonstrate XMCD at the L2,3 absorption edges of Ni with an 800 ns helicity flip. Rapidly switching the helicity of polarised x-rays is desirable for studying magnetic dynamics via circular dichroism spectroscopy, but has not yet been realised from synchrotron radiation sources. Here, switching between ultrafast emission of monoenergetic, circularly polarized x-rays with opposite helicity is shown within a few nanoseconds.