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

Abstract

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 L 2 , 3 absorption edges of Ni with an 800 ns helicity flip.

Fast and versatile polarization control of X-ray by segmented cross undulator at SPring-8

“…This is in contrast to the switching with the kickers, where the polarization is changed by switching the IDs placed at different positions that produce different polarizations, so the position of the light source is generally shifted by the order of meter. Recently, a different approach for the faster polarization switching was proposed [37], but this also changes the light source position during the polarization switching. It should be emphasized that the advantage of the SCU is that the effect on the electron orbit is very small, almost negligible, and that the position of the light source does not change.…”

Split or unsplit electron beams?

“…Finally, it is possible to generalise what was done in [39] and to propose an application that provides fast switching (on a turn-by-turn basis and even faster) of general x-ray properties, not only helicity as reported in [39]. Two configurations can be envisaged, namely, electrons inside a single island of a resonance of order r, or electrons with an AC dipole that excites a periodic orbit of period r. In both cases, at a given location of the ring, the electrons pass at different positions each turn, and each position repeats regularly with a periodicity of r turns.…”

“…For a list of current applications and benefits to synchrotron light source beamlines see Refs. [17,38,39]. In this section, a few novel proposals are discussed.…”

Attosecond Polarisation Modulation of X-ray Radiation in a Free Electron Laser

“…Simulations are carried out here in the soft X-ray generating radiation pulse trains with alternate orthogonal polarisation pulse timescales of tens of attoseconds. This is approximately four orders of magnitude faster than any other method [4] and eight orders of magnitude faster than polarisation switching available at X-ray wavelengths [5].…”

“…In electron accelerator based light sources, which can generate light into the hard X-ray, the motion of the radiating electrons propagating though magnetic undulators determines the polarisation of the photon beam. For example, circular polarisation modulation has been demonstrated in a synchrotron by controlling electron bunch orbits through twin undulators [5]. Although this modulates the X-ray polarisation, the modulation rate is limited to less than 500MHz.…”