First Observation of Undulator Photon Beams Carrying Orbital Angular Momentum

For the first time researchers of HZB observed 99eV photons carrying Orbital Angular Momentum (OAM). The photons were detected in the 2nd harmonic off-axis radiation of a helical undulator. The measurements confirm a theoretical prediction from five years ago. In the visible regime these singular photon beams or OAM-photons can be generated from laser light utilizing phase modulating techniques. The proof-of-principle experiment at BESSY II demonstrates the availability of OAM-photons up to the X-ray regime in low emittance machines, with helical undulators being used for the generation of circularly polarized radiation similar to today’s 3rd generation light sources.

Under normal operation conditions – electron energy of 1.72 GeV - the BESSY II beam emittance (i.e. the electron bunch phase space volume) is too large for the detection of the singular phase structure of OAM-photons. The emittance scales quadratically with the energy, and, thus, at 917 MeV (energy of a former PTB- electron optic) the emittance is reduced roughly by a factor of four which is sufficient for a clear well-defined experiment.  In January 2013 the BESSY II storage ring was ramped down for a 24h shift from 1.72 GeV to 917 MeV. An injection at 917 MeV was not possible and, hence, all storage ring magnets had to be tuned simultaneously during this delicate procedure in order minimize electron losses.

The interference experiment with the double undulator UE56-2 was conducted by a team around Dr. Johannes Bahrdt. One undulator was tuned to helical mode generating OAM photons in the 2nd harmonic. The other undulator served as a reference undulator providing linearly polarized light. The transverse distribution of both photon beams was mapped with a pinhole in front of the 1st optical element. The longitudinal overlap and the interference happened only behind the monochromator. The direct detection of the singular phase distribution of OAM beams would require a complicated wavefront sensor whereas the interference experiment permitted an intensity detection with a simple diode. The signature of the OAMs is a spiral intensity distribution where the orientation is determined by the helicity of the helical undulator and the spiral orientation is a measure of the phase advance between both undulators.

The joint effort of people from several HZB-groups (P. Kuske, P. Schmid: G-IA; R. Müller: NP-ABS, K. Holldack: G-ISRR and J. Bahrdt, M. Scheer: G-AUND) resulted in the success of this experiment, particularly, because it required specific machine conditions and the beam current was only 1mA. Photons carrying orbital angular momentum will be available permanently in synchrotron radiation light sources of the next generation such as Energy Recovery Linacs, Ultimate Storage Rings or Free Electron Lasers. The additional degree of freedom may trigger new spectroscopic experiments which are not possible at existing light sources.

Physical Review Letters (DOI: 10.1103/PhysRevLett.111.034801)