Light’s Orbital Angular Momentum Carried by Helical Undulator Harmonics

Abstract

Spiral interference patterns between two different harmonic radiations from two tandem-aligned helical undulators were observed by a scanning fiber multi-channel spectrometer and a UV-CCD camera placed at the end of beamline downstream of S1 straight section in UVSOR-III. Using these measurements, various interference patterns such as single, double, and triple spirals were observed which concur with the theoretical predictions for every mode in the right or left circular polarization. The rotation of an interference pattern by rotating a polarizer was also observed. In this article, a theoretical background and a series of experimental results are described for the exotic property of undulator harmonics.