Polarization switching demonstration using crossed-planar undulators in a seeded free-electron laser

Polarization switching of light sources is required over a wide spectral range to investigate the symmetry of matter. In this paper, we report the first experimental demonstration of the crossed-planar undulators technique at a seeded free-electron laser, which holds great promise for the full control and fast switching of the polarization of short-wavelength radiation. In the experiment, the polarization state of the coherent radiation at the secondharmonic of the seed laser is switched successfully. The experimental results confirm the theory and pave the way for applying the crossed-planar undulators technique for seeded x-ray free-electron lasers.