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

Abstract

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 second harmonic 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.

Control of the Polarization of a Vacuum-Ultraviolet, High-Gain, Free-Electron Laser

“…At each measured wavelength, all the experimental diagnostics confirm that, as expected from standard single-particle emission theory [39], FERMI's output polarization state may be controlled and adjusted directly via the undulator polarization. Our findings are in accordance with earlier polarization measurements made on visible and UV FEL light, both for a storagering FEL [40] and for low-gain, single-pass seeded FELs [27,[41][42][43] where a very high degree of polarization was found both at the resonant FEL wavelength and at the higher harmonics [42]. They also agree with a recent FERMI user experiment [35] that provided independent evidence at a single VUV wavelength (25.6 nm) of a similar high degree of polarization for the output light of FERMI's FEL-1.…”

“…Here too, though, both the peak power and total pulse energy are extremely small compared to what is commonly produced by the aforementioned FEL sources such as LCLS and SACLA. Although methods for partial control of the output x-ray polarization for these nominally linearly polarized sources by means of either polarizer filters [23] or via crossed-field undulators [24,25] have been proposed and in part demonstrated, in principle [26,27], none has been operationally implemented yet; both SACLA and LCLS are working toward possible solutions that would allow users to choose the FEL polarization. Very recently, by means of specially designed diamond-crystal optics that can be used in the hard x ray as a phase retarder, the SACLA team has measured a high degree of circular and also vertically polarized radiation at about 1 angstrom [28].…”

Coherent photon beam based diagnostics for a seeded extreme ultraviolet free-electron laser

“…The SDUV-FEL is a multipurpose test facility for novel FEL principle studies, where echo-enabled harmonic generation FEL [26], crossed-planar undulators [46] and two-stage HGHG [47][48][49] have been successfully demonstrated. During the experiment of coherent photon beam diagnostics, the beam energy of SDUV-FEL was set to be 148 MeV.…”

Polarization Characterization of Soft X-Ray Radiation at FERMI FEL-2

“…An alternative to produce a polarized hard X-ray beam by means of phase-retarder crystals has the drawbacks of a pulse intensity reduced by several orders of magnitude and of a poor degree of polarization, typically below 60% [15,16]. Another alternative for radiation polarization control in facilities based on linearly polarized undulators relies on the superposition of two orthogonally polarized pulses [17][18][19]. Finally, in seeded FELs, where the amplification process is expedited by the use of an external seed, the required undulator length is reduced and the use of variable polarization undulators is a viable solution.…”