VUV and X-ray coherent light with tunable polarization from single-pass free-electron lasers

摘要

Tunable polarization over a wide spectral range is a required feature oflight sources employed to investigate the properties of local symmetry in bothcondensed and low-density matter. Among new-generation sources, free-electronlasers possess a unique combination of very attractive features, as they allowto generate powerful and coherent ultra-short optical pulses in the VUV andX-ray spectral range. However, the question remains open about the possibilityto freely vary the light polarization of a free-electron laser, when the latteris operated in the so-called nonlinear harmonic-generation regime. In suchconfiguration, one collects the harmonics of the free-electron laserfundamental emission, gaining access to the shortest possible wavelengths thedevice can generate. In this letter we provide the first experimentalcharacterization of the polarization of the harmonic light produced by afree-electron laser and we demonstrate a method to obtain tunable polarizationin the VUV and X-ray spectral range. Experimental results are successfullycompared to those obtained using a theoretical model based on the paraxialsolution of Maxwell's equations. Our findings can be expected to have a deepimpact on the design and realization of experiments requiring full control oflight polarization to explore the symmetry properties of matter samples.