Poincaré beams from a FEL
In Poincaré beams, light polarization varies across the transverse profile, exhibiting rich spatial structures. For the first time, Poincaré beams have been realized in the extreme-ultraviolet spectral region at the FERMI FEL in collaboration with researchers from LCLS.
Morgan et al., Nature Photonics, Vol. 19 - 9, pp. 946-951 (2025).
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A "star" Poincaré beam has been realized directly from the source of the FERMI FEL, by combining two pulses of opposite circular polarization, one carrying a helical phase and a ring-shaped intensity profile and another in a Gaussian mode. In the radiator, five helical undulators induce nonlinear harmonic bunching, resulting in circularly polarized, helically phased second harmonic emission radiated off-axis. The final undulator is tuned to produce a Gaussian beam with opposite circular polarization at the second harmonic of earlier undulators. A "star" Poincaré beam is generated by the superposition of the two modes, with a relative phase controlled by a small magnetic chicane (phase shifter). |
The Poincaré beam shows a Gaussian intensity profile, but, after a linear polarizer (reflection near Brewster’s angle) the structured polarization emerges as a spiral pattern, whose orientation rotates with the modes relative phase. Retrieve articlePoincaré beams from a free electron laser.
Morgan J., Rebernik Ribič P., Capotondi F., Brynes A., Manfredda M., De Ninno G., Novinec L., Pancaldi M., Pedersoli E., Simoncig A., Spezzani C., Zangrando M., Hemsing E. |
