Multilayer-based Soft–X-Ray Polarimetry

AulaTouschek (Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati)


Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati

Via Enrico Fermi 40 00044 Frascati

Soft x-ray synchrotron radiation with variable polarisation is a sophisticated probe of the physical properties of matter. Many of the most advanced experiments take advantage of the inherently high degree of linear and/or circular polarisation of such a source which is in general an elliptical undulator. Polarimeters designed to deliver the four Stokes parameters of a source rely on a phase retarder and analyser in combination /1/. Because of the enhancement of the multilayer performance near absorption edges most multilayer phase retarders have been designed to be used near the 2p absorption edges of the constituting materials (Mo/Si, Cr/C, Cr/Sc, Ni/Ti, and Ni/V) /1/. Thus these optical elements are monoenergetic in the range between 100 eV and 600 eV. At best they can operate at two distinctive energies (e.g. Sc 397 eV and Cr 550 eV) /2/. Here I will show data on newly developed Cr/Sc phase plates to be used as two-band quarter-wave plates and on the performance of a newly developed W/B4C phase retarder designed to work from ~600 eV to ~1000 eV. This range was previously not accessible for circular polarisation analysis without further modelling /3/. The measured phase shift is slowly variing and sufficiently large over this photon energy range to enable quantitative and self- calibrating soft x-ray polarimetry in the range of the 2p edges of Fe, Co and Ni, where most of the polarisation-sensitive magneto-optical investigations are being carried out.

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