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Polarizing x-ray optics for synchrotron radiation

Bragg reflecting perfect crystals such as silicon are doubly birefringent and dichroic. Phase coherence is preserved during Bragg reflection and therefore all the polarizing optics familiar in the visible band can be realized for x rays. Design principles are discussed for three key devices; tuneable high intensity linear polarizers with extinction ratios greater than 106; Bragg/Fresnel Rhombs that are the only high efficiency tuneable quarter wave plates so far described; and x-ray phase compensators. The linear x-ray polarizers and the quarter wave rhomb both rely on multiple Bragg reflections in channel cut crystals. Less efficient linear polarizers, which rely on x-ray linear dichroism to separate orthogonal polarization states, have been used in some experiments as have dichroic wave plates for the conversion of linearly polarized x-rays into elliptically polarized beams. These are likely to be less useful than bending magnets (for linear polarization) or helical insertion devices (for circular polarization) in practice as their efficiency and polarization ratio is rather poor.