Bragg-case x-ray dynamical diffraction propagator in SRW: application to thin crystal phase retarders

A Bragg-case X-ray dynamical diffraction propagator has already been integrated into the “Synchrotron Radiation Workshop” (SRW) physical optics simulation software package. Previous benchmarking tests on crystal monochromators assumed thicknesses many times the extinction length, for which transmission is negligible. This paper reports tests of this propagator applied to thin crystals in transmission. The chosen example is a phase retarder, which allows users to alter the polarization of an X-ray beam. Phase retarders are often applied to studies of solid-state materials with hard X-rays, which current medium-energy storage ring synchrotron sources typically produce only with linear polarization. Correct designs of phase retarders require the accurate determination of both the intensity and the phase of the diffracted wave in all polarization states. First, to approximate an incident plane wave, SRW is used to simulate the passage of a Gaussian beam of very large radius of curvature through the phase retarder. Then, the phase retarder’s effects on a typical undulator beam are simulated and the results are compared. Because X-ray phase retarders are highly sensitive to angular alignment, tolerances in misalignment are also determined. SRW simulations are compared with experimental data from the Integrated In Situ and Resonant Hard X-ray Studies (ISR) beamline at NSLSII. The design of phase retarders can therefore be optimized for X-ray beamlines that must combine variable polarization with focusing or other properties.