Towards perfectly linearly polarized x-rays

In recent years, high-precision x-ray polarimeters have become a key method for the investigation of fundamental physical questions from solid-state physics to quantum optics. Here, we report on the verification of a polarization purity of better than $8\times {10}^{-11}$ at an x-ray free-electron laser, which implies a suppression of the incoming photons to the noise level in the crossed polarizer setting. This purity provides exceptional sensitivity to tiny polarization changes and offers intriguing perspectives for fundamental tests of quantum electrodynamics.

Figure 1

Scheme of the experimental setup used to create perfect linear polarization. The XFEL radiation with a photon energy of 6.457 keV was collimated to a high degree by a beryllium lens, followed by mirrors, which guided the beam to the experiment. A high-heat load monochromator (mono) selected and stabilized the photon energy and reduced the heat load on the polarizer. The polarimeter consisted of two so-called channel-cut crystals made from silicon. Each of the crystals supported six consecutive reflections with a scattering angle of ${90}^{\circ }$. The analyzer crystal could be rotated around the beam to analyze the created polarization state.

Figure 2

(a) Dependence of the intensity behind the polarimeter when the analyzer was rotated from the parallel polarizer setting at $0^{\circ }$ to the crossed polarizer setting at ${90}^{\circ }$. The data points are the integrals of the rocking curve normalized to the value at $0^{\circ }$. The solid line represents the ${cos}^2$ behavior expected for perfect polarizers. Insets (b) and (c) show the count rate when the analyzer crystal is rocked in the parallel and the crossed setting, respectively. In the latter, no rocking curve above the noise level can be detected. The dotted lines mark the region in which the curve is expected.

Figure 3

Variation of the count rate close to the crossed polarizer setting. At exactly ${90}^{\circ }$, the count rate lies within the background noise (gray-shaded area). The values were obtained from the data of an energy- and space-resolving timepix detector. The corresponding intensity distributions are shown for specific positions at which in the upper one the white rectangle highlights the region of interest used for integration. The blue solid line describes the behavior of perfect polarizers. Error bars represent the statistical error.