Abstract: We report on the realization of an extremely sensitive x-ray polarization microscope, allowing to detect tiniest polarization changes of 1 in 100 billion (10(-11)) with a mu m-size focused beam. The extreme degree of polarization purity places the most stringent requirements on the orientation of the polarizer and analyzer crystals as well as the composition and the form fidelity of the lenses, which must not exhibit any birefringence. The results show that these requirements are currently only met by polymer lenses. Highly sensitive scanning x-ray polarization microscopy thus is established as a new method. It can provide new insights in a wide range of applications ranging from quantum electrodynamics and quantum optics to x-ray spectroscopy, materials research, and laser physics.
Abstract: High-brilliance synchrotron radiation sources have opened new avenues for x-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear x-ray polarizers in a crossed setting, polarization extinction ratios down to 10⁻¹⁰ can be achieved. This renders the method sensitive to probe the tiniest optical anisotropies that would occur, for example, in strong-field quantum electrodynamics due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La₂CuO₄ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and x-ray lasers, where polarization extinction ratios of 10⁻¹² can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)
Abstract: Magnetism, superconductivity, and other macroscopic quantum effects are based on symmetry breaking in solids. Their atomic and molecular structure can be studied using linearly polarized X-rays, where a change of the polarization state of the transmitted beam enables conclusions about electronic anisotropies in the material. Responsible for a change of the polarization state are the optical effects dichroism and birefringence. While X-ray absorption spectroscopy is a well-established method for the detection of dichroism, the effect of birefringence in the vicinity of an X-ray absorption edge is little studied. This work presents the first comprehensive experimental and theoretical investigation of X-ray birefringence and dichroism at the Cu K-absorption edge for two different model substances, CuO and La2CuO4. For this purpose, high-precision X-ray polarimetry, which detects changes of the polarization state with utmost sensitivity, was further developed into a spectroscopic method.