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X-ray polarization offers unique insights into the electronic structure of matter

01/11/2021

We receive visual information about our world through light. A less “obvious” property of light, besides color and brightness, is its polarization. The polarization of light is the plane in which the electric field of the light wave oscillates. The plane of oscillation of light can change (e.g., rotate) when light is scattered by a material. This is called optical activity of the material. The rotation of the plane of polarization of the light contains valuable information about the properties of the illuminated material. For example, it allows insights about the preferred orientation of electrons in atoms (orbitals) and the directions along which electrons can move in a material. Such preferred directions are called anisotropies and often form the basis for specific properties of modern functional materials, e.g. directed current transport, magnetic alignment of nanostructures, optical birefringence and many more.

Physicists from the Helmholtz Institute Jena and the Friedrich Schiller University, DESY in Hamburg and research institutes in Grenoble and Paris have now discovered a new way to detect polarization changes of X-rays with particularly high sensitivity and thereby investigate anisotropies in the electronic properties of materials with high accuracy. To do this, the researchers used the extremely luminous PETRA III synchrotron radiation source at DESY in Hamburg. The samples studied were the metal oxides CuO and La2CuO4. These are starting materials for high-temperature superconductors, in which the electrons in the orbitals of the copper atom play a major role in superconductivity. The results of this study were recently published in the journal Optica.

The key to the new measurement method is a technique that has been under development in Jena for many years, namely precision polarimetry with X-rays: If X-ray light is allowed to reflect several times at an angle of incidence of 45 degrees on a perfect single crystal of silicon, then this light is almost completely linearly polarized after reflection, i.e. it oscillates practically exclusively in one plane. Scattering of this light by an optically active material causes deviations from this perfection, which become apparent in a slight rotation of the plane of oscillation of the light. This rotation of polarization can be detected by reflecting the scattered light off a second, identical crystal whose reflection plane is rotated 90 degrees with respect to the first crystal. Due to the high perfection of silicon crystals, sensitivities of one in a billion can be achieved, i.e. a clear measurement signal is obtained even if only a tiny fraction – about one billionth (10-9) – of the light scattered by the sample is rotated.

Two optical effects contribute to the rotation of the polarization: the selective attenuation of the light by the sample (dichroism), and the direction-dependent refraction of the light (birefringence) in the material.

In this experiment, the scientists succeeded in separating the contributions of these two effects and attributing them to the interaction of the X-ray light with specific orbitals of the copper atom in the materials studied (cf. the figure on the right). This now provides a highly sensitive method for tracking down the electronic anisotropies in complex materials and investigating the role they play, for example, in novel forms of superconductivity, magnetism and optical properties, reports Annika Schmitt, first author of this study.

Further development of the method at X-ray sources such as the European X-ray laser XFEL or the future diffraction-limited synchrotron radiation source PETRA IV promises to increase the detection sensitivity by a factor of 1000 to one in a trillion (10-12). This could even make it possible to detect the optical birefringence of the vacuum, an effect that was predicted by Heisenberg and Euler as early as 1936, but which has so far eluded experimental proof. A corresponding experiment at the European X-ray laser is currently in preparation, reports Professor Gerhard G. Paulus, who holds the Chair of Nonlinear Optics at the University of Jena. If a dichroism of the vacuum were also detected in such an experiment, this could be a direct indication of particles beyond the Standard Model. Ralf Röhlsberger, professor for X-ray physics at Friedrich Schiller University and a scientist at DESY, is convinced that the method of high-purity X-ray polarimetry opens up fascinating applications, not only for studying the electronic properties of complex solids, but also for fundamental studies of the light—matter interaction.

Original publication: Annika T. Schmitt, Yves Joly, Kai S. Schulze, Berit Marx-Glowna, Ingo Uschmann, Benjamin Grabiger, Hendrik Bernhardt, Robert Loetzsch, Amelie Juhin, Jerome Debray, Hans-Christian Wille, Hasan Yavas, Gerhard G. Paulus, and Ralf Röhlsberger: Disentangling x-ray dichroism and birefringence via high-purity polarimetry. Optica 8, 56–61 (2021), DOI: 10.1364/OPTICA.410357

Channel cut crystal, made from silicon, and a core component of the polarizer–analyzer setup

Comparison of observed data with simulated dichroism and birefringence contributions