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.
Abstract: We report on the use of synthetic single-crystal diamonds for high definition x-ray polarimetry. The diamonds are precision mounted to form artificial channel-cut crystals (ACCs). Each ACC supports four consecutive reflections with a scattering angle 2ΘB of 90°. We achieved a polarization purity of 3.0×10−10 at beamline ID18 of the European Synchrotron Radiation Facility (ESRF). When the x-ray beam's horizontal divergence was reduced through additional collimation from 17 to 8.4μrad, the polarization purity improved to 1.4×10−10. Precision x-ray polarimetry thus has reached the limit, where the purity is determined by the divergence of the beam. In particular, this result is important for polarimetry at fourth generation x-ray sources, which provide diffraction-limited x-ray beams. The sensitivity expected as a consequence of the present work will pave the way for exploring new physics such as the investigation of vacuum birefringence.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)
Abstract: The dissertation describes the development and application of several diamond crystal x-ray polarizers. The polarizers are based on the channel-cut principle, in which an X-ray beam is diffracted several times under a Bragg angle of 45° and linearly polarized. The diamond crystals were characterized and the effect of defects (dislocations and stacking faults) on X-ray polarimetry were investigated. Since the diamonds were unsuitable for the fabrication of monolithic channel-cut crystals, special quasi-channel cuts (QCC's) out of invar alloy and mirror mounts were developed. With these QCC's up to four diamonds could be adjusted parallel to each other with a precision of sub-μrad. These diamond QCC’s were used in experiments at the European synchrotron in Grenobel, where an unprecedented polarization purity of 1.3 x 10^(-10) was achieved. As a further result, it was proved that the polarization purity is limited by the divergence of the synchrotron and that a better purity can be measured with reduced divergence. Thus, even better polarization purity can be achieved at x-ray sources with lower divergence, e.g. Synchrotron 4th generation and X-ray lasers. This is an important result for the measurement of vacuum birefringence in future. Al in al the dissertation shows that even diamond crystals with dislocation densities in the range of 10^4 to 10^6 cm^-2 are suitable for high-precision X-ray polarimetry and the production of highly pure linear polarized X-ray beams.
Abstract: We report on the use of synthetic single-crystal diamonds for high purity x-ray polarimetry to improve the polarization purity of present-day x-ray polarimeters. The polarimeter setup consists of a polarizer and an analyzer, each based on two parallel diamond crystals used at a Bragg angle close to 45°. The experiment was performed using one (400) Bragg reflection on each diamond crystal and synchrotron undulator radiation at an x-ray energy of 9838.75 eV. A polarization purity of 8.9 × 10−10 was measured at the European Synchrotron Radiation Facility, which is the best value reported for two-reflection polarizer/analyzer setups. This result is encouraging and is a first step to improve the resolution of x-ray polarimeters further by using diamond crystal polarizers and analyzers with four or six consecutive reflections.
Abstract: The advent of third-generation synchrotron radiation sources and X-ray free-electron lasers has opened up the opportunity to perform quantum optical experiments with high-energy X-rays. The prime atomic system for experiments in this energy range is the strongly nuclear resonant 57Fe Mössbauer isotope. Experiments have included measurements of the collective Lamb shift, observation of electromagnetically induced transparency, subluminal propagation of X-rays and spontaneously generated coherences. In these experiments, however, the nuclei were only weakly coupled to the light field. Collective strong coupling of nuclei and X-rays, which is desirable for many quantum optical applications, has eluded researchers so far. Here, we observe collective strong coupling between X-rays and matter excitations in a periodic array of alternating 57Fe and 56Fe layers. Our experiment extends the range of methods for X-ray quantum optics and paves the way for the observation and exploitation of strong-coupling-related phenomena at X-ray energies.
Abstract: We present a possible fabrication scheme of anisotropic nanoparticles grown in a crystal high-index material (SrTiO3). Different ellipsoidal Au nano-antennas were formed by changing the Au seed layer thickness and subsequent embedding in SrTiO3, prepared by pulsed laser deposition. Prior to the SrTiO3 deposition, a temperature-induced dewetting process of the thin Au films results in different particle sizes and size distributions, which are the basis for anisotropic particle formation after embedding in a crystalline SrTiO3 matrix. The dependence of the anisotropy on the Au seed layer thickness was investigated by X-ray diffraction (XRD) measurements. At this was noticed a stronger increase in size in c-axis direction than in a/b-axis direction for an increase of the Au seed layers. Additionally, the optical response of the particles was detected via the plasmon resonance shift in extinction and scattering spectra.
Abstract: Crystalline Au nanoparticles embedded in epitaxially grown SrTiO₃ layers were prepared by an annealing and coating procedure of Au seed layers on SrTiO₃ (STO) substrates. X-ray diffraction and transmission electron microscopy measurements were performed to investigate the size, shape, and deformation of the particles and their crystal orientation. The shape and size of the crystalline Au nanoparticles can be tuned by controlling the Au seed layer thickness and single crystalline elliptically shaped Au nanoparticles have been generated. Furthermore, the orientation of the surrounding SrTiO₃ matrix changes significantly from homoepitaxially grown (001) to secondary (111) and (011) orientations for Au seed layers that are thicker than 4 nm. This is of great interest for modifying the electrical properties of SrTiO₃ layers, whereas the anisotropically shaped crystalline particles are relevant for optical applications, due to localized surface plasmon resonances.
Abstract: Self-organized monocrystalline Au nanoparticles with potential applications in plasmonics are grown in a SrTiO3 matrix by a novel two-step deposition process. The crystalline preferred orientation of these Au nanoparticles is investigated by synchrotron hard x-ray diffraction. Nanoparticles preferentially align with the (111) direction along the substrate normal (001), whereas two in-plane orientations are found with SrTiO3∥Au and SrTiO3∥Au. Additionally, a smaller diffraction signal from nanoparticles with the (001) direction parallel to the substrate normal (001) is observed; once again, two in-plane orientations are found, with SrTiO3∥Au and SrTiO3∥Au. The populations of the two in-plane orientations are found to depend on the thickness of the gold film deposited in the first step of the growth.
Abstract: The authors present a novel in-situ method of fabricating crystalline gold nanoparticles by self-organization. This nanoparticles are grown and modified in a surrounding thin film matrix using two different host materials (YBa2Cu3O7-δ and SrTiO3) prepared by a pulsed laser deposition technique. The crystalline Au nanoparticles are formed out of a gold seed layer whereby the thickness of the initial seed layer influences the particle size and their distribution density. As we will show, using a matrix based preparation technique offers several advantages over conventional preparation methods. On the one hand, nanoparticle size and the distribution density can be controlled individually. On the other hand, by choosing an appropriate matrix material as well as suitable growth conditions also the shape of the resulting particles can be modified. Thus, also anisotropic nanoparticles can be prepared without using highly sophisticated methods like electron beam lithography or focused ion beam techniques. As one might have to extract the nanoparticles or at least theirs tips from the surrounding matrix material to realize photonic applications, we will show that an extraction is easily possible by selectively etching the matrix. This extraction process does not influence the particle distribution, i.e. particles can be prepared and extracted at distinct positions on the substrate utilizing a patterning of the Au seed layer. A spectral characterization of extracted as well as embedded particles will be presented based on microspectroscopy as well as on measurements using an integrating sphere.