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Publikationen von
Dr. Robert Lötzsch

Alle Publikationen des HI Jena

2022

B. Marx-Glowna, B. Grabiger, R. Loetzsch, I. Uschmann, A. T. Schmitt, K. S. Schulze, A. Last, T. Roth, S. Antipov, H.-P. Schlenvoigt, I. Sergueev, O. Leupold, R. Roehlsberger, and G. G. Paulus
Scanning high-sensitive x-ray polarization microscopy
New Journal of Physics 24, 053051 (2022)

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.

K. Schulze, B. Grabiger, R. Loetzsch, B. Marx-Glowna, A. Schmitt, A. Garcia, W. Hippler, L. Huang, F. Karbstein, Z. Konopková, H.-P. Schlenvoigt, J.-P. Schwinkendorf, C. Strohm, T. Toncian, I. Uschmann, H.-C. Wille, U. Zastrau, R. Röhlsberger, T. Stöhlker, T. Cowan, and G. Paulus
Towards perfectly linearly polarized x-rays
Physical Review Research 4, 013220 (2022)

Abstract: 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 8×10−11 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.

E. Eftekhari-Zadeh, M. S. Blümcke, Z. Samsonova, R. Loetzsch, I. Uschmann, M. Zapf, C. Ronning, O. N. Rosmej, D. Kartashov, and C. Spielmann
Laser energy absorption and x-ray generation in nanowire arrays irradiated by relativistically intense ultra-high contrast femtosecond laser pulses
Physics of Plasmas 29, 013301 (2022)

Abstract: We report here on the results of comparative experimental measurements of laser energy absorption in a bulk and different morphology nanowire arrays interacting with relativistically intense, ultra-high temporal contrast femtosecond laser pulses. We compare polished, flat bulk samples with vertically and randomly oriented nanowires made of ZnO semiconductor material. The optical absorption of the 45° incident laser pulses of ∼40 fs duration with a central wavelength of 400 nm at intensities above 1019Wcm2 was determined using an integrating Ulbricht sphere. We demonstrate an almost twofold enhancement of absorption in both nanowire morphologies with an average of (79.6±1.9)% in comparison to the flat bulk sample of (45.8±1.9)%. The observed substantially enhanced absorption in nanowire arrays is also confirmed by high-resolution x-ray emission spectroscopy. The spectral analysis of the K-shell x-ray emission lines revealed that the He-like resonance line emission from highly ionized Zn (Zn28+) is only present in the case of nanowire arrays, whereas, for the flat bulk samples, only neutral and low charge states were observed. Our numerical simulations, based on radiative-collisional kinetic code FLYCHK, well reproduce the measured He-like emission spectrum and suggest that high charge state observed in nanowire arrays is due to substantially higher plasma temperature. Our results, which were measured for the first time with femtosecond laser pulses, can be used to benchmark theoretical models and numerical codes for the relativistic interaction of ultrashort laser pulses with nanowires.

2021

K. S. Schulze, R. Lötzsch, R. Rüffer, I. Uschmann, R. Röhlsberger, and G. G. Paulus
X-ray dichroism in polyimide caused by non-resonant scattering
Journal of Synchrotron Radiation 28, 176 (2021)

Abstract: Dichroism is one of the most important optical effects in both the visible and the X-ray range. Besides absorption, scattering can also contribute to dichroism. This paper demonstrates that, based on the example of polyimide, materials can show tiny dichroism even far from electronic resonances due to scattering. Although the effect is small, it can lead to a measurable polarization change and might have influence on highly sensitive polarimetric experiments.

A. T. Schmitt, Y. Joly, K. S. Schulze, B. Marx-Glowna, I. Uschmann, B. Grabiger, H. Bernhardt, R. Lötzsch, A. Juhin, J. Debray, H.-C. Wille, H. Yavaş, G. G. Paulus, and R. Röhlsberger
Disentangling x-ray dichroism and birefringence via high-purity polarimetry
Optica 8, 56 (2021)

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.

L. Wollenweber, T. R. Preston, A. Descamps, V. Cerantola, A. Comley, J. H. Eggert, L. B. Fletcher, G. Geloni, D. O. Gericke, S. H. Glenzer, S. Goede, J. Hastings, O. S. Humphries, A. Jenei, O. Karnbach, Z. Konopkova, R. Lötzsch, B. Marx-Glowna, E. E. McBride, D. McGonegle, G. Monaco, B. K. Ofori-Okai, C. A. J. Palmer, C. Plückthun, R. Redmer, C. Strohm, I. Thorpe, T. Tschentscher, I. Uschmann, J. S. Wark, T. G. White, K. Appel, G. Gregori, and U. Zastrau
High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the Free-Electron Laser
Review of Scientific Instruments 92, 013101 (2021)

Abstract: We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.

2020

B. Grabiger, B. Marx-Glowna, I. Uschmann, R. Loetzsch, G. Paulus, and K. Schulze
A highly sensitive imaging polarimeter in the x-ray regime
Applied Physics Letters 117, 201102 (2020)

Abstract: We report on the development of a highly sensitive imaging polarimeter that allows for the investigation of polarization changing properties of materials in the x-ray regime. By combining a microfocus rotating anode, collimating multilayer mirrors, and two germanium polarizer crystals, we achieved a polarization purity of the two orthogonal linear polarization states of 8 × 10−8. This enables the detection of an ellipticity on the same order or a rotation of the polarization plane of 6 arcsec. The high sensitivity combined with the imaging techniques allows us to study the microcrystalline structure of materials. As an example, we investigated beryllium sheets of different grades, which are commonly used for fabricating x-ray lenses, with a spatial resolution of 200 μm, and observed a strong degradation of the polarization purity due to the polycrystalline nature of beryllium. This makes x-ray lenses made of beryllium unsuitable for imaging polarimeter with higher spatial resolution. The results are important for the development of x-ray optical instruments that combine high spatial resolution and high sensitivity to polarization.

A. Descamps, B. Ofori-Okai, K. Appel, V. Cerantola, A. Comley, J. Eggert, L. Fletcher, D. Gericke, S. Göde, O. Humphries, O. Karnbach, A. Lazicki, R. Loetzsch, D. McGonegle, C. Palmer, C. Plueckthun, T. Preston, R. Redmer, D. Senesky, C. Strohm, I. Uschmann, T. White, L. Wollenweber, G. Monaco, J. Wark, J. Hastings, U. Zastrau, G. Gregori, S. Glenzer, and E. McBride
An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser
Scientific Reports 10, 14564 (2020)

Abstract: We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8 % for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

H. Bernhardt, A. Schmitt, B. Grabiger, B. Marx-Glowna, R. Loetzsch, H.-C. Wille, D. Bessas, A. Chumakov, R. Rüffer, R. Röhlsberger, T. Stöhlker, I. Uschmann, G. Paulus, and K. Schulze
Ultra-high precision x-ray polarimetry with artificial diamond channel cuts at the beam divergence limit
Physical Review Research 2, 023365 (2020)

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.

T. Gassner, A. Gumberidze, M. Trassinelli, R. Heß, U. Spillmann, D. Banaś, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, M. Lestinsky, D. Liesen, Y. Litvinov, R. Lötzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. Sanjari, K. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. Winters, N. Winters, E. Ziegler, and H. Beyer
High-resolution wavelength-dispersive spectroscopy of K-shell transitions in hydrogen-like gold
X-Ray Spectrometry 49, 204 (2020)

Abstract: We present a measurement of K‐shell transitions in H‐like gold (Au78+) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high‐resolution wavelength‐dispersive measurement of a K‐shell transition in a high‐Z H‐like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.

2019

G. Becker, M. Schwab, R. Lötzsch, S. Tietze, D. Klöpfel, M. Rehwald, H.-P. Schlenvoigt, A. Sävert, U. Schramm, M. Zepf, and M. Kaluza
Characterization of laser-driven proton acceleration from water microdroplets
Scientific Reports 9, 17169 (2019)

Abstract: We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 mm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 mm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser's angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet's surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.

S. Creutzburg, E. Schmidt, P. Kutza, R. Loetzsch, I. Uschmann, A. Undisz, M. Rettenmayr, F. Gala, G. Zollo, A. Boulle, A. Debelle, and E. Wendler
Defects and mechanical properties in weakly damaged Si ion implanted GaAs
Physical Review B 99, 245205 (2019)

Abstract: Damage formation is investigated in GaAs implanted with 1 MeV Si ions to ion fluences from 3 x 10(12) to 5 x 10(15) cm(-2) at room temperature. Under the conditions applied, amorphization of the implanted layers does not occur. The weakly damaged layers are studied by applying different experimental techniques including Rutherford backscattering spectrometry in channeling configuration, x-ray diffraction, in situ curvature measurement, optical subgap spectroscopy, and transmission electron microscopy. The results are evaluated and quantitatively connected with each other. Damage formation is described as a function of the ion fluence using a common defect evolution model. Point defects and defect clusters have to be taken into account in the ion fluence range of main interest up to 2 x 10(15) cm(-2). Point defects contribute by a factor of about 8 more to both perpendicular strain and in-plane stress than defect clusters. When the concentration of point defects or the induced strain reaches a critical value, defect clusters form, which ensures that no further increase of perpendicular strain occurs. This reveals a clear driving force for cluster formation. The microstructure of the defect clusters cannot be determined from the results. As3Ga2 interstitial clusters are supposed. A remarkable decrease of the shear modulus of the implanted layers below the value of pristine GaAs by approximate to -35% is observed. Surprisingly, the change of shear modulus already sets in at a very low damage level of a few percent.

2018

O. N. Rosmej, Z. Samsonova, S. Höfer, D. Kartashov, C. Arda, D. Khaghani, A. Schoenlein, S. Zähter, A. Hoffmann, R. Loetzsch, A. Saevert, I. Uschmann, M. E. Povarnitsyn, N. E. Andreev, L. P. Pugachev, M. C. Kaluza, and C. Spielmann
Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction
Physics of Plasmas 25, 083103 (2018)

Abstract: We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 10^23 cm^{−3} electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.

T. Gassner, M. Trassinelli, R. Heß, U. Spillmann, D. Banas, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. Grisenti, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, Ch. Kozhuharov, M. Lestinsky, D. Liesen, Yu. A. Litvinov, R. Loetzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. S. Sanjari, K. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. I. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. Winters, N. Winters, E. Ziegler, and H. Beyer
Wavelength-dispersive spectroscopy in the hard x-ray regime of a heavy highly-charged ion: the 1s Lamb shift in hydrogen-like gold
New Journal of Physics 20, 073033 (2018)

Abstract: Accurate spectroscopy of highly-charged high-Z ions in a storage ring is demonstrated to be feasible by the use of specially adapted crystal optics. The method has been applied for the measurement of the is Lamb shift in hydrogen-like gold (Au78+) in a storage ring through spectroscopy of the Lyman x-rays. This measurement represents the first result obtained for a high-Z element using high-resolution wavelength-dispersive spectroscopy in the hard x-ray regime, paving the way for sensitivity to higher-order QED effects.

2016

H. Bernhardt, B. Marx-Glowna, K. Schulze, B. Grabiger, J. Haber, C. Detlefs, R. Lötzsch, T. Kämpfer, R. Röhlsberger, E. Förster, Th. Stöhlker, I. Uschmann, and G. G. Paulus
High purity x-ray polarimetry with single-crystal diamonds
Applied Physics Letters 109, 121106 (2016)

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.

J. Colgan, A. Ya. Faenov, S. A. Pikuz, E. Tubman, N. M. H. Butler, J. A. jr., R. J. Dance, T. A. Pikuz, I. Yu. Skobelev, M. A. Alkhimova, N. Booth, J. Green, C. Gregory, A. Andreev, R. Lötzsch, I. Uschmann, A. Zhidkov, R. Kodama, P. McKenna, and N. Woolsey
Evidence of high-n hollow-ion emission from Si ions pumped by ultraintense x-rays from relativistic laser plasma
Europhysics Letters 114, 35001 (2016)

Abstract: We report on the first observation of high- n hollow ions (ions having no electrons in the K or L shells) produced in Si targets via pumping by ultra-intense x-ray radiation produced in intense laser-plasma interactions reaching the radiation dominant kinetics regime (RDKR). The existence of these new types of hollow ions in high-energy density plasma has been found via observation of highly resolved x-ray emission spectra of silicon plasma. This has been confirmed by plasma kinetics calculations, underscoring the ability of powerful radiation sources to fully strip electrons from the innermost shells of light atoms. Hollow-ions spectral diagnostics provide a unique opportunity to characterize powerful x-ray radiation of laboratory and astrophysical plasmas. With the use of this technique we provide evidence for the existence of the RDKR via observation of asymmetry in the observed radiation of hollow ions from the front and rear sides of the target.

J. Haber, K. Schulze, K. Schlage, R. Loetzsch, L. Bocklage, T. Gurieva, H. Bernhardt, H.-C. Wille, R. Rüffer, I. Uschmann, G. G. Paulus, and R. Röhlsberger
Collective strong coupling of X-rays and nuclei in a nuclear optical lattice
Nature Photonics 10, 445 (2016)

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.

2015

H. F. Beyer, T. Gassner, M. Trassinelli, R. Heß, U. Spillmann, D. Banaś, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. E. Grisenti, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, C. Kozhuharov, M. Lestinsky, D. Liesen, Y. A. Litvinov, R. Loetzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. S. Sanjari, K. S. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. I. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. F. A. Winters, N. Winters, and E. Ziegler
Crystal optics for precision x-ray spectroscopy on highly charged ions—conception and proof
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144010 (2015)

Abstract: The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.

K. Heeg, J. Haber, D. Schumacher, L. Bocklage, H.-C. Wille, K. Schulze, R. Loetzsch, I. Uschmann, G. Paulus, R. Rüffer, R. Röhlsberger, and J. Evers
Tunable Subluminal Propagation of Narrow-band X-Ray Pulses
Physical Review Letters 114, 203601 (2015)

Abstract: Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Subluminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of Fe57 Mössbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.

R. Loetzsch, A. Lübcke, F. Zamponi, T. Kämpfer, I. Uschmann, and E. Förster
Time-Resolved X-ray Diffraction of Cryogenic Samples Using a Laser Based Plasma Source
in: Davide Bleiner, John Costello, Francois Dortan, Gerry O'Sullivan, Ladislav Pina, Alan Michette (ed.): Short Wavelength Laboratory Sources : Principles and Practices (ESF (European Science Foundation)) (2015)

Abstract: In this chapter, the microscopic characteristics of a bright, short-pulsed source of Ti Kα radiation are studied. This x-ray emission is generated from fast electrons that are generated when a relativistically intense laser pulse interacts with a solid metal surface. The electrons have average energies significantly exceeding the ionization threshold of the K-shell (5 keV) and give rise to K-radiation when the K-shell recombines with a lifetime of a few femtoseconds only. Hence the duration of the Kα emission is dominantly determined by the time these fast electrons are present. But at the same time, the electrons also generate a solid-density plasma state at several tens of electronvolts temperature (e.g., several 100 000 K). This alters the emission probabilities of the Kα source, potentially effecting the brightness of the x-ray source. These mechanisms and possible optimizations are subject of this chapter.

2013

U. Zastrau, A. Woldegeorgis, E. Förster, R. Loetzsch, H. Marschner, and I. Uschmann
Characterization of strongly-bent HAPG crystals for von-Hámos x-ray spectrographs
Journal of Instrumentation 8, P10006 (2013)

Abstract: The properties of two strongly bent Highly Annealed Pyrolytic Graphite (HAPG) crystals with different thicknesses of 40 μm and 100 μm are studied at all possible reflection orders using x-rays at 4.5 keV and 8 keV photon energies. Typical reflecting areas within 50% reflectivity drop boundaries have sizes of about ≤ 1 mm. These domains are mis-oriented by ≤ 1 minutes of arc to each other. The mosaicity was measured to be ~ 0.06° on a 1 × 1 mm 2 scale, whereas it amounts to ~ 0.14° when the probed area becomes > 2 × 1 mm 2 . We find that the integrated reflectivity of the reflection (004) is in good agreement with the kinematical diffraction theory, while a maximum value of 2.3 mrad is achieved for 8 keV and reflection (002). The highest spectral resolution is obtained with an x-ray source of ≤ 50 μm size and a 40 μm thin graphite coating, which amounts to E /Δ E ≥ 1000 for 4.5 keV and 8 keV. In the case of 8 keV and reflection (008), the resolving power exceeds E /Δ E = 2000. In von-Hámos geometry, it was found that > 60% of the reflected photons are confined in a central 500 μm wide profile where high spectral resolution is pertained. Ray tracing simulations reveal that in order to pertain a certain resolution, a larger mosaicity would result in less contributing photons. Thus the efficiency of the crystal drops significantly when the mosaicity is increased and could not be increased by large crystal opening angles.

B. Marx, K. S. Schulze, I. Uschmann, T. Kämpfer, R. Lötzsch, O. Wehrhan, W. Wagner, C. Detlefs, T. Roth, J. Härtwig, E. Förster, Th. Stöhlker, and G. G. Paulus
High-Precision X-Ray Polarimetry
Physical Review Letters 110, 254801 (2013)

Abstract: The polarization purity of 6.457- and 12.914-keV x rays has been improved to the level of 2.4×10-10 and 5.7×10-10. The polarizers are channel-cut silicon crystals using six 90° reflections. Their performance and possible applications are demonstrated in the measurement of the optical activity of a sucrose solution.

2012

R. Lötzsch, O. Jäckel, S. Höfer, T. Kämpfer, J. Polz, I. Uschmann, M. C. Kaluza, E. Förster, E. Stambulchik, E. Kroupp, and Y. Maron
K-shell spectroscopy of silicon ions as diagnostic for high electric fields
Review of Scientific Instruments 83, 113507 (2012)

Abstract: We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in μm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von Hámos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.

R. Lötzsch, I. Uschmann, and E. Förster
Spatially resolved twin domain distribution and lattice parameter variations in the near-surface region of SrTiO_3 single crystals
Applied Physics B 106, 563 (2012)

Abstract: We investigated the twin domain distribution and lattice parameter variations associated with the displacive phase transition in SrTiO_3 by means of X-ray diffraction with high spatial resolution. By using 4.5-keV photons, the probed region is the first micrometer near the surface. We find a very inhomogeneous domain distribution, showing both regions with large monodomains and highly twinned regions, as well as large needle domains. Also, the lattice parameters in these different regions vary substantially.

R. Lötzsch
Bent crystal X-ray optics for the diagnosis and applications of laser-produced plasmas
Dissertation
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2012)

Abstract: The present work deals with x-ray optics based on bent crystals. Such crystals are used for monochromatic imaging and high-resolution x-ray spectroscopy of laser-produced plasmas. In this thesis, the reflection properties of perfect, elastically bent crystals are investigated and it is shown, that the elastic deformations of these crystals depends not only on the depth in the crystal, as hitherto considered, but also on the lateral coordinates.
Beneath these fundamental investigations, the thesis presents a variety of x-ray optics, which demonstrate their application potential. This includes two optics, which are used in the field high repetition rate x-ray sources based on laser-produced plasmas.
Furthermore, a new application of toroidally bent crystals presented. These crystals allow for a scheme to measure crystal rocking curves with both great angular and spatial resolution. With this technique, it is possible to detect lateral variations of strain in the order of 10-5 and with lateral resolution better than 20 µm.
The last part of the thesis presents an experiment from the field of x-ray spectroscopy of laser-produced plasmas. X-ray emission of ions in high electric fields is analyzed. Therefor the emission of these ions has to be recorded at laser intensities of 1020 W/cm² with high dynamics. To this end, a new spectrometer is developed, which allows to detect the transient subtle changes in the spectra caused by electric fields in the order of TV/m, which are created in laser-plasma experiments.

2011

K. S. Schulze, T. Kämpfer, I. Uschmann, S. Höfer, R. Lötzsch, and E. Förster
Laser-excited acoustical phonons probed by ultrashort pulses from a laser-driven x-ray diode
Applied Physics Letters 98, 141109 (2011)

Abstract: We demonstrate that an ultrashort-pulse laser-driven x-ray diode can be used for time-resolved experiments on a picosecond timescale. Hence, acoustical phonons in germanium are observed after ultrashort laser-excitation and the results are compared with calculations according to a microphysical model. We also show the advantages of this kind of picosecond x-ray source compared to other sources on the basis of its properties.

B. Marx, I. Uschmann, S. Höfer, R. Lötzsch, O. Wehrhan, E. Förster, M. C. Kaluza, T. Stöhlker, H. Gies, C. Detlefs, T. Roth, J. Hartwig, and G. G. Paulus
Determination of high-purity polarization state of X-rays
Optics Communications 284, 915 (2011)

Abstract: We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^{-9} is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.

L. A. Gizzi, S. Betti, E. Förster, D. Giulietti, S. Höfer, P. Köster, L. Labate, R. Lötzsch, A. P. L. Robinson, and I. Uschmann
Role of resistivity gradient in laser-driven ion acceleration
Physical Review Special Topics - Accelerators and Beams 14, 011301 (2011)

Abstract: It was predicted that, when a fast electron beam with some angular spread is normally incident on a resistivity gradient, magnetic field generation can occur that can inhibit beam propagation [A. R. Bell et al. Phys. Rev. E 58 2471 (1998)]. This effect can have consequences on the laser-driven ion acceleration. In the experiment reported here, we compare ion emission from laser irradiated coated and uncoated metal foils and we show that the ion beam from the coated target has a much smaller angular spread. Detailed hybrid numerical simulations confirm that the inhibition of fast electron transport through the resistivity gradient may explain the observed effect.