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Publications by
Dr. Ingo Uschmann

All publications of 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

F. Karbstein, C. Sundqvist, K. S. Schulze, I. Uschmann, H. Gies, and G. G. Paulus
Vacuum birefringence at x-ray free-electron lasers
New Journal of Physics 23, 095001 (2021)

Abstract: We study the perspectives of measuring the phenomenon of vacuum birefringence predicted by quantum electrodynamics using an x-ray free-electron laser (XFEL) alone. We devise an experimental scheme allowing two consecutive XFEL pulses to collide under a finite angle, and thus act as both pump and probe field for the effect. The signature of vacuum birefringence is encoded in polarization-flipped signal photons to be detected with high-purity x-ray polarimetry. Our findings for idealized scenarios underline that the discovery potential of solely XFEL-based setups can be comparable to those involving optical high-intensity lasers. For currently achievable scenarios, we identify several key details of the x-ray optical ingredients that exert a strong influence on the magnitude of the desired signatures.

R. Hollinger, E. Haddad, M. Zapf, V. Shumakova, P. Herrmann, R. Roeder, I. Uschmann, U. Reisloehner, A. Pugzlys, A. Baltuska, F. Legare, M. Zuerch, C. Ronning, C. Spielmann, and D. Kartashov
Role of free-carrier interaction in strong-field excitations in semiconductors
Physical Review B 104, 035203 (2021)

Abstract: The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 mu m (1 eV) and 10 mu m (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.

B. Marx-Glowna, I. Uschmann, K. Schulze, H. Marschner, H.-C. Wille, K. Schlage, T. Stöhlker, R. Röhlsberger, and G. Paulus
Advanced X-ray polarimeter design for nuclear resonant scattering
Journal of Synchrotron Radiation 28, 120 (2021)

Abstract: This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of Fe-57. By using four asymmetric reflections at asymmetry angles of alpha(1) = -28 degrees, alpha(2) = 28 degrees, alpha(3) = -28 degrees and alpha(4) = 28 degrees, the degree of polarization purity could be improved to 2.2 x 10(-9). For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.

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.

R. Hollinger, P. Herrmann, V. Korolev, M. Zapf, V. Shumakova, R. Roeder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO
Nanomaterials 11, 4 (2021)

Abstract: The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 mu m. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

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.

R. Hollinger, E. Haddad, M. Zapf, V. Shumakova, P. Herrmann, R. Roder, I. Uschmann, U. Reislohner, A. Pugzlys, A. Baltuska, F. Legare, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Contribution of free carriers to light absorption upon intense light-semiconductor interaction
(2021)

Abstract: In this work we investigated the role of free carriers in the interaction of a wide-band gap semiconductor with strong light fields at long wavelengths. Motivated by the beneficial scaling law of the pondermotive potential (U p ~ Iλ 2 ), the interaction of intense long wavelength laser pulses with condensed matter has attracted huge attention over the last decade [Kruchinin] . After excitation of quasi free electrons in the conduction band (CB) via multiphoton absorption or tunnelling the strong pondermotive force leads to highly energetic free electrons. Bound electrons can be collisionally excited if the energy of the free electrons exceed the band gap energy. Here, we use the onset of near ultraviolet (NUV) stimulated emission in ZnO thin films ( Fig. 1a ) to study off-resonance light absorption and the role of free carriers thereby.

P. Herrmann, R. Hollinger, V. Korolev, M. Zapf, V. Shumakova, R. Roder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Ellipticity dependent excitation and high harmonic generation from intense mid-IR laser pulses in ZnO
(2021)

Abstract: In this contribution we experimental examine the ellipticity dependence of strong off resonance absorption and high harmonic generation (HHG) in the wide band gap semiconductor ZnO upon irradiation with intense mid-IR laser pulses. HHG in semiconductors originates from nonlinear intraband currents and interband transitions of the electrons driven by the strong mid-IR fields [Ghimire]. Here, we show that the intra- and interband contribution to HHG in a ZnO thin film are affected differently by the driving laser ellipticity.

2020

R. Hollinger, D. Gupta, M. Zapf, M. Karst, R. Röder, I. Uschmann, U. Reislöhner, D. Kartashov, C. Ronning, and C. Spielmann
Polarization dependent multiphoton absorption in ZnO thin films
Journal of Physics D: Applied Physics 53, 055102 (2020)

Abstract: We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

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

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.

Z. Samsonova, S. Höfer, V. Kaymak, S. Ališauskas, V. Shumakova, A. Pugžlys, A. Baltuška, T. Siefke, S. Kroker, A. Pukhov, O. Rosmej, I. Uschmann, C. Spielmann, and D. Kartashov
Relativistic Interaction of Long-Wavelength Ultrashort Laser Pulses with Nanowires
Physical Review X 9, 021029 (2019)

Abstract: We report on experimental results in a new regime of relativistic light-matter interaction employing midinfrared (3.9-mu m wavelength) high-intensity femtosecond laser pulses. In the laser-generated plasma, electrons reach relativistic energies already for rather low intensities due to the fortunate lambda(2) scaling of the kinetic energy with the laser wavelength. The lower intensity efficiently suppresses optical field ionization and creation of the preplasma at the rising edge of the laser pulse, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. Numerical simulations, which are in a good agreement with experimental results, suggest that approximate to 80% of the incident laser energy has been absorbed resulting in a long-living, key-temperature, high-charge-state plasma with a density more than 3 orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

2018

Z. Samsonova, S. Höfer, R. Hollinger, T. Kämpfer, I. Uschmann, R. Röder, L. Trefflich, O. Rosmej, E. Förster, C. Ronning, D. Kartashov, and C. Spielmann
Hard X-ray generation from ZnO nanowire targets in a non-relativistic regime of laser-solid interactions
Applied Sciences 8, 1728 (2018)

Abstract: We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5E16 W/cm^2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance the development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in high energy density (HED) physics.

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.

J. Körner, T. Lühder, J. Reiter, I. Uschmann, H. Marschner, V. Jambunathan, A. Lucianetti, T. Mocek, J. Hein, and M. C. Kaluza
Spectroscopic investigations of thulium doped YAG and YAP crystals between 77 K and 300 K for short-wavelength infrared lasers
Journal of Luminescence 202, 427 (2018)

Abstract: We present detailed measurements of laser relevant cross sections of thulium doped yttrium-aluminum-garnet (Tm:YAG) and yttrium-aluminum-perovskite (Tm:YAP), including the absorption cross sections for the H63 to H43 transition near 800nm, and the absorption and emission cross sections for the transitions between the H63 and F43 manifolds in the short-wavelength infrared region. For Tm:YAP we present data for all polarization axes. The measurements were carried out at temperatures ranging from 80 K to 300 K. Furthermore, re-absorption free fluorescence lifetimes of the F43 to H63 transition at 77 K, 200 K and 29 5K were obtained using the pinhole method. We observed a significant enhancement of the fluorescence lifetime when cooling from room temperature to 77 K. The lifetime was increased from 9.42 ms to 15.22 ms in Tm:YAG and from 3.81 ms to 4.93 ms in Tm:YAP. This indicates that lifetime quenching is present at room temperature, which can be overcome, at least partially, by cryogenic cooling. These data are presented with the scope to qualify these materials for their use in a new generation of cryogenically cooled, short-wavelength infrared, high-energy class diode pumped solid state lasers utilizing the cross relaxation mechanism for pumping.

2017

Z. Samsonova, S. Höfer, A. Hoffmann, B. Landgraf, M. Zürch, I. Uschmann, D. Khaghani, O. Rosmej, P. Neumayer, R. Röder, L. Trefflich, C. Ronning, E. Förster, C. Spielmann, and D. Kartashov
X-ray emission generated by laser-produced plasmas from dielectric nanostructured targets
AIP Conference Proceedings 1811, 180001 (2017)

Abstract: We present an experimental study of X-ray generation from nanostructured ZnO targets. Samples of different morphology ranging from nanowires to polished surfaces are irradiated by relativistically intense femtosecond laser pulses. X-ray emission of plasma is generated by 45 fs 130 mJ laser pulses at 400 nm with picosecond temporal contrast better than 1E−9 interacting with an array of ZnO nanowires. The measured spectra indicate the existence of highly ionized states of Zn (up to He-like Zn). The obtained flux of ∼1E10 photons per laser shot at the neutral Zn Kα energies around 8.65 keV and at the Zn Heα energies around 9 keV is almost 3 times higher for nanostructured targets compared to the reference polished sample and implies 1E−4 conversion efficiency from the laser energy to the total energy of the emitted X-ray photons.

L. Deák, L. Bottyán, T. Fülöp, D. G. Merkel, D. L. Nagy, S. Sajti, K. S. Schulze, H. Spiering, I. Uschmann, and H.-C. Wille
Realizing total reciprocity violation in the phase for photon scattering
Scientific Reports 7, 43114 (2017)

Abstract: Reciprocity is when wave or quantum scattering satisfies a symmetry property, connecting a scattering process with the reversed one. While reciprocity involves the interchange of source and detector, it is fundamentally different from rotational invariance, and is a generalization of time reversal invariance, occurring in absorptive media as well. Due to its presence at diverse areas of physics, it admits a wide variety of applications. For polarization dependent scatterings, reciprocity is often violated, but violation in the phase of the scattering amplitude is much harder to experimentally observe than violation in magnitude. Enabled by the advantageous properties of nuclear resonance scattering of synchrotron radiation, we have measured maximal, i.e., 180-degree, reciprocity violation in the phase. For accessing phase information, we introduced a new version of stroboscopic detection. The scattering setting was devised based on a generalized reciprocity theorem that opens the way to construct new types of reciprocity related devices.

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.

M. Lestinsky, V. Andrianov, B. Aurand, V. Bagnoud, D. Bernhardt, H. Beyer, S. Bishop, K. Blaum, A. Bleile, At. Borovik, F. Bosch, C. Bostock, C. Brandau, A. Bräuning-Demian, I. Bray, T. Davinson, B. Ebinger, A. Echler, P. Egelhof, A. Ehresmann, M. Engström, C. Enss, N. Ferreira, D. Fischer, A. Fleischmann, E. Förster, S. Fritzsche, R. Geithner, S. Geyer, J. Glorius, K. Göbel, O. Gorda, J. Goullon, P. Grabitz, R. Grisenti, A. Gumberidze, S. Hagmann, M. Heil, A. Heinz, F. Herfurth, R. Heß, P.-M. Hillenbrand, R. Hubele, P. Indelicato, A. Källberg, O. Kester, O. Kiselev, A. Knie, C. Kozhuharov, S. Kraft-Bermuth, T. Kühl, G. Lane, Y. Litvinov, D. Liesen, X. Ma, R. Märtin, R. Moshammer, A. Müller, S. Namba, P. Neumayer, T. Nilsson, W. Nörtershäuser, G. G. Paulus, N. Petridis, M. Reed, R. Reifarth, P. Reiß, J. Rothhardt, R. Sanchez, M. Sanjari, S. Schippers, H. Schmidt, D. Schneider, P. Scholz, R. Schuch, M. Schulz, V. Shabaev, A. Simonsson, J. Sjöholm, Ö. Skeppstedt, K. Sonnabend, U. Spillmann, K. Stiebing, M. Steck, T. Stöhlker, A. Surzhykov, S. Torilov, E. Träbert, M. Trassinelli, S. Trotsenko, X. Tu, I. Uschmann, P. Walker, G. Weber, D. Winters, P. Woods, H. Zhao, and Y. Zhang
Physics book: CRYRING@ESR
European Physical Journal Special Topics 225, 797 (2016)

Abstract: The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm and at the Test Storage Ring (TSR) at Heidelberg. For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei.

S. Höfer, T. Kämpfer, E. Förster, T. Stöhlker, and I. Uschmann
The formation of rarefaction waves in semiconductors after ultrashort excitation probed by grazing incidence ultrafast time-resolved x-ray diffraction
Structural Dynamics 3, 051101 (2016)

Abstract: We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mA ° and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.

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.

O. Culfa, G. J. Tallents, A. K. Rossall, E. Wagenaars, C. P. Ridgers, C. D. Murphy, R. J. Dance, R. J. Gray, P. McKenna, C. D. R. Brown, S. F. James, D. J. Hoarty, N. Booth, A. P. L. Robinson, K. L. Lancaster, S. A. Pikuz, A. Ya. Faenov, T. Kampfer, K. S. Schulze, I. Uschmann, and N. C. Woolsey
Plasma scale-length effects on electron energy spectra in high-irradiance laser plasmas
Physical Review E 93, 043201 (2016)

Abstract: An analysis of an electron spectrometer used to characterize fast electrons generated by ultraintense (10^20 Wcm^−2) laser interaction with a preformed plasma of scale length measured by shadowgraphy is presented. The effects of fringing magnetic fields on the electron spectral measurements and the accuracy of density scale-length measurements are evaluated. 2D EPOCH PIC code simulations are found to be in agreement with measurements of the electron energy spectra showing that laser filamentation in plasma preformed by a prepulse is important with longer plasma scale lengths (>8 μm).

T. Kämpfer, I. Uschmann, Z. W. Wu, A. Surzhykov, S. Fritzsche, E. Förster, and G. G. Paulus
Linear polarization of the characteristic x-ray lines following inner-shell photoionization of tungsten
Physical Review A 93, 033409 (2016)

Abstract: The linear polarization of the characteristic lines Lα1 (3d5/2→2p3/2) and Lα2 (3d3/2→2p3/2), following inner-shell photoionization of neutral tungsten, is analyzed both experimentally and theoretically. In the experiment, a tungsten target is photoionized by the primary emission of an x-ray tube with incident photon energies in the range of 10.2–30 keV. The σ and π components of the emitted fluorescence are measured by using a spectropolarimeter, based on x-ray diffraction at Bragg angles close to 45∘. The degree of linear polarization of the Lα1 and Lα2 lines is determined to be +(1.6±0.5)% and −(7±2)%, respectively. In addition, this degree of polarization is calculated within the framework of the density-matrix theory as a function of the incident photon energy. These calculations are in good agreement with the experimental results and show only a weak dependence of the degree of polarization on the energy of the incident photoionizing photon.

2015

B. Marx-Glowna, K. Schulze, I. Uschmann, T. Kämpfer, G. Weber, C. Hahn, H.-C. Wille, K. Schlage, R. Röhlsberger, E. Förster, Th. Stöhlker, and G. Paulus
Influence of higher harmonics of the undulator in X-ray polarimetry and crystal monochromator design
Journal of Synchrotron Radiation 22, 1151 (2015)

Abstract: The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel-cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.

H. Bernhardt, R. Diener, P. Sungur, C. Katzer, G. Schmidl, U. Hübner, I. Uschmann, W. Fritzsche, and F. Schmidl
Engineering crystalline Au nanoparticles of anisotropic shape in epitaxially grown high-index SrTiO3
Journal of Materials Science 50, 5562 (2015)

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.

M. Schnell, A. Sävert, I. Uschmann, O. Jansen, M. C. Kaluza, and C. Spielmann
Characterization and application of hard x-ray betatron radiation generated by relativistic electrons from a laser-wakefield accelerator
Journal of Plasma Physics 81, 1 (2015)

Abstract: The necessity for compact table-top x-ray sources with higher brightness, shorter wavelength and shorter pulse duration has led to the development of complementary sources based on laser-plasma accelerators, in contrast to conventional accelerators. Relativistic interaction of short-pulse lasers with underdense plasmas results in acceleration of electrons and in consequence in the emission of spatially coherent radiation, which is known in the literature as betatron radiation. In this article, we report on our recent results in the rapidly developing field of secondary x-ray radiation generated by high-energy electron pulses. The betatron radiation is characterized with a novel setup allowing to measure the energy, the spatial energy distribution in the far-field of the beam and the source size in a single laser shot. Furthermore, the polarization state is measured for each laser shot. In this way, the emitted betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very subtle information of the electron dynamics within the plasma wave. Parallel to the experimental work, 3D particle-in-cell simulations were performed, proved to be in good agreement with the experimental results.

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.

H. Bernhardt, C. Katzer, A. Undisz, M. Drüe, M. Rettenmayr, I. Uschmann, and F. Schmidl
Characterization of self-organized crystalline Au nanoparticles embedded in epitaxially grown SrTiO₃
Journal of Materials Research 30, 973 (2015)

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.

D. Pincini, C. Mazzoli, H. Bernhardt, C. Katzer, F. Schmidl, I. Uschmann, and C. Detlefs
Crystallographic investigation of Au nanoparticles embedded in a SrTiO3 thin film for plasmonics applications by means of synchrotron radiation
Journal of Applied Physics 117, 105305 (2015)

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 [110]SrTiO3∥[110]Au and [100]SrTiO3∥[110]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 [100]SrTiO3∥[100]Au and [100]SrTiO3∥[110]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.

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.

2014

B. Marx, K. S. Schulze, I. Uschmann, T. Kämpfer, O. Wehrhan, H. C. Wille, K. Schlage, R. Röhlsberger, E. Weckert, E. Förster, Th. Stöhlker, and G. G. Paulus
High precision measurement of undulator polarization in the regime of hard x-rays
Applied Physics Letters 105, 024103 (2014)

Abstract: We have measured the polarization purity of undulator radiation at 12.9 keV, with hitherto unachievable precision. We could measure a polarization purity of 1.8 × 10−4 by using a silicon channel-cut crystal with six Bragg reflections at 45° as analyzer.

K. S. Schulze, B. Marx, I. Uschmann, E. Förster, Th. Stöhlker, and G. G. Paulus
Determination of the polarization state of x rays with the help of anomalous transmission
Applied Physics Letters 104, 151110 (2014)

Abstract: Besides intensity and direction, the polarization of an electromagnetic wave provides characteristic information on the crossed medium. Here, we present two methods for the determination of the polarization state of x rays by polarizers based on anomalous transmission (Borrmann effect). Using a polarizer-analyzer setup, we have measured a polarization purity of less than 1.5 × 10^−5, three orders of magnitude better than obtained in earlier work. Using the analyzer crystal in multiple-beam case with slightly detuned azimuth, we show how the first three Stokes parameters can be determined with a single angular scan. Thus, polarization analyzers based on anomalous transmission make it possible to detect changes of the polarization in a range from degrees down to arcseconds.

T. G. White, N. J. Hartley, B. Borm, B. J. B. Crowley, J. W. O. Harris, D. C. Hochhaus, T. Kaempfer, K. Li, P. Neumayer, L. K. Pattison, F. Pfeifer, S. Richardson, A. P. L. Robinson, I. Uschmann, and G. Gregori
Electron-Ion Equilibration in Ultrafast Heated Graphite
Physical Review Letters 112, 145005 (2014)

Abstract: We have employed fast electrons produced by intense laser illumination to isochorically heat thermal electrons in solid density carbon to temperatures of ∼10 000  K. Using time-resolved x-ray diffraction, the temperature evolution of the lattice ions is obtained through the Debye-Waller effect, and this directly relates to the electron-ion equilibration rate. This is shown to be considerably lower than predicted from ideal plasma models. We attribute this to strong ion coupling screening the electron-ion interaction.

O. Culfa, G. J. Tallents, E. Wagenaars, C. P. Ridgers, R. J. Dance, A. K. Rossall, R. J. Gray, P. McKenna, C. D. R. Brown, S. F. James, D. J. Hoarty, N. Booth, A. P. L. Robinson, K. L. Lancaster, S. A. Pikuz, A. Ya. Faenov, T. Kampfer, K. S. Schulze, I. Uschmann, and N. C. Woolsey
Hot electron production in laser solid interactions with a controlled pre-pulse
Physics of Plasmas 21, 043106 (2014)

Abstract: Hot electron generation plays an important role in the fast ignition approach to inertial confinement fusion (ICF) and other applications with ultra-intense lasers. Hot electrons of temperature up to 10–20 MeV have been produced by high contrast picosecond duration laser pulses focussed to intensities of ∼10^20 W cm^(−2) with a deliberate pre-pulse on solid targets using the Vulcan Petawatt Laser facility. We present measurements of the number and temperature of hot electrons obtained using an electron spectrometer. The results are correlated to the density scale length of the plasma produced by a controlled pre-pulse measured using an optical probe diagnostic. 1D simulations predict electron temperature variations with plasma density scale length in agreement with the experiment at shorter plasma scale lengths ( <7.5μ<7.5μ<7.5μ m), but with the experimental temperatures (13–17 MeV) dropping below the simulation values (20–25 MeV) at longer scale lengths. The experimental results show that longer interaction plasmas produced by pre-pulses enable significantly greater number of hot electrons to be produced.

E. Stambulchik, E. Kroupp, Y. Maron, U. Zastrau, I. Uschmann, and G. G. Paulus
Absorption-aided x-ray emission tomography of planar targets
Physics of Plasmas 21, 033303 (2014)

Abstract: Suggested is a tomography-like method for studying properties of solid-density plasmas with cylindrical symmetry, such as formed in the interaction of high-power lasers with planar targets. The method is based on simultaneous observation of the target x-ray fluorescence at different angles. It can be applied for validation of existing hypotheses and lately for reconstruction of the plasma properties with three-dimensional resolution. The latter becomes straightforward if the resonance x-ray self-absorption is negligible. The utility of the method is demonstrated by examples.

S. B. Hansen, J. Colgan, A. Ya. Faenov, J. Abdallah, S. A. Pikuz, I. Yu. Skobelev, E. Wagenaars, N. Booth, O. Culfa, R. J. Dance, G. J. Tallents, R. G. Evans, R. J. Gray, T. Kämpfer, K. L. Lancaster, P. McKenna, A. K. Rossall, K. S. Schulze, I. Uschmann, A. G. Zhidkov, and N. C. Woolsey
Detailed analysis of hollow ions spectra from dense matter pumped by X-ray emission of relativistic laser plasma
Physics of Plasmas 21, 031213 (2014)

Abstract: X-ray emission from hollow ions offers new diagnostic opportunities for dense, strongly coupled plasma. We present extended modeling of the x-ray emission spectrum reported by Colgan et al. [Phys. Rev. Lett. 110, 125001 (2013)] based on two collisional-radiative codes: the hybrid-structure Spectroscopic Collisional-Radiative Atomic Model (SCRAM) and the mixed-unresolved transition arrays (MUTA) ATOMIC model. We show that both accuracy and completeness in the modeled energy level structure are critical for reliable diagnostics, investigate how emission changes with different treatments of ionization potential depression, and discuss two approaches to handling the extensive structure required for hollow-ion models with many multiply excited configurations.

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.

S. Pikuz, A. Faenov, J. Colgan, R. Dance, J. Abdallah, E. Wagenaars, N. Booth, O. Culfa, R. Evans, R. Gray, T. Kämpfer, K. Lancaster, P. McKenna, A. Rossall, I. Skobelev, K. Schulze, I. Uschmann, A. Zhidkov, and N. Woolsey
Measurement and simulations of hollow atom X-ray spectra of solid-density relativistic plasma created by high-contrast PW optical laser pulses
High Energy Density Physics 9, 560 (2013)

Abstract: Abstract K-shell spectra of solid Al excited by petawatt picosecond laser pulses have been investigated at the Vulcan PW facility. Laser pulses of ultrahigh contrast with an energy of 160 J on the target allow studies of interactions between the laser field and solid state matter at 10^{20} W/cm^{2}. Intense X-ray emission of KK hollow atoms (atoms without n = 1 electrons) from thin aluminum foils is observed from optical laser plasma for the first time. Specifically for 1.5 μm thin foil targets the hollow atom yield dominates the resonance line emission. It is suggested that the hollow atoms are predominantly excited by the impact of X-ray photons generated by radiation friction to fast electron currents in solid-density plasma due to Thomson scattering and bremsstrahlung in the transverse plasma fields. Numerical simulations of Al hollow atom spectra using the ATOMIC code confirm that the impact of keV photons dominates the atom ionization. Our estimates demonstrate that solid-density plasma generated by relativistic optical laser pulses provide the source of a polychromatic keV range X-ray field of 10^{18} W/cm^{2} intensity, and allows the study of excited matter in the radiation-dominated regime. High-resolution X-ray spectroscopy of hollow atom radiation is found to be a powerful tool to study the properties of high-energy density plasma created by intense X-ray radiation.

M. Schnell, A. Sävert, I. Uschmann, M. Reuter, M. Nicolai, T. Kämpfer, B. Landgraf, O. Jäckel, O. Jansen, A. Pukhov, M. C. Kaluza, and C. Spielmann
Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator
Nature Communications 4, 2421 (2013)

Abstract: Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.

V. Hilbert, A. Blinne, S. Fuchs, T. Feigl, T. Kämpfer, C. Rödel, I. Uschmann, M. Wünsche, G. Paulus, E. Förster, and U. Zastrau
An extreme ultraviolet Michelson interferometer for experiments at free-electron lasers
Review of Scientific Instruments 84, 095111 (2013)

Abstract: We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.

K. Heeg, H.-C. Wille, K. Schlage, T. Guryeva, D. Schumacher, I. Uschmann, K. S. Schulze, B. Marx, T. Kämpfer, G. Paulus, R. Röhlsberger, and J. Evers
Vacuum-Assisted Generation and Control of Atomic Coherences at X-Ray Energies
Physical Review Letters 111, 073601 (2013)

Abstract: The control of light-matter interaction at the quantum level usually requires coherent laser fields. But already an exchange of virtual photons with the electromagnetic vacuum field alone can lead to quantum coherences, which subsequently suppress spontaneous emission. We demonstrate such spontaneously generated coherences (SGC) in a large ensemble of nuclei operating in the x-ray regime, resonantly coupled to a common cavity environment. The observed SGC originates from two fundamentally different mechanisms related to cooperative emission and magnetically controlled anisotropy of the cavity vacuum. This approach opens new perspectives for quantum control, quantum state engineering and simulation of quantum many-body physics in an essentially decoherence-free setting.

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.

J. Colgan, J. Abdallah, A. Ya. Faenov, S. A. Pikuz, E. Wagenaars, N. Booth, O. Culfa, R. J. Dance, R. G. Evans, R. J. Gray, T. Kämpfer, K. L. Lancaster, P. McKenna, A. L. Rossall, I. Yu. Skobelev, K. S. Schulze, I. Uschmann, A. G. Zhidkov, and N. C. Woolsey
Exotic Dense-Matter States Pumped by a Relativistic Laser Plasma in the Radiation-Dominated Regime
Physical Review Letters 110, 125001 (2013)

Abstract: In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n=1 electrons) from thin Al foils was observed at pulse intensities of 3 × 10^(20)  W/cm^2. The observations of spectra from these exotic states of matter are supported by detailed kinetics calculations, and are consistent with a picture in which an intense polychromatic x-ray field, formed from Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface, drives the KK hollow atom production. We estimate that this x-ray field has an intensity of > 5 × 10^(18)  W/cm^2 and is in the 3 keV range.

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.

U. Zastrau, T. Burian, J. Chalupsky, T. Döppner, T. W. J. Dzelzainis, R. R. Fäustlin, C. Fortmann, E. Galtier, S. H. Glenzer, G. Gregori, L. Juha, H. J. Lee, R. W. Lee, C. L. S. Lewis, N. Medvedev, B. Nagler, A. J. Nelson, D. Riley, F. B. Rosmej, S. Toleikis, T. Tschentscher, I. Uschmann, S. M. Vinko, J. S. Wark, T. Whitcher, and E. Förster
XUV spectroscopic characterization of warm dense aluminum plasmas generated by the free-electron-laser FLASH
Laser and Particle Beams 30, 45 (2012)

Abstract: We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 µJ are focused to intensities ranging between 10^(13) and 10^(17) W/cm^2. We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations.

2011

E. Kroupp, D. Osin, A. Starobinets, V. Fisher, V. Bernshtam, L. Weingarten, Y. Maron, I. Uschmann, E. Förster, A. Fisher, M. E. Cuneo, C. Deeney, and J. L. Giuliani
Ion Temperature and Hydrodynamic-Energy Measurements in a Z-Pinch Plasma at Stagnation
Physical Review Letters 107, 105001 (2011)

Abstract: The time history of the local ion kinetic energy in a stagnating plasma was determined from Doppler-dominated line shapes. Using independent determination of the plasma properties for the same plasma region, the data allowed for inferring the time-dependent ion temperature, and for discriminating the temperature from the total ion kinetic energy. It is found that throughout most of the stagnation period the ion thermal energy constitutes a small fraction of the total ion kinetic energy; the latter is dominated by hydrodynamic motion. Both the ion hydrodynamic and thermal energies are observed to decrease to the electron thermal energy by the end of the stagnation period. It is confirmed that the total ion kinetic energy available at the stagnating plasma and the total radiation emitted are in balance, as obtained in our previous experiment. The dissipation time of the hydrodynamic energy thus appears to determine the duration (and power) of the K emission.

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.