Peer-Review Publications

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.

Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, 220MeV/u. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U91+). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum j=1/2 and j=3/2 states of the L shell is determined directly from the obser-ved yields of Lyα1 and Lyα2 radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.

Nonlinear Compton scattering in ultrashort intense laser pulses is discussed with the focus on angular distributions of the emitted photon energy. This is an observable which is easily accessible experimentally. Asymmetries of the azimuthal distributions are predicted for both linear and circular polarization. We present a systematic survey of the influence of the laser intensity, the carrier envelope phase, and the laser polarization on the emission spectra for single-cycle and few-cycle laser pulses. For linear polarization, the dominant direction of the emission changes from a perpendicular pattern with respect to the laser polarization at low-intensity to a dominantly parallel emission for high-intensity laser pulses.

Chlorine atom densities in the (3p^5)^2P_1/2^0 spin–orbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl2. The atoms were excited by two photons at 235.702 nm to the (4p)^4S_3/2^0 state and detected by fluorescence to the (4s) ^4P_5/2 state at 726 nm. The population of this state relative to that in the (3p^5)^2P_3/2^0 ground state, n_P_1/2/n_P_3/2 was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. n_P_1/2/n_P_3/2 was found to increase continuously with radio-frequency power (50–500 W), whereas with Cl2 pressure (5–90 mTorr) it passes through a maximum at 10 mTorr, reaching ~30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models.

We demonstrate an approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold. Both the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold. The physical working principle is discussed at the light of the recently published theoretical explanations of mode instabilities.

Magnesium aluminosilicate glasses doped with 0.2 mol% Sm2O3 (1 x 10^20 Sm3+ cm^-3) have been prepared in a very broad compositional range. The effect of the MgO, Al2O3 and SiO2 concentrations as well as the effect of partial substitution of MgO by CaO, SrO, BaO, ZnO or MgF2 have been studied. Increasing the network modifier concentration results in decreasing the glass transformation temperature and increasing the coefficient of thermal expansion due to the formation of non-bridging oxygen sites and decreasing glass network connectivity. Although the network connectivity is changed substantially by the addition of network modifier oxides, the maximum phonon energy and the fluorescence lifetime of Sm3+ are not affected. Equimolar replacement of up to 9 mol% MgO by MgF2 results in increasing Sm3+ fluorescence lifetimes without increasing the coefficient of thermal expansion or decreasing the glass forming ability. Glasses with fairly small thermal expansion coefficients (≤ 3.2 x 10^-6 K^-1), low thermal stress values (≤ 0.5 MPa K^-1), broad fluorescence emission peaks and fluorescence lifetimes in the range from 2.4 to 2.8 ms are obtained. Such glasses are interesting candidates for laser host materials in ultrahigh peak power laser systems.

Temperature dependent absorption and emission cross-sections of 5 at-% Yb3+ doped yttrium lanthanum oxide (Yb:YLO) ceramic between 80K and 300K are presented. In addition, we report on the first demonstration of ns pulse amplification in Yb:YLO ceramic. A pulse energy of 102mJ was extracted from a multi-pass amplifier setup. The amplification bandwidth at room temperature confirms the potential of Yb:YLO ceramic for broad bandwidth amplification at cryogenic temperatures.

We report on a femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier. With this system, a pulse energy of 1.3 mJ and a peak power of 1.8 GW are achieved at 400 kHz repetition rate. The corresponding average output power is as high as 530 W. Additionally, an excellent beam quality and efficiency of the combination have been obtained. To the best of our knowledge, such a parameter combination, i.e., gigawatt pulses with half a kilowatt average power, has not been demonstrated so far with any other laser architecture.

Relativistic electrons with small energy spread propagating through undulators produce monochromatic radiation with high spectral intensity. The working principle of undulators requires a small energy spread of the electron beam in the order of ΔE/E ~ 0.1%. Laser-wakefield accelerators can produce electron bunches with an energy of several 100 MeV within a few millimeters acceleration length, but with a relatively large energy spread (ΔE/E ~ 1 - 10%). In order to produce monochromatic undulator radiation with these electrons, a novel scheme involving transverse-gradient superconducting undulators was proposed in an earlier work. This paper reports on the design-optimization and construction of an iron-free cylindrical superconducting undulator tailored to the particular beam properties of the laser-wakefield electron accelerator at the University of Jena, Germany.

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.

Detailed calculations are carried out for the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the 1s2s^(2) 2p_(3/2)J = 2 excited state of highly-charged beryllium-like ions by using a fully relativistic distorted-wave (RDW) method. The contributions of the Breit interaction to the linear polarization of the 1s2s^(2) 2p_(3/2)J = 2 → 1s^(2) 2s^(2)J = 0 magnetic quadrupole (M2) line are investigated systematically for the beryllium isoelectronic sequence with 42 ≤ Z ≤ 92. It is found that the Breit interaction depolarizes significantly the linear polarization of the M2 fluorescence radiation and that these depolarization effects increase as the incident electron energy and/or the atomic number is enlarged.

We report our observation of the resonant fluorescence from highly charged uranium ions. Using the resonant coherent excitation (RCE) technique, the 2s-2p_(3/2) transition in 191.68 MeV/u Li-like U^(89+) ions was excited at 4.5 keV with a resonance width of 4.4 eV. The result demonstrated that the RCE can be applied to resonant fluorescence spectroscopy of high-Z ions up to uranium with high efficiency and resolution.

The generation of ultrarelativistic positron beams with short duration (τ_e+≃30  fs), small divergence (θ_e+≃3  mrad), and high density (n_e+≃10^14–10^15  cm−3) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Standard Model extensions often predict low-mass and very weakly interacting particles, such as the axion. A number of small-scale experiments at the intensity/precision frontier are actively searching for these elusive particles, complementing searches for physics beyond the Standard Model at colliders. Whilst a next generation of experiments will give access to a huge unexplored parameter space, a discovery would have a tremendous impact on our understanding of fundamental physics.

The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses, which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules. Despite numerous spectacular developments in the new field of attoscience the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy, or even video-rate acquisition for spatially resolved pump–probe measurements.

We have performed a measurement of the spectral shape from the two-photon decay of the 1s2s 1S0 state in He-like uranium. The two-photon emission followed the ionization of initially Li-like uranium ions in collisions with a N2 gas-jet target. The measured shape of the two-photon energy distribution shows good agreement with results of the relativistic calculations that take into account the electron-electron interaction rigorously up to the first order in quantum electrodynamic perturbation expansion. From the full width at half maximum of the measured two-photon energy distribution, we confirm the theoretically predicted modification of the shape due to the relativistic effects.

Solid density matter at temperatures ranging from 150 eV to < 5 eV has been created by irradiating thin wire targets with high-energy laser pulses at intensities ≈ 10^18 W/cm^2 . Energy deposition and transport of the laser-produced fast electrons are inferred from spatially resolved Kα-spectroscopy. Time resolved x-ray radiography is employed to image the target mass density up to solid density and proves isochoric heating. The subsequent hydrodynamic evolution of the target is observed for up to 3 ns and is compared to radiation-hydrodynamic simulations. At distances of several hundred micrometers from the laser interaction region, where temperatures of 5–20 eV and small temperature gradients are found, the hydrodynamic evolution of the wire is a near axially symmetric isentropic expansion, and good agreement between simulations and radiography data confirms heating of the wire over hundreds of micrometers.

The K shell excitation of H-like uranium (U91+) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.

The linear polarization of the characteristic photon emission from few-electron ions is studied for its sensitivity with regard to the nuclear spin and magnetic moment of the ions. Special attention is paid, in particular, to the Kα1 (1s2p3/2 1,3P1,2→1s2 1S0) decay of selected heliumlike ions following the radiative electron capture into initially hydrogenlike species. Based on the density matrix theory, a unified description is developed that includes both the many-electron and hyperfine interactions as well as the multipole-mixing effects arising from the expansion of the radiation field. It is shown that the polarization of the Kα1 line can be significantly affected by the mutipole mixing between the leading M2 and hyperfine-induced E1 components of 1s2p 3P2,Fi→1s2 1S0,Ff transitions. This E1-M2 mixing strongly depends on the nuclear properties of the considered isotopes and can be addressed experimentally at existing heavy-ion storage rings.

High Energy Density (HED) physics spans over numerous areas of basic and applied physics, for example, astrophysics, planetary physics, geophysics, inertial fusion and many others. Due to this reason, it has been a subject of very active research over the past many decades. Static as well as dynamic methods have been applied to generate samples of HED matter in the laboratory. The most commonly used tool in the static techniques is the diamond anvil cell while the dynamic methods involve shock compression of matter. During the past fifteen years, great progress has been made on the development of bunched intense particle beams that have emerged as an additional new tool for studying HED physics. In this paper we present two experiment designs that have been worked out for HED physics studies at the Facility for Antiprotons and Ion Research (FAIR) at Darmstadt. This facility has entered into construction phase and will provide one of the largest and most powerful particle accelerators in the world. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An optical technique to study the longitudinal distribution of ions in a bunched ion beam circulating in a storage ring is presented. It is based on the arrival-time analysis of photons emitted after collisional excitation of residual gas molecules. The beam-induced fluorescence was investigated in the ultraviolet regime with a channeltron and in the visible region using a photomultiplier tube. Both were applied to investigate the longitudinal shape of bunched and electron-cooled (209)^Bi^(80+) ion beams at about 400 MeV/u in the experimental storage ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Bunch lengths were determined with an uncertainty of about 0.5 m using the UV-sensitive channeltron and with slightly lower accuracy from the photomultiplier data due to the slower transitions in the red region of the spectrum. The Gaussian shape of the longitudinal distribution of ions inside the bunch was confirmed. With the information of the transverse beam size that can be measured simultaneously by a newly installed ionization profile monitor (IPM) at the ESR, an accurate determination of the ion density in the bunched beam will be allowed.

Beam divergences of high-order extreme ultraviolet harmonics from intense laser interactions with steep plasma density gradients are studied through experiment and Fourier analysis of the harmonic spatial phase. We show that while emission due to the relativistically oscillating mirror mechanism can be explained by ponderomotive surface denting, in agreement with previous results, the divergence of the emission due to the coherent wake emission mechanism requires a combination of the dent phase and an intrinsic emission phase. The temporal dependence of the divergences for both mechanisms is highlighted while it is also shown that the coherent wake emission divergence can be small in circumstances where the phase terms compensate each other.

High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.

Spatially and spectrally resolved imaging (S2) is a very sensitive, robust and elegant method to measure the power of excited modes in a fiber. For the common reconstruction technique an approximation is necessary, which is based on dominant fundamental mode content. In this work we present several algorithms that significantly improve the accuracy of modal reconstruction for weak excited fundamental mode content. We show that in some cases general analytical solutions exist that can completely overcome the former limitation. In addition, we introduce an iterative procedure to improve the mode modal reconstruction independent of the specific used algorithm.

A tunable source of intense ultra-short hard X-ray pulses represents a novel tool for the structural analysis of complex systems with unprecedented temporal and spatial resolution. With the simultaneous availability of a high power short-pulse laser system this provides unique opportunities at the forefront of relativistic light–matter interactions. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) we demonstrated the principle of such a light source (PHOENIX – Photon Electron collider for Narrow bandwidth Intense X-Rays) by colliding picosecond electron bunches from the ELBE linear accelerator with counter-propagating femtosecond laser pulses from the 150 TW Draco Ti:Sapphire laser system. The generated narrowband X-rays are highly collimated and can be reliably adjusted from 12 keV to 20 keV by tuning the electron energy (24–30 MeV). Ensuring the spatial–temporal overlap at the interaction point and suppressing the Bremsstrahlung background a signal to noise ratio of greater than 300 was reached.