Peer-Review Publications

2011

F. Fratini, S. Trotsenko, S. Tashenov, T. Stöhlker, and A. Surzhykov
Photon-photon polarization correlations as a tool for studying parity nonconservation in heliumlike uranium
Phys. Rev. A, 83 :052505 (May 2011)
Abstract:
Due to electron-nucleus weak interaction, atomic bound states with different parities turn out to be mixed. We discuss a prospective method for measuring the mixing parameter between the nearly degenerate metastable states 1s_(1/2) 2s_(1/2):J=0 and 1s_(1/2) 2p_(1/2):J=0 in heliumlike uranium. Our analysis is based on the polarization properties of the photons emitted in the two-photon decays of such states.
D. B. Thorn, A. Gumberidze, S. Trotsenko, D. Banaś, H. Beyer, C. J. Bostock, I. Bray, W. Chen, R. DuBois, C. J. Fontes, S. Fritzsche, D. V. Fursa, R. Grisenti, S. Geyer, S. Hagmann, S. Hess, M. Hegewald, C. Kozhuharov, R. Märtin, I. Orban, N. Petridis, R. Reuschl, A. Simon, U. Spillmann, A. Surzhykov, M. Trassinelli, G. Weber, D. F. A. Winters, N. Winters, H. L. Zhang, and T. Stöhlker
Polarization and anisotropic emission of K-shell radiation from heavy few electron ions
Can. J. Phys., 89 :513 (May 2011)
Abstract:
The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.
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
Appl. Phys. Lett., 98 :141109 (April 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.
S. Hädrich, H. Carstens, J. Rothhardt, J. Limpert, and A. Tünnermann
Multi-gigawatt ultrashort pulses at high repetition rate and average power from two-stage nonlinear compression
Opt. Express, 19 :7546 (April 2011)
Abstract:
We present simple and compact (1.5 m x 0.5 m footprint) post-compression of a state-of-the-art fiber chirped pulse amplification system. By using two stage nonlinear compression in noble gas filled hollow core fibers we shorten 1 mJ, 480 fs, 50 kHz pulses. The first stage is a 53 cm long, 200 µm inner diameter fiber filled with xenon with subsequent compression in a chirped mirror compressor. A 20 cm, 200 µm inner diameter fiber filled with argon further broadens the spectrum in a second stage and compression is achieved with another set of chirped mirrors. The average power is 24.5 W / 19 W after the first / second stage, respectively. Compression to 35 fs is achieved. Numerical simulations, agreeing well with experimental data, yield a peak power of 5.7 GW at a pulse energy of 380 µJ making this an interesting source for high harmonic generation at high repetition rate and average power.
T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, J. Rothhardt, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann
Preferential gain photonic-crystal fiber for mode stabilization at high average powers
Opt. Express, 19 :8656 (April 2011)
Abstract:
We report on the design and experimental investigation of a preferential gain photonic-crystal fiber with a mode-field diameter of 47 µm. This few-mode fiber design confines the doping of Ytterbium-ions just to the center of the core and, therefore, promotes fundamental mode operation. In a chirped-pulse amplification system we extracted up to 303 W of average power from this fiber with a measured M^2 value of 1.4.
H. Redlin, A. Al-Shemmary, A. Azima, N. Stojanovic, F. Tavella, I. Will, and S. Duesterer
The FLASH pump-probe laser system: Setup, characterization and optical beamlines
Nucl. Instr. Meth. Phys. Res. A, 635 :S88 (April 2011)
Abstract:
We describe the optical pump–probe laser system at the XUV free-electron laser FLASH at DESY. A growing interest in optical laser pulses combined with the free-electron laser (FEL) beam for time-resolved experiments raise high demands on the stability of operation and knowledge on the status of synchronization between the XUV and optical pulses. In this publication we describe this system, the characterization of the optical pulses, synchronization and timing schemes as well as beamlines that transport beam to experimental stations.
I. Hofmann, J. Meyer-ter-Vehn, X. Yan, A. Orzhekhovskaya, and S. Yaramyshev
Collection and focusing of laser accelerated ion beams for therapy applications
Phys. Rev. ST AB, 14 :031304 (March 2011)
Abstract:
Experimental results in laser acceleration of protons and ions and theoretical predictions that the currently achieved energies might be raised by factors 5 - 10 in the next few years have stimulated research exploring this new technology for oncology as a compact alternative to conventional synchrotron based accelerator technology. The emphasis of this paper is on collection and focusing of the laser produced particles by using simulation data from a specific laser acceleration model. We present a scaling law for the “chromatic emittance” of the collector - here assumed as a solenoid lens - and apply it to the particle energy and angular spectra of the simulation output. For a 10 Hz laser system we find that particle collection by a solenoid magnet well satisfies requirements of intensity and beam quality as needed for depth scanning irradiation. This includes a sufficiently large safety margin for intensity, whereas a scheme without collection - by using mere aperture collimation - hardly reaches the needed intensities.
G. Saathoff, S. Reinhardt, R. Holzwarth, T. Hänsch, Th. Udem, D. Bing, D. Schwalm, A. Wolf, S. Karpuk, G. Huber, C. Novotny, B. Botermann, C. Geppert, W. Nörtershäuser, T. Kühl, T. Stöhlker, and G. Gwinner
Comment on: “Lorentz violation in high-energy ions” by Santosh Devasia
Eur. Phys. J. C, 71 :1 (March 2011)
Abstract:
In an article “Lorentz violation in high-energy ions” by S. Devasia published in this Journal [EPJ C 69, 343 (2010)], our recent Doppler shift experiments on fast ion beams are reanalyzed. Contrary to our analysis, Devasia concludes that our results provide an “indication of Lorentz violation”. We argue that this conclusion is based on a fundamental misunderstanding of our experimental scheme and reiterate that our results are in excellent agreement with Special Relativity.
F. Tavella, N. Stojanovic, G. Geloni, and M. Gensch
Few-femtosecond timing at fourth-generation X-ray light sources
Nat. Photonics, 5 :162 (March 2011)
Abstract:
Fourth-generation X-ray light sources are being developed to deliver laser-like X-ray pulses at intensities and/or repetition rates that are beyond the reach of table-top devices. An important class of experiments at these new facilities comprises pump–probe experiments, which are designed to investigate chemical reactions and processes occurring on the molecular or even atomic level, and on the timescale of a few femtoseconds. Good progress has been made towards the generation of ultrashort X-ray pulses (for example, at FLASH or LCLS), but experiments suffer from the intrinsic timing jitter between the X-ray pulses and external laser sources3. In this Letter, we present a new approach that provides few-femtosecond temporal resolution. Our method uses coherent terahertz radiation generated at the end of the X-ray undulator by the same electron bunch that emits the X-ray pulse. It can therefore be applied at any advanced light source working with ultrashort electron bunches and undulators.
F. Stutzki, F. Jansen, T. Eidam, A. Steinmetz, C. Jauregui, J. Limpert, and A. Tünnermann
High average power large-pitch fiber amplifier with robust single-mode operation
Opt. Lett., 36 :689 (March 2011)
Abstract:
Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 µm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.
J. Limpert, S. Hädrich, J. Rothhardt, M. Krebs, T. Eidam, T. Schreiber, and A. Tünnermann
Ultrafast fiber lasers for strong-field physics experiments
Laser Photon. Rev., 5 :634 (March 2011)
Abstract:
The recent demonstration of rare-earth-doped fiber lasers with a continuous-wave output power approaching the 10-kW level with diffraction-limited beam quality proves that fiber lasers constitute a scalable solid-state laser concept in terms of average power. In order to generate high peak power pulses from a fiber several fundamental limitations have to be overcome. This can be achieved by novel experimental strategies and fiber designs that offer an enormous potential towards ultrafast laser systems combining high average powers (> kW) and high peak power (> GW). In this paper the challenges, achievements and perspectives of ultrashort pulse generation and amplification in fibers are reviewed. This kind of laser system will have a tremendous impact on strong-field physics experiments, such as the generation of coherent light by high-harmonic generation. So far, applications in the interesting EUV spectral range suffer from the very low photon count leading to nonrelevant integration times with highly sophisticated detection schemes. High repetition rate high average power fiber lasers can potentially solve this issue. First demonstrations of high repetition-rate strong-field physics experiments using novel fiber laser systems will be discussed.
J. Rothhardt, T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, T. Gottschall, T. Andersen, J. Limpert, and A. Tünnermann
135 W average-power femtosecond pulses at 520 nm from a frequency-doubled fiber laser system
Opt. Lett., 36 :316 (February 2011)
Abstract:
We present a high-average-power femtosecond laser system at 520 nm central wavelength. The laser system delivers sub- 500 fs pulses with 135 W average power at a pulse repetition rate of 5.25 MHz. Excellent beam quality is provided by high power fiber amplifiers and maintained during frequency doubling, resulting in a beam quality factor of M^2 < 1.2. To our knowledge, the system presented here is the highest average power green laser source generating femtosecond pulses with diffraction-limited beam quality.
E. Seise, A. Klenke, S. Breitkopf, M. Plötner, J. Limpert, and A. Tünnermann
Coherently combined fiber laser system delivering 120 mu J femtosecond pulses
Opt. Lett., 36 :439 (February 2011)
Abstract:
We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 μm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hänsch-Couillaud polarization detection system. Pulses with 120 μJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 μJ coming from each single amplifier, results in a combining efficiency as high as 91%.
B. Marx, I. Uschmann, S. Höfer, R. Lötzsch, O. Wehrhan, E. Förster, M.C. Kaluza, T. Stöhlker, H. Gies, C. Detlefs, T. Roth, J. Härtwig, and G.G. Paulus
Determination of high-purity polarization state of X-rays
Opt. Commun., 284 :915 (February 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.
O. Schmidt, M. Rekas, C. Wirth, J. Rothhardt, S. Rhein, A. Kliner, M. Strecker, T. Schreiber, J. Limpert, R. Eberhardt, and A. Tünnermann
High power narrow-band fiber-based ASE source
Opt. Express, 19 :4421 (February 2011)
Abstract:
In this paper we describe a high power narrow-band amplified spontaneous emission (ASE) light source at 1030 nm center wavelength generated in an Yb-doped fiber-based experimental setup. By cutting a small region out of a broadband ASE spectrum using two fiber Bragg gratings a strongly constrained bandwidth of 12±2 pm (3.5±0.6 GHz) is formed. A two-stage high power fiber amplifier system is used to boost the output power up to 697 W with a measured beam quality of M2≤1.34. In an additional experiment we demonstrate a stimulated Brillouin scattering (SBS) suppression of at least 17 dB (theoretically predicted ~20 dB), which is only limited by the dynamic range of the measurement and not by the onset of SBS when using the described light source. The presented narrow-band ASE source could be of great interest for brightness scaling applications by beam combination, where SBS is known as a limiting factor.
S. Hädrich, S. Demmler, J. Rothhardt, C. Jocher, J. Limpert, and A. Tünnermann
High-repetition-rate sub-5-fs pulses with 12 GW peak power from fiber-amplifier-pumped optical parametric chirped-pulse amplification
Opt. Lett., 36 :313 (February 2011)
Abstract:
An optical parametric chirped-pulse amplification system delivering pulses with more than 12 GW peak power is presented. Compression to sub- 5 fs, 87 μJ and 5.4 fs, 100 μJ is realized at the 30 kHz repetition rate. A high-energy fiber chirped-pulse amplification system operating at 1 mJ pulse energy and nearly transform-limited pulses is used to achieve ultrabroadband amplification in two 2mm beta-barium borate crystals. Precise pulse shaping is used to compress the pulses to a few percentages of their transform limit. Assuming diffraction limited focusing (d < 2 μm), peak intensities as high as 10^(18) W/cm2 can be reached.
A. M. Sayler, T. Rathje, W. Müller, C. Kürbis, K. Rühle, G. Stibenz, and G.G. Paulus
Real-time pulse length measurement of few-cycle laser pulses using above-threshold ionization
Opt. Express, 19 :4464 (February 2011)
Abstract:
The pulse lengths of intense few-cycle (4 - 10 fs) laser pulses at 790 nm are determined in real-time using a stereographic above-threshold ionization (ATI) measurement of Xe, i.e. the same apparatus recently shown to provide a precise, real-time, every-single-shot, carrier-envelope phase measurement of ultrashort laser pulses. The pulse length is calibrated using spectral-phase interferometry for direct electric-field reconstruction (SPIDER) and roughly agrees with calculations done using quantitative rescattering theory (QRS). This stereo-ATI technique provides the information necessary to characterize the waveform of every pulse in a kHz pulse train, within the Gaussian pulse approximation, and relies upon no theoretical assumptions. Moreover, the real-time display is a highly effective tool for tuning and monitoring ultrashort pulse characteristics.
A. Kumar, S. Trotsenko, A. V. Volotka, D. Banaś, H. F. Beyer, H. Bräuning, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, R. Reuschl, U. Spillmann, M. Trassinelli, G. Weber, and T. Stöhlker
Spectral distribution of the 2S → 1S two-photon transition in atoms and few-electron ions
Pramana - J. Phys., 76 :331 (February 2011)
Abstract:
The two-photon decay of the 2S state to the ground state in dressed atoms and one- or two-electron ions has been studied for several decades. Relativistic calculations have shown an Z-dependence of the spectral shape of this two-photon transition in one- or two-electron ions. We have measured the spectral distribution of the 1s2s 1^S_0 → 1s2 1^S_0 two-photon transition in He-like tin at the ESR storage ring using a new approach for such experiments. In this method, relativistic collisions of initially Li-like projectiles with a gaseous target were used to populate exclusively the first excited state, 1s2s, of He-like tin, which provided a clean two-photon spectrum. The measured two-photon spectral distribution was compared with fully relativistic calculations. The obtained results show very good agreement with the calculations for He-like tin.
R. Geithner, R. Neubert, W. Vodel, P. Seidel, K. Knaack, S. Vilcins, K. Wittenburg, O. Kugeler, and J. Knobloch
Dark current measurements on a superconducting cavity using a cryogenic current comparator
Rev. Sci. Instrum., 82 :013302 (January 2011)
Abstract:
his paper presents nondestructive dark current measurements of tera electron volt energy superconducting linear accelerator cavities. The measurements were carried out in an extremely noisy accelerator environment using a low temperature dc superconducting quantum interference device based cryogenic current comparator. The overall current sensitivity under these rough conditions was measured to be 0.2 nA/Hz^1/2, which enables the detection of dark currents of 5 nA.
T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann
Fiber chirped-pulse amplification system emitting 3.8 GW peak power
Opt. Express, 19 :255 (January 2011)
Abstract:
We report on the experimental demonstration of a fiber chirped- pulse amplification system capable of generating nearly transform-limited sub 500 fs pulses with 2.2 mJ pulse energy at 11 W average power. The resulting record peak power of 3.8 GW could be achieved by combining active phase shaping with an efficient reduction of the acquired nonlinear phase. Therefore, we used an Ytterbium-doped large-pitch fiber with a mode field diameter of 105 µm as the main amplifier.
M. Baumgartl, F. Jansen, F. Stutzki, C. Jauregui, B. Ortac, J. Limpert, and A. Tünnermann
High average and peak power femtosecond large-pitch photonic-crystal-fiber laser
Opt. Lett., 36 :244 (January 2011)
Abstract:
We report on the generation of high-average-power and high-peak-power ultrashort pulses from a mode-locked fiber laser operating in the all-normal-dispersion regime. As gain medium, a large-mode-area ytterbium-doped large-pitch photonic-crystal fiber is used. The self-starting fiber laser delivers 27 W of average power at 50.57 MHz repetition rate, resulting in 534 nJ of pulse energy. The laser produces positively chirped 2 ps output pulses, which are compressed down to sub - 100 fs , leading to pulse peak powers as high as 3.2 MW.
F. Ferro, A. Artemyev, A. Surzhykov, and T. Stöhlker
Hyperfine interaction effects on the (1s2p)P-3(1)-(1s2s)S-1(0) energy splitting in He-like ions for parity nonconservation studies
Can. J. Phys., 89 :73 (January 2011)
Abstract:
Accurate theoretical knowledge of the (1s2p)^3P_1–(1s2s)^1S_0 splitting in He-like ions is demanded for future experimental studies of the nuclear spin-dependent part of the weak interaction. In this paper we perform a calculation of the hyperfine structure of (1s2p)^3P_1–(1s2s)^1S_0 within 3 ≤ Z ≤ 35, Z being the atomic number. In this Z range parity nonconservation (PNC) effects are amplified by the close energy proximity of the opposite parity levels (1s2p)^3P_1 and (1s2s)^1S_0. We find that the hyperfine structure is relevant for Z > 12, and produces splitting among the hyperfine sublevels as large as 150 meV for medium Z He-like ions (Z  ~ 35).
W. Nörtershäuser, R. Sanchez, G. Ewald, A. Dax, J. Behr, P. Bricault, B. A. Bushaw, J. Dilling, M. Dombsky, G. W. F. Drake, S. Götte, H.-J. Kluge, T. Kühl, J. Lassen, C. D. P. Levy, K. Pachucki, M. Pearson, M. Puchalski, A. Wojtaszek, Z.-C. Yan, and C. Zimmermann
Isotope-shift measurements of stable and short-lived lithium isotopes for nuclear-charge-radii determination
Phys. Rev. A, 83 :012516 (January 2011)
Abstract:
Changes in the mean square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8-ms-lifetime isotope with production rates on the order of only 10 000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope 11^Li at the on-line isotope separators at GSI, Darmstadt, Germany, and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.
A. M. Sayler, T. Rathje, W. Müller, K. Rühle, R. Kienberger, and G.G. Paulus
Precise, real-time, every-single-shot, carrier-envelope phase measurement of ultrashort laser pulses
Opt. Lett., 36 :1 (January 2011)
Abstract:
In this Letter we demonstrate a method for real-time determination of the carrier-envelope phase of each and every single ultrashort laser pulse at kilohertz repetition rates. The technique expands upon the recent work of Wittmann and incorporates a stereographic above-threshold laser-induced ionization measurement and electronics optimized to produce a signal corresponding to the carrier-envelope phase within microseconds of the laser interaction, thereby facilitating data-tagging and feedback applications. We achieve a precision of 113 mrad (6.5°) over the entire 2π range.
B. Botermann, C. Novotny, D. Bing, C. Geppert, G. Gwinner, T. W. Hänsch, G. Huber, S. Karpuk, T. Kühl, W. Nörtershäuser, S. Reinhardt, G. Saathoff, D. Schwalm, T. Stöhlker, and A. Wolf
Preparatory measurements for a test of time dilation in the ESR
Can. J. Phys., 89 :85 (January 2011)
Abstract:
We present preparatory measurements for an improved test of time dilation at the experimental storage ring (ESR) at GSI in Darmstadt. A unique combination of particle accelerator experiments and laser spectroscopy is used to perform this test with the highest precision. 7^Li+ ions are accelerated to 34% of the speed of light at the GSI Helmholtzzentrum für Schwerionenforschung and stored in the experimental storage ring. The forward and backward Doppler shifts of an electric dipole transition of these ions are measured with laser spectroscopy techniques. From these Doppler shifts, both the ion velocity β = ν/c and the time dilation factor can be derived for testing Special Relativity. Two laser systems have been developed to drive the 3^S_1 → 3^P_2 transition in 7^Li+. Moreover, a detector system composed of photomultipliers, both to monitor the exact laser ion beam overlap as well as to optimize fluorescence detection, has been set up and tested. We investigate optical-optical double-resonance spectroscopy on a closed Λ-type three-level system to overcome Doppler broadening. A residual, broadened fluorescence background caused by velocity-changing processes in the ion beam is identified, and a background subtraction scheme implemented. At the present stage the experimental sensitivity, although already comparable with previous measurements on slower ion beams at the TSR storage ring that led to  < 8.4 × 10^–8, suffer from a poor signal-to-noise ratio. Modifications of the ion source as well as the detection system are discussed that promise to improve the sensitivity by one order of magnitude.