Peer-Review Publications


J. Rothhardt, S. Hädrich, F. Röser, J. Limpert, and A. Tünnermann
500 MW peak power degenerated optical parametric amplifier delivering 52 fs pulses at 97 kHz repetition rate
Opt. Express, 16 :8981 (June 2008)
We present a high peak power degenerated parametric amplifier operating at 1030 nm and 97 kHz repetition rate. Pulses of a state-of-the art fiber chirped-pulse amplification (FCPA) system with 840 fs pulse duration and 410 µJ pulse energy are used as pump and seed source for a two stage optical parametric amplifier. Additional spectral broadening of the seed signal in a photonic crystal fiber creates enough bandwidth for ultrashort pulse generation. Subsequent amplification of the broadband seed signal in two 1 mm BBO crystals results in 41 µJ output pulse energy. Compression in a SF 11 prism compressor yields 37 µJ pulses as short as 52 fs. Thus, pulse shortening of more than one order of magnitude is achieved. Further scaling in terms of average power and pulse energy seems possible and will be discussed, since both concepts involved, the fiber laser and the parametric amplifier have the reputation to be immune against thermo-optical effects.
S. N. Andreev, I. L. Beigman, I. Y. Tolstikhina, V. P. Shevelko, G. Baur, and T. Stöhlker
Ionization of Heavy Ions in Collisions with Neutral Atoms at Relativistic Energies
Bull. Lebedev Phys. Inst., 35 :89 (March 2008)
Ionization processes of heavy ions colliding with atoms and ions at relativistic energies are considered. Formulaes for calculating ionization cross sections in the Born approximation using momentum-transfer representation without regard to magnetic interactions are given as well as those in dipole and impulse approximations. Using the LOSS-R [25] and HERION codes, calculations of relativistic cross sections are performed for H-like multiply changed ions with the nuclear charge Z approximate to 80-90, colliding with neutral atoms and for multiply changed uranium ions colliding with protons and carbon atoms. The results of calculations are compared with available experimental data and calculations performed by other authors.
O. Schmidt, J. Rothhardt, T. Eidam, F. Röser, J. Limpert, A. Tünnermann, K. P. Hansen, C. Jakobsen, and J. Broeng
Single-polarization ultra-large-mode-area Yb-doped photonic crystal fiber
Opt. Express, 16 :3918 (March 2008)
We report on an ytterbium-doped single-transverse-mode rod-type photonic crystal fiber that combines the advantages of low nonlinearityand intrinsic polarization stability. The mode-field-area of the fundamental mode is as large as 2300 µm2. An output power of up to 163 W with adegree of polarization better than 85% has been extracted from a simple fiber laser setup without any additional polarizing element within the cavity than the fiber itself. The beam quality has been characterized by a M2 value of 1.2. The single-polarization window ranges from 1030 to 1080 nm, hence possesses an excellent overlap with the gain profile of ytterbium-doped silica fibers. To the best of our knowledge this fiber design has the largest mode-field-diameter ever reported for polarizing or even polarization maintaining rare-earth-doped double-clad fibers.


J. Rothhardt, S. Hädrich, D. Schimpf, J. Limpert, and A. Tünnermann
High repetition rate fiber amplifier pumped sub-20 fs optical parametric amplifier
Opt. Express, 15 :16729 (December 2007)
We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) based on a cavity dumped Ti:Sapphire oscillator providing the signal, and an Ytterbium-doped fiber amplifier pumping the device. Temporally synchronized NOPA pump pulses are created via soliton generation in a highly nonlinear photonic crystal fiber. This soliton is fiber amplified to high pulse-energies at high repetition rates. The broadband Ti:Sapphire laser pulses are parametrically amplified either directly or after additional spectral broadening. The approach of fiber-based pump-pulse generation from a femtosecond laser, that emits in the spectral region of NOPA-gain, offers enhanced long-term stability and pulse quality compared to conventional techniques, such as signal pulse generation from a high power laser system via filamentation in bulk media. The presented system produces high-energy ultra-short pulses with pulse-durations down to 15.6 fs and pulse-energies up to 500 nJ at a repetition rate as high as 2 MHz.
F. Röser, T. Eidam, J. Rothhardt, O. Schmidt, D. Schimpf, J. Limpert, and A. Tünnermann
Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system
Opt. Lett., 32 :3495 (December 2007)
We report on an ytterbium-doped fiber chirped-pulse amplification (CPA) system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses are as short as 800 fs. As the main amplifier, an 80 µm core diameter short length photonic crystal fiber is employed, which allows the generation of pulse energies up to 1.45 mJ with a B-integral as low as 7 at a stretched pulse duration of 2 ns. A stretcher-compressor unit consisting of dielectric diffraction gratings is capable of handling the average power without beam and pulse quality distortions. To our knowledge, we present the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.
D. N. Schimpf, J. Rothhardt, J. Limpert, A. Tünnermann, and D. C. Hanna
Theoretical analysis of the gain bandwidth for noncollinear parametric amplification of ultrafast pulses
J. Opt. Soc. Am. B, 24 :2837 (November 2007)
The choice of optimum phase-matching conditions for noncollinear optical parametric amplifiers is usually made on the basis of the linear spectral dispersion characteristics of the anisotropic nonlinear crystal. However, for high-peak-power operation, where pump depletion is involved, it is shown that the tolerance of the parametric gain with regard to k-vector mismatch is to change the optimum phase-matching parameters. Our calculations show that, with the revised parameters, an enhancement in peak power approaching 50% could be achieved.
R. DuBois, O. d. Lucio, M. Thomason, G. Weber, T. Stöhlker, K. Beckert, P. Beller, F. Bosch, C. Brandau, A. Gumberidze, S. Hagmann, C. Kozhuharov, F. Nolden, R. Reuschl, J. Rzadkjewicz, P. Spiller, U. Spillmann, M. Steck, and S. Trotsenko
Beam lifetimes for low-charge-state heavy ions in the GSI storage rings
Nucl. Instr. Meth. Phys. Res. B, 261 :230 (August 2007)
Lifetimes for 10 – 50 MeV/u U28+ ions were measured for base vacuum conditions in the ESR storage ring at GSI-Darmstadt. The lifetimes are due to total electron loss from U28+ resulting from interactions with background gases in the ring. Lifetimes were also measured for interactions with H2 and N2 targets. These data provide information about the relative magnitudes and energy dependences of the stripping cross-sections resulting from interactions with H2 and N2, gases which represent the primary constituents in high and ultra-high vacuum environments.
O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, and A. Tünnermann
Millijoule pulse energy Q-switched short-length fiber laser
Opt. Lett., 32 :1551 (June 2007)
We report on a Q-switched short-length fiber laser producing 100 W of average output power at 100 kHz repetition rate and pulse durations as short as 17 ns. Up to 2 mJ of energy and sub-10-ns pulse duration are extracted at lower repetition rates. This performance is obtained by employing a rod-type ytterbium-doped photonic crystal fiber with a 70 microm core as gain medium, allowing for very short pulse durations, high energy storage, and emission of a single-transverse-mode beam.
C. Aguergaray, T. Andersen, D. Schimpf, O. Schmidt, J. Rothhardt, T. Schreiber, J. Limpert, E. Cormier, and A. Tünnermann
Parametric amplification and compression to ultrashort pulse duration of resonant linear waves
Opt. Express, 15 :5699 (April 2007)
We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation. Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal generation is a promising alternative to white light generation in bulky glass or sapphire plates. We demonstrate a novel and successful signal engineering implemented in a setup for parametric amplification and subsequent recompression of resonant linear waves resulting from soliton fission in a highly nonlinear photonic crystal fiber. The applied pump source is a high repetition rate ytterbium-doped fiber chirped pulse amplification system. The presented approach results in the generation of \~50 fs pulses at MHz repetition rate. The potential of generating even shorter pulse duration and higher pulse energies will be discussed.
S. Trotsenko, T. Stöhlker, D. Banaś, C. Z. Dong, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, P. Indelicato, C. Kozhuharov, M. Nofal, R. Reuschl, J. Rzadkiewicz, U. Spillmann, A. Surzhykov, M. Trassinelli, and G. Weber
Investigation of the Decay Properties of the 1s(2s) 2 State in Li-Like Uranium
J. Phys.: Conf. Ser., 58 :141 (January 2007)
We report on an experiment aiming for a study of the radiative decay modes of the 1s (2s)2 level in Li-like uranium. The experiment was performed of initially Be-like uranium colliding with N2 molecules at an energy of 90 MeV/u. By measuring the x-ray production associated with K-shell ionization of the projectile, a high selectivity for the production of the 1s (2s)2 level is observed.
G. Weber, T. Stöhlker, D. Banaś, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, M. Nofal, U. Popp, R. Reuschl, U. Spillmann, A. Surzhykov, and S. Trotsenko
Radiative processes studied for bare uranium ions in collisions with H_2
J. Phys.: Conf. Ser., 58 :243 (January 2007)
Radiative processes occurring in collision of decelerated bare uranium ions and molecular hydrogen are studied at the heavy-ion storage ring ESR. The combination of the deceleration technique and the narrow Compton profile of molecular hydrogen allowed us to resolve a multitude of REC transitions into the bound states of the projectile and to resolve unambiguously the tip region of primary bremsstahlung. For this purpose, a supersonic molecular hydrogen jet-target, precooled with liquid nitrogen and optimized for long-term stability, was applied.
R. Reuschl, D. Banaś, H. F. Beyer, S. Chatterjee, A. Gumberidze, S. Hess, T. Krings, D. Liesen, D. Protić, U. Spillmann, T. Stöhlker, M. Trassinelli, S. Trotsenko, and G. Weber
Recent experimental developments for the Lamb shift investigation in heavy ions
J. Phys.: Conf. Ser., 58 :407 (January 2007)
The latest commissioning experiment of a two arm transmission crystal x-ray spectrometer along with high-performance position-sensitive microstrip germanium detectors is presented. The goal of the experiment was to observe with high resolution the Ly-α-transitions of H-like Pb 81+ produced in collisions with Kr atoms. Due to a photon efficiency of only 10^−8 the position sensitivity as well as the energy and time resolution of segmented solid state Germanium detectors are absolutely essential for experiments using crystal x-ray spectrometers dealing with beams of heavy ions. A detector system with the desired properties has become available through a collaboration with the Forschungszentrum Jülich.


J. Limpert, O. Schmidt, J. Rothhardt, F. Röser, T. Schreiber, A. Tünnermann, S. Ermeneux, P. Yvernault, and F. Salin
Extended single-mode photonic crystal fiber lasers
Opt. Express, 14 :2715 (April 2006)
We report on an ytterbium-doped photonic crystal fiber with a core diameter of 60 microm and mode-field-area of ~2000 µm^2 of the emitted fundamental mode. Together with the short absorption length of 0.5 m this fiber possesses a record low nonlinearity which makes this fiber predestinated for the amplification of short laser pulses to very high peak powers. In a first continuous-wave experiment a power of 320 W has been extracted corresponding to 550 W per meter. To our knowledge this represents the highest power per unit length ever reported for fiber lasers. Furthermore, the robust single-transverse-mode propagation in a passive 100 microm core fiber with a similar design reveals the potential of extended large-mode-area photonic crystal fibers.


F. Röser, J. Rothhardt, B. Ortac, A. Liem, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann
131 W 220 fs fiber laser system
Opt. Lett., 30 :2754 (October 2005)
We report on an ytterbium-doped photonic-crystal-fiber-based chirped-pulse amplification system delivering 131 W average power 220 fs pulses at 1040 nm center wavelength in a diffraction-limited beam. The pulse repetition rate is 73 MHz, corresponding to a pulse energy of 1.8 µJ and a peak power as high as 8.2 MW.


P. Beiersdorfer, S. R. Elliott, A. Osterheld, T. Stöhlker, J. Autrey, G. V. Brown, A. J. Smith, and K. Widmann
Search for 1s2s S-3(1)-1s2p P-3(2) decay in U90+
Phys. Rev. A, 53 :4000 (June 1996)
We present high-resolution crystal-spectrometer measurements that cover the wavelength range 3.0-3.2 Angstrom containing the electric-dipole-allowed 2s(1/2)-2p(3/2) transitions in highly charged uranium ions. Strong features from 2s(1/2)-2p(3/2) transitions in lithiumlike, berylliumlike, and boronlike uranium were observed. In addition, a weak feature with intensity just above the level of background fluctuations was observed at the predicted location of the 1s2s S-3(1)-1s2p P-3(2) transition in heliumlike U90+. The feature was observed in spectra where the intensity of the 2s(1/2)-2p(3/2) transition in lithiumlike uranium, and thus the ionization balance, was optimized; it appeared absent in spectra where the average ionization balance was lower. The measured energy of the feature is 4510.05+/-0.24 eV, which agrees closely with the values predicted for the 1s2s S-3(1)-1s2p P-3(2) transition by recent theories. Detailed spectral modeling calculations indicate the possibility that a weak transition in berylliumlike U88+ is situated within 4 eV of the-location of the heliumlike S-3(1)-P-3(2) transition. This transition connects the level 1s (2)2p(1/2)2p(3/2) (3)p(1) with the 1s (2)2s(1/2)2p(1/2) P-3(0) metastable ground level in berylliumlike uranium. The accuracy with which the energy of the berylliumlike transition can be predicted is insufficient to rule out a blend with the heliumlike S-3(1)-P-3(2) transition.
P. H. Mokler, and T. Stöhlker
The physics of highly charged heavy ions revealed by storage/cooler rings
Adv. Atom. Mol. Opt. Phy, 37 :297 ( 1996)
This chapter presents an overview of the current advances in the rapidly developing field of heavy ion accelerator technology. Now, essential aspects in this area are accessible. In the meantime, great progress already has been made in the fundamental physics in this field. This is particularly true for achievements in the atomic physics of highly charged heavy ions. There are two general domains to be considered in the atomic physics of highly charged heavy ions: the fields of collisions and of atomic structure. Both aspects have to be explored equally because they are strongly interconnected. The interaction processes has to be investigated to know, for instance, the population of excited states to help answer questions on the atomic structure; conversely, the structure has to be known to understand the interactions. In both the fields, fundamental principles can be studied uniquely. This is in particular true for the heaviest ion species with only a few- or even zero-electrons left.


R. E. Marrs, S. R. Elliott, and T. Stöhlker
Measurement of 2-Electron Contributions to the Ground-State Energy of Helium-Like Ions
Phys. Rev. A, 52 :3577 (November 1995)
We report direct measurements of two-electron contributions to the ground-state energy of high-Z heliumlike ions. The difference in radiative-recombination x-ray energy (i.e., ionization potential of the recombined ion) was measured for bare and hydrogenlike target ions trapped in an electron-beam ion trap for six elements ranging from Z = 32 to Z = 83. The achieved uncertainties (as small as 1.6 eV) test the second-order many-body contributions to the energy and approach the size of the two-electron (screened) Lamb shift: We also report a measurement of the ground-state ionization potential of heliumlike bismuth relative to heliumlike xenon. All results are in agreement with available theories.
P. H. Mokler, T. Stöhlker, R. W. Dunford, A. Gallus, T. Kandler, G. Menzel, H. T. Prinz, P. Rymuza, Z. Stachura, P. Swiat, and A. Warczak
The X-Ray-Spectrum of H-Like Uranium
Z. Phys. D, 35 :77 (August 1995)
By decelerating highly-charged, very heavy ions to low energies in the storage and cooler ring, ESR, a new brilliant source for projectile x-rays has been obtained. In a first precision spectroscopy experiment up to 2*10^7 stored bare U92+ ions have been decelerated down to 49 MeV/u before detecting the characteristic projectile x-rays associated with one-electron capture at the ESR gas target. The Lyman, Balmer and Paschen series are distinctly observed for hydrogenic U91+ ions.
H. F. Beyer, G. Menzel, D. Liesen, A. Gallus, F. Bosch, R. Deslattes, P. Indelicato, T. Stöhlker, O. Klepper, R. Moshammer, F. Nolden, H. Eickhoff, B. Franzke, and M. Steck
Measurement of the Ground-State Lambshift of Hydrogenlike Uranium at the Electron Cooler of the ESR
Z. Phys. D, 35 :169 (July 1995)
X-rays are emitted with the radiative recombination of free electrons in an electron cooler of a heavy-ion storage ring. Due to a small width of the X-ray lines, an observation angle close to 0 degrees and an accurate determination of the ion velocity, the ground-state Lambshift of hydrogenlike uranium (470 +/- 16) eV could be measured to an accuracy of 3.4%. A re-evaluation of a measurement of the 1S(1/2) Lambshift in hydrogenlike gold gave a new value of (202.3 +/- 7.9) eV as compared to the former value of (212 +/- 15) eV. The results are in excellent agreement with QED calculations and are more precise than any other measurements previously reported for a high-Z, hydrogenlike ion.
T. Kandler, T. Stöhlker, P. H. Mokler, C. Kozhuharov, H. Geissel, C. Scheidenberger, P. Rymuza, Z. Stachura, A. Warczak, R. W. Dunford, J. Eichler, A. Ichihara, and T. Shirai
Photon Angular-Distribution of Radiative Electron-Capture into the M-Shell of He-Like Uranium Ions at 110-140 MeV/u
Z. Phys. D, 35 :15 (July 1995)
Measurements of the photon angular distribution of Radiative Electron Capture into the M shell have been performed with He-like uranium ions in the range 110-140 MeV/u. In addition, L REC was studied at a projectile energy of 140 MeV/u. In both cases, the experimental data show an asymmetry around 90° and agree well with a fully relativistic theory.
T. Stöhlker, C. Kozhuharov, P. H. Mokler, A. Warczak, F. Bosch, H. Geissel, R. Moshammer, C. Scheidenberger, J. Eichler, A. Ichihara, T. Shirai, Z. Stachura, and P. Rymuza
Radiative Electron-Capture Studied in Relativistic Heavy-Ion Atom Collisions
Phys. Rev. A, 51 :2098 (March 1995)
The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard sin^2(θ) distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.


P. Rymuza, T. Stöhlker, C. L. Cocke, H. Geissel, C. Kozhuharov, H. Mokler, R. Moshammer, F. Nickel, C. Scheidenberger, Z. Stachura, J. Ullrich, and A. Warczak
Deviation from 1st-Order Perturbation-Theory Observed at Intermediate Relativistic Velocities for the Ionization of Highly-Charged Heavy Projectiles
J. Phys. B, 26 :L169 (April 1993)
Projectile K-shell ionization cross sections were measured for 80-200 MeV/u H- and He-like Bi ions incident on thin C, Al and Ni targets. The results are compared with predictions of first-order perturbation theories, RSCA and PWBA, as well as with experimental results published for Xe and U ions at similar ratios eta of projectile velocity to K-shell electron velocity. All experimental data lie a factor of 1.5 above theory for 0.6 < eta < 1.5.


T. Stöhlker, C. Kozhuharov, P. H. Mokler, R. E. Olson, Z. Stachura, and A. Warczak
Single and Double Electron-Capture in Collisions of Highly Ionized, Decelerated Ge Ions with Ne
J. Phys. B, 25 :4527 (November 1992)
Experimental cross sections for non-radiative single and double electron capture from Ne target into decelerated H-like Ge ions at collision energies of (4-12) MeV u-1 are presented. The results are compared with theoretical calculations and an empirical scaling rule. Information concerning the impact parameter dependence of electron capture is extracted using classical trajectory Monte Carlo calculations.
T. Stöhlker, P. H. Mokler, H. Geissel, R. Moshammer, P. Rymuza, E. M. Bernstein, C. L. Cocke, C. Kozhuharov, G. Münzenberg, F. Nickel, C. Scheidenberger, Z. Stachura, J. Ullrich, and A. Warczak
Ground-State Transitions in One-Electron and 2-Electron Bi Projectiles
Phys. Lett. A, 168 :285 (August 1992)
By using segmented solid state X-ray detectors and applying X-ray/ charge-state selective particle coincidences, the ionic structures of Bi-83(82+) and Bi-83(81+) have been studied separately at 82 MeV/u under single collision conditions. The high granularity of the Ge(i) X-my detectors used allowed a partial Doppler correction for the X-ray events while maintaining a large total solid angle. An absolute precision of K-transition energies of 10(-3) is feasible; the relative accuracy is better than 30 eV. The experimental values compare very well with the theoretical transition energies in H- and He-like Bi ions.
T. Stöhlker, C. Kozhuharov, A. E. Livingston, P. H. Mokler, Z. Stachura, and A. Warczak
Radiative Electron-Capture into the K-Shell, L-Shell and M-Shell of Decelerated, Hydrogenic Ge Projectiles
Z. Phys. D, 23 :121 (June 1992)
Radiative Electron Capture (REC) in 4 to 12 MeV/u Ge^31+ —> H2 collisions has been studied using an X-ray/particle coincidence technique. This technique allowed a systematic investigation of K-shell REC as well as a separation of REC into the projectile L- and M-shells. The cross sections are discussed within a general scaling picture based on the reduced projectile velocity.