Peer-Review Publications

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

The propagation of fast electrons produced in the interaction of relativistically intense, picosecond laser pulses with solid targets is experimentally investigated using K α emission as a diagnostic. The role of fast electron refluxing within the target, which occurs when the electrons are reflected by the sheath potentials formed at the front and rear surfaces, is elucidated. The targets consist of a Cu fluorescence layer of fixed thickness at the front surface backed with a propagation layer of CH, the thickness of which is varied to control the number of times the refluxing fast electron population transits the Cu fluorescence layer. Enhancements in the K α yield and source size are measured as the thickness of the CH layer is decreased. Comparison with analytical and numerical modelling confirms that significant refluxing occurs and highlights the importance of considering this phenomenon when deriving information on fast electron transport from laser–solid interaction experiments involving relatively thin targets.

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.

Single-shot carrier-envelope-phase (CEP) tagging is combined with a reaction mircoscope (REMI) to investigate CEP-dependent processes in atoms. Excellent experimental stability and data acquisition longevity are achieved. Using this approach, we study the CEP effects for nonsequential double ionization of argon in 4-fs laser fields at 750 nm and an intensity of 1.6 × 10^14 W/cm2. The Ar^(2+) ionization yield shows a pronounced CEP dependence which compares well with recent theoretical predictions employing quantitative rescattering theory [S. Micheau et al. Phys. Rev. A 79 013417 (2009)]. Furthermore, we find strong CEP influences on the Ar^(2+) momentum spectra along the laser polarization axis.

We report on a novel approach of performance scaling of ultrafast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

We report the observation of an interference between the electric dipole (E1) and the magnetic quadrupole (M2) amplitudes for the linear polarization of the Ly-alpha(1) (2p(3/2) -> 1s(1/2)) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-alpha(1) radiation due to the E1-M2 amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the E1 and the M2 transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the 2p(3/2) state.

We lay out a new approach to solving the two-centre Dirac eigenvalue problem. It is based on the application of B-spline basis sets constructed in Cassini coordinates. The approach provides a very promising way for the theoretical description of various atomic processes such as charge transfer, excitation, ionization and electron-positron pair production that accompany slow collisions of heavy ions. Moreover, yielding a (quasi-) complete set of eigensolutions, the finite basis set method allows a systematic analysis of the quantum electrodynamics (QED) corrections to the energy levels of heavy quasi-molecules formed in these collisions.

The high-harmonic generation (HHG) yield depends on several important experimental parameters and can be described with models including single atom response and propagation effects. In our recent paper we extended this description by adding a stimulated emission process and named it self-seeded X-ray parametric amplification (XPA).

The nonperturbative electron-positron pair production (Schwinger effect) is considered for space- and time-dependent electric fields E(x,t). Based on the Dirac-Heisenberg-Wigner formalism, we derive a system of partial differential equations of infinite order for the 16 irreducible components of the Wigner function. In the limit of spatially homogeneous fields the Vlasov equation of quantum kinetic theory is rediscovered. It is shown that the quantum kinetic formalism can be exactly solved in the case of a constant electric field E(t)=E_0 and the Sauter-type electric field E(t)=E_0 sech^2 (t/τ). These analytic solutions translate into corresponding expressions within the Dirac-Heisenberg-Wigner formalism and allow to discuss the effect of higher derivatives. We observe that spatial field variations typically exert a strong influence on the components of the Wigner function for large momenta or for late times.

We present a novel, cascaded acceleration scheme for the generation of spectrally controlled ion beams using a laser-based accelerator in a 'double-stage' setup. An MeV proton beam produced during a relativistic laser-plasma interaction on a thin foil target is spectrally shaped by a secondary laser-plasma interaction on a separate foil, reliably creating well-separated quasi-monoenergetic features in the energy spectrum. The observed modulations are fully explained by a one-dimensional (1D) model supported by numerical simulations. These findings demonstrate that laser acceleration can, in principle, be applied in an additive manner.