Peer-Review Publications

2016

N. Dimitrov, L. Stoyanov, I. Stefanov, A. Dreischuh, P. Hansinger, and G. Paulus
Pulse front tilt measurement of femtosecond laser pulses
Opt. Commun., 371 :51 (July 2016)
Abstract:
In this work we report experimental investigations of an intentionally introduced pulse front tilt on femtosecond laser pulses by using an inverted field correlator/interferometer. A reliable criterion for the precision in aligning (in principle) dispersionless systems for manipulating ultrashort pulses is developed, specifically including cases when the pulse front tilt is a result of a desired spatio-temporal coupling. The results obtained using two low-dispersion diffraction gratings are in good qualitative agreement with the data from a previously developed analytical model and from an independent interferometric measurement.
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M. S. Ebrahimi, N. Stallkamp, W. Quint, M. Wiesel, M. Vogel, A. Martin, and G. Birkl
Superconducting radio-frequency resonator in magnetic fields up to 6 T
Rev. Sci. Instrum., 87 :075110 (July 2016)
Abstract:
We have measured the characteristics of a superconducting radio-frequency resonator in an external magnetic field. The magnetic field strength has been varied with 10 mT resolution between zero and 6 T. The resonance frequency and the quality factor of the resonator have been found to change significantly as a function of the magnetic field strength. Both parameters show a hysteresis effect which is more pronounced for the resonance frequency. Quantitative knowledge of such behaviour is particularly important when experiments require specific values of resonance frequency and quality factor or when the magnetic field is changed while the resonator is in the superconducting state.
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T. Scholtes, S. Pustelny, S. Fritzsche, V. Schultze, R. Stolz, and H.-G. Meyer
Suppression of spin-exchange relaxation in tilted magnetic fields within the geophysical range
Phys. Rev. A, 94 :013403 (July 2016)
Abstract:
We present a detailed experimental and theoretical study on the relaxation of spin coherence due to the spin-exchange mechanism arising in the electronic ground states of alkali-metal vapor atoms. As opposed to the well-explored formation of a stretched state in a longitudinal geometry (magnetic field parallel to the laser propagation direction) we employ adapted hyperfine-selective optical pumping in order to suppress spin-exchange relaxation. By comparing measurements of the intrinsic relaxation rate of the spin coherence in the ground state of cesium atoms with detailed density-matrix simulations we show that the relaxation due to spin-exchange collisions can be reduced substantially even in a tilted magnetic field of geomagnetic strength, the major application case of scalar magnetic surveying. This explains the observed striking improvement in sensitivity and further deepens the understanding of the light-narrowed Mx magnetometer, which was presented recently. Additionally, new avenues for investigating the dynamics in alkali-metal atoms governed by the spin-exchange interaction and interacting with arbitrary external fields open up.
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G. Matthäus, S. Demmler, M. Lebugle, F. Küster, J. Limpert, A. Tünnermann, S. Nolte, and R. Ackermann
Ultra-broadband two beam CARS using femtosecond laser pulses
Vib. Spectrosc., 85 :128 (July 2016)
Abstract:
Femtosecond (fs)-CARS is a promising approach for gas spectroscopy under high pressure and temperature conditions, as it allows probing molecular states on a time scale which is significantly shorter than the typical decay time induced by interfering collisions. Usually, fs-CARS is performed in a three beam setup, which requires maintaining spatial and temporal overlap of the pulses at the focal point. This is a challenging task, especially in harsh environments such as in a combustion chamber. In this study, we present an alternative approach, which uses two beams in a collinear configuration. An ultra-broadband, sub 7 fs laser pulse acts as pump and Stokes pulse, and a ∼500 fs pulse is used for probing. We show that this configuration is suitable for measuring the gas temperature and concentration. Furthermore, possible single shot measurements of the gas temperature are evaluated.
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M. L. Bissell, T. Carette, K. T. Flanagan, P. Vingerhoets, J. Billowes, K. Blaum, B. Cheal, S. Fritzsche, M. Godefroid, M. Kowalska, J. Krämer, R. Neugart, G. Neyens, W. Nörtershäuser, and D. T. Yordanov
Cu charge radii reveal a weak sub-shell effect at N=40
Phys. Rev. C, 93 :064318 (June 2016)
Abstract:
Collinear laser spectroscopy on Cu58–75 isotopes was performed at the CERN-ISOLDE radioactive ion beam facility. In this paper we report on the isotope shifts obtained from these measurements. State-of-the-art atomic physics calculations have been undertaken in order to determine the changes in mean-square charge radii δ⟨r2⟩A,A′ from the observed isotope shifts. A local minimum is observed in these radii differences at N=40, providing evidence for a weak N=40 sub-shell effect. However, comparison of δ⟨r2⟩A,A′ with a droplet model prediction including static deformation deduced from the spectroscopic quadrupole moments, points to the persistence of correlations at N=40.
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M. Schöffler, X. Xie, P. Wustelt, M. Möller, S. Roither, D. Kartashov, A. Sayler, A. Baltuska, G.G. Paulus, and M. Kitzler
Laser-subcycle control of sequential double-ionization dynamics of helium
Phys. Rev. A, 93 :063421 (June 2016)
Abstract:
We present measured momentum distributions on the double ionization of helium with intense, near-circularly-polarized few-cycle laser pulses with a known carrier-envelope offset phase (CEP). The capability of obtaining CEP-resolved momentum distributions enables us to observe signatures of the various combinations of laser-half-cycle two-electron emissions. By comparison to semiclassical trajectory simulations, we succeed in assigning the corresponding structures in the measured distributions to certain two-electron emission dynamics. Based on this possibility, we demonstrate that the sequential double-ionization dynamics can be sensitively controlled with the pulse duration and the laser peak intensity. For the shortest pulse durations and not too high intensities we find that the two electrons are dominantly emitted with a delay of roughly a laser half cycle. For a just slightly increased intensity we find evidence that at least one of the two electrons is surprisingly likely emitted in between the peaks of the laser field oscillations rather than at the field maxima. The simulations reproduce the signatures of these kinds of two-electron emissions overall relatively well.
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Z. W. Wu, A. V. Volotka, A. Surzhykov, C. Z. Dong, and S. Fritzsche
Level sequence and splitting identification of closely spaced energy levels by angle-resolved analysis of fluorescence light
Phys. Rev. A, 93 :063413 (June 2016)
Abstract:
The angular distribution and linear polarization of the fluorescence light following the resonant photoexcitation is investigated within the framework of density matrix and second-order perturbation theory. Emphasis has been placed on “signatures” for determining the level sequence and splitting of intermediate (partially) overlapping resonances, if analyzed as a function of photon energy of incident light. Detailed computations within the multiconfiguration Dirac–Fock method have been performed, especially for the 1s^2 2s^2 2p^6 3s,Ji=1/2+γ1 → (1s^2 2s 2p^6 3s)_1 3p3/2,J=1/2,3/2 → 1s^2 2s^2 2p^6 3s,Jf=1/2+γ2 photoexcitation and subsequent fluorescence emission of atomic sodium. A remarkably strong dependence of the angular distribution and linear polarization of the γ2 fluorescence emission is found upon the level sequence and splitting of the intermediate (1s^2 2s 2p^6 3s)_1 3p3/2,J=1/2,3/2 overlapping resonances owing to their finite lifetime (linewidth). We therefore suggest that accurate measurements of the angular distribution and linear polarization might help identify the sequence and small splittings of closely spaced energy levels, even if they cannot be spectroscopically resolved.
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A. N. Grum-Grzhimailo, E. V. Gryzlova, S. Fritzsche, and N. M. Kabachnik
Photoelectron angular distributions and correlations in sequential double and triple atomic ionization by free electron lasers
\u200eJ. Mod. Opt., 63 :334 (June 2016)
Abstract:
We present a review of theoretical studies of the simplest nonlinear photoprocesses detected in the XUV range with the use of free electron lasers: sequential double and triple ionization of atoms by two and three XUV photons. Photoelectron angular distributions and angular correlations between emitted electrons are considered. A comparison of the calculated results with recent angle-resolved photoelectron spectroscopy experiments is discussed.
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B. Faatz, E. Plönjes, S. Ackermann, A. Agababyan, V. Asgekar, V. Ayvazyan, S. Baark, N. Baboi, V. Balandin, N. v. Bargen, Y. Bican, O. Bilani, J. Bödewadt, M. Böhnert, R. Böspflug, S. Bonfigt, H. Bolz, F. Borges, O. Borkenhagen, M. Brachmanski, M. Braune, A. Brinkmann, O. Brovko, T. Bruns, P. Castro, J. Chen, M. K. Czwalinna, H. Damker, W. Decking, M. Degenhardt, A. Delfs, T. Delfs, H. Deng, M. Dressel, H.-T. Duhme, S. Düsterer, H. Eckoldt, A. Eislage, M. Felber, J. Feldhaus, P. Gessler, M. Gibau, N. Golubeva, T. Golz, J. Gonschior, A. Grebentsov, M. Grecki, C. Grün, S. Grunewald, K. Hacker, L. Hänisch, A. Hage, T. Hans, E. Hass, A. Hauberg, O. Hensler, M. Hesse, K. Heuck, A. Hidvegi, M. Holz, K. Honkavaara, H. Höppner, A. Ignatenko, J. Jäger, U. Jastrow, R. Kammering, S. Karstensen, A. Kaukher, H. Kay, B. Keil, K. Klose, V. Kocharyan, M. Köpke, M. Körfer, W. Kook, B. Krause, O. Krebs, S. Kreis, F. Krivan, J. Kuhlmann, M. Kuhlmann, G. Kube, T. Laarmann, C. Lechner, S. Lederer, A. Leuschner, D. Liebertz, J. Liebing, A. Liedtke, L. Lilje, T. Limberg, D. Lipka, B. Liu, B. Lorbeer, K. Ludwig, H. Mahn, G. Marinkovic, C. Martens, F. Marutzky, M. Maslocv, D. Meissner, N. Mildner, V. Miltchev, S. Molnar, D. Mross, F. Müller, R. Neumann, P. Neumann, D. Nölle, F. Obier, M. Pelzer, H.-B. Peters, K. Petersen, A. Petrosyan, G. Petrosyan, L. Petrosyan, V. Petrosyan, A. Petrov, S. Pfeiffer, A. Piotrowski, Z. Pisarov, T. Plath, P. Pototzki, M. J. Prandolini, J. Prenting, G. Priebe, B. Racky, T. Ramm, K. Rehlich, R. Riedel, M. Roggli, M. Röhling, J. Rönsch-Schulenburg, J. Rossbach, V. Rybnikov, J. Schäfer, J. Schaffran, H. Schlarb, G. Schlesselmann, M. Schlösser, P. Schmid, C. Schmidt, F. Schmidt-Föhre, M. Schmitz, E. Schneidmiller, A. Schöps, M. Scholz, S. Schreiber, K. Schütt, U. Schütz, H. Schulte-Schrepping, M. Schulz, A. Shabunov, P. Smirnov, E. Sombrowski, A. Sorokin, B. Sparr, J. Spengler, M. Staack, M. Stadler, C. Stechmann, B. Steffen, N. Stojanovic, V. Sychev, E. Syresin, T. Tanikawa, F. Tavella, N. Tesch, K. Tiedtke, M. Tischer, R. Treusch, S. Tripathi, P. Vagin, P. Vetrov, S. Vilcins, M. Vogt, Z. Wagner, T. Wamsat, H. Weddig, G. Weichert, H. Weigelt, N. Wentowski, C. Wiebers, T. Wilksen, A. Willner, K. Wittenburg, T. Wohlenberg, J. Wortmann, W. Wurth, M. Yurkov, I. Zagorodnov, and J. Zemella
Simultaneous operation of two soft x-ray free-electron lasers driven by one linear accelerator
New J. Phys., 18 :062002 (June 2016)
Abstract:
Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs — dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.
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J. Körner, F. Yue, J. Hein, and M.C. Kaluza
Spatially and temporally resolved temperature measurement in laser media
Opt. Lett., 41 :2525 (June 2016)
Abstract:
A technique to measure the spatially resolved temperature distribution in a laser medium is presented. It is based on the temperature dependence of the absorption cross section close to the zero-phonon line of the active medium. Since other materials in the beam path exhibit a high (and constant) transmission at this wavelength, the method can easily be applied in realistic amplifier setups. The method was successfully tested on three different samples, which were pumped by a pulsed laser diode with up to 150 W average power: side-cooled Yb:YAG and Yb:fluoride-phosphate glass at room temperature and face-cooled Yb:CaF2 at 120 K.
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C. Shah, P. Amaro, R. Steinbrügge, C. Beilmann, S. Bernitt, S. Fritzsche, A. Surzhykov, J. Crespo López-Urrutia, and S. Tashenov
Strong higher-order resonant contributions to x-ray line polarization in hot plasmas
Phys. Rev. E, 93 :061201 (June 2016)
Abstract:
We studied angular distributions of x rays emitted in resonant recombination of highly charged iron and krypton ions, resolving dielectronic, trielectronic, and quadruelectronic channels. A tunable electron beam drove these processes, inducing x rays registered by two detectors mounted along and perpendicular to the beam axis. The measured emission asymmetries comprehensively benchmarked full-order atomic calculations. We conclude that accurate polarization diagnostics of hot plasmas can only be obtained under the premise of inclusion of higher-order processes that were neglected in earlier work.
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V. Yu. Kharin, D. Seipt, and S. G. Rykovanov
Temporal laser-pulse-shape effects in nonlinear Thomson scattering
Phys. Rev. A, 93 :063801 (June 2016)
Abstract:
The influence of the laser-pulse temporal shape on the nonlinear Thomson scattering on-axis photon spectrum is analyzed in detail. Using the classical description, analytical expressions for the temporal and spectral structure of the scattered radiation are obtained for the case of symmetric laser-pulse shapes. The possibility of reconstructing the incident laser pulse from the scattered spectrum averaged over interference fringes in the case of high peak intensity and symmetric laser-pulse shape is discussed.
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A. A. Peshkov, V. G. Serbo, S. Fritzsche, and A. Surzhykov
Absorption of twisted light by a mesoscopic atomic target
Phys. Scripta, 91 :064001 (May 2016)
Abstract:
The excitation of a hydrogen-atom target by a twisted Bessel light beam is investigated. The atoms are assumed to have a Gaussian spatial distribution in the target. Theoretical analysis is performed within a nonrelativistic framework using a first-order perturbation approach and density matrix formalism. By using this theory, we derive the expressions for excitation cross sections and for alignment parameters of the excited atomic state. In particular, we make calculations for the 1s -> 2p transition caused by the interaction of Bessel beams with the atomic target. For this transition, we analyze the population of magnetic sublevels for the excited 2p state and study how it is affected by the projection of the total angular momentum of incident light. The calculations indicate that the projection of the total angular momentum of the incident Bessel beam affects the alignment of atoms for sufficiently small targets with size less than 200 nm. This can be observed experimentally by measuring the linear polarization of the subsequent fluorescent light.
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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
Nat. Photonics, 10 :445 (May 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.
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M. Kübel, C. Burger, N. Kling, T. Pischke, L. Beaufore, I. Ben-Itzhak, G.G. Paulus, J. Ullrich, T. Pfeifer, R. Moshammer, M. F. Kling, and B. Bergues
Complete characterization of single-cycle double ionization of argon from the nonsequential to the sequential ionization regime
Phys. Rev. A, 93 :053422 (May 2016)
Abstract:
Selected features of nonsequential double ionization have been qualitatively reproduced by a multitude of different (quantum and classical) approaches. In general, however, the typical uncertainty of laser pulse parameters and the restricted number of observables measured in individual experiments leave room for adjusting theoretical results to match the experimental data. While this has been hampering the assessment of different theoretical approaches leading to conflicting interpretations, comprehensive experimental data that would allow such an ultimate and quantitative assessment have been missing so far. To remedy this situation we have performed a kinematically complete measurement of single-cycle multiple ionization of argon over a one order of magnitude range of intensity. The momenta of electrons and ions resulting from the ionization of the target gas are measured in coincidence, while each ionization event is tagged with the carrier-envelope phase and intensity of the 4-fs laser pulse driving the process. The acquired highly differential experimental data provide a benchmark for a rigorous test of the many competing theoretical models used to describe nonsequential double ionization.
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G. Ma, W. Yu, M. Y. Yu, S. Luan, and D. Wu
Control of transmission of right circularly polarized laser light in overdense plasma by applied magnetic field pulses
Phys. Rev. E, 93 :053209 (May 2016)
Abstract:
The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.
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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
Europhys. Lett., 114 :35001 (May 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.
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N. A. Zubova, A. V. Malyshev, I. I. Tupitsyn, V. M. Shabaev, Y. S. Kozhedub, G. Plunien, C. Brandau, and T. Stöhlker
Isotope shifts of the 2p₃/₂-2p₁/₂ transition in B-like ions
Phys. Rev. A, 93 :052502 (May 2016)
Abstract:
Isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions are evaluated for a wide range of the nuclear charge number: Z=8–92. The calculations of the relativistic nuclear recoil and nuclear size effects are performed using a large-scale configuration-interaction Dirac-Fock-Sturm method. The corresponding QED corrections are also taken into account. The results of the calculations are compared with the theoretical values obtained with other methods. The accuracy of the isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions is significantly improved.
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F. Wagner, O. Deppert, C. Brabetz, P. Fiala, A. Kleinschmidt, P. Poth, V. A. Schanz, A. Tebartz, B. Zielbauer, M. Roth, T. Stöhlker, and V. Bagnoud
Maximum Proton Energy above 85 MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH₂ Targets
Phys. Rev. Lett., 116 :205002 (May 2016)
Abstract:
We present a study of laser-driven ion acceleration with micrometer and submicrometer thick plastic targets. Using laser pulses with high temporal contrast and an intensity of the order of 10^20  W/cm2 we observe proton beams with cutoff energies in excess of 85 MeV and particle numbers of 109 in an energy bin of 1 MeV around this maximum. We show that applying the target normal sheath acceleration mechanism with submicrometer thick targets is a very robust way to achieve such high ion energies and particle fluxes. Our results are backed with 2D particle in cell simulations furthermore predicting cutoff energies above 200 MeV for acceleration based on relativistic transparency. This predicted regime can be probed after a few technically feasible adjustments of the laser and target parameters.
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E. Siminos, S. Skupin, A. Sävert, J. M. Cole, S. P. D. Mangles, and M.C. Kaluza
Modeling ultrafast shadowgraphy in laser-plasma interaction experiments
Plasma Phys. Contr. F., 58 :065004 (May 2016)
Abstract:
Ultrafast shadowgraphy is a new experimental technique that uses few-cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.
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C. Kohlfürst, and R. Alkofer
On the effect of time-dependent inhomogeneous magnetic fields in electron–positron pair production
Phys. Lett. B, 756 :371 (May 2016)
Abstract:
Electron–positron pair production in space- and time-dependent electromagnetic fields is investigated. Especially, the influence of a time-dependent, inhomogeneous magnetic field on the particle momenta and the total particle yield is analyzed for the first time. The role of the Lorentz invariant E^2-B^2, including its sign and local values, in the pair creation process is emphasized.
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A. Otto, T. Nousch, D. Seipt, B. Kämpfer, D. Blaschke, A. D. Panferov, S. A. Smolyansky, and A. I. Titov
Pair production by Schwinger and Breit–Wheeler processes in bi-frequent fields
J. Plasma Phys., 82 :65582030 (May 2016)
Abstract:
Counter-propagating and suitably polarized light (laser) beams can provide conditions for pair production. Here, we consider in more detail the following two situations: (i) in the homogeneity regions of anti-nodes of linearly polarized ultra-high intensity laser beams, the Schwinger process is dynamically assisted by a second high-frequency field, e.g. by an XFEL beam; and (ii) a high-energy probe photon beam colliding with a superposition of co-propagating intense laser and XFEL beams gives rise to the laser-assisted Breit–Wheeler process. The prospects of such bi-frequent field constellations with respect to the feasibility of conversion of light into matter are discussed.
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R. H. Pratt, R. A. Müller, and A. Surzhykov
Sum rules for the polarization correlations in photoionization and bremsstrahlung
Phys. Rev. A, 93 :053421 (May 2016)
Abstract:
The polarization correlations in doubly differential cross sections are investigated for photoionization and ordinary bremsstrahlung. These correlations describe the polarization transfer between incident light and ejected photoelectrons as well as between an incoming electron beam and bremsstrahlung light, respectively. They are characterized by a set of seven real parameters Cij. We show that the squares of these parameters are connected by simple “sum rules.” These sum rules can be applied for both one-electron systems and also for atoms, if the latter are described within the independent particle approximation. In particular, they are exact in their simplest form (i) for the photoionization of K-, LI,II-, and MI,II-atomic shells, as well as (ii) for bremsstrahlung in which the electron is scattered into s1/2 or p1/2 states, as in the tip (bremsstrahlung) region. Detailed calculations are performed to verify the derived identities and to discuss their possible applications for the analysis of modern photoionization and bremsstrahlung experiments. In particular, we argue that the sum rules may help to determine the entire set of (significant) polarization correlations in the case when not all Cij are available for experimental observation.
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J. Rothhardt, C. Rothhardt, M. Müller, A. Klenke, M. Kienel, S. Demmler, T. Elsmann, M. Rothhardt, J. Limpert, and A. Tünnermann
100  W average power femtosecond laser at 343  nm
Opt. Lett., 41 :1885 (April 2016)
Abstract:
We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2<1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.
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C. Serrat, D. Roca, J. M. Budesca, J. Seres, E. Seres, B. Aurand, A. Hoffmann, S. Namba, T. Kuehl, and C. Spielmann
Avalanche of stimulated forward scattering in high harmonic generation
Opt. Express, 24 :8028 (April 2016)
Abstract:
Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.
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