Elastic and inelastic scattering of 58Ni +90,94Zr

Pure elastic and inelastic scattering cross sections have been measured for the systems ⁵⁸Ni +^90,94Zr at energies near the Coulomb barrier where not only quasi-elastic and fusion but also deep-inelastic process come into play. Coupled channels calculations including both projectile and target inelastic excitations can successfully explain the elastic and inelastic scattering angular distributions with an energy-independent semi-empirical bare potential. The calculation reproduces also the sum of the total quasi-elastic, fusion and deep-inelastic cross sections. Received: 14 September 1998 / Revised version: 21 October 1998     

Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Photorefractive multichannel correlator based on stoichiometric LiTaO3

We demonstrate a Vander Lugt type correlator based on photorefractive stoichiometric LiTaO₃ as the volume holographic material. Accurate correlator output was obtained for binary phase-modulated input images. The correlator is tested with binary amplitude- and phase-modulated input images. Phase-modulated images show a high-quality correlation, whereas amplitude-modulated images suffer in part from a photorefractive self-focusing effect. Using 100 previously stored holograms as filters, a rate of 10000 correlations/s are demonstrated. A paraxial theoretical analysis of the correlator, giving analytic expressions for the shift invariance, is also described in detail. Received: 22 November 2000 / Revised version: 9 February 2001 / Published online: 20 April 2001     

Performance of a diode-pumped 5 W Nd3+:GdVO4 microchip laser at 1.06 μm

4 as a host for neodymium has several advantages for diode pumping in comparison with other crystals. The absorption cross section of neodymium in GdVO₄ is considerably stronger and broader than in YAG. This allows for the construction of very compact monolithical microchip lasers. In our paper, we report for the first time on a diode-pumped monolithical Nd³⁺([%at.]1.3):GdVO₄ microchip laser at 1.06 μm. A maximum output power of 5 W is achieved. The temporal and the spectral emission properties are described. The beam propagation properties are studied in detail. Received: 23 July 1998 / Revised version: 9 November 1998 / Published online: 24 February 1999     

EUV and fast ion emission from cryogenic liquid jet target laser-generated plasma

A liquid jet of either nitrogen or argon of 20 μm diameter was exposed to intense laser fields with pulse durations between 70 fs and 250 ps, leading to intensities of 10¹⁶ W cm^-2 and 10¹³ W cm^-2, respectively. The emission of extreme UV light and soft X-rays shows the characteristic lines of hydrogen-like nitrogen and carbon-like argon. For nitrogen the emitted photon flux at 250 ps was about two orders of magnitude higher than for 70 fs pulses. A weak dependence on the laser polarization with respect to the liquid jet axis was found. The kinetic energy of the emitted ions easily exceeded 100 keV for nitrogen and 200 keV for argon for a pulse duration close to 2 ps. Received: 21 August 2000 / Revised version: 20 December 2000 / Published online: 22 March 2001     

Emission properties of a Tm30.1mm+:GdVO4 microchip laser at 1.9 μm

4 as a host for thulium has several advantages for diode pumping in comparison with other crystals. The absorption cross section of thulium in GdVO₄ is considerably stronger and broader than in YAG and YLF and the spectrum is shifted closer to the emission wavelength of commercially available AlGaAs laser diodes. In our paper, we report on the temporal and spectral emission properties of a monolithical Tm^30.01mm+([%at.]6.9):GdVO₄ microchip laser at 1.9 μm. The laser can be adjusted to emit either cw or in an oscillating regime. Slope efficiencies up to 47% are achieved. This value exceeds the Stokes limit of 42%. Received: 31 March 1998/Revised version: 02 June 1998     

High-dynamic range pulse-contrast measurements of a broadband optical parametric chirped-pulse amplifier

We report on the pulse contrast-ratio characterization of a few-cycle 0.2 TW optical parametric chirped pulse amplifier system operating at 20 Hz. A specially designed third-order correlator was used to characterize and optimize the contrast of the system. We demonstrate that the pulse contrast depends much more on the temporal overlap between the pump and the seed pulse than the shape of the amplified spectrum. The best amplified pulse contrast-ratio was 10^-4 at Δt=±25 ps and >10^-9 at Δt=±150 ps delays. PACS 42.65.Vj; 42.65.Re     

Comparative study of UV absorption changes induced in germanosilicate glass by high-intensity femtosecond pulses at 267, 400 and 800 nm

We investigated UV absorption changes induced in 3.5 mol% Ge-doped fused silica at high-intensity (∼10¹¹-10¹³ W/cm²) femtosecond (130 fs) irradiation at 267, 400 and 800 nm. We have shown that the induced spectra in the region 190-300 nm are similar in all three cases. At 800 nm irradiation, in addition to the UV absorption changes, we observed small-scale damage due to self-focusing. This damage appears when the incident pulse fluence value of about 1 J/cm² (pulse intensity of about 7.5 × 10¹² W/cm²) is overcome, while the threshold for the induced absorption changes is twice lower.     

Spectroscopic analysis and laser parameters of Nd3+ in Ca3Sc2Ge3O12 garnet crystals

3 Sc₂Ge₃O₁₂ (CaSGG) single crystals with the garnet structure have been grown by means of the flux growth technique. The doping with Nd³⁺ and Mg²⁺ (as charge compensator) yields samples suitable for optical spectroscopy experiments. The absorption and emission properties have been measured at temperatures ranging from 10 to 298 K. The emission spectra give evidence of the presence of non-equivalent Nd³⁺ sites. The decay time of the 1.06-μm emission band has been measured as a function of temperature and incident power. The intensities of the 298 K absorption transitions have been analyzed by means of the Judd–Ofelt theory. The radiative lifetimes, the branching ratios (β), and the spontaneous emission probabilities have been evaluated for the ⁴F_3/2 excited state using the calculated intensity parameters. The stimulated-emission cross sections and the branching ratios have been estimated from the experimental data for the most important laser transitions indicating that this crystal can be considered an interesting material for solid-state laser applications. Received: 2 June 1998 / Revised version: 28 October 1998 / Published online: 24 February 1999     

Excited state absorption and stimulated emission of Nd3+ in crystals. Part 2: YVO4, GdVO4, and Sr5(PO4)3F

3+ ion in the crystals YVO₄, GdVO₄, and Sr₅(PO₄)₃F. The measurements were performed in the spectral region of the main laser transitions ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2by a continuous wave pump and probe technique. The calibrated gain and ESA spectra are presented and possible implications of ESA on the laser performance are estimated. It is shown that ESA can be a small loss factor to the laser emission near 1060 nm but does considerably diminish the effective emission cross sections near 1340 nm especially in Nd:YVO₄ and Nd:GdVO₄. Received: 29 January 1998/Revised version: 8 May 1998     

Soft and hard Pomeron in the structure function of the proton at low x and low Q^2

We study inclusive electroproduction on the proton at low x and low using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, , proposed by López and Ynduráin. This extension allows to consider both the case and the region of higher on the basis of the same parametrization. A good simultaneous fit to the data on and on the cross section of real photoproduction is obtained for . With four parameters we achieve a for 222 data points. In addition, we use our model of the inclusive interaction to compute the longitudinal structure function . Received: 6 December 1998 / Revised version: 19 April 1999 / Published online: 18 June 1999     

Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

2 . Dynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation threshold, the ablated mass, and the particle size were investigated. The ablation plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm² the average initial velocity of the ejected particles was deduced from the extension of the plume to range from 120–400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopically determined absorption coefficient of cornea α_cornea. Where absorption due to proteins is high (at λ=6.2 and 6.5 μm), ablated mass, velocity and recoil momentum behave according to α_cornea. For the first time, variations of the ablation plume from pulse to pulse were observed. Those, as well as the particle size, not only depend on the absorption coefficient, but also on the predominant absorber. Received: 4 November 1997/Revised version: 7 September 1998     

Spectroscopic characterization of a Tm3+:SrGdGa3O7 crystal

3 O₇ crystal have been recorded at room temperature and at approximately 8 K. Room-temperature luminescence lifetimes of ¹D₂, ¹G₄, ³H₄, and ³F₄ states have been measured for Tm concentration ranging from [%at.]0.5 to [%at.]9. Based upon these data the crystal field splitting of luminescent states has been derived and radiative transitions rates have been evaluated. Strong self-quenching of luminescence originating in the ¹G₄ and ³H₄ states has been found in this activator–host combination. Peak emission cross section of the potential laser transition at 1800 nm was determined to be 3.9×10^-21 cm² and the gain coefficient versus wavelength was estimated and discussed. Received: 14 April 1998/Revised version: 10 August 1998     

Correlator of heavy- quark currents at small q2 in the large-$\beta_0$ limit

The correlator of vector heavy-quark currents at small q ² is considered in the large- limit. The leading IR renormalon ambiguity of the sum of the perturbative series is canceled by the UV renormalon ambiguity of the gluon condensate. The asymptotic behavior of the perturbative series is obtained in a model-independent way, up to a single unknown normalization factor. Gluon-virtuality distribution functions for the perturbative correction are calculated.Received: 3 December 2004, Published online: 21 February 2005     

Triple differential cross section of electron impact ionisation of Na(3s, 3p, 3d)

The fivefold differential cross section (5DCS) of the ionisation by electron impact of atomic sodium is determined theoretically for its fundamental 3s(² S) state and the excited 3p(² P) and 3d(² D) states by a procedure which employs in the transition matrix element of the first order Born approximation, the correlated double continuum (3C) wave function. This permits us to determine the statistical M-state population and the orientation and alignment tensors in (e,2e) detection. It is also shown that, the use of Gamow correlation term, in the independent particle (2C) model, reproduces, only in some situations, the shape of the angular distribution of the 5DCS obtained by the (3C) wave function. Received: 17 November 1997 / Received in final form: 16 March 1998 / Accepted: 21 March 1998     

An interpolation of the vacuum polarization function for the evaluation of hadronic contributions to the muon anomalous magnetic moment

We propose a simple parameterization of the two-point correlator of hadronic electromagnetic currents for the evaluation of the hadronic contributions to the muon anomalous magnetic moment. The parameterization is explicitly done in the Euclidean domain. The model function contains a phenomenological parameter which provides an infrared cutoff to guarantee the smooth behavior of the correlator at the origin in accordance with experimental data in e ⁺e^- annihilation. After fixing a numerical value for this parameter from the leading order hadronic contribution to the muon anomalous magnetic moment, the next-to-leading order results related to the vacuum polarization function are accurately reproduced. The properties of the four-point correlator of hadronic electromagnetic currents as for instance the so-called light-by-light scattering amplitude relevant for the calculation of the muon anomalous magnetic moment are briefly discussed. Received: 14 December 2001 / Published online: 7 June 2002     

Passive Q-switching of 1.44 μm and 1.34 μm diode-pumped Nd:YAG lasers with a V:YAG saturable absorber

Spectroscopic data of a V³⁺:YAG passive Q-switch crystal were measured. The absorption recovery time was determined to be of 37±7 ns and the ground state absorption cross section was estimated to be 0.7×10^-18 cm² at 1.44 μm and 3.5×10^-18 cm² at 1.34 μm. Passively Q-switched operation of diode pumped 1.44 μm and 1.3 μm Nd:YAG lasers was demonstrated using this crystal as saturable absorber. Average output powers of 1.42 W (1.44 μm) and 1.56 W (1.34 μm) and pulse energies of 24 μJ (1.44 μm) and 25 μJ (1.34 μm) were observed, respectively. Received: 19 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6281, E-mail: kretschmann@physnet.uni-hamburg.de     

Crystal lattice quantum computer

31 P nucleus can be used to represent a quantum bit (‘qubit’) with a relatively long relaxation time. In a CeP crystal lattice, ³¹P nuclei are periodically situated in three dimensions at distances of about 6 Å. The application of a static magnetic field gradient in one direction causes differences in the Zeemanfrequencies of separate nuclei. This allows thousands of distinct qubits to be individually addressed. Initializations of the qubits can be done efficiently by the Pound–Overhauser double resonance effect on the nuclear spins and the antiferromagnetically ordered 4f electron spins of cerium ions. Logic operations can be performed by simple pulse sequences, and computational results after logic operations can be measured by the nuclear magnetic resonance of neighboring nuclei, or the electron resonance of neighboring 4f electrons of cerium ions. Received: 26 October 1998/Accepted: 29 October 1998     

Properties of an ultrashort-pumped near-IR high gain dye amplifier

An ultra-short-pumped optical dye amplifier operating in the near-IR (720 nm) has been developed. We present an experimental study of the input–output characteristics of this simple device in a traveling wave collinear configuration. In essence, the combined effects of the ultrafast nature of the anisotropy induced by the exciting laser pulse along with the short transit time across the length of the cell allow for optimal output conditions given the number of molecules in the active volume. Two distinct amplifying regimes have been observed depending on photon density. Typical gain values of 10⁴ of the narrow bandwidth (∼9 nm) subpicosecond output signal were measured. The 8-mm² beam cross section enables this photon amplifier to be utilized in time-gated imaging applications. Received: 12 December 2001 / Revised version: 13 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-819/564-5442, E-mail: Daniel.Houde@Usherbrooke.ca     

Collinear and non-collinear sum-frequency mixing inβ-BBO for a tunable 195–198 nm all-solid-state laser system

We attained tunable UV laser radiation between 195 and 198 nm by sum-frequency mixing two synchronized flashlamp-pumped solid-state Q-switch lasers, a Nd:YAG laser frequency quadrupled to 266 nm and a tunable (730–770 nm) alexandrite laser. UV pulse energies of 0.12 mJ with repetition rates of 10 Hz were attained in collinear, as well as non-collinear sum-frequency interaction in a-Barium Borate (BBO) crystal with a conversion efficiency of 2.5%. Theoretical models for the non-collinear phase-matching interaction were investigated at UV wavelengths below 200 nm.