Femtosecond pump-probe experiments on non-stoichiometric sodium-fluoride clusters

In this paper we present two-color pump and probe spectroscopy on Na₂F, the smallest of the non-stoichiometric sodium-fluoride clusters (Na_nF_n-1), in molecular beams by employing femtosecond laser pulses. The molecules were pumped into the first excited state by one photonic transition and consecutively ionized from there by the second photon. We resolved the wavepacket oscillatory motion involving periodical structural rearrangements in the first excited state of Na₂F with a period of 185 fs. The time-resolved experiments show that sodium fluoride clusters provide interesting features which can be manipulated in optimal control experiments.     

X-ray laser experiments using double foil nickel targets

New target geometries for collisional excitation X-ray laser experiments (in nickel) were proposed, analyzed, and experimentally studied on the Glass Development Laser (GDL). Experiments using a short line focus lens with new target geometries showed general agreement with predictions. The new geometries are designed to yield a higher gain and reduced refraction due to: (1) a higher plasma density, (2) a wider lateral density profile, and (3) a concave lateral density profile. These new geometries were: (1) two parallel exploding (thin) foils, irradiated from one side only, (2) two ablating (thick) foils, one of which is irradiated on its inner face, and (3) an exploding foil in front of an ablating foil, irradiated by a single laser beam incident on the thin foil. New experiments with a long line focus are in progress. The intensity ratio of Ne-like and F-like Ni lines is used to deduce the temperature, and these results together with the absolute intensity, yield the density profile. The results show improvement achieved with the new target geometries: the density is higher (leading to a higher gain), and the concave density profile results in collimation rather than divergence of the X-ray laser beam. Theoretical developments included: (1) prediction of high gain on new type transitions in neon like ions involving the excitation of an inner (2s) electron, and (2) development of a ray tracing code for an amplifying medium of varying (e.g., collimating) lateral density profile. Results using this code are shown for concave lateral density profiles.     

Intense laser induced shock generation in gold targets

Abstract

Experiments carried out demonstrate the production due tosoft X-rays of enhanced ablation pressure. The targets used were planar gold foils. Aluminium targets were also used to demonstrate the relative effect.     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Two-color pump-probe experiments in helium using high-order harmonics

A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p ¹P and 1s3p ¹P states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of τ(1s2p) = 0.57 ns and τ(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6×10^-5 mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p ¹P and He 1s3p ¹P states in the threshold region are determined by measuring the saturation of the ionization process with a precision of ∼ 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections. Received 3 June 2002 / Received in final form 14 August 2002 Published online 22 October 2002 RID="a" ID="a"Present address: Department of Biophysics, Leiden University, 2333 CA Leiden, The Netherlands. RID="b" ID="b"Present address: Continuum Electro-Optics Inc., 3150 Central Expressway, Santa Clara, CA 95051, USA. RID="c" ID="c"Present address: CEA/DRECAM/SPAM, CEN Saclay, 91105 Gif-sur-Yvette, France. RID="d" ID="d"Present address: Department of Physics, National University of Rwanda, Butare, Rwanda. RID="e" ID="e"Present address: CELIA, Université Bordeaux 1, 33405 Talence, France. RID="f" ID="f"Present address: MAX-Lab, Lund University, Box 118, 221 00 Lund, Sweden. RID="g" ID="g"e-mail: anne.lhuillier@fysik.lth.se     

A proposed semiconductor laser pump-probe source

A model is presented which determines the output from a semiconductor laser as a function of the direct current (d.c.) and radio frequency (r.f.) current applied to the laser. It is shown that by adjusting these two components the output optical pulse from a laser can be shifted in time without change to the pulse shape or height. Experimental results are presented which support the model. It is suggested how these results can be used to produce a semiconductor pump-probe source, which is essential for the development of optical probing.     

Formation of microbumps and nanojets on gold targets by femtosecond laser pulses

Femtosecond laser-induced sub-wavelength microstructuring of a thin gold layer coated onto a quartz glass substrate is investigated. Formation of microbumps (microbubbles) and nanojets under single pulse laser irradiation is observed. Discussion of these effects and demonstration of their dependencies on the laser pulse energy and gold layer thickness are presented. PACS 42.62-b; 42.65.Re; 52.38.Mf     

Time-resolved stimulated emission spectroscopy in the ultrashort domain through pump-probe experiments

The photoemission properties of fluorescent chromophores have a widespread application in many fields ranging from chemical-physics and biology to organic light emitting devices. These systems usually display high fluorescence conversion efficiency, which makes them suitable for transient/gain experiments also in liquid solutions, thin films and eventually in protein environments.Pump and probe methods have been widely employed for wavelength-resolved spectroscopy in the subpicosecond time scale. In our group, we have recently assembled a new experimental setup for pump and probe spectroscopy: preliminary tests on the Rhodamine B dye in ethanol have been performed in order to optimize the setup. The dynamic response of photoinduced changes of the chromophore dispersed into a suitable solvent has been studied with a subpicosecond time resolution.The optically prepared initial state of the Rhodamine B in ethanol solution appears to evolve on a timescale of few picoseconds into a successive state, which could be attributed to an intramolecular charge transfer state.     

Effect of a plasma grating on pump-probe experiments near the ionization threshold in gases

Calculations are performed of the phase shift caused by the spatial modulation in the plasma density due to interference between a strong pump pulse and a weak probe pulse. It is suggested that a recent experiment [Opt. Express 17, 13429 (2009)] observed an effective birefringence from this plasma grating rather than from the higher-order Kerr effect.     

Theory of pump-probe experiments of metallic metamaterials coupled to a gain medium

We establish a new approach for pump-probe simulations of metallic metamaterials coupled to the gain materials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, we have studied light amplification of arrays of metallic split-ring resonators with a gain layer underneath. We find that the differential transmittance ΔT/T can be negative for split-ring resonators on the top of the gain substrate, which is not expected, and ΔT/T is positive for the gain substrate alone. These simulations agree with pump-probe experiments and can help to design new experiments to compensate for the losses of metamaterials.     

The analysis of surface processes on solid electrons by combined modulated ellipsometric and reflectometric experiments

The response of reflectometric and ellipsometric parameters to small changes in the properties of the metal-solution interphase is analysed by the use of linear approximation expressions. It is shown that the changes in R₆ and R_⊥ are generally closely related, and the ratio (δR/R_⊥ /δR/R_⊥∥ may be insensitive to the value of n?_film. On the other hand, the ratio δΔ/(δR/R_⊥)₆ is shown to have always good sensitivity to ^n?film. For two surface processes on solid electrodes, gold surface oxidation and hydrogen adsorption on platinum, it is demonstrated that the information extracted from the combined measurements of Δ and R transients is more detailed and informative compared with the evidence obtained by reflectometry alone. A fast component is resolved in the process of gold surface oxidation, and shown to result optically in an apparent metallic film, which probably reflects changes in the properties and structure of the metal surface atoms induced as the oxygen uptake commences.     

Filamentation on laser irradiated spherical targets

Filamentary structures have been observed in the corona plasma of spherical targets irradiated uniformly at intensities of 10¹²-10¹⁴ W cm^-2 by a 1.5 ns Nd glass laser pulse and probed by a 50 ps (FWHM) Raman shifted second harmonic probe pulse.     

Ultrashort-pulse sources based on single-mode rare-earth-doped fibers

An overview of ultrashort-pulse sources based on single-mode rare-earth-doped fibers is given. A wide range of pulse-generation schemes comprising mode-locked fiber lasers, parametric pulse sources and hybrid diode-fiber amplifier sources are discussed. Both actively and passively mode-locked fiber lasers are described and their specific merits and operation regimes are elucidated. Techniques for improving the spectral quality and the output powers of diode-based systems based on amplification in rare-earth-doped fibers are also reviewed. Finally, applications are discussed and directions for future research are indicated.     

The interpretation of radiation damage experiments on gold

The paper shows that the conclusions drawn from recent experiments on electron irradiation of gold at about 100° K should be reversed, i.e. that these experiments speak against the free thermally activated migration of interstitials at the irradiation temperature. It is shown that this viewpoint is in agreement with several related experiments, which may all be interpreted by a dynamic propagation of crowdions over distances between 1000 Å and 2000 Å. At intermediate and high defect concentrations the dynamic propagation mechanism is suppressed.     

Radioactive ion beam experiments with active targets

By the very nature of secondary beams, their intensity is limited, particularly for beams of the highest interest --farthest away from stability. Active targets, which can be described as time projection chamber (TPC)-like detectors in which the detector gas is the target, have been shown to have the highest sensitivity for quantitative high-resolution studies of rare events. The physics cases that can be addressed with these devices are reviewed and some of the first results obtained with first-generation active targets are detailed. Finally some general ideas on the next generation of active targets are presented     

The pump-probe approach to coherent backscattering of intense laser light from cold atoms

We present a new approach to near-resonant coherent backscattering of light from cold two-level atoms. In the dilute regime, where the distance between atoms is much larger than the laser wavelength, this approach is able to account for multi-photon scattering processes between many atoms through solutions of single-atom optical Bloch equations. We elaborate the method for double scattering from two atoms, and discuss the way of its extension for dilute, cold atomic clouds.     

Ultrashort-pulse Yb3+-fiber-based laser and amplifier system producing >25-W average power

We have demonstrated an ultrashort-pulse Yb3+-fiber laser and amplifier system that produces >400-nJ pulses at a repetition rate of 62 MHz (>25-W average power). The output pulses were recompressed to a duration of 110 fs with good pulse quality by use of a standard bulk grating-based compressor.