Efficient cascaded difference frequency conversion in periodically poled Ti:LiNbO3 waveguides using pulsed and cw pumping

Using an FM-mode-locked Ti:Er:LiNbO₃ waveguide laser as the fundamental source, wavelength conversion by cascaded χ⁽²⁾:χ⁽²⁾-difference frequency generation with a conversion efficiency of up to +3(-4.6) dB was demonstrated at a pulse repetition rate of about 2 (10) GHz. In addition, multi-channel conversion was demonstrated with a fully packaged wavelength converter using a continuous fundamental source. Received: 29 May 2001 / Revised version: 10 August 2001 / Published online: 2 November 2001     

Matrix elements of four-quark operators relevant to life time difference \Delta \Gamma _{B_s} from QCD sum rules

We extract the matrix elements of four-quark operators relevant to the and life time difference from QCD sum rules. We find that the vacuum saturation approximation works reasonably well, i.e., within 10%. We discuss the implications of our results and compare them with a recent lattice QCD determination. Received: 23 April 2001 / Revised version: 27 May 2001 / Published online: 19 July 2001     

Additive-pulse mode locking of a diode-pumped Nd3+:YVO4 laser

We demonstrate self-starting additive-pulse mode locking of a diode-pumped Nd³⁺:YVO₄ laser. Pulse durations of 2.7 ps are measured at a repetition frequency of 90.7 MHz and at an average output power of 1.1 W. This corresponds to a peak power of 4.5 kW. Received: 27 June 2001 / Revised version: 10 October 2001 / Published online: 29 November 2001     

Towards controlled coupling between a high-Q whispering-gallery mode and a single nanoparticle

We discuss our recent experiments that aim at the realization of coupling between a nano-emitter that is placed at the extremity of a sharp glass-fiber tip and a high-Q whispering-gallery mode. We quantify the influence of the tip using different probes and modes of a microsphere with different quality factors and mode extensions. Our measurements show that a micron-sized tip results in a substantial perturbation of the modes. On the contrary, by using a tip of diameter about 100 nm it should be possible to couple a nanoparticle to the most-confined modes of a microsphere without spoiling quality factors even as high as 10⁸. Received: 10 August 2001 / Revised version: 17 October 2001 / Published online: 23 November 2001     

500-Hz two-photon Ramsey fringes with a SF6 beam: towards a new frequency standard in the 30-THz spectral region

A two-photon Ramsey-fringe experiment with a supersonic beam of SF₆ has been performed with an interzone distance of up to 50 cm. Using a He-seeded beam with 50% of SF₆, the two-photon transition P(4)E⁰ in the 2ν₃ band reveals its magnetic hyperfine structure and the periodicity of the fringes is 500 Hz. The strength of the central fringe of the main hyperfine component corresponds to a flux of 10¹⁰–10¹¹useful molecules/s, which is very promising for a new frequency standard in the 30-THz spectral region. Received: 27 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Intensity clamping and re-focusing of intense femtosecond laser pulses in nitrogen molecular gas

We present results of measurements of fluorescence spectra due to the interaction of a Ti:sapphire laser pulse with N₂ molecules at different gas pressures and pulse energies. The analysis of the data together with the results of numerical simulations, using a propagation model, reveal signatures of the phenomena of intensity clamping and of re-focusing of the laser pulse at high gas pressure. The laser pulse energy for intensity clamping as a function of the gas pressure is determined. Received: 21 May 2001 / Revised version: 10 July 2001 / Published online: 19 September 2001     

An experimental confirmation of longitudinal electrodynamic forces by N. Graneau,T. Phipps Jr,and D. Roscoe

Eur. Phys. J. D 15, 87 (2001) Received 12 January 2001 Published online 10 December 2002 RID="a" ID="a"e-mail: aerobson@cstone.net     

Electrically switchable reflection holograms formed using two-photon photopolymerization

Two-photon holographic photopolymerization was used to form switchable Bragg gratings composed of layers of phase-separated liquid-crystal (LC) domains interspersed with cured, crosslinked polymer. These holographic polymer-dispersed liquid crystals form a periodic structure which diffracts red light due to nanostructured planes ∼250 nm in spacing. These structures were formed by interfering two 90-fs pulses coherently upon a reactive syrup consisting of acrylate monomer, liquid crystal, and a two-photon dye. The large two-photon cross-section allows excitation of the two-photon dye that results in electron transfer between this dye and the monomer. Diffraction efficiencies of approximately 10% were obtained, which can be modulated using an electric field applied across the film. Switching speeds below 1 ms were observed due in part to the small size of the LC domains. Received: 10 April 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Effect of gas pressure on the growth and structure of carbon nanotubes by chemical vapor deposition

The effect of gas pressure on the structure of carbon nanotubes (CNTs) has been systematically investigated in the chemical vapor deposition process. The yield of CNTs (defined as the weight ratio of CNTs vs. catalyst) increases significantly with the gas pressure, reaches 600% at 600 Torr, then decreases with further increase of gas pressure. At low reacting gas pressure the CNTs have completely hollow cores, whereas at high pressure the CNTs have a bamboo structure. The density of the compartments in the bamboo-structured CNTs increases dramatically with the increase of the gas pressure. This result shows that the structure and yield of carbon nanotubes are strongly affected by the growth gas pressure. Received: 10 May 2001 / Accepted: 10 May 2001 / Published online: 20 June 2001     

Chaotic sources and percolation of strings in heavy ion collisions

The chaoticity parameter of Bose–Einstein correlations is studied as a tool for analyzing the interaction between color strings in multiparticle production at high energies. Different scenarios of this interaction lead to a different behavior of with energy and atomic number of the participants. Comparison to the present experimental data favors the percolation of strings scenario. The one of its versions in which shows a peculiar dependence on the string density, very similar to the dependence of the fractional average cluster size, looks particularly attractive. Received: 19 February 2001 / Revised version: 5 July 2001 / Published online: 10 August 2001     

Optical limiting in cobalt-doped polyvinylpyrrolidone

The results of optical limiting investigations of cobalt-doped polyvinylpyrrolidone solutions are presented. The optical limiting studies have shown that this process is due to self-defocusing at the wavelength of 1064 nm and also due to reverse saturable absorption and self-defocusing at the wavelength of 532 nm. The results of measurements on the non-linear optical characteristics of organometallic complexes are presented. Received: 10 July 2001 / Revised version: 15 October 2001 / Published online: 29 November 2001     

Optical oscillation modes of plasmon particles observed in direct space by phase-contrast near-field microscopy

Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (<10 nm) and with simultaneous amplitude and phase contrast. The images depict the optical oscillation patterns of single plasmon particles. Examples are presented of particles excited in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for non-linear and single-molecule spectroscopy applications. Received: 20 June 2001 / Revised version: 3 August 2001 / Published online: 19 September 2001     

The role of sample rotation and oblique ion incidence on quantum-dot formation by ion sputtering

Starting from a theory recently proposed by Kahng et al. that explains the formation of ordered dots by ion sputtering under normal ion incidence, it was demonstrated that extending this theory to ion sputtering under oblique ion incidence with simultaneous sample rotation offers the self-organized formation of dots by ion sputtering for a large variety of different ion/material combinations. While for sputtering under normal incidence the shape of the collision cascade must be anisotropic, where the lateral straggling exceeds the longitudinal straggling, this constraint is not necessary under oblique incidence. Received: 16 May 2001 / Accepted: 10 September 2001 / Published online: 17 October 2001     

Probabilities of rapidity gaps in high energy interactions

We show that high energy hadronic reactions which contain a rapidity gap and a hard subprocess have a specific dependence on the kinematic variables, which results in a characteristic behaviour of the survival probability of the gap. We incorporate this mechanism in a two-channel eikonal model to make an essentially parameter-free estimate of diffractive dijet production at the Tevatron, given the diffractive structure functions measured at HERA. The estimates are in surprising agreement with the measurements of the CDF collaboration. We briefly discuss the application of the model to other hard processes with rapidity gaps. Received: 16 May 2001 / Revised version: 12 July 2001 / Published online: 10 August 2001     

Heterolytic dissociation under hot-electron bombardment of the silanic bond at the Si-SiO2 interface

An atomistic model is proposed to account for the threshold-voltage instability of mos transistors under hot-electron bombardment. High-level density-functional-theory calculations for a set of model molecules simulating the Si-SiO₂ interface show that in the presence of one adsorbed water molecule the diradical center (resulting from the homolytic cleavage by hot-electron impact of the silanic bond at the hydrogen-passivated interface trap) evolves spontaneously to the formation of a positively charged defect at the oxide side of the Si-SiO₂ interface. Received: 10 January 2001 / Accepted: 19 June 2001 / Published online: 30 August 2001     

Generation of high-quality 10-femtosecond laser pulses with intracavity spatial and spectral control

Spatial and spectral control, using an intracavity capillary and a slit, is applied to improve the output pulse quality of a Ti:sapphire laser. Satellite-free 10-fs optical pulses with a smooth spectral and spatial profile have been generated. Employing a root-mean-square formalism for pulse characterization, spatial, spectral and temporal intensity distributions are analyzed for laser pulses with a duration as short as three to four optical cycles. Received: 11 June 2001 / Published online: 18 July 2001     

Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

Construction of Quasi-Periodic Breathers¶via KAM Technique

By developing a KAM theorem which involves an infinitely multiple normal frequency, it is shown that there are plenty of breathers, quasi-periodic in time and super-exponentially localized in space, for the networks of weakly coupled oscillators. This answers an open problem by Aubry [A2] in case the linearized system has no continuous spectrum. Received: 29 March 2001 / Accepted: 10 October 2001     

Pulsed laser deposition of conductive metallo-dielectric optical filters

We describe the fabrication by room-temperature pulsed laser deposition of a transparent conductor comprising alternating layers of silver and aluminum oxide, forming a metallo-dielectric filter. Transmittances of 0.7 over specific wavelength bands were achieved with resistivities as low as 6.0×10^-6 Ω cm, almost two orders of magnitude lower than that of the best single-substrate thin films, such as indium tin oxide. The resistivity can be predicted without adjustable parameters and designed using a simple parallel-circuit model; the optical properties are well described by standard matrix transmission calculations. This demonstrates that pulsed laser deposition may be used to fabricate prototypes of high-quality transparent conductors with predictable properties for conducting windows where low-temperature deposition is critical, as in organic light-emitting diodes and for non-linear optical films. Received: 10 June 2001 / Accepted: 9 August 2001 / Published online: 20 December 2001     

Kilohertz extreme-ultraviolet light source based on femtosecond high-order harmonic generation from noble gases

A compact, versatile table-top kilohertz source of coherent extreme-ultraviolet (XUV) radiation in the wavelength region 18–100 nm, based on high-order harmonic generation from noble gases induced by a 40-femtosecond Ti:sapphire laser system, is presented. The XUV beamline delivers at its output 10⁸ photons/s at a wavelength of 23 nm. The monochromatized XUV radiation is directly focused onto a 10^-2-mm² spot by a toroidal grating, allowing one to reach intensities higher than 10⁶ W/cm². Optimization results are presented for a new XUV-generating geometry, utilizing a ‘semi-infinite’ quasi-static gas cell and strong focusing. In those conditions, we observe an anomalous inversion between the cutoffs of argon and krypton, with the krypton spectrum extending to much higher orders than expected in an adiabatic limit. Received: 9 July 2001 / Revised version: 1 August 2001 / Published online: 7 November 2001