Spatial beam profiles of femtosecond X-ray pulses generated by laser Compton scattering from a low-emittance electron beam

Polarization-dependent spatial beam profiles of femtosecond X-ray pulses generated by a laser Compton scheme were measured. The X-ray pulses were generated by the interaction at an angle of 90° between 100-fs laser light and a 3-ps, 3π-mm mrad electron beam. The polarization of the laser light was linear in two different directions, either parallel or perpendicular to the electron beam axis. The measured profiles showed good agreement with theoretical results. Received: 5 July 2002 / Revised version: 17 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-424/684477, E-mail: msf_yorozu@shi.co.jp     

Characterization and control of phase fluctuations in elongated Bose–Einstein condensates

Quasi-one-dimensional Bose–Einstein condensates (BECs) in elongated traps exhibit significant phase fluctuations even at very low temperatures. We present recent experimental results on the dynamic transformation of phase fluctuations into density modulations during time of flight and show the excellent quantitative agreement with the theoretical prediction. In addition we confirm that, under our experimental conditions, in the magnetic trap density modulations are strongly suppressed even when the phase fluctuates. We also discuss our theoretical results on control of the condensate phase by employing a time-dependent perturbation. Our results set important limitations on future applications of BECs in precision atom interferometry and atom optics, but at the same time suggest pathways to overcome these limitations. Received: 17 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/762-3023, E-mail: Helge.Kreutzmann@ITP.uni-hannover.de     

Simple parameterization of dark-resonance line shapes

We have studied the dispersion and absorption line shapes of coherent population trapping resonances (dark resonances) in thermal cesium vapor. Outside the time-of-flight regime the line shapes can be described by a simple phenomenological model: a sum of a dispersive and an absorptive Lorentzian of identical width and position, whose relative weights depend on the detuning of the laser frequencies from the optical resonance. This functional form can be derived from a theoretical model for the full multilevel structure of an alkali atom. The analytical expressions for the parameters of the line shape model show the same behavior as the experimental data. Received: 4 October 2002 / Revised version: 30 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Fundamental limits for coherent manipulation on atom chips

The limitations for the coherent manipulation of neutral atoms with fabricated solid-state devices, so-called ‘atom chips’, are addressed. Specifically, we examine the dominant decoherence mechanism, which is due to the magnetic noise originating from the surface of the atom chip. It is shown that the contribution of fluctuations in the chip wires at the shot-noise level is not negligible. We estimate the coherence times and discuss ways to increase them. Our main conclusion is that future advances should allow for coherence times as long as 1 s, a few μm away from the surface. Received: 22 August 2002 / Revised version: 30 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. E-mail: carsten.henkel@quantum.physik.uni-potsdam.de     

100-W average-power, high-energy nanosecond fiber amplifier

We report on the fiber-based amplification of a Q-switched Nd:YAG thin-disk laser. At repetition rates between 3 and 50 kHz output powers up to 100 W are generated. Pulse energies up to 4 mJ, with diffraction-limited beam quality, are generated in a 30-μm Yb-doped large-mode-area fiber, furthermore pulse energies up to 8 mJ are achieved from a multimode fiber amplifier. Received: 8 May 2002 / Revised version: 10 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/65-7680, E-mail: Jens.Limpert@uni-jena.de     

Laser-induced nano-patterning by means of interference subpatterns generated by microspheres

Interference patterns generated by a regular lattice of SiO₂ microspheres on a transparent support are used for the surface patterning of polyimide (PI) foils. Using 248 nm excimer-laser radiation, thousands to millions of holes with a full width at half maximum (FWHM) of 160±40 nm can be generated with a single laser pulse. Received: 1 March 2002 / Accepted: 4 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at     

High-contrast Mach–Zehnder lithium-atom interferometer in the Bragg regime

We have constructed an atom interferometer of the Mach–Zehnder type, operating with a supersonic beam of lithium. Atom diffraction uses Bragg diffraction on laser standing waves. With first-order diffraction, our apparatus has given a large signal and a very good fringe contrast (74%), which we believe to be the highest ever observed with thermal atom interferometers. This apparatus will be applied to high-sensitivity measurements. Received: 22 November 2001 / Revised version: 10 February 2002 / Published online: 24 April 2002     

Bose–Einstein condensates in magnetic waveguides

In this article, we describe an experimental system for generating Bose–Einstein condensates and controlling the shape and motion of a condensate by using miniaturised magnetic potentials. In particular, we describe the magnetic trap setup, the vacuum system, the use of dispenser sources for loading a high number of atoms into the magneto-optical trap, the magnetic transfer of atoms into the microtrap, and the experimental cycle for generating Bose–Einstein condensates. We present first results on outcoupling of condensates into a magnetic waveguide and discuss influences of the trap surface on the ultra-cold ensembles. Received: 21 August 2002 / Revised version: 10 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-7071/295-829, E-mail: fortagh@pit.uni-tuebingen.de     

Geometry,commutation relations and the quantum fictitious force

We express the commutation relation between the operators of the momentum and the radial unit vectors in D dimensions in differential and integral form. We connect this commutator with the quantum fictitious potential emerging in the radial Schr?dinger equation of an s-wave. Received: 6 August 2002 / Revised version: 30 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-3086, E-mail: markus.cirone@physik.uni-ulm.de     

Hybrid laser processing for microfabrication of glass

Hybrid laser processing for the precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and a medium on the material surface leads to effective ablation and modification, is reviewed. A major role of the medium is to produce strong absorption of the conventional laser beam by the material. Simultaneous irradiation by a vacuum ultraviolet (VUV) laser beam that possesses an extremely small laser fluence and an ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused silica (VUV-UV multiwavelength excitation process). The metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials by the same laser beam, resulting in microstructuring, cutting, color marking, printing, and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion presented here includes the ablation mechanism of hybrid laser processing. Received: 18 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: ksugioka@postman.riken.go.jp     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

High-peak-power diode-pumped passively Q-switched Nd:YVO<Subscript>4</Subscript> laser

We report on a passively Q-switched diode-pumped Nd:YVO₄ laser polarized along the a axis (corresponding to the smallest value of emission cross section at 1064 nm), generating 157-μJ pulses with 6.0-ns time duration (>20 kW peak power) and 3.6 W of average power at 1064 nm with good beam quality (M²<1.4). The selection of the polarization was performed by a novel technique relying on the birefringence of the laser crystal and on the misalignment sensitivity of the resonator. Received: 30 September 2002 / Revised version: 22 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +39-382/422583, E-mail: agnesi@ele.unipv.it     

3-D microstructuring inside photosensitive glass by femtosecond laser excitation

We show that a femtosecond laser enables us to produce true three-dimensional (3-D) microstructures embedded in a photosensitive glass, which has superior properties of transparency, hardness and chemical and thermal resistances. The photosensitivity arises from the cerium in the glass. After exposure to a focused laser beam, latent images are written. Modified regions are developed by a post-baking process and then preferentially etched away in a 10% dilute solution of hydrofluoric acid at room temperature. We have measured the critical dose for modification of the photosensitive glass, and fabricated 3-D microstructures with microcells and hollow microchannels embedded in the glass based on the critical dose. Received: 12 August 2002 / Accepted: 13 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/468-4682, E-mail: mmasudaw@postman.riken.go.jp     

Trace-gas sensing in the 3.3-μm region using a diode-based difference-frequency laser photoacoustic system

A compact, diode-based difference-frequency laser system combined with a photoacoustic detection scheme is presented for trace-gas sensing. It features a broad, continuous tuning range (3.2–3.7 μm), a narrow line width (154 MHz), and room-temperature operation, and thus allows numerous gas species to be measured both isolated and in mixtures of different gases. Several trace-gas species of environmental interest were detected, and gas mixtures were analysed. The detection limits are in the low-ppmV range, e.g. 1.3 ppmV for methane, 1.8 ppmV for ethane, and 1.2 ppmV for hydrogen chloride. Received: 10 April 2002 / Revised version: 5 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: Sigrist@iqe.phys.ethz.ch     

Seven-terawatt Ti:sapphire laser system operating at 50 Hz with high beam quality for laser Compton femtosecond X-ray generation

We present a 7-TW Ti:sapphire laser system operating at 50 Hz for laser Compton femtosecond X-ray generation. This laser system delivers 8.4 W of average output power at a repetition rate of 50 Hz with a pulse width of 24 fs. It demonstrates successful management using a dynamically stable resonator in the regenerative amplifier and compensation for thermal lensing by a convex mirror in a ring-type four-pass power amplifier. We also present the results of closed-loop corrections for distorted wavefronts of amplified and compressed laser pulses, using an adaptive optical system consisting of a Shack–Hartmann-type wavefront sensor and a deformable mirror. This closed-loop correction results in dramatic improvements, reducing wavefront distortions below 0.05 λ rms. Received: 31 October 2002 / Revised version: 3 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-3349, E-mail: ito@festa.or.jp     

Optical switching of 2-(2<Superscript>′</Superscript>-hydroxyphenyl) benzoxazole in different solvents

All-optical switching and beam deflection of 2-(2^′-hydroxyphenyl) benzoxazole (HBO) in three species of solvent (cyclohexane, ethanol and dimethyl sulfoxide) have been investigated by using third-harmonic generation (355 nm) of a mode-locked Nd:YAG laser as a pump beam and a continuous-wave He-Ne laser (632.8 nm) as a probe beam. The nonlinear refractive indices of HBO in different solvents are determined by using the Z-scan technique. The optical switching and beam-deflection effects are due to the change of the refractive index of HBO under the pump beam. Through the study of the absorption spectra and the fluorescence spectra of HBO in different solvents, we conclude that the principal reason for the change of the refractive index of HBO is not the thermal effect because of absorption of the pump beam, but the excited-state intramolecular proton-transfer (ESIPT) effect of HBO under the pump beam. As the ESIPT process is very fast, HBO might be an excellent material for high-speed optical switching. Received: 28 October 2002 / Revised version: 18 March 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-22/2350-1242, E-mail: zhanggl@nankai.edu.cn     

The laser pulse from a rf waveguide CO2 laser

The behaviour of a rf-excited waveguide CO₂ laser in the pulse regime is studied experimentally. The specific time sections of the pulse have been defined and described in detail. The output pulse evolution versus input power, pressure of the laser mixture, output coupling, repetition frequency and pulse width has been investigated. The spectral content of the pulse has been analysed. A model explaining the effect of laser tuning during the pulse duration is given. The ultrahigh pulse (UHP), defined as the pulse for which the bulge effect occurs, is predicted. Received: 22 October 2002 / Revised version: 13 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +48-71/3203-189, E-mail: eda@zr.ita.pwr.wroc.pl     

Table-top KrF amplifier delivering 270 fs output pulses with over 9 W average power at 300 Hz

Applying the combination of a solid-state Ti:Sa laser system and a newly developed wide-aperture, discharge-pumped KrF amplifier, output pulses with over 9 W average power at 300 Hz have been achieved in a single output beam. The frequency-tripled seed pulses of the Ti:Sa system – delivering approximately 10 μJ energy at 248 nm – were amplified to over 30 mJ using a 3-pass off-axis amplification scheme. The optical set-up has been fitted to the amplifier’s parameters, and stored-energy measurements were carried out with different parameters in order to optimize the operational conditions of the device for the highest average power. Received: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503-599, E-mail: jbekesi@llg.gwdg.de     

Early stages of laser graphitization of diamond

A mechanism for photographitization of a free diamond surface is proposed. The quantum-kinetic rate of this process is determined. The graphitization rate is close to zero if the activation energy of the graphitization process is taken as being equal to the binding energy of a carbon atom with the surface (i.e. equal to the sublimation energy of a carbon atom). On the contrary, if the activation energy is close to the energy of C–C bonds, the graphitization process may occur at a noticeable rate and be observed under ‘relatively smooth’ experimental conditions. The temperature rise leads to a considerable increase in the graphitization rates. Preliminary experimental data on the low-rate laser ablation of diamond are presented to support the proposed model of photographitization. An early stage of laser-induced graphitization in the bulk of diamond is also considered. It is found that the nucleation of a ‘tiny graphite drop’ is possible in the bulk of the diamond inside the focal area of a laser beam; the ‘graphite drop’ growth causing the appearance of mechanical stresses in the surrounding regions. The maximum size of the graphite drop is determined, which, when exceeded, leads to mechanical damage of the sample and to a change in the mechanism of laser graphitization. An evident mechanical criterion for laser-induced damage of diamond is proposed. Received: 2 October 2002 / Accepted: 5 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail: stvn@stankin.ru     

Ultrahigh-efficiency TEM<Subscript>00</Subscript> diode-side-pumped Nd:YVO<Subscript>4</Subscript> laser

Ultrahigh-efficiency TEM₀₀ operation is demonstrated in a diode-pumped Nd:YVO₄ laser in a bounce amplifier geometry using a specially designed astigmatically optimised cavity configuration. Optical efficiency >68% is demonstrated and up to 27.1 W of output power for multimode operation. For single-mode TEM₀₀ operation, an output power of 23.1 W for 39.5 W of diode pumping was produced with beam propagation parameters of M_x ²=1.3 and M_y ²=1.1. Received: 10 October 2002 / Revised version: 9 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: a.minassian@ic.ac.uk