Magnetic field effect on the induction of an electric microfield in potassium vapor partially ionized by a laser pulse

A laser-induced electric microfield in potassium vapor partially ionized by laser radiation and placed in an external magnetic field has been studied experimentally. It was found that the dependence of the electric field rise time on the magnetic field strength had a peak value when the plasma frequency is equal to the Larmor frequency. This finding allows us to measure the photoelectron density in weakly ionized potassium vapor. The results are explained in terms of a simple model that shows how the length of the photoelectron trajectory changes with the magnetic field. Received: 4 February 2002 / Revised version: 20 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +374-32/31172, E-mail: rdramp@ipr.sci.am     

Thermal and carrier transport originating from photon recycling and non-radiative recombination in laser micromachining of GaAs thin films

Coupled thermal and carrier transports (electron/hole generation, recombination, diffusion and drifting) in laser photoetching of GaAs thin film is investigated. A new volumetric heating mechanism originating from SRH (Shockley–Read–Hall) non-radiative recombination and photon recycling is proposed and modeled based on recent experimental findings. Both volumetric SRH heating and Joule heating are found to be important in the carrier transport, as well as the etching process. SRH heating and Joule heating are primarily confined within the space-charge region, which is about 20 nm from the GaAs surface. The surface temperature rises rapidly as the laser intensity exceeds 10⁵ W/m². Below a laser intensity of 10⁵ W/m², the thermal effect is negligible. The etch rate is found to be dependent on the competition between photovoltaic and photothermal effects on surface potential. At high laser intensity, the etch rate is increased by more than 100%, due to SRH and Joule heating. Received: 24 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-310/206-2302, E-mail: xiang@seas.ucla.edu     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Thermal tuning and modulation of a DFB fibre laser with a thin-film heater

Results are given for thermal tuning and modulation of a 1556-nm distributed feedback fibre laser by resistive heating of a thin silver film chemically deposited on the fibre. Without reaching the limits of performance, linear tuning is demonstrated at a rate of 1.72 pm/mW up to about 200 pm, and a peak-to-peak modulation of 100 MHz up to modulation frequencies of 60 Hz. The heat flow is analyzed, and the coated fibre is characterized in terms of the static and dynamic wavelength response to the applied electric power. The performance of the scheme is tested by recording part of the ν₁+ν₃ combination band spectrum of ¹³C₂H₂ with thermal modulation and scanning of the fibre laser. Received: 12 March 2002 / Revised version: 24 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +45/4593-1137, E-mail: sus@dfm.dtu.dk     

Studies of high-repetition-rate laser plasma EUV sources from droplet targets

The water droplet laser plasma source has been shown to have many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray applications. Through a dual experimental and theoretical study, we analyze the interaction physics between the laser light and the target. The hydrodynamic laser plasma simulation code, Medusa103 is used to model the electron density distribution for comparison to electron density distributions obtained through Abel inversion of plasma interferograms. In addition, flat field EUV spectra are compared to synthetic spectra calculated with the atomic physics code RATION. Received: 31 October 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Present address: Naval Reseach Laboratory, Washington D.C. RID="**" ID="**"Present address: Xtreme Technologies, G?ttingen, Germany. RID="***" ID="***"Corresponding author. Fax: +1-407/823-3570, E-mail: mrichard@mail.ucf.edu     

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     

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength. Received: 3 September 2002 / Accepted: 4 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-778/62-3306, E-mail: yasuma@fukui-nct.ac.jp     

Observation of light filaments induced in air by visible picosecond laser pulses

We report the observation of light filaments produced by a picosecond laser pulse in the visible. The pulse trapped in the filamentary mode experiences large-scale self-phase modulation, with almost 100-fold spectral broadening along with apparent shortening of the leading edge. Spatial-temporal properties of the light filament reveal rather complex propagation dynamics. Received: 7 August 2002 / Revised version: 12 September 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +370-2/3660-06, E-mail: audrius.dubietis@ff.vu.lt     

Transmission electron microscopy studies of femtosecond laser induced modifications in quartz

Cross-sectional transmission electron microscopy investigations of femtosecond laser induced sub-micrometer structural modifications inside crystalline quartz were carried out. Modifications from single laser shots and from lines built of overlapping shots were imaged. Both single laser shot modifications and line structures show an amorphous core surrounded by a disturbed crystalline structure. A strong strain field surrounding the central, irradiated, core is responsible for an increase of the refractive index. Finite element method calculations of the strain field show maxima on both sides of the irradiated core, which are in good agreement with optical measurements of the refractive-index change. Received: 29 September 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/947792, E-mail: tatiana.gorelik@rz.uni-jena.de     

Laser induced decomposition of a designed and a commercial polymer studied by ns-interferometry and shadowgraphy

Commercially available polymers often exhibit quite poor laser ablation properties for irradiation wavelengths around 248 nm. At these wavelengths, the absorption is due to photostable aromatic groups. Photolabile triazene polymers were developed to compare the influence of a photolabile group on the laser ablation process. The photochemically active triazene group has a strong absorption band at 332 nm, whereas the second absorption maximum at 220 nm is due to the photostable aromatic group. By irradiating at 308 nm and 193 nm, the influence of the photochemically active group on the ablation process can be studied. The etching of the triazene polymer starts and ends with the laser pulse. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than with 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer compared with polyimide. Received: 28 August 2002 / Accepted: 20 September 2002 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/310-4412, E-mail: thomas.lippert@psi.ch     

Hot-electron-induced plasma formation on the rear surface of a foil

The plasma jet formed on the rear surface of a foil in laser–solid interaction is investigated by laser probing. The rear plasma jet, which is in line with the laser, formed a few picoseconds after the incidence of the focused laser, is due to a beam of fast electrons propagating through the target and is collimated by a strong magnetic field in the plasma. Received: 14 January 2003 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531 E-mail: jzhang@aphy.iphy.ac.cn     

Spectral broadening of 40-fs Ti:sapphire laser pulses in photonic-molecule modes of a cobweb-microstructure fiber

Photonic-molecule modes of a cobweb-microstructure fiber allow efficient nonlinear optical spectral broadening of nanojoule femtosecond light pulses. Spectral widths of approximately 200 nm are achieved at the output of a 6-cm sample of such a fiber for 40-fs Ti:sapphire laser pulses with an energy of a few nanojoules coupled into photonic-molecule modes. Higher values of the group index and a lower group-velocity dispersion, attainable with higher-order photonic-molecule modes, allow the efficiency of spectral broadening of femtosecond laser pulses to be increased relative to the efficiency of spectral broadening in the fundamental photonic-molecule mode. Received: 9 June 2002 / Revised version: 29 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/939-3959, E-mail: zheltikov@top.phys.msu.su     

Growth of a textured quasicrystalline phase in Ti-Ni-Zr films prepared by pulsed laser deposition

The preparation in thin film form of the known icosahedral phase in Ti-Ni-Zr bulk alloys has been investigated as a function of substrate temperature. Films were deposited by pulsed laser deposition on sapphire substrates at temperatures ranging from room temperature to 350 °C. Morphological and structural modifications have been followed by grazing-incidence and θ–2θ X-ray diffraction, transmission electron diffraction and imaging. Chemical composition has been analyzed by electron probe microanalysis. The in-depth variation of composition has been studied by secondary neutral mass spectroscopy. We show that pulsed laser deposition at 275 °C makes the formation of a 1-μm-thick film of Ti-Ni-Zr quasicrystalline textured nanocrystallites possible. Received: 7 June 2001 / Accepted: 18 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +33-3/8357-6300, E-mail: brien@mines.u-nancy.fr     

Time-resolved photoemission electron microscopy of magnetic field and magnetisation changes

Owing to its parallel image acquisition, photoemission electron microscopy is well suited for real-time observation of fast processes on surfaces. Pulsed excitation sources like synchrotron radiation or lasers, fast electric pulsers for the study of magnetic switching, and/or time-resolved detection can be utilised. A standard approach also being used in light optical imaging is stroboscopic illumination of a periodic (or quasi-periodic) process. Using this technique, the time dependence of the magnetic field in a pulsed microstrip line has been imaged in real time exploiting Lorentz-type contrast. Similarly, the corresponding field-induced changes in the magnetisation of cobalt microstructures deposited on the microstrip line have been observed exploiting magnetic X-ray circular dichroism as a contrast mechanism. The experiment has been performed at the UE 56/1-PGM at BESSY II (Berlin) in the single-bunch mode. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: krasyuk@mail.uni-mainz.de     

Theory of optical bistability in a non-linear quasi-waveguide

Optical bistable behavior in a quasi-waveguide containing non-linear film has been experimentally investigated in several publications in the past years; however, the physical mechanism for optical bistability has not been theoretically explained. In the present letter, we propose a theoretical model and successfully explain the observed optical bistability of both the reflected light and the scattered light (m-lines) in a non-linear quasi-waveguide. The optical bistability in the non-linear quasi-waveguide is due to the scattering-type wavevector mismatch mechanism. Received: 10 July 2002 / Revised version: 10 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: htwang@nju.edu.cn     

Poincaré representation of polarization-shaped femtosecond laser pulses

The usage of Poincaré phase space for the representation of polarization-shaped femtosecond laser pulses is discussed. In these types of light fields the polarization state (i.e. ellipticity and orientation) changes as a function of time within a single laser pulse. Such deliberate variation can be achieved by frequency-domain femtosecond pulse shaping in which two polarization components are manipulated individually. Here it is shown how these light pulses can be represented as temporal trajectories through the ellipticity-orientation (Poincaré) phase space, whereas conventional light (either continuous-wave or pulsed) is determined by only one specific Poincaré location. General properties of parametric Poincaré trajectories are discussed, and their relation to experimentally accessible pulse-manipulation parameters (i.e. amplitudes and phases) determined. Specifically, it is shown how the maximum rate by which a given polarization state can be turned into a different one (at significant intensity levels) is limited by the spectral laser bandwidth. Apart from their direct usage in polarization-shaped pulse representation, Poincaré trajectories also form the basis for intuitive quasi-three-dimensional renderings of the electric field profile. There, the temporal evolution of polarization, intensity, and chirp is directly apparent in a single illustration. Received: 10 December 2002 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-931/888-4906, E-mail: brixner@physik.uni-wuerzburg.de     

Steady-state vectorial self-diffraction on a non-local photorefractive grating in a crystal of symmetry \overline{4}3m at symmetrical transmitting geometry

The steady-state two-wave interaction in a cubic crystal of the symmetry group 3m with the non-local photorefractive response in the absence of an external electric field is considered for the case of arbitrary interaction orientation with respect to the crystallographic coordinate system and for arbitrary intensities and polarization states of incident light waves. The self-diffraction problem is described on the basis of four coupled-wave equations in terms of the complex scalar amplitudes of components of the light waves with orthogonal linear polarization. The derived conservation laws are valid for the non-linear dependency of the photorefractive-grating amplitude on the modulation coefficient of the interference light pattern. It follows from these laws that the two non-unidirectional energy fluxes can form the total energy exchange between the two interacting light waves. A set of independent conservation laws allows us to decouple the coupled-wave equations and to obtain their analytical solution, at least, in the form of quadrature formulae. For example, such a solution is derived for the case of linearly polarized incident light waves and for the linearized dependency of the photorefractive-grating amplitude on the modulation coefficient. The explicit analytical expressions for the scalar amplitudes are obtained for the transversal electro-optic configuration of interaction. The possibility of polarization-state transformation of light waves without energy exchange between them is shown. Received: 30 July 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +7-3822/414321, E-mail: litvinov@ed.rk.tusur.ru     

Optical target and spiral patterns in a single-mirror feedback scheme

Spatio-temporal structures with a regular time dependence are observed in a nonlinear optical system consisting of a sodium-vapour cell and a single feedback mirror. Target and spiral patterns appear spontaneously in the intensity of the transmitted light field as a result of a self-organisation process. In contrast to the results obtained in most other pattern-forming systems, the radial motion of the waves is directed towards the centre. The emergence of oscillatory structures is traced back to a Hopf bifurcation at a finite wave number. The preference for a radial drift motion is due to a nonlinear guiding effect which results from phase gradients created by the inhomogeneous pump profile. The direction of the drift can be reversed by externally applied phase gradients. The system is a striking example of a case in which the radial variation of the pump profile has a decisive influence on pattern formation. Received: 31 July 2002 / Revised version: 30 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-251/83-33513, E-mail: huneus@uni-muenster.de     

Narrow-bandwidth diode-laser-based blue and ultraviolet light source

A compact, tunable and narrow-bandwidth laser source for blue and ultraviolet radiation is presented. A grating-stabilized diode laser at 922 nm is frequency-stabilized to below 100 Hz relative to a reference resonator. Injection of the diode-laser light into a tapered amplifier yields a power of 0.5 W. In a first frequency-doubling stage, more than 200 mW of blue light at 461 nm is generated by use of a periodically poled KTP crystal. Subsequent second-harmonic generation employing a BBO crystal leads to about 1 mW of ultraviolet light at 231 nm. Received: 12 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-89/32905-200, E-mail: christian.schwedes@mpq.mpg.de RID="**" ID="**"Present address: PTB, Bundesallee 100, 38116 Braunschweig, Germany     

Spontaneous transverse pattern formation in a microchip laser excited by a doughnut pump profile

A transition from a pure Laguerre–Gaussian (LG) mode to a pattern of optical vortex lattices in a large-Fresnel-number microchip laser is experimentally demonstrated by controlling the cavity Q-factor. The cooperative frequency locking of nearly degenerate modes is found to be a primary process for the generation of the optical vortex lattices in a class-B laser. When the cavity Q-factor is high enough, a LG-like mode and a structure of optical vortex lattices are found to coexist. Competition between coexisting transverse patterns of different symmetry gives rise to chaotic fluctuations. Received: 20 March 2002 / Revised version: 20 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +886-35/729134, E-mail: yfchen@cc.nctu.edu.tw