Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca     

Deep UV laser induced fluorescence in fluoride thin films

Fluorescence experiments have been performed to study the interaction of 193-nm laser radiation with dielectric thin films of LaF₃, AlF₃, and MgF₂. Spectral- and time-resolved measurements reveal the presence of cerium in LaF₃ and the influence of hydrocarbons in MgF₂ and LaF₃. Virtually no fluorescence response is observable in the case of AlF₃. Supplementary measurements on multilayer stacks confirm the contribution of hydrocarbon and cerium emission in high-reflective UV mirrors upon ArF excimer laser irradiation. Energy density dependent measurements indicate a linear absorption process as the origin of UV laser induced fluorescence in LaF₃. Luminescence calculations are applied as a helpful tool in order to account for interference effects that are inherently to be found in the multilayer emission spectra. Received: 21 May 2002 / Accepted: 23 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/807-601, E-mail: heber@iof.fraunhofer.de     

Laser-induced etching and deposition of W using a-SiO<Subscript>2</Subscript> microspheres

A regular lattice of a-SiO₂ microspheres on a quartz support is used as a microlens array for laser-induced surface patterning by etching and deposition of W in atmospheres of WF₆ and WF₆+H₂, respectively. Received: 22 July 2002 / Accepted: 30 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, Email: dieter.baeuerle@jku.at     

Laser-induced fluorescence detection of acetylene in low-pressure propane and methane flames

Laser-induced fluorescence is used to detect and record profiles of acetylene formed as an intermediate species in 10-Torr premixed propane and methane flames. In low-temperature regions of the flames, excitation spectra confirm acetylene as the spectral carrier. The spectra of acetylene overlap those of O₂ and NO in terms of both excitation and detection wavelengths, however, acetylene can be detected with relatively little interference in the vicinity of 228 nm, using a detection wavelength of 260 nm. The fluorescence lifetime of acetylene in the flame conditions studied is approximately 20 ns, much shorter than the radiative lifetime, due to a high quenching rate for all the colliders investigated. This can be exploited in low-pressure flames to avoid interference from acetylene in monitoring nitric oxide. The acetylene mole fraction in propane flames reaches its peak value at nearly the same location as that of HCO, slightly closer to the burner than the peak CH mole fraction. The acetylene fluorescence signal is easily detected in propane flames over equivalence ratios from 0.6 to 1.2, although it increases under fuel-rich conditions. In methane flames, the acetylene signal is much weaker and is undetectable for fuel-lean conditions. Received: 5 August 2002 / Revised version: 30 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/767-1716, E-mail: brad@code6185.nrl.navy.mil     

150-times phase conjugation by degenerate four-wave mixing in a continuous-wave Nd:YVO4 amplifier

The high phase-conjugate reflectivity of 150, by four-wave mixing in a diode-pumped Nd:YVO₄, has been demonstrated. The phase-conjugate mirror was capable of correcting the phase aberration of a probe beam. The experimental diffraction efficiency of the transmission gratings was 60. Received: 8 April 2002 / Revised version: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +81/43-290-3490, E-mail: omatsu@image.tp.chiba-u.ac.jp     

Reduced space-charge fields in near-stoichiometric LiTaO3 for blue, violet, and near-ultraviolet light beams

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO₃ crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO₃ crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in the photoconductivity relative to the photogalvanic effect. Especially, at near-UV wavelengths, the saturated space-charge field of the order of 0.1 kV/cm reaches a minimum that is two orders of magnitude lower than typical values observed in undoped near-stoichiometric LiNbO₃. Received: 11 July 2002 / Revised version: 2 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1056, E-mail: mojca@fiz.uni-lj.si     

WS<Subscript>2</Subscript> layer formation on multi-walled carbon nanotubes

Time-dependent powder X-ray-diffraction analyses reveal that the conversion of WO₃ into WS₂ on carbon nanotube surfaces in the presence of H₂S is a one-step process. The WS₂ layers grow simultaneously along the tube in the radial and axial directions. Received: 17 June 2002 / Accepted: 19 June 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-1273/677-196, E-mail: d.walton@sussex.ac.uk     

Removal of oxygen atoms from a SiO2 surface by incoherent vacuum ultraviolet excimer irradiation

We have demonstrated the non-thermal removal of oxygen atoms from an oxidized silicon surface (SiO₂) on a silicon wafer by the use of a low-power (0.3 mW cm^-2) incoherent vacuum ultraviolet (VUV) light source at 126 nm. X-ray photoelectron spectroscopy (XPS) has shown that a maximum Si concentration of 80% appears at the surface after a 20-h irradiation with 9.8 eV photons, as a result of oxygen removal from the SiO₂ matrix. The surface morphology, however, indicates no damage or melting on the surface even after the irradiation. Received: 15 April 2002 / Accepted: 17 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Permanent address: Nano-Tech Photon Inc., Shimotomita 4132-1, Shintomi, Miyazaki, 889-1404, Japan RID="**" ID="**"Corresponding author. Fax: +81-985/583-899, E-mail: kubodera@opt.miyazaki-u.jp     

Additive-pulse mode locking of a diode-pumped Nd:KGd(WO4)2 laser

Self-starting additive-pulse mode locking of a diode-pumped Nd:KGd(WO₄)₂ laser is demonstrated for the first time. An output power of 0.85 W is achieved with 2.3-ps pulses at a repetition frequency of 76.5 MHz. The shortest pulse generated had a duration of 1.9 ps. Received: 2 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +44-141/5482891, E-mail: a.major@phys.strath.ac.uk     

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     

Study of range distribution parameters for bismuth-ion implantation in silver gallium diselenide

Range distributions for bismuth ions implanted in AgGaSe₂ in the energy range 80–300 keV were investigated by using 2.1-MeV He²⁺ Rutherford backscattering spectrometry (RBS). A convolution calculation method was used to extract the true distributions of bismuth from the measured RBS spectra. The range distribution parameters, R_p and ΔR_p, were obtained and compared with those obtained from Monte Carlo simulation. The experimental R_p values agree with the Monte Carlo simulation values very well, but the experimental ΔR_p values are systematically larger than those from the theoretical simulation. Received: 28 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-531/856-5167, E-mail: xdliu@sdu.edu.cn     

Interaction of two light beams with different polarizations in TGS

By ellipsometric measurements we have observed the polarization variations of the primary light beam, with wavelength λ₁=5145 ?, interacting with the secondary light beam of controlled variable polarization, transmitted through a TGS crystal layer. We present a theoretical explanation based on the second-order optical nonlinearity of TGS. Received: 4 December 2001 / Revised version: 28 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +48-22/660-5447, E-mail: petyk@if.pw.edu.pl     

Diode-pumped Nd:YVO<Subscript>4</Subscript> and Nd:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> 1.3 μm lasers passively Q-switched with PbS-doped glass

Diode-pumped Nd:YVO₄ and Nd:KGW lasers passively Q-switched with PbS-quantum dot-doped phosphate glass were demonstrated. For the Nd:YVO₄ laser, pulses 110 ns in duration with a 13% Q-switching efficiency were obtained. The absorption recovery time of the PbS-doped glass was measured to be 27±4 ps. Some recommendations for more efficient use of PbS-doped glasses for Q-switching of diode-pumped lasers are suggested on the basis of our analysis. Received: 29 July 2002 / Revised version: 18 September 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +375-17/232-6286, E-mail: savitski@eudoramail.com     

Excitation of Tm<Superscript>+3</Superscript> at a wavelength of 1064 nm

We report on the experimental study of the excitation mechanism of fluorescence in a 10000-ppm wt. Tm⁺³-doped ZBLAN fiber. Visible (at 453 nm and 480 nm) and near-infrared (∼800 nm) radiations were studied under excitation at 1.064 μm. The up-conversion mechanisms responsible are investigated, based on experimental data. The results show that the³ H ₄ and the ¹ D ₂ levels are predominantly excited by ion-ion cross-relaxation processes. Received: 19 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +41/31-631-3765, E-mail: Reda.El-Agmy@iap.unibe.ch     

Self-adaptive Nd:YLF holographic laser with selectable wavelength operation

We demonstrate, for the first time to our knowledge, operation of a self-adaptive holographic laser with wavelength selection, which uses Nd:YLF as the gain medium. The holographic laser was self-starting and, by virtue of the temporal dynamics in writing the gain grating, passively Q-switched the cavity to produce pulses with 20–50 ns duration. A pulse energy of 184 mJ was obtained from the system with beams having a TEM₀₀ spatial profile. Single-longitudinal-mode operation was observed with near-transform-limited bandwidth. Received: 30 July 2002 / Revised version: 1 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: s.y.lam@ic.ac.uk     

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     

Fabrication of submicron gratings in fused silica by F<Subscript>2</Subscript>-laser ablation

Submicron surface-relief gratings were fabricated on fused silica by F₂-laser ablation with nanosecond duration pulses from a high-resolution 157-nm optical processing system. A 157 nm wavelength projection mask was prepared by ArF-laser ablation to form a 20-μm period grating of equal lines and spaces. A 25-fold demagnification of the mask by a Schwarzschild objective generated gratings of an 830-nm period and a 250 nm modulation depth, as characterized by SEM, AFM and HeNe-laser beam diffraction. Received: 24 April 2002 / Accepted: 25 April 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503 599, E-mail: jihle@llg.gwdg.de     

Large-scale synthesis of SnO2 nanobelts

Tin dioxide (SnO₂) nanobelts have been successfully synthesized in bulk quantity by a simple and low-cost process based on the thermal evaporation of tin powders at 800 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the nanobelts are uniform, with lengths from several-hundred micrometers to a few millimeters, widths of 60 to 250 nm and thicknesses of 10 to 30 nm. X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and selected-area electron diffraction analysis (SAED) indicate that the nanobelts are tetragonal rutile structure of SnO₂. The SnO₂ nanobelts grow via a vapor–solid (VS) process. Received: 3 June 2002 / Accepted: 10 June 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-551/559-1434, E-mail: gwmeng@mail.issp.ac.cn     

Experimental investigation of non-linear selective reflection at the interface of Cs atomic vapor and quartz

We present an experimental study of non-linear selective reflection (SR) at a quartz–Cs-vapor interface in a V-type three-level scheme. The non-linear selective reflection at the Cs D₂ resonance line (⁶ S _1/2F=4→⁶ P _3/2) is monitored with and without pumping. The sub-Doppler reflection spectrum is observed and the effect of pumping on the signal of the selective reflection is investigated. The experimental result is in agreement with the theoretical calculation. Received: 16 April 2002 / Revised version: 12 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-351/701-1500, E-mail: zhaojm@sxu.edu.cn     

Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation. Received: 29 May 2002 / Revised version: 5 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-650/723-1748, E-mail: jkoch@stanford.edu RID="**" ID="**"E-mail: hanson@me.stanford.edu