Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Characteristics of SrBi2Ta2O9 ferroelectric films on GaAs with a TiO2 buffer layer

Mainly [115]-oriented SrBi₂Ta₂O₉ (SBT) films were prepared on GaAs(100) substrates with TiO₂ buffer layers. Both the SBT films and the TiO₂ buffer layers were deposited by pulsed laser deposition (PLD) using a KrF excimer laser. The depth profile of the constituent elements observed by Auger electron spectrometry (AES) shows no remarkable diffusion at both the interfaces between SBT and TiO₂ and between TiO₂ and the GaAs substrate. The electrical characteristics of the Pt/SBT/TiO₂/GaAs(100) structures show a ferroelectric hysteresis loop with a small remanent polarization (∼0.5 μC/cm²). Received: 1 March 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: xhliu81@hotmail.com RID="**" ID="**"Present address: Data Storage Institute, DSI Building, 5, Engineering Drive 1 (off Kent Ridge Crescent, NUS) 117608 Singapore     

Maskless photoassisted etching of N-type Ga0.47In0.53As in basic solutions

The maskless photoassisted etching of n-type Ga_0.47In_0.53As is examined for basic KOH solutions in comparison with GaAs and InP material. The etch rate increases with laser intensity and with carrier concentration up to a saturation value. The best etch rate is obtained with molar KOH in ethyl alcohol (7 ms^–1 for laser intensity 10⁴ W cm^–2). Selective etching have been realized on heterojunction in order to isolate p-n junctions without the help of masks.     

Portable difference-frequency laser-based cavity leak-out spectrometer for trace-gas analysis

We report a portable mid-infrared spectrometer for trace-gas analysis which is based on an all-solid-state difference-frequency-generation laser. The spectrometer provides in situ absorption path lengths of more than 3 km by means of the cavity leak-out method, a cw variant of the cavity ring-down technique. The design, performance, and application of this spectrometer are presented. The light source utilizes difference-frequency generation in a periodically poled lithium niobate (PPLN) crystal pumped by two single-frequency solid-state lasers. A maximum power of 27 μW in the wavelength region near 3.3 μm is achieved using a pump power of 20 mW at 808 nm, a signal power of 660 mW at 1064 nm, and a 50-mm-long PPLN crystal. This corresponds to a conversion efficiency of 0.42 mW/(W² cm). We demonstrate that this portable laser system is suitable as a light source in a cavity leak-out spectrometer. We achieved a minimum detectable absorption coefficient of 1×10^-8/cm (integration time: 2 s), corresponding, for example, to a detection limit of 1 part per billion ethane. This compact trace-gas analyzer with high sensitivity and specificity is promising for various environmental and medical applications. Received: 8 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-211/811-3121, E-mail: muertz@uni-duesseldorf.de     

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     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Oxygen plasma and high pressure H<Subscript>2</Subscript>O vapor heat treatments used to fabricate polycrystalline silicon thin film transistors

Oxygen plasma and high pressure H₂O vapor heat treatment were applied to fabrication of n-channel polycrystalline silicon thin film transistors (poly-Si TFTs). 13.56 MHz-oxygen-plasma treatment at 250 °C, 100 W for 5 min effectively reduced defect states of 25-nm-thick silicon films crystallized by 30 ns-pulsed XeCl excimer laser irradiation. 1.3×10⁶-Pa-H₂O vapor heat treatment at 260 °C for 3 h was carried out in order to improve electrical properties of SiO_x gate insulators and SiO_x/Si interfaces. A carrier mobility of 470 cm²/V s and a low threshold voltage of 1.8 V were achieved for TFTs fabricated with crystallization at 285 mJ/cm². Received: 18 November 2002 / Accepted: 25 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-42/388-7109, E-mail: tsamesim@cc.tuat.ac.jp     

PbS-doped phosphate glasses saturable absorbers for 1.3-μm neodymium lasers

Passive mode locking and saturable absorber Q-switching of neodymium lasers at 1.3 μm with PbS-doped phosphate glasses are demonstrated. Q-switched pulses of 120 ns (0.1 μJ) in duration (energy) and the average output power of 3 mW from a quasi-cw diode-pumped Nd³⁺:KGW laser and ultrashort pulses of a maximum of 250 μJ in energy and 150 ps in duration from a Nd³⁺:YAP laser were obtained. The bleaching decay rate of the samples was found to increase with the Quantum Dot’s size decreasing due to the enhancement of quantum confinement effects for smaller dots and stronger overlapping of the electron and trap state wave functions. Received: 23 January 2002 / Revised version: 2 April 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +375-17/232-6286, E-mail: savitski@eudoramail.com     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr     

Photoacoustic characteristics of dynamical adhesion of films on a metallic substrate

In this communication, we report a numerical model that predicts the mechanical deformations associated with the pulsed laser irradiation of a film surface, based on thermal diffusion theory. The model is consequently advanced to produce a method for evaluating film adhesion strength. The epicenter surface displacements within the irradiated fields have been measured using a heterodyne interferometer. The comparison of the experimental data and the displacements calculated by the model shows good agreement. By investigating the propagating acoustic modes under non-destructive and destructive modes, we reveal that, with or without interface delamination, the phase structure of the longitudinal waves will be altered due to the change of reflection mode at the interface. Applying shock dynamics theory, we evaluate the adhesion strength of the TiN/stainless interface. We also indicate the strain rate can be up to 10⁵∼10⁶ s^-1 during film interface delamination. Received: 5 April 2002 / Accepted: 24 June 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +86-511/879-1919, E-mail: mzhou@ujs.edu.cn     

In situ observation of dynamics of plasma self-channeling and bulk modification in silica glasses induced by a high-intensity femtosecond laser

The time-resolved dynamics of plasma self-channeling and refractive index bulk modification in silica glasses were first observed in situ using a high-intensity femtosecond (110 fs) Ti:sapphire laser (λ_p=790 nm). Plasma channeling is induced in silica glass at an irradiation higher than an input intensity of 1.5×10¹² W/cm² and photoinduces either the refractive-index modification or optical crack modification. In the domain of refractive-index modification, the lifetime of induced plasma self-channeling was 20 ps and the structural transition time for forming the refractive-index change was 10 ps. In the domain of optical cracks, however, the lifetime of induced plasma formation was 30 ps and the structural transition time for forming the optical cracks was 40 ps. According to electron spin resonance spectroscopic (ESP) measurement, it was found that the defect concentration of the SiE^′ center increased significantly in the refractive index modification region. A maximum value of the refractive-index change Δn was measured to be 1.6×10^-2. The intensity profile of the output beam transmitted through the refractive-index modification showed that the bulk modification produced a permanent optical waveguide. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: shcho@riken.go.jp     

Compact diode-side-pumped Nd:YVO<Subscript>4</Subscript> laser in grazing-incidence configuration

The analysis of compact CW diode-side-pumped grazing-incidence-geometry Nd:YVO₄ laser designs is presented. An output power of 5 W (λ=1064 nm) was produced at 17 W of diode pump (conversion efficiency of 30%) in single transverse TEM₀₀ mode operation at high laser beam quality (M_x ²≈1.05 and M_y ²≈1.01). The resonator geometry was analyzed by applying generalized 4×4 matrix modeling of the spatial mode size, including the impact on the laser operation of cavity astigmatism and a thermal lens in the laser slab. The simplicity and compactness of the laser cavities allow their use for technological applications. Received: 31 July 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: m.damzen@ic.ac.uk     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere

We describe a near-infrared in situ tunable diode laser spectrometer developed for atmospheric measurements of CH₄ in the upper troposphere and lower stratosphere (UT/LS). The instrument is designed to provide fast-response (0.5–1 Hz) measurements and operate autonomously on the NASA WB-57F high-altitude aircraft. A single-mode InGaAsP distributed feedback laser diode operating at 1.6537 μm scans continuously over the R(3) rotation–vibration transition in the 2ν₃ band. We use a direct absorption technique incorporating a custom-designed long path length (252 m) low-volume (3.6 L) astigmatic Herriott cell. The present detection sensitivity is 5×10¹⁰ molecules cm^-3, corresponding to ∼20 ppbv in the UT/LS, with the main limit to instrument precision being background optical interference fringes. In-flight performance is demonstrated by presentation of recent data. Received: 25 January 2002 / Revised version: 5 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5373, E-mail: richard@al.noaa.gov     

Solar cells with porous silicon: modification of surface-recombination velocity

Laser-induced transient-grating measurements were performed to monitor the influence of porous silicon on the surface recombination of a highly doped n⁺-silicon emitter of solar cells. With this technique, photocarrier diffusion and recombination with a time resolution of some tens of picoseconds can be studied. Using pulses of the second- and third-harmonic radiation from an Nd³⁺:YAG laser (quantum energy 2.34 and 3.51 eV, respectively), two different-depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in the bulk of the emitter, surface-recombination velocities in a series of samples typical for each successive operation of solar-cell technology with different surface-doping level and surface preparation were evaluated. From the analysis, we conclude that porous silicon formed on the emitter passivates the surface of the silicon layer, i.e. reduces the rate of surface recombination at the porous silicon–crystalline silicon interface. Ytterbium as a co-dopant of the emitter increases the surface-recombination velocity. Received: 26 June 2000 / Accepted: 4 December 2000 / Published online: 26 April 2001     

Methane analyzer based on TDL’s for measurements in the lower stratosphere: design and laboratory tests

This paper describes a tunable diode laser spectrometer for the in situ measurement of methane by high-resolution absorption spectroscopy of roto-vibrational lines, using a distributed feedback laser emitting in the near infrared (1.65 μm) and a multi-pass cell, with a two-tone detection scheme. The instrument was designed to be installed on the high-altitude aircraft M55 Geophysica, for high-sensitivity and high-temporal-resolution measurement of CH₄, both as a greenhouse gas and as a tracer of air-mass motion. The instrument-design criteria to achieve the compactness, low weight, and ruggedness necessary for automatic operation on an unpressurized high-altitude aircraft are reported. Results of the laboratory testing are also shown and discussed. Received: 1 April 2002 / Revised version: 28 May 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +39-050/313-7597, E-mail: damato@scintec.it     

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     

Rapid formaldehyde monitoring in ambient air by means of mid-infrared cavity leak-out spectroscopy

We report the spectroscopic detection of formaldehyde in ambient air using cavity leak-out spectroscopy, a cw variant of cavity ring-down spectroscopy. This technique proved to be suitable for a real-time quantitative analysis of polluted air without any preprocessing of the air sample. Using a tunable CO-overtone sideband laser for the λ=3 μm spectral region and a ring-down cell with R=99.95% mirrors, we achieved a detection limit of 2 parts per billion formaldehyde in ambient air, corresponding to a minimum detectable absorption coefficient of 7×10^-9/cm (sampling time: 2 s). Calibration problems arising from the polarity of the molecule and due to HITRAN database uncertainties are discussed. Received: 28 March 2002 / Revised version: 7 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-211/811-3121, E-mail: muertz@uni-duesseldorf.de     

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     

High power single-shot laser ablation of silicon with nanosecond 355 nm

We report on high intensity single-shot laser ablation of monocrystalline silicon with a nanosecond Nd:YAG at 355 nm. It is shown that for incident laser intensities exceeding ∼11.5 GW/cm² on the silicon surface, unusually high etch depths can be achieved reaching values up to 60 μm. The results support previous observations of dramatic increase in etch rates in single-shot laser ablation at 266 nm. A laser-induced explosive boiling mechanism together with secondary plasma heating is believed to be associated with this effect.