Tungsten microcone arrays grown using nanosecond pulsed-Nd:YAG laser in a low-pressure He-gas atmosphere

Tungsten microcone arrays with a high aspect ratio are formed by the cumulative nanosecond pulsed-Nd:YAG laser irradiation of single-crystal tungsten under low pressure in an inert atmosphere. The morphology of the microcones and their density were strongly affected by the number of laser pulses. The microcones grew to a length of 20 μm with a diameter of about 1.5 μm at the tip after irradiation with more than 1200 pulses under our experimental conditions. They may have potential applications for emission cathodes in a field-emission display (FED) and in microelectronic devices. Received: 8 January 2001 / Accepted: 13 June 2001 / Published online: 2 October 2001     

Short-wavelength far-infrared laser cavity yielding new laser emissions in CD3OH

A significantly improved far-infrared laser has been used to generate optically pumped laser emissions from 26 to 150 μm for CD₃OH. Using an X–V-pumping geometry, several new laser emissions have been found for CD₃OH. In addition, an increase in power, by factors from 10 to 1000, for many of the previously known shorter-wavelength laser lines, below 100 μm, has been observed. Frequency measurements for several lines have also been performed and have been reported to a fractional uncertainty up to ±2×10^-7, permitting the spectroscopic assignment of the laser transition. One of the frequency-measured lines, 44.256 μm observed using the 10R34 pump, has confirmed the assignment of the previously reported FIR emission (n,K;J)=(1,7;20)?(0,8;20)A in the ground vibrational state. Received: 26 October 2000 / Published online: 7 February 2001     

Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Combined dielectrophoretic field cages and laser tweezers for electrorotation

Symmetric rotating octode field cages do not allow simultaneous particle trapping and electrorotation. Laser-trapped particles can stably be held in inhomogeneous rotating electric fields. Therefore, in a combination of laser tweezers and field-cage (electro-optical cage) dielectric single-particle spectroscopy (DSPS) of particles and cells can be carried out at low electric field strength. The electro-optical cage overcomes the limitation of low enough effective particle conductivity ensuring repelling dielectrophoretic forces in conventional field cages. At low power, forces in field cages and laser tweezers hardly interfere and can be used to calibrate each other. Received: 3 May 1999 / Revised version: 16 August 1999 / Published online: 27 October 1999     

Laser polishing of diamond plates

Results are reported on laser polishing of 150–400-μm-thick free-standing diamond films with either a copper vapor laser (510 nm wavelength) or an ArF excimer laser (193 nm wavelength). Studies were focused on three particular goals. First, we aimed at a choice of optimum conditions for laser polishing of thick diamond films. It was shown that the laser polishing conditions and the resulting surface roughness were controlled by varying the angle of incidence of a scanning laser beam and by polishing time. Second, the laser ablation technique was applied to remove a defective layer from the “substrate” side of the diamond plates in order to reduce optical losses due to absorption in this layer. Third, the structure of the laser-graphitized diamond surface was studied using UV, visible, and IR optical spectroscopy techniques in the course of the “step-by-step” oxidative removal of the graphitic layer with increasing temperature of the oxidation in ambient air. Once the graphitic layer was removed, the optical transmission in the UV-visible-IR spectral range of the diamond films polished under optimum conditions was measured and compared with the optical transmission of the mechanically polished diamond films. It was shown that the optical quality (in the long-wave infrared region) of the laser-polished diamond plates was sufficient to reach the transmittance value very close to the theoretical limit. Received: 20 October 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Characterization of particulates accompanying laser ablation of pressed polytetrafluorethylene (PTFE) targets

We present observations of sub-micron- to micron-sized particles generated by high fluence (≈2 J/cm²) 248-nm laser ablation of pressed polytetrafluorethylene (PTFE) targets in air at atmospheric pressure. The original target material was hydrostatically compressed ≈7 μm PTFE powder, sintered at 275 °C. Collected ejecta due to laser irradiation consists of four basic particle morphologies ranging from small particles 50–200 nm in diameter to larger particles ≈10 μm in diameter. Many particles formed in air carry electric charge. Using charged electrodes we are able to collect charged particles to determine relative numbers of ± charge. We observe roughly equal numbers of positively and negatively charged particles except for the largest particles which were predominantly negative. For a range of particle sizes we are able to measure the sign and magnitude of this charge with a Millikan-oil-drop technique and determine surface charge densities. The implications of these observations with respect to pulsed laser deposition of PTFE thin films and coatings are discussed. Received: 15 January 1999 / Accepted: 18 January 1999 / Published online: 7 April 1999     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Hard tissue ablation with a free running Er:YAG and a Q-switched CO2 laser: a comparative study

The Er:YAG and the CO₂ laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length _L of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO₂ lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO₂ laser (9.6 μm, τ_L=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO₂ laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO₂ laser at an energy density of 9 J/cm ² and a broad shallow minimum in the range of 10 to 70 J/cm² for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures. Received: 19 July 2000 / Revised version: 1 November 2000 / Published online: 30 November 2000     

Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization threshold. Received: 20 October 1997/Revised version: 16 February 1998     

New short-wavelength laser emissions from partially deuterated methanol isotopes

The partially deuterated isotopes of methanol, CH₂DOH and CHD₂OH, have been reinvestigated as sources of far-infrared (FIR) laser emissions using an optically pumped molecular laser (OPML) system recently designed for wavelengths below 150 μm. With this system, 10 new FIR laser emissions from these isotopes ranging from 32.8 to 174.6 μm have been discovered. This includes the shortest known OPML emission from CHD₂OH, at 32.8 μm. These lines are reported with their operating pressure, polarizations relative to the CO₂ pump laser and wavelengths, measured to ±0.5 μm. In addition, polarizations for three previously observed FIR laser lines from CHD₂OH were measured for the first time. This paper is dedicated to the memory of Dr. K.M. Evenson, a pioneer in the field for his role in the development of optically pumped molecular lasers and their use in laser frequency measurements and the laser magnetic resonance technique. His scientific expertise, guidance, mentoring and friendship will be greatly missed. Received: 27 March 2002 / Published online: 8 May 2002     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

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     

Stereoscopic flow-tagging velocimetry

A laser-based method for measuring the three components of the velocity in a plane simultaneously and instantaneously without seed particles is presented. This is achieved by combining a laser flow-tagging technique with stereoscopic detection, in which the tagged flow is viewed from two different directions. A single CCD camera is employed for this purpose by using a new optical detection system. The flow tagging is performed by two consecutive laser pulses, i.e., “write” and “read” laser pulses. The write laser creates a grid of tracer molecules (NO) by inducing a photodissociation process. The three-dimensional motion of the tracer molecules is measured by a thick read laser sheet. Received: 22 July 1999 / Revised version: 5 August 1999 / Published online: 30 September 1999     

Self-organized structuring of W/C multilayers on Si substrate

After aging at room temperature for several months W/C multilayers (20 periods, single layer thicknesses in the nanometer range) grown on Si-(111) substrates by pulsed laser deposition (PLD) developed homogeneously wrinkled surfaces. Their structures were studied by optical microscopy, atomic force microscopy and X-ray diffractometry. Typical dimensions of debonded areas are some 100 μm in length, about 40 μm in width and 2–3 μm in height. The formation of wrinkles is accompanied by an increase in the free surface by 1–2%. Stress relaxation is considered the driving force of this phenomenon. Received: 26 July 1999 / Accepted: 29 July 1999 / Published online: 16 September 1999     

Strong amplified spontaneous emission and lasing from a narrow-gap RF-excited Ar-He-Xe gas mixture

The performance of a RF excited cw atomic xenon laser at wavelengths of 2.03 μm and 2.65 μm was studied theoretically and experimentally as a function of electrode distance. Results for inter-electrode distances from 2 to 0.25 mm are presented. A high pumping rate resulted in strong 40 mW cw amplified spontaneous emission at 2.65 μm wavelength from the configuration with the smallest distance of 0.25 mm between the electrodes. The maximum laser output of 2.7 W (0.24 W/cm³) was obtained with an active medium volume of 2×15×370 mm³ whereas the maximum specific output of 1.9 W/cm³ was received for an active medium volume of 0.25×2.25×370 mm³. A fluid model of the RF discharge was developed to analyze the laser behavior for different distances between the electrodes. Received: 30 November 1999 / Revised version: 21 April 2000 / Published online: 6 September 2000     

Simultaneous detection of methane, oxygen and water vapour utilising near-infrared diode lasers in conjunction with difference-frequency generation

An all-diode-laser-based spectrometer is used for the simultaneous detection of methane, oxygen and water vapour. This is accomplished using a 760-nm diode laser and a 980-nm diode laser in conjunction with difference-frequency generation to 3.4 μm in a periodically poled lithium niobate crystal. Each of the output wavelengths is resonant with one of the molecular species. Simultaneous recordings over a 15-m open path of laboratory air are demonstrated. The recording scheme shows the wide applicability of a diode-laser-based difference-frequency spectrometer for the detection of molecular species in different wavelength ranges. By increasing the frequency of the 760-nm diode laser and decreasing the frequency of the 980-nm diode laser, a maximum continuous tuning range in the mid infrared of 3.6 cm^-1 is achieved. This enables the recording of several methane lines at atmospheric pressure. Pressure-dependence studies of methane lineshapes are also performed in an absorption cell. An indoor-air methane background level of 3 ppm is measured. The signal-to-noise ratio in the recorded methane spectra indicates that sub-ppm detection of methane at atmospheric pressure is feasible. Received: 6 March 2000 / Revised version: 19 June 2000 / Published online: 11 October 2000     

Portable fiber-coupled diode-laser-based sensor for multiple trace gas detection

Tunable narrowband mid-infrared radiation from 3.25 to 4.4 μm is generated by a compact fiber-coupled, difference-frequency-based spectrosopic source. A 20-mW external cavity diode laser (with a tuning range from 814 to 870 nm) and a 50-mW distributed-Bragg-reflector diode-laser-seeded ytterbium-doped fiber amplifier operating at 1083 nm are difference-frequency mixed in a multi-grating, temperature-controlled periodically poled LiNbO₃ crystal. A conversion efficiency of 0.44 mW/(W² cm) (corresponding to a power of ≈3 μW at 3.3 μm) represents the highest conversion efficiency reported for a portable device. Performance characteristics of such a sensor and its application to spectroscopic detection of CO₂, N₂O, H₂CO, HCl, NO₂, and CH₄ will be reported in this work. Received: 14 May / Revised version: 24 June 1999 / Published online: 30 September 1999     

CdTe epilayers for uses in optical waveguides

CdTe epilayers have been grown by vapor phase epitaxy (VPE) on glass, MgO, sapphire, LiNbO₃ and mica substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies show the good structural quality of the epilayers. In these epilayers, a few optical modes were excited with a 1.33-μm laser. The measured propagation losses were in the range between 5 dB/cm and less than 0.5 dB/cm. From dark-mode m-lines, the epilayer thickness was found to be in the 1–3 μm range, in good accord with that obtained by SEM measurements. The refractive index obtained from the fitting is also in good accord with that of bulk CdTe. Received: 7 October 1999 / Accepted: 13 March 2000 / Published online: 5 July 2000     

X-ray emission from femtosecond laser micromachining

We characterize the spectral properties of X-rays generated from selected metal and semiconductor targets when 120-fs laser pulses are focused to intensities of∼10¹⁴–3×10¹⁵ W/cm² during laser micromachining in air. High fluxes of multi-keV-energy X-rays could be obtained with 280-μJ pulses at a 1 kHz repetition rate. The yield and spectral composition of the X-rays are found to depend sensitively on the processing conditions, and thus the X-ray emission is expected to be a novel indicator of optimal laser machining. Received: 17 July 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001