Ring beam shaping optics fabricated with ultra-precision cutting for YAG laser processing

In this study, a method for generating ring intensity distribution at a refraction-type lens with an aspheric element was proposed, and the beam shaping optical element was finished using only ultra-precision cutting. The shape of the optical element and its irradiance pattern were determined from numerical calculation based on its geometrical and physical optics. An ultra-precision lathe was employed to fabricate beam shaping optical elements, and acrylic resin was used as the material. The transmittance of an optical element (a rotationally symmetrical body) with an aspheric surface fabricated using a single-crystal diamond tool was over 98%, and its surface roughness was 9.6 nm Ra. The method enabled the formation of a circular melting zone on a piece of stainless steel with a thickness of 300 μm through pulse YAG laser (λ 1:06 μm) processing such that the average radius was 610 μm and the width was 100–200 μm. Circular processing using a ring beam shaping optical element can be realized by single-pulse beam irradiation without beam scanning.     

Sub-wavelength micromachining of silica glass by irradiation of CO<Subscript>2</Subscript> laser with Fresnel diffraction

We investigated a simple and productive micromachining method of silica glass by ablation using a TEA CO₂ laser (10.6 μm) with a spatial resolution down to sub-wavelength scale. The silica glass was irradiated by the TEA CO₂ laser light through a copper grid mask with square apertures of 20×20 μm² attached to the silica glass surface. After the irradiation, circular holes with a diameter of several μm were formed on the silica glass surface at the centers of the apertures due to the Fresnel diffraction effect. The minimum diameter of the holes was 3.4 μm. The characteristics of the micromachining are discussed based on the electric field distributions of the CO₂ laser light under the mask using a three-dimensional full-wave electromagnetic field simulation.     

Depth profiles of Ag nanoparticles in silicate glass

Silver nanoparticle composite glass has been synthesized by ion exchange and a subsequent thermal treatment method. Transmission electron microscopy (TEM) and ultraviolet-visible spectrometry were used to study the depth profiles of Ag nanoparticles in silicate glass. Results indicate that Ag nanoparticles are nearly spherical in shape with a single-crystalline structure. Also, the volume fraction of Ag nanoparticles is very high in the surface layer of 5 μm, and then decreases slowly across the depth of 30 μm, thereafter the volume fraction reduces quickly to zero at the depth of 100 μm. The plasmon resonance bi-absorptions in the surface layer of the annealed specimen at 600 °C can be attributed to bi-modal distributions of Ag nanoparticles in the layer. The bi-modal distributions can be explained by the Ostwald ripening theory. PACS 81.05.Pj; 81.07.-b; 68.37.Lp     

Rotation rate of a three-wing rotor illuminated by upward-directed focused beam in optical tweezers

The optical torque and the trapping position (focal point) in optical tweezers are analyzed for upward-directed focused laser illumination using a ray optics model, considering that laser light is incident at not only the lower surface but also the side surface of a 3-wing rotor. The viscous drag force due to the pressure and the shearing stress on all surfaces of the rotor is evaluated using computational fluid dynamics. The rotation rate is simulated in water by balancing the optical torque with the drag force, resulting in 500 rpm for an SU-8 rotor with 20 μm diameter at a laser power of 200 mW. The trapping position is estimated to be 7.6 μm in the rotor with an upward-directed laser at 200 mW via an objective lens having a numerical aperture of 1.4. Both the rotation rate and the trapping position agree well with the values obtained in the experiment.     

Quantitative visualization of micro-tube flow using micro-PIV

This paper describes PIV measurements ofthe flow field in a micro round tube with an internal diameter of 100 μm in order to examine micro-scale effects. Since the refractive index of the micro tube almost corresponds to that of water, the inner flow in the tube can be observed clearly. The micro PIV system has been developed using a microscope, a high sensitive CCD camera, a double pulsed Nd:YAG laser and optics. Applying the micro PIV technique to the flow, the velocity distributions with spatial resolution of 1.8 × 1.8 μm were measured even near the wall in the center plane of the round tube. It was found that the velocities near the tube wall were smaller than the theoretical values calculated by using Poiseuille’s law. It is believed that this disparity is due to micro-scale effects such as interference between particles and the wall, friction at the wall, surface tension and so on.     

Influence of particle size and concentration on the second-harmonic signal generated at colloidal surfaces

Second harmonic generation (SHG) spectroscopy is a recently developed technique for the investigation of surface properties of particles. To apply the method to technical colloidal systems, the dependences of several experimental parameters on the signal have to be studied. In this work the influence of particle concentration on the SHG signal from the surfaces of colloids (polystyrene beads in a size range of 0.1 μm to 2.9 μm) is investigated. A simple model, based on Lambert–Beer’s law, to describe the measured dependences is derived. The model agrees with the experimental observations for particles smaller 1.1 μm and with a small modification also for larger particles. Based on the new model an analytical equation for determining the optimum concentration, where highest signals in colloidal SHG spectroscopy measurements are obtained, is derived. PACS 42.25.Fx; 42.65.-k; 82.70.Dd     

Measurement of low retardation using reflective optical system based on the photoelastic principle

A new measuring technique that can measure retardation and can output magnitude and direction of plane stress in each glass of a panel composed of double transparent pieces of glass has been developed using reflective confocal optics. The linear polarized probe beam is incident to the glass and we can detect a reflected beam converted to orthogonal polarization caused by the photoelastic phenomenon. Using the high extinction ratio (10⁶) beam-displacing prism as a polarization discriminator, we can measure the photoelasticity by rotating the polarization of the probe beam from 0 to π rad without disturbing the optical axis. This system has the ability to measure retardation. The lowest one is estimated as nearly 0.066 nm for 700 μm thickness glass which corresponds to 0.03 MPa stress from our calibration line.     

Influence of fractal surface dimension on the dissolution process of sparingly soluble CaCO<Subscript>3</Subscript> microparticles

The dissolution process of sparingly soluble CaCO₃ microparticles and how the fractal surface dimension of the particles changes during dissolution is analyzed. The particles and the dissolution process are studied using scanning electron microscopy, X-ray diffraction, nitrogen adsorption, laser diffraction and conductance measurements. Ball milling of the particles is shown to maintain the particle crystallinity, and to introduce an increased fractal surface dimension in the 1–10 μm size range. Dissolution is found to increase the surface dimension of initially smooth particles and to maintain the fractal surface roughness of milled particles. The dissolution process increases the relative number of small particles (50 nm–1 μm) whereas the larger ones decrease in size. The solubility of the milled fractal particles was ∼1.8 times higher than that for the initially smooth ones. The presented findings show that developing methods for increasing the fractal surface roughness of particles should be of interest for improving the solubility of poorly soluble drug candidates.     

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm². The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm⁻¹, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.     

Formation of optical color and fluorescence centers in polytetrafluoroethylene under γ-irradiation

Fluorescent properties and colors of polytetrafluoroethylene (PTFE) samples were studied as functions of absorbed dose of γ-radiation at a temperature above the melting temperature of the crystallites (327°C) in the dose range 0.2–0.8 MGy. The samples were irradiated at low pressure (10^–6 Torr) in glass ampuls and at atmospheric pressure in a chamber purged with argon. Samples in ampuls acquired an off-white tint and fluoresced weakly. The fluorescence intensity increased slowly as the radiation dose increased from 0.2 to 0.6 MGy and decreased at 0.8 MGy. Samples irradiated in the chamber fluoresced more strongly and acquired colors that changed from gray (0.2 MGy) to dark-brown (0.8 MGy). Color centers were formed only in the sample surface layer. Their appearance was associated with the adsorption of particles of an unknown nature from the environment. The argon-purged samples turned white upon removing their surface layer (50 μm). The intensity and shape of fluorescence bands emitted by these samples remained essentially unaltered, i.e., depended weakly on the absorbed dose in the range 0.2–0.8 MGy.     

Infrared steam laser cleaning

Steam Laser Cleaning with a pulsed infrared laser source is investigated. The infrared light is tuned to the absorption maximum of water (λ=2.94 μm, 10 ns), whereas the substrates used are transparent (glass, silicon). Thus a thin liquid water layer condensed on top of the contaminated substrate is rapidly heated. The pressure generated during the subsequent phase explosion generates a cleaning force which exceeds the adhesion of the particles. We examine the cleaning threshold in single shot experiments for particles sized from 1 μm down to 300 nm.     

Intense formation of intermetallic phases during implantation of aluminum ions in titanium

The results from microstructure and phase composition investigations of titanium in different structural states (with average grain sizes of 0.3 μm, 1.5 μm, and 17 μm) are presented following Al ion implantation using the Mevva-V.RU source (irradiating dose, 10¹⁸ ion/cm²). The implanted multiphase layers are found to form on the base of α-Ti grains. The size, shape, and localization of the formed phases (TiO₂, Ti₂O, TiC, Ti₃Al, Al₃Ti) depend strongly on the grain size of titanium target. It is shown that the nanostructural particles of TiO₂ phase are located mainly on dislocations in the body of target grains. A Ti₂O surface layer is found to arise in titanium with a grain size of 17 μm. It is established that an ordered Ti₃Al phase is located at a depth of more than 200 nm in the implanted layer along the bounaries of the titanium grains.     

Electron microscopy investigation of AlMg6 aluminum alloy surface defects caused by microorganisms extracted in space stations

The surfaces of AMg6 (aluminum-magnesium) alloy samples that have passed accelerated biocorrosion tests have been investigated in a Quanta-3D scanning electron microscope. The alloy samples have been treated with the Ulocladium botrytis Preuss fungus, which is an active destructive fungus and was previously extracted on surfaces of the International Space Station. Biocorrosion pits 2–10 μm in diameter, cavities the depths of which can reach 70–250 μm depth, and spots of modified color are found to be the most typical defects. The surfaces of large cavities consist of faceted cubic particles that are formed when the acid products of fungus metabolism interact with the alloy surface. The particles have an average size of 30 μm, which is close to the size of alloy grains. The microstructure of a biocorrosion layer has been investigated in a Quanta-3D microscope with the use of a focused Ga⁺ ion beam. The samples of 12-μm-wide transverse slices are obtained near large cavities and investigated in a Tecnai G₂₃0ST transmission electron microscope. The X-ray microanalysis of the defective layer has revealed the high concentration of oxygen in this layer. Obtained images indicate that the corrosion cavity surface has a complex porous structure.     

Experimental and Monte Carlo investigation of visible diffuse-reflectance imaging sensitivity to diffusing particle size changes in an optical model of a bladder wall

We have investigated the influence of scatterer size changes on the laser light diffusion, induced by collimated monochromatic laser irradiation, in tissue-like optical phantoms using diffuse-reflectance imaging. For that purpose, three-layer optical phantoms were prepared, in which nano- and microsphere size varied in order to simulate the scattering properties of healthy and cancerous urinary bladder walls. The informative areas of the surface diffuse-reflected light distributions were about 15×18 pixels for the smallest scattering particles of 0.05 μm, about 21×25 pixels for the medium-size particles of 0.53 μm, and about 25×30 pixels for the largest particles of 5.09 μm. The computation of the laser spot areas provided useful information for the analysis of the light distribution with high measurement accuracy of up to 92%. The minimal stability of 78% accuracy was observed for superficial scattering signals on the phantoms with the largest particles. The experimental results showed a good agreement with the results obtained by the Monte Carlo simulations. The presented method shows a good potential to be useful for a tissue-state diagnosis of the urinary bladder.     

Excimer laser-induced microbumps on preheated BK7-glass

Microbumps on BK7-glass are made by nanosecond KrF-excimer laser irradiation. Bumps with heights in the micrometer-range and a diameter of about 20 μm are reproducibly obtained in the case of a preheated glass substrate. Preheating and laser-induced local heating act together to exceed the glass transition temperature on a volume forming a bump due to the action of surface tension. This process may open up a route to the rapid fabrication of microlens arrays.     

Emission of submicron particles upon rock deformation

Microparticle emission upon rock deformation is established experimentally. The results from experiments on the uniaxial compression of samples with a through hole as a stress concentrator with the simultaneous recording of emissions of mineral particles using laser spectrometry are presented. Quantitative estimates of the emission of particles in the size range of 0.3–5.0 μm with respect to the acting stresses are obtained.     

Aggregation modeling of calcium carbonate particles by Monte Carlo simulation

The mechanism on aggregation of spindle granular particles of calcite was investigated for the carbonation of calcium hydroxide in aqueous suspension for the purpose of controlling morphology of CaCO₃. The experimental carbonation process was carried out in a semi-batch bubble column reactor under different conditions. Although, fine rhombic nano-particles diameter ranged from 100 to 200 nm were obtained at 291 K, a higher temperature of 300 K provided spindle granular particles with a length of 1.0–1.5 μm and a width of 0.3–0.5 μm. The average crystallite size was 28 nm for the fine rhombic nano-particles and 43 nm for the spindle granules. Zeta potential measurement for the spindle granules indicated that the suspension tended to be aggregated during the carbonation process. The effect of the degree of particle aggregation on the shape of the obtained calcite particles was studied by Monte Carlo simulations. Our simulation results elucidated the dependence of aggregation on unit particles, i.e., primary particles, on the experiment carbonation condition where the spindle granules were formed out of the unit particles under the same condition as the experiments. In addition, the formation mechanism of the granules was investigated by applying classical nucleation theory to the present simulations.     

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     

Effective 2.5-μm ZnSe:Cr<Superscript>2+</Superscript> laser with transverse laser pumping

Effective lasing is obtained in the transversely pumped ZnSe single crystals that are doped with Cr²⁺ ions using diffusion methods. A Q-switched Er³⁺-doped glass laser with a radiation wavelength of 1.54 μm is used for pumping. The resulting laser energy is 150 μJ at an absorbed pump energy of 600 μJ, so that the efficiency is 25% and the slope efficiency is 29%. An increase in the gain (up to the superluminescence level) due to the application of the transverse pumping of the active element with a substantially non-uniform distribution of the dopant is discussed.     

Scanning force microscopy of catalytic nickel particles prepared from carbon nanotubes

The method of scanning force microscopy (SFM) is used to study catalytic nickel nanoparticles deposited on a substrate of quartz glass by decomposition of carbon nanotubes. The SFM images so obtained were computer processed using an original numerical deconvolution algorithm which allowed us to determine the actual dimensions and shape of the nanoparticles. Nonoverlapping particles with diameters from 20 to 200 nm were recorded. Analysis of the SFM images revealed that the shape of the nickel particles is nearly spherical, which is in good agreement with transmission electron microscopy data. Fiz. Tverd. Tela (St. Petersburg) 39, 2065–2071 (November 1997)