Laser Focal Point Research Articles

Sample concentration using optical chromatography

Optical chromatography is a technique for the separation of particles that capitalizes on the balance between optic and fluidic forces. When microscopic particles in a fluid flow encounter a laser beam propagating in the opposite direction, they are trapped axially along the beam. They are then optically pushed upstream from the laser focal point to rest at a point where the optic and fluidic forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, both coarse and fine separations are possible. We describe how an optical chromatography beam directed into a tailored flow environment, has been adapted to operate as an optical filter for the concentration / bioenrichment of colloidal and biological samples. In this work, the demonstrated ability to concentrate spores of the biowarfare agent, Bacillus anthracis, may have significant impact in the biodefense arena. Application of these techniques and further design of fluidic and optical environments will allow for more specific identification, concentration and separation of many more microscopic particle and biological suspensions.

Application of Response Surface Methodology in Describing the Residual Stress Distribution in CO2 Laser Welding of AISI304

Abstract: Laser welding is a low‐distortion welding process. Despite this advantage some detrimental residual stresses are still introduced during the process, which reduce the service life of the welded component. Therefore, it is important to estimate the magnitude and distribution of the residual stress. This study aims to create mathematical models to determine the relationship between laser welding parameters and the magnitude of the residual stress at different locations by using response surface methodology. In this study, the process input parameters are laser power, travel speed and focal point position. Laser butt‐welding of AISI304 plates joints were investigated using a 1.5 kW continuous wave CO2 Rofin laser as a welding source. Incremental hole‐drilling method was employed to measure the magnitude and the distribution of the maximum residual stress. The experiment was developed using a three‐factor, five‐level central composite design with full replication. Analysis of variance and other adequacy measures were employed to check the adequacy of the developed models using the statistical package Design‐Expert. Twenty‐one models were developed to demonstrate the magnitude and distribution of the residual stress. The main effect of each factor and the interaction effect with other factors were determined quantitatively and presented graphically.

Single Cell Control Based on Femtosecond Laser-induced Nonlinear Phenomena

When a near-infrared (NIR) femtosecond laser is focused in water, several kinds of nonlinear phenomena, such as shockwave propagation, cavitation bubble generation, and jet flow, are induced in the vicinity of the laser focal point. Such phenomena are also induced by near-infrared nanosecond and picosecond lasers, but femtosecond irradiation allows a better localization. Moreover NIR femtosecond laser focussing permits to perform a local processing of cell and tissue because of its efficient multi-photon absorption and photo-mechanical ablation mechanism. In this study, the impulsive force generated in cell culture medium by these nonlinear phenomena was applied to individual control of single animal cells. Mouse NIH-3T3 fibroblast cells cultured on a substrate were independently isolated by the impulsive force, and patterned by transferring them to another substrate. Furthermore nanoparticles and biological molecules are transfected to the cell by the local processing of the cell membrane. These demonstrations are promising for future femtosecond laser methodology for bioscience and biotechnology.

Compositionally graded Ti6Al4V + TiC made by direct laser fabrication using powder and wire

Ti6Al4V reinforced with TiC has been fabricated as compositionally graded material by direct laser fabrication using TiC powder and Ti6Al4V wire which were fed simultaneously into the laser focal point. The microstructure along the length of the sample has been characterised using X-ray diffraction and scanning electron microscopy. The results show that the composition along the length changes as expected from the imposed changes in feed rate when allowance is made for the different capture efficiency for the powder and the wire. Some unmelted TiC has been observed in regions where the TiC fraction was high, but along most of the length of the samples TiC was completely melted and formed primary TiC, eutectic TiC and secondary TiC. Some preliminary tribological properties of the compositionally graded material were obtained using a sliding wear test which showed that the tribological properties of Ti6Al4V are improved by the reinforced TiC particles with the optimum frictional behaviour being found with approximately 24 vol% of TiC.

Residual Stresses Prediction for CO<sub>2</sub> Laser Butt-Welding of 304-Stainless Steel

Residual stresses are an integral part of the total stress acting on any component in service. It is important to determine and/or predict the magnitude, nature and direction of the residual stress to estimate the life of important engineering parts, particularly welded components. This work aims to introduce experimental models to predict residual stresses in the heat-affected zone (HAZ). These models specify the effect of laser welding input parameters on maximum residual stress and its direction. The process input variables considered in this study are laser power (1.03 - 1.368 kW), travel speed (26.48 – 68.52 cm/min) and focal point position (- 1 to 0 mm). Laser butt-welding of 304 stainless steel plates of 3 mm thick were investigated using a 1.5 kW CW CO2 Rofin laser as a welding source. Hole-drilling method was employed to measure the magnitude, and direction of the maximum principal stress in and around the HAZ, using a CEA-06- 062UM-120 strain gauge rosette, which allows measurement of the residual stresses close to the weld bead. The experiment was designed based on Response Surface Methodology (RSM). Fifteen different welding conditions plus 5 repeat tests were carried out based on the design matrix. Maximum principal residual stresses and their directions were calculated for the twenty samples. The stepwise regression method was selected using Design-expert software to fit the experimental responses to a second order polynomial. Sequential F test and other adequacy measures were then used to check the models adequacy. The experimental results indicate that the proposed mathematical models could adequately describe the residual stress within the limits of the factors being studied. Using the models developed, the main and interaction effect of the process input variables on the two responses were determined quantitatively and presented graphically. It is observed that the travel speed and laser power are the main factors affecting the behavior of the residual stress. It is recommended to use the models to find the optimal combination of welding conditions that lead to minimum distortion.

Microstructure study of direct laser fabricated Ti alloys using powder and wire

A compositionally graded material has been fabricated using direct laser fabrication (DFL). Two types of feedstock were fed simultaneously into the laser focal point, a burn resistant (BurTi) alloy Ti–25V–15Cr–2Al–0.2C powder and a Ti–6Al–4V wire. The local composition of the alloy was changed by altering the ratio of powder to wire by varying the feed rate of the powder whilst maintaining a fixed feed rate of wire-feed. For the range of compositions between about 20% and 100% BurTi only the beta phase was observed and the composition and lattice parameter varied monotonically. The grain size was found to be much finer in these functionally graded samples than in laser fabricated Ti64. Some samples were made using the wire-feed alone, where it was found that the microstructure is different from that found when using powder feed alone. The results are discussed in terms of the power requirements for laser fabrication of powder and wire samples.

1414 集束レーザ光を用いた三次元微細形状の加工(S80-2 特殊加工プロセス(2),S80 特殊加工プロセス)

Development of a high-speed microfabrication technique using focused laser beam was conducted. Conventional CAD/CAM system was used and the beam was scanned mechanically by attaching both optical mirror arrays and a focal lens to the column of an ultraprecision machining center. Measured groove width and pocket depth obtained by grooving and pocket machining were used as a tool diameter and an axial depth of cut. A glass-like carbon having over 80% laser beam absorptivity and 650MHv hardness was employed as a workpiece. It was verified through the machining test that pseudo 3 dimensional concave prisms could be fabricated by digging square pockets with different square sizes and the same pocket depth to negative Z direction. Shifting laser focal point from the work surface could control a depth of the square pocket, so that a high-speed roughing by adjusting focal point on the work surface could be conducted. In addition, the laser micromachining system could be adapted to a high-speed finishing by shifting focal point from the work surface.

Cluster-assisted multiple ionization of methyl iodide by a nanosecond laser: Influence of laser intensity on the kinetic energy and peak profile of multicharged ions

The dependences of kinetic energies and peak profiles of multicharged ions of I q+ ( q = 2–3) and C 2+ on the laser intensity have been studied in detail by time-of-flight mass spectrometry, those multicharged ions are produced by irradiation of methyl iodide cluster beam with a nanosecond 532 nm Nd-YAG laser. Our experiments show that the kinetic energies released of multicharged ions increase linearly with the laser intensity in the range of 3 × 10 9–2 × 10 11 W/cm 2. The peaks of multicharged ions are split to forward ions and backward ions, and the ratio of the backward ions to forward ions decreases exponentially with laser intensity. The decreasing of backward ions is probably due to Coulomb scattering by the heavier I + ions when they turn around through the laser focus point. The linear dependence of kinetic energy of multicharged ions on laser intensity is interpreted by the ionization mechanism, in which the laser induced inverse bremsstrahlung heating of electron is the rate-limiting step.

Catalytic Three-Dimensional Protein Architectures

We demonstrate a strategy for microfabricating catalytically active, three-dimensional matrixes composed of cross-linked protein in cellular and microfluidic environments. In this approach, a pulsed femtosecond laser is used to excite photosensitizers via multiphoton absorption within three-dimensionally defined volumes, a process that promotes cross-linking of protein residue side chains in the vicinity of the laser focal point. In this manner, it is possible to fabricate protein microparticles with dimensions on the order of the multiphoton focal volume (less than 1 microm(3)) or, by scanning the position of a laser focal point relative to a specimen, to generate surface-adherent matrixes or cables that extend through solution for hundreds of micrometers. We show that protein matrixes can be functionalized either through direct cross-linking of enzymes, by decoration of avidin matrixes with biotinylated enzymes, or by cross-linking biotinylated proteins that then are linked to biotinylated enzymes via an avidin couple. Several formats are explored, including microparticles that can be translocated to desired sites of action (including cytosolic positions), protein pads that generate product gradients within cell cultures, and on-column nanoreactors for microfluidic systems. These biomaterial fabrication technologies offer opportunities for studying a variety of cell functions, ranging from single-cell biochemistry and development to perturbation and analysis of small populations of cultured cells.

Effect of laser welding parameters on the heat input and weld-bead profile

Laser butt-welding of medium carbon steel was investigated using CW 1.5 kW CO 2 laser. The effect of laser power (1.2–1.43 kW), welding speed (30–70 cm/min) and focal point position (−2.5 to 0 mm) on the heat input and the weld-bead geometry (i.e. penetration ( P), welded zone width ( W) and heat affected zone width ( W HAZ)) was investigated using response surface methodology (RSM). The experimental plan was based on Box–Behnken design. Linear and quadratic polynomial equations for predicting the heat input and the weld-bead geometry were developed. The results indicate that the proposed models predict the responses adequately within the limits of welding parameters being used. It is suggested that regression equations can be used to find optimum welding conditions for the desired criteria.

Laser-induced cavitation bubbles for cleaning of solid surfaces

When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency.

Electrical Probe Measurements of the Late Stage of the Plasma Produced by Laser Irradiation in an Argon Gas-puff Target

The electron temperature and the expansion velocity of the late stage of an laser-induced argon plasma in an argon gas-puff target was measured using a Langmuir probe. The typical electron temperature at a distance of 100 mm from the laser focal point was found to be less than 1.0 eV and the expansion velocity of the plasma was about 1600 m/s.

Rydberg Matter of K and N 2: angular dependence of the time-of-flight for neutral and ionized clusters formed in Coulomb explosions

Laser fragmentation via Coulomb explosions in a cloud of Rydberg Matter (RM) is shown to give an angular dependence of the time-of-flight (TOF) observations for neutral RM clusters. The RM cloud is formed by emission from a K doped FeO catalyst sample at 700–900 K and is formed of K ∗ atoms, or N 2 ∗ molecules after nitrogen gas adsorption in the emitter. Close to the surface normal of the emitter, relatively large clusters from high excitation states in the RM dominate, while at large angles (40–50°) smaller clusters and monomers with high kinetic energy release from lower excitation states are observed more frequently. N 2 RM usually gives smaller clusters and more monomers than K RM. The extension of the RM cloud in the apparatus is easily observed by the different TOFs for the various particles at different angular settings of the detector, and by direct shifting of the laser focus point. The TOF results show well defined Rydberg states and RM clusters with very low translational temperature, below 100 K even for the monomers.

Thermal gradient control at the solid–liquid interface in the laser-heated pedestal growth technique

The laser-heated pedestal growth technique, which is used for preparing single crystal fibers, involves very large temperature gradients at the growth interface. While this is often useful, it can also be problematic for some material systems. By introducing an additional optical component into the laser beam path, we were able to substantially decrease the axial temperature gradients. In this new configuration, the CO 2 laser beam divergence was altered by using a ZnSe beam expander. This created an optical aberration at the laser focal point, thereby redistributing the thermal energy over a larger melt surface area. With this modification the thermal gradients were decreased from (3800±100)°C/cm to (2700±100)°C/cm during the growth of Bi–Sr–Ca–Cu–O (2 2 1 2) single crystal fibers, showing that it is possible to control the thermal gradients by manipulating the laser beam, before it enters the growth chamber.

Generation of calcium waves in living cells by pulsed-laser-induced photodisruption

Here, we present an optical technique that can induce waves of calcium ion concentration in live biological cells. Ca2+ waves were induced by femtosecond pulsed-laser illumination. Living HeLa cells were exposed to focused 140 fs pulses of 780 nm wavelength at 30 mW average power. Ca2+ waves were imaged by fluorescence and were observed to propagate from the laser focal point inside the cell. Photoinduced generation of Ca2+ waves can be performed at any point inside the cell, an improvement over previous mechanical or biochemical stimulation techniques.

Determination of trace elements in quartz glass by use of LINA-Spark-ICP-MS as a new method for bulk analysis of solid samples.

The determination of trace elements in pure quartz glass samples has been performed by coupling an ICP quadrupole mass spectrometer with the LINA-Spark-Atomizer, an IR laser ablation system dedicated to direct bulk and surface analysis of solid samples. Linear calibration curves were obtained for nine elements (Na, Al, Ca, Ti, Cr, Mn, Zr, Ba, and Pb) in the ng g(-1) range with detection limits of less than 10 ng g(-1) for Ca, Cr, Mn, Zr, Ba, and Pb and in the range of 120-220 ng g(-1) for Na, Al, and Ti. The distance between the laser focal point and the sample surface has a significant influence on signal intensity and precision, both of which can be improved by a factor of approximately two by focusing the laser 15 mm behind the sample surface. Aerosol moistening reduced the standard deviation of the signal intensity by a factor of 2-4. Signal instability, which resulted from different ablation rates or variations in the transmission of the mass spectrometer, were compensated by use of the simultaneously measured SiAr+ ion as an internal standard. Under these conditions precision was usually better than 5% RSD. The results were compared with those obtained by use of a commercial LA-ICP-MS system. With this instrumentation linear calibration curves were achieved for three elements only (Al, Ti, and Pb), showing that LA-ICP-MS is less appropriate for bulk analysis in the ng g(-1) range.

Acceleration of re-ignition of high pressure mercury discharge lamps using photo-pre-ionization

High Intensity Discharge (HID) lamps are difficult to re-ignite rapidly because of the high pressure of metal vapor at high temperatures. We have succeeded in reducing the re-ignition time of high-pressure mercury lamps by using a pre-ionization step where an excess of electrons are generated by UV laser irradiation into the lamp. The effect of changing the laser focal point of the UV laser in the lamp and the position of an auxiliary electrode were also investigated. The time interval for re-ignition was reduced from 230s to 100s by laser irradiation near to the cathode. The results clearly showed that the effect of accelerating the re-ignition time by the pre-ionization step was determined by the behavior of excess electrons generated during the UV laser irradiation.

Sheath fluid control to permit stable flow in rapid mix flow cytometry.

Flow cytometry is a potentially powerful tool to analyze the kinetics of ligand binding, cell response and molecular assembly. The difficulty in adding reactant to cells, achieving adequate mixing, delivering those cells to the laser focal point and establishing stable flow, has historically limited flow cytometry to systems with reactions times longer than 5 s. With the advent of automated syringes and flow injection methods, sample injection times shorter than 1 s have become routine. However, an inherent problem in acquiring time courses starting under 1 s is that rapid sample introduction through the flow tip to the detection point perturbs laminar flow. The purpose of this work was to determine if stable flow could be reestablished more quickly if the sheath flow was reduced during sample introduction, returning to normal sheath and sample rates afterward. We used programmable syringes and valves to control sample mixing as well as sheath and sample delivery through the flow tip to the detection point for stream-in-air detection. Stable flow was monitored by mean particle fluorescence during sample introduction. With no sheath reduction, stable flow recovered after more than 1 s. By reducing sheath flow during the short period (300 msec) of sample mixing and delivery, stable laminar flow recovered within 200 msec. This use of automated syringes to control both sheath and sample flow provides a potential for robust sample handling applicable to kinetic as well as high throughput flow cytometric analysis.

Direct laser trapping of single DNA molecules in the globular state.

A sharply focused laser is able to trap small particles at the laser focal point due to the difference in refractive index of the particles and that of the surrounding medium. This technique, called laser trapping, can be used to manipulate animal or bacterial cells without any contact and has been widely applied in biological research. However, it has been difficult to trap biological macromolecules such as DNA molecules, because these molecules give a low difference in refractive index and cannot overcome Brownian motion. DNA molecules can be transformed to a condensed globular state. This transformation results in a higher refractive index of DNA due to its increased density. We demonstrate in this paper that a single DNA molecule can be optically trapped using a Nd:YAG laser (1064 nm wavelength) upon transformation from the coiled state to the globular state.

Picoliter sample preparation in MALDI-TOF MS using a micromachined silicon flow-through dispenser.

This paper presents a picoliter sample preparation technique utilizing the flow-through principle, allowing on-line coupling of chromatographic systems to be made. The work was performed in order to investigate the characteristics and the physicochemical properties of the sample preparation using typical mobile phase conditions from mu-CLC (column liquid chromatography) separations. The device presented here is a pressure pulse-driven dispenser, formed by two silicon structures processed by conventional micromachining. The pressure pulse is generated in the flow-through channel by a piezoceramic element. Depending on the orifice size, the droplets ejected range between 30 and 200 pL. The maximum ejection frequency is 500 Hz, limited by resonances within the unit. A pyramid-shaped nozzle improves the directivity of the droplets since it reduces the wetting of the orifice front surface area. The risk of particles sticking close to the orifice is also minimized. The analyses of the deposited sample spots were carried out on a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer with delayed extraction. It was possible to detect attomole amounts (159-248 amol) of various proteins (cytochrome c, ribonuclease A, lysozyme, and myoglobin) from a single droplet of matrix:analyte 1:1 (drop volume approximately 110 pL). Additionally, it was found that sample enrichment could be carried out using multiple depositions on the same spot; i.e., 31 nM of insulin was easily detected when more than four depositions were made on the same spot, while no detection was possible without sample enrichment. Size optimization of the MALDI sample spot gave target zones of 100-500-micron diameter that matched the size of the laser focal point and resulted in a considerably increased sample throughput.