Increasing Number Of Pulses Research Articles

The ripples and nanoparticles on silicon irradiated by femtosecond laser

The surface configuration evolvement of silicon with the increasing pulse number irradiated by femtosecond laser has been studied. The ripples and nanoparticles are formed in the irradiated region. The period of the ripples decreases with the increasing pulse number, while the diameter of the visible structured region increases with the increasing pulse number. The interference of laser with surface plasmons is proposed to explain the formation of the ripple. The decreasing of the spatial period for surface ripples is due to the decrease of the surface plasmon wavelength with the decreasing electron number density in the plasma.

Surface Treatment of 316L Stainless Steel by High Current Pulsed Electron Beam

High current pulsed electron beam has been developing as a useful tool for surface modification of materials. This paper presents our research on the surface treatment of 316L stainless steel with an equipment of working parameters as electron energy 20-30keV, pulse duration 1.5µs and energy density ~6J/cm2. The surface microstructure was characterized with metallography, X-ray diffractometry and electron backscatter diffraction (EBSD) techniques. It was found that the modified samples showed significant improvement on corrosion resistance when using increased pulse number and higher energy density. This result is discussed in relation to the coupled temperature-stress fields formed after the absorption of electron beam energy.

Physical Modeling Study on Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Back-attack Phenomenon of Gas Streams with Horizontal Side Blowing and Its Influence on Erosion and Wear of Refractory Lining

The back-attack phenomenon of the gas side blowing streams and its influence on the erosion and wear of the refractory lining during the combined side and top blowing AOD refining process of stainless steel were investigated on a water model unit of a 120 t AOD converter. Sufficiently full kinetic similarity between the model and its prototype was maintained. The influences of the gas flow rates for side and top blowing, the side tuyere position and number were examined. The preliminary industrial experiments were conducted. The results indicated that the presence of a gas top blowing jet and using of multiple tuyeres did not change the basic features of the back-attack, but could give it some distinctive behaviors. The back-attack could indeed bring about the evident and uneven erosion and wear of the lining. On the back-attack, the gas streams of the main tuyeres had a decisive role, and the subtuyere streams showed a certain suppression and alleviation effect. At a given tuyere number and position, its frequency and pressure and the total average pulse number in per unit time in the processes of this work increased with an increase in the gas side blowing rate. At a given tuyere position and gas side blowing rate, the back-attack and its influence on the erosion and wear of the lining enhanced with decreasing the tuyere number (increasing the gas flow rate for single tuyere). The gas top blowing jet could make the back-attack become more uniform and its frequency decrease, and its intensity and the total average pulse number increase; and it could reduce the eroded and worn rate of the lining at a given tuyere number and position and gas side blowing rate. The increased amplitude of the back-attack intensity and the extension of the damaged area of the lining caused by the buoyancy in a combined blowing were smaller and lower than those in a simple side blowing. As another important reason resulted in the back-attack and the erosion and wear of the lining, the effectiveness of the circulatory motion of the liquid in a combined blowing is different from that in a simple side blowing. At a given tuyere number, properly increasing the angle between each tuyere could be beneficial to alleviating the back-attack and to slowing down the erosion and wear of the lining. Under the conditions of this work, the back-attack actions and the lining eroded and worn extents and rates with 7 tuyeres and 22.5° or 6 tuyeres and 27° were all gentler and lower than those with the other tuyere equipments and arrangements. The results obtained from the physical modeling studies on the refining process were reliable, believable and valid. Suitably increasing the angle between each tuyere of the 120 t AOD converter could raise the life of its lining by a big margin, and remarkably improve the technical and economic indications of the process.

Estimation of temperature rise from trapped field gradient on superconducting bulkmagnetized by multi-pulse technique

Trapped field profiles of the GdBaCuO superconducting bulk of 65 mm in diameter have beenmeasured as magnetized using a successive pulsed-field application (SPA) and a subsequentiterative pulsed-field magnetization with reducing amplitude (IMRA). The trapped field gradientdBT/dx on the bulk periphery increases concomitantly with increasing pulse number in the IMRA part. ThedBT/dx valueon the bulk magnetized by field-cooled magnetization (FCM) was also measured at each temperatureTF. The maximumtemperature Tmax for each pulse was estimated by comparing thedBT/dx value for the IMRA part with that for the FCM at each temperature. Furthermore,Tmax was measured directly for the SPA part in the drilled hole in the bulk. The estimatedTmax was nearly equal tothe measured Tmax, whichthereby confirmed that Tmax after applying the pulsed field can be estimated using thedBT/dx value in the bulk periphery. The enhancement of the total trapped fluxΦTP inthe IMRA part with increasing pulse number results from enhancement of the critical current densityJc because of thereduction in Tmax.

Experimental study of infrared nanosecond laser ablation of silicon: The multi-pulse enhancement effect

Experimental study has been performed on nanosecond (ns) laser ablation of silicon at 1064 nm, through which a so-called “multi-pulse enhancement effect” has been revealed, which has been rarely reported in literature. The major features of this effect are: (1) for multi-pulse laser ablation at the same spatial location, the ablation efficiency increases as the pulse number increases and the pulse-to-pulse temporal distance decreases; (2) for multi-pulse ablation performed sequentially at a group of locations, the ablation quality and efficiency starting from the second location can be significantly enhanced if the distance between adjacent locations is sufficiently small. Further study is needed to confirm and understand the underlying physical mechanism for the multi-pulse enhancement effect, which can be utilized to significantly improve the quality and efficiency of laser silicon micromachining using the low-cost and low-energy-consumption infrared ns lasers. This may decrease the cost and energy consumption of many relevant areas, such as the solar cell industry.

Improved wear resistance of magnesium alloys AZ91 by high current pulsed electron beam treatment

Surface modification of magnesium alloy AZ91 by high current pulsed electron beam (HCPEB) was investigated in the present work. After irradiated by HCPEB, crater-like defects were found scattering on the modified surface, when increasing pulse number, the density of craters sharply reduced. According to XRD analysis, hard intermetallic phase Mg17Al12 would sappear after several pulse of HCPEB treatment that is accordance with the improvement of wear resistance, but more pulses will lead to the solution of Mg17Al12 phase. Ultimately, due to the thickest depth of modified layer, sample treated with 30 pulses and accelerating voltage of 27 kV exhibited the best wear resistance, the depth of wear track decreased from 12.75 µm in initial state to 7.82 μm.

EXPERIMENTAL ANALYSIS OF MULTIVARIATE FEMALE CHOICE IN GRAY TREEFROGS (HYLA VERSICOLOR): EVIDENCE FOR DIRECTIONAL AND STABILIZING SELECTION

Even simple biological signals vary in several measurable dimensions. Understanding their evolution requires, therefore, a multivariate understanding of selection, including how different properties interact to determine the effectiveness of the signal. We combined experimental manipulation with multivariate selection analysis to assess female mate choice on the simple trilled calls of male gray treefrogs. We independently and randomly varied five behaviorally relevant acoustic properties in 154 synthetic calls. We compared response times of each of 154 females to one of these calls with its response to a standard call that had mean values of the five properties. We found directional and quadratic selection on two properties indicative of the amount of signaling, pulse number, and call rate. Canonical rotation of the fitness surface showed that these properties, along with pulse rate, contributed heavily to a major axis of stabilizing selection, a result consistent with univariate studies showing diminishing effects of increasing pulse number well beyond the mean. Spectral properties contributed to a second major axis of stabilizing selection. The single major axis of disruptive selection suggested that a combination of two temporal and two spectral properties with values differing from the mean should be especially attractive.

Laser-induced backside dry and wet etching of transparent materials using solid and molten tin as absorbers

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm2. A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (slLIBDE=0.111 nm/mJ cm−2, slLIBDE=0.127 nm/mJ cm−2) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm2, respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.

Femtosecond pulsed laser damage characteristics of 7% Y2O3-ZrO2 thermal barrier coating

Ablation characteristics of 7% yttria stabilized zirconia thermal barrier coating, deposited by the electron beam physical vapor deposition method, was studied using a femtosecond pulsed laser. The single-pulse damage threshold fluence for the coating was determined to be 1.64 J cm-2. The threshold fluence was not appreciably dependent on the initial roughness of the coating surface. An incubation effect, i.e., a decrease in the damage threshold fluence with an increase in number of pulses incident at a given location, was observed for this coating. Based on the damage morphology generated by single and multi-pulse interactions, a mechanism for the incubation effect was proposed.

Standing electron plasma wave mechanism of void array formation inside glass by femtosecond laser irradiation

We investigate the mechanism of formation of periodic void arrays inside fused silica and BK7 glass irradiated by a tightly focused femtosecond (fs) laser beam. Our results show that the period of each void array is not uniform along the laser propagation direction, and the average period of the void array decreases with increasing pulse number and pulse energy. We propose a mechanism in which a standing electron plasma wave created by the interference of a fs-laser-driven electron wave and its reflected wave is responsible for the formation of the periodic void arrays.

Characterization of rebound depolarization in hippocampal neurons

Rebound depolarization (RD) following hyperpolarizing pulses is found in several neuronal cell types where it takes part in the regulation of neuronal firing behavior. During whole-cell current and voltage clamp recordings in slice preparations, we investigated the modulation of RD by different stimulation patterns and its underlying ionic currents in rat CA1 pyramidal cells. RD was mainly carried by the hyperpolarization-activated cation current I h (about two-third) and T-type calcium currents (about one-third), respectively. RD increased with increasing hyperpolarizing amplitude and stimulation frequency, whereas RD substantially decreased with longer pulse duration and, less pronounced, with increasing pulse number. The pulse duration-related decrease of RD was due to a decrease of the driving force of I h. In conclusion, we showed that RD is differentially modulated by precedent hyperpolarization. Since RD amplitude was high enough to generate action potentials, RD may serve, even under physiologic conditions, as an inhibition-excitation converter.

Inactivation of naturally occurring microorganisms in tomato juice using pulsed electric field (PEF) with and without antimicrobials

The combination of different hurdles such as moderately high temperatures (<50 °C), antimicrobial compounds and pulsed electric field (PEF), to reduce naturally occurring microbes in tomato juice was explored. The microbial count decreased with the increase in pulse number and treatment temperature at constant field strength. At field strength of 80 kV/cm, 20 pulses, 50 °C, and in the presence of nisin (100 U/mL), there was about 4.4 log reductions in microbial counts. There was significant difference ( P < 0.05) in the reduction of microbial count when the temperature was increased from 45 to 50 °C. There was no Vitamin C reduction due to the treatment. Treated juice, without aseptically filled, was stored at 4 °C for 28 days without any significant microbial growth. Enzyme polygalacturonase was unaffected by PEF, but the activity of pectin methyl esterase was reduced by 55%. Antimicrobials such as clove oil and mint extract provide large microbial decay at low concentration and mild heat without PEF.

Compressive Residual Stress Generation ProcessBy Laser Peening Without Pre-coating

Laser peening without pre-coating has been applied to water-immersed specimens of high tensile strength steel. In order to understand the generation process of compressive residual stress, specimens with various laser irradiation patterns, i.e., single spot, line scanning and area scanning, were prepared. Detailed distributions of residual stress on the specimens were measured using synchrotron radiation. Large tensile residual stresses, which might be caused by the thermal effect of the laser pulse, were observed in the center region on the single spot. It decreased towards the edge of the spot, and changed to compression around the spot. The compression became larger with the increasing pulse numbers irradiated on the same spot. In the line scanning, a tensile residual stress was observed in the final spot of the line, which decreased and changed to compression as the distance from the final spot increased. The residual stress on the area scanning was compression as a whole. It was estimated that the compressive residual stress in the area scanning would be generated from the overlapping effect of the compressive field made around each laser spot. The residual stress generated by laser peening without pre-coating is considered to be the superimposition of the tensile and the compressive components due to thermal effect and plastic deformation, respectively.

On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.

Topography and roughness evolution of microstructured surfaces at laser-induced backside wet etching

The topography and roughness evolution of surfaces etched by laser-induced backside wet etching (LIBWE) is investigated in detail. The etching of sub-μm gratings with a period of 760 nm into flat surfaces by means of interfering laser beams shows a saturation of the grating depth within 20 pulses. The over-etching of already microstructured surfaces results in the change of the cross section and in reduction of the microstructure height with increasing pulse number. The decrease in height of sub-micron gratings from 125 to less than 10 nm within 15 laser pulses causes a substantial roughness reduction. The depth limitations in etching of the gratings as well as the height reduction of microstructures are the result of the influence of the surface topography to the heat flow. The more efficient heating of surface peaks in contrast to the valleys results in higher etch rates and probably causes the smooth surfaces observed in LIBWE processing. The thermal diffusion length determines the structure dimension influenced by this “smoothing” effect.

Adsorbed layer etching of fused silica by excimer laser with nanometer depth precision

High quality etching of transparent materials for applications in micro- and nano-structuring as well as in precision engineering is still a challenge for current laser processing techniques. The recently developed LESAL technique allows precise etching of transparent materials, e.g., fused silica, at low laser fluences with small etch rates in the nanometer scale. The etch process is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. A constant etch rate of 1.4 nm/pulse was measured in a fluence range from 2.0 to 4.5 J/cm 2 for fused silica. The etched surfaces with a depth of more than 1 μm feature a roughness of below 1 nm rms. With increasing pulse number the rising roughness is mainly caused by the growing waviness and spikes. Mask projection techniques were applied for the fabrication of surface structures in the micron range with nanometer depth precision and a low roughness.

Dynamics of excitation pulses with attractive interaction: Kinematic analysis and chemical wave experiments

We present a theoretical analysis of stacking and destacking wave trains in excitable reaction-diffusion systems with anomalous velocity-wavelength dependence. For linearized dispersion relations, kinematic analysis yields an analytical function that rigorously describes front trajectories. The corresponding accelerations have exactly one extremum that slowly decays with increasing pulse number. For subsequent pulses these maxima occur with a lag time equal to the inverse slope of the linearized dispersion curve. These findings are reproduced in experiments with chemical waves in the 1,4-cyclohexanedione Belousov-Zhabotinsky reaction but should be also applicable to step bunching on crystal surfaces and certain traffic phenomena.

UV laser micromachining of piezoelectric ceramic using a pulsed Nd:YAG laser

UV laser (λ=355 nm) ablation of piezoelectric lead zirconate titanate (PZT) ceramics in air has been investigated under different laser parameters. It has been found that there is a critical pulse number (N=750). When the pulse number is smaller than the critical value, the ablation rate decreases with increasing pulse number. Beyond the critical value, the ablation rate becomes constant. The ablation rate and concentrations of O, Zr and Ti on the ablated surface increase with the laser fluence, while the Pb concentration decreases due to the selective evaporation of PbO. The loss of the Pb results in the formation of a metastable pyrochlore phase. ZrO2 was detected by XPS in the ablated zone. Also, the concentrations of the pyrochlore phase and ZrO2 increase with increasing laser fluence. These results clearly indicate that the chemical composition and phase structure in the ablated zone strongly depend on the laser fluence. The piezoelectric properties of the cut PZT ceramic samples completely disappear due to the loss of the Pb and the existence of the pyrochlore phase. After these samples were annealed at 1150 °C for 1 h in a PbO-controlled atmosphere, their phase structure and piezoelectric properties were recovered again. Finally, 1–3 and concentric-ring 2–2 PZT/epoxy composites were fabricated by UV laser micromachining and their thickness modes were measured by impedance spectrum analysis and a d33 meter. Both composites show high piezoelectric properties.

Laser annealing of silicon nanocrystal films formed by pulsed-laser deposition

We report the laser annealing effects on the structures and properties of silicon nanocrystal films fabricated by pulsed-laser deposition in inert argon gas. The surface morphology of the as-deposited films is greatly determined by the angle relative to the laser ablation spot. Photoluminescence (PL) peaked at 620 nm corresponding to 2.0 eV is observed in the as-deposited films. After single-pulse laser annealing, better crystallinity and increased PL intensity are obtained. The PL peak remains at 620 nm. High laser fluence causes damage to the films and the threshold fluence for laser ablation is around 100 mJ/cm2. Multiple-pulse laser annealing with increasing pulse number at fluences below the threshold of laser ablation can provide better crystallization and property improvement than single-pulse annealing. There are a certain number of pulses for the best multiple-pulse annealing effect before damage or laser ablation occur.

Decrements in per pulse release of norepinephrine, antagonist potentiation of release and presynaptic receptor theory

In recent decades the theory that amine transmitter release at nerve terminals is routinely regulated through negative feedback systems sensing and responding to the instantaneous perineuronal concentration of previously liberated transmitter has assumed pre-eminence. However, observations indicate a major drop off in per pulse transmitter release when only two or four stimulation pulses are administered, reflecting the unexpectedly prompt operation of feedback inhibition. We explored this quandary in our understanding of control of transmitter release by axonal depolarization versus terminal feedback using isotopic norepinephrine and in vitro slices of rabbit hippocampus. A technique of continuous collection of superfusate over a 30 min cycle of stimulation utilizing a wide range of intervals between individual stimulation pulses was used. Following simulation with single pulses even 60 s apart or pseudo one pulses 150 s apart a marked decline in per pulse release was noted. The deficits in per pulse release were not related to the number of pulse delivered at any time over the course of a 30 min stimulation period. The pulse decrements were independent of the activity of neuronal uptake and of superfusion flow rate or even individual pulse duration and frequency. Presynaptic receptor antagonists, potentiated efflux near maximally with the second of only two pulses. Potentiations were independent of pulse number, pulse duration, or frequency. No linkage between perineuronal transmitter concentrations and the antagonist potentiation of release was found. However, the decrements in per pulse release with multiple pulses and the potentiation by alpha presynaptic antagonists occurred under the same test conditions. We conclude that the pulse deficit can be looked at as largely attributable to an enhanced efflux with delivery of the first pulse in a train of pulses, rather then to a pattern of progressively declining efflux linked to increasing extracellular transmitter levels as frequency and pulse number increase.