Effect Of Laser Repetition Rate Research Articles

Performance improvement of iron chalcogenide films grown at high laser repetition rates by pulsed laser deposition

AbstractFor mass production, a high deposition rate is necessary. A powerful approach is to deposit Fe(Se, Te) (FST) superconducting films at a high laser repetition rate by pulsed laser deposition. Hence, the FST films were grown on flexible metallic tapes using pulsed laser deposition at different laser repetition rates to explore the effect of laser repetition rate on the properties. It was found that as the laser repetition rate increases, the crystallinity and stoichiometry of FST films deteriorate, resulting in a degeneration in superconductivity. The critical temperature of the FST film deposited at a high repetition rate of 120 Hz dropped to 12.89 K. Furthermore, high‐performance FST film was successfully deposited at 120 Hz by increasing the substrate temperature with = 16.96 K and = 18.32 K. The self‐field critical current density can reach 1.3 MA/cm2 at 4.2 K. The analysis of the pinning mechanism reveals that normal surface pinning centers play a dominant part in the FST film and are independent of the temperature. The collective pinning theory states that the FST film exhibits δl pinning. The results may offer valuable information for rapid deposition of FST‐coated conductors for industrial production.

The effect of laser repetition rate in second harmonic generation imaging and intensity detection

Here we revealed that lasers with lower repetition rates (5 kHz and 50 kHz) from an amplifier were more efficient in SHG imaging than a laser with high repetition rate (80 MHz) from an oscillator, demonstrating that peak power played a more important role than the repetition rate for high SHG yield. Although lasers with relatively high peak power damaged the cancer cells used in the imaging at a relatively lower laser power, better signal/noise ratio was achieved before the occurrence of the adverse damage. It was also revealed that photobleaching was avoided with the lower repetition rate lasers. Furthermore, the three lasers were applied in analyzing the scattered SHG intensity from colloidal BaTiO3 nanoparticles using photon counting method. All tests with the three lasers reached satisfactory results. Importantly, the lasers with low repetition rates showed great potential in the detection of weak nonlinear scattering.

Effect of laser repetition rate on the fluorescence characteristic of a long-distance femtosecond laser filament.

In this paper, the effect of the laser repetition rate on the long-distance femtosecond laser filament in air is investigated by measuring the fluorescence characteristic of the filament. A femtosecond laser filament emits fluorescence due to the thermodynamical relaxation of the plasma channel. Experimental results show that as the repetition rate of femtosecond laser increases, the fluorescence of the filament induced by a single laser pulse weakens, and the position of the filament moves away from the focusing lens. These phenomena may be attributed to the slow hydrodynamical recovery process of air after being excited by a femtosecond laser filament, whose characteristic time is on the millisecond time scale and comparable to the inter-pulse duration of the femtosecond laser pulse train. This finding suggests that at a high laser repetition rate, to generate an intense laser filament, the femtosecond laser beam should scan across the air to eliminate the adverse effect of slow air relaxation, which is beneficial to laser filament remote sensing.

Effect of laser repetition rate on the growth of Sc2O3 via pulsed laser deposition

This paper reports a study of the effect of laser pulse repetition rate and substrate temperature on the properties of crystalline Sc2O3 films grown on <0001>-oriented sapphire via pulsed laser deposition (PLD). For the range of substrate laser-heating powers investigated, optimum <111>-film growth was obtained at the highest available repetition rate of 100 Hz. Scanning electron microscopy and X-ray diffraction measurements revealed that a decrease in the repetition rate favours highly textured/island growth. The repetition rate was also proven to affect the lattice constant of the films, with a difference up to 0.3% between the films grown at high and low repetition rates. The general trend indicates that the out-of-plane lattice constant of the film shifts closer to the bulk value when the pulse repetition rate is increased. However, the lattice constant could also be reduced through in-situ post-growth annealing, which had the additional benefit of reducing the width of the (222) rocking curve peak. This work provides further evidence that energetic-PLD dynamics leads to higher quality PLD-grown crystalline films and for this material undermines the old adage that lower pulse repetition rates lead to higher quality thin-film crystalline growth.

Quantitative photochemical scaling model for femtosecond laser micromachining of ophthalmic hydrogel polymers: effect of repetition rate and laser power in the four photon absorption limit

We report on the effect of laser repetition rate on the refractive index (RI) change or phase change induced by the femtosecond micromachining hydrogel-based ophthalmic materials. A repetition rate tunable femtosecond ytterbium (Yb) doped fiber laser was used to study the relation between the laser-induced phase change and the pulse repetition rate in the range of 1 MHz to 60 MHz. We present both the qualitative and quantitative results of the laser-induced phase change obtained in hydrogel-based contact lenses at different repetition rates and discuss the effect of repetition rate on the magnitude of the phase shift, the optical damage threshold and the maximum achievable phase change just below the optically-induced damage threshold. A photochemical model derived in the four photon absorption limit with pulse overlapping is employed to fit the experimental data obtained at four different repetition rates, 5 MHz, 10 MHz, 15 MHz and 60 MHz. This work contributes to the current knowledge of the response of hydrogel polymers to various laser irradiation parameters and the optimization of laser repetition rates enables femtosecond micromachining of ophthalmic materials at a lower average power and a faster writing speed.

Effect of laser repetition rate in the synthesis of colloidal zinc nanoparticles by pulse laser ablation method

Synthesis of Zinc nanoparticles (ZnNPs) has been carried out using pulse laser ablation method. In this method neodymium-doped yttrium aluminum garnet (Nd:YAG) laser pulse are used. Effect of laser repetition rate pulse was examined to produced ZnNPs. Spherical shapes of ZnNPs was produced when pulse laser beam (1064 nm, 7 ns) was bombarded onto a high purity (99.95%) zinc plate surface. The averaged diameters of ZnNPs produced in this research were 12.1 nm with standard deviation 4.4 nm for the laser repetition rate 10 Hz, the diameter of ZnNPs was 5.6 with standard deviation 4.7 nm.The spectrum of ZnNPs was obtained by ultraviolet visible (UV-Vis) spectroscopy shows that the surface plasma resonance (SPR) at the center of 300 nm wavelength, which certifies that te ZnNPs produced has the spherical shape.

Effects of picosecond laser repetition rate on ablation of Cr12MoV cold work mold steel

In this paper, the effects of pulse repetition rate on ablation efficiency and quality of Cr12MoV cold work mold steel have been studied using a picosecond (ps) pulse Nd:YVO4 laser system at λ=1064nm. The experimental results of area ablation on target surface reveal that laser repetition rate plays a significant role in controlling ablation efficiency and quality. Increasing the laser repetition rate, while keeping a constant mean power improves the ablation efficiency and quality. For each laser mean power, there is an optimal repetition rate to achieve a higher laser ablation efficiency with low surface roughness. A high ablation efficiency of 42.29, 44.11 and 47.52μm3/mJ, with surface roughness of 0.476, 0.463 and 0.706μm could be achieved at laser repetition rate of 10MHz, for laser mean power of 15, 17 and 19W, respectively. Scanning electron microcopy images revels that the surface morphology evolves from rough with numerous craters, to flat without pores when we increased the laser repetition rate. The effects of laser repetition rate on the heat accumulation, plasma shield and ablation threshold were analyzed by numerical simulation, spectral analysis and multi-laser shot, respectively. The synergetic effects of laser repetition rate on laser ablation rate and machining quality were analyzed and discussed systemically in this paper.

Epitaxial growth of GaN films on lattice-matched ScAlMgO4substrates

High-quality GaN films have been epitaxially grown on ScAlMgO4 (SCAM) (0001) substrates with an in-plane epitaxial relationship of GaN[1−100]//SCAM[1−100] by pulsed laser deposition (PLD). The effect of laser repetition rate on the qualities of GaN epitaxial films is studied in depth. It is found that as the laser repetition rate increases from 10 to 40 Hz, the qualities of as-grown ∼300 nm-thick GaN epitaxial films increase first and then decrease, and show the optimized values at a laser repetition rate of 30 Hz. The ∼300 nm-thick GaN epitaxial films grown with the laser repetition rate of 30 Hz present very high crystalline quality with full-width at half-maximum values for GaN(0002) and GaN(10−12) X-ray rocking curves of 0.18° and 0.40°, and reveal a very smooth surface with a root-mean-square surface roughness of 1.5 nm. The as-grown GaN films also show an in-plane tensile stress of 0.51 GPa. Meanwhile, cross-sectional transmission electron microscopy confirms the presence of sharp and abrupt GaN/SCAM hetero-interfaces.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.

The effects of laser repetition rate on femtosecond laser ablation of dry bone: a thermal and LIBS study.

The aim of this study is to understand the effect of varying laser repetition rate on thermal energy accumulation and dissipation as well as femtosecond Laser Induced Breakdown Spectroscopy (fsLIBS) signals, which may help create the framework for clinical translation of femtosecond lasers for surgical procedures. We study the effect of repetition rates on ablation widths, sample temperature, and LIBS signal of bone. SEM images were acquired to quantify the morphology of the ablated volume and fsLIBS was performed to characterize changes in signal intensity and background. We also report for the first time experimentally measured temperature distributions of bone irradiated with femtosecond lasers at repetition rates below and above carbonization conditions. While high repetition rates would allow for faster cutting, heat accumulation exceeds heat dissipation and results in carbonization of the sample. At repetition rates where carbonization occurs, the sample temperature increases to a level that is well above the threshold for irreversible cellular damage. These results highlight the importance of the need for careful selection of the repetition rate for a femtosecond laser surgery procedure to minimize the extent of thermal damage to surrounding tissues and prevent misclassification of tissue by fsLIBS analysis.

Effects of Laser Repetition Rate and Fluence on Micromachining of TiC Ceramic

Micromachining of titanium carbide (TiC) ceramic is very difficult because of its high hardness and brittleness. Femtosecond pulsed laser was employed to process circular rings on the surface of TiC ceramic. The interaction area between femtosecond laser pulses and TiC at different laser repetition rates and fluences was studied. Morphology and composition of irradiated area were analyzed by scanning electron microscope, energy dispersive spectroscopy, and Raman spectrum. The results indicated that the radius of outer circle was close to the intended radius. Laser fluence had obvious effects on the radius and width of circular rings, compared to laser repetition rate. The width of circular rings increased rapidly with increasing laser fluence from 2.55 × 10−2 to 1.27 × 10−1 J/mm2, and then stabilized at around 40 µm when laser fluence was above 7.64 × 10−1 J/mm2. The surface of circular rings was characterized by ripples at the lower laser fluence. With increasing laser fluence, four kinds of typical morphology were observed, including ripples, cauliflower-like particles, ball-like particles, and deposited oxide layer. Ball-like particles contained high concentration of titanium, which came from melt ball splashing from ablation area. The others came from the different oxidation stages occurred on the surface of TiC sample.

Effects of Laser Repetition Rate on the Structural and Superconducting Properties of YBCO films Deposited by PLD on NiW Tapes

YBa2Cu3O7−x (YBCO) films were fabricated at various laser repetition rates on CeO2/YSZ/CeO2 buffered Ni-W tapes by pulsed laser deposition (PLD). The dependence of structural and superconducting properties on the laser repetition rates was investigated. The microstructure and surface morphology of YBCO films were characterized by X-ray diffraction and atomic force microscopy. And the critical current (Ic) of YBCO films was measured by the conventional four-probe method. The results show that with increasing laser repetition rate the island density decreased and the island size increased. It was found that the texture and Ic of YBCO films were largely dependent on the laser repetition rate. And furthermore it was observed that the film thickness was not in directly proportional to laser repetition rate. Under optimum experimental condition, high quality YBCO films with critical current Ic above 500 A/cm at 77 K and 0T were obtained.

The effect of laser repetition rate on the LASiS synthesis of biocompatible silver nanoparticles in aqueous starch solution

Laser ablation-based nanoparticle synthesis in solution is rapidly becoming popular, particularly for potential biomedical and life science applications. This method promises one pot synthesis and concomitant bio-functionalization, is devoid of toxic chemicals, does not require complicated apparatus, can be combined with natural stabilizers, is directly biocompatible, and has high particle size uniformity. Size control and reduction is generally determined by the laser settings; that the size and size distribution scales with laser fluence is well described. Conversely, the effect of the laser repetition rate on the final nanoparticle product in laser ablation is less well-documented, especially in the presence of stabilizers. Here, the influence of the laser repetition rate during laser ablation synthesis of silver nanoparticles in the presence of starch as a stabilizer was investigated. The increment of the repetition rate does not negatively influence the ablation efficiency, but rather shows increased productivity, causes a red-shift in the plasmon resonance peak of the silver–starch nanoparticles, an increase in mean particle size and size distribution, and a distinct lack of agglomerate formation. Optimal results were achieved at 10 Hz repetition rate, with a mean particle size of ~10 nm and a bandwidth of ~6 nm ‘full width at half maximum’ (FWHM). Stability measurements showed no significant changes in mean particle size or agglomeration or even flocculation. However, zeta potential measurements showed that optimal double layer charge is achieved at 30 Hz. Consequently, Ag–NP synthesis via the laser ablation synthesis in solution (LASiS) method in starch solution seems to be a trade-off between small size and narrow size distributions and inherent and long-term stability.

Effect of Laser Repetition Rate on Silver Nanoparticles Colloid

The silver nanoparticles colloid was prepared by pulsed laser ablation in distilled water under various laser repetition rates. The particles size, morphologies and absorption spectroscopy of the obtained nanoparticles colloids were characterized by ultraviolet to visible (UV-Vis) spectrometer and transmission electron microscopy (TEM). The average diameter and its distribution were analyzed by Image-ProPlus software. The results showed that the average diameter of the silver nanoparticles prepared at the laser repetition rate of 10 HZ was the smallest (D=29.75 nm), also, the distribution of particle size decreases with increasing the laser repetition rate.

Effects of laser repetition rate on corneal tissue ablation for 193‐nm excimer laser light

The goal of the present work is to assess whether bovine corneal ablations generated at laser repetition rates of up to 400 Hz are comparable to ablations performed at rates consistent with current clinical laser systems. A combination of experiments was used to assess a comprehensive range of ablation parameters, including ablation plume dynamics via imaging and transmission, corneal ablation profiles via scanning interferometry, and high-resolution electron microscopy of collagen structure following ablation. Using white-light interferometry analysis, no statistical difference was found between corneal ablation profiles created at 60 and 400 Hz, with an average rate of 0.94 microm/pulse at 60 Hz versus 0.92 microm/pulse at 400 Hz. In addition, based on plume imaging and transmission studies, the bulk ablation plume was found to dissipate on a time-scale less than the pulse-to-pulse separation for a laser repetition rate up to about 400 Hz. A persistent, diffuse gas-phase component of the ablation products was observed and concluded to be comparable at both repetition rates. Finally, SEM and TEM analysis revealed no signs of differential thermal tissue damage, including collagen fibril analysis, for laser repetition rates up to 400 Hz. In summary, investigation of the relative effects of excimer laser repetition rate on the overall corneal ablation metrics revealed no measurable difference under conditions typical of clinical refractive procedures. This study suggests that increases in ArF laser repetition rates for clinical applications (up to approximately 400 Hz) appear feasible.

LASER DICING OF SILICON WAFER

The effect of various laser processing parameters on the kerf width and cut quality of Si wafer as well as encapsulated Si wafer is investigated. The parameters are then optimized to minimize the heat affect zone and obtain the best possible cut quality. It has been found that oxygen is the most suitable assist gas for laser dicing and that the highest gas pressure may not produce the best cut quality. The effect of laser repetition rate, pump energy, feed rate, and number of passes are also studied. Under optimized parameters, the cut quality of Si wafer using laser dicing is found to be comparable to diamond saw dicing.

Effect of laser repetition rate on the melting and ablation of Ni24Zr76 alloy ribbon

An amorphous Ni24Zr76 metallic ribbon 35 μm thick, prepared by planar flow casting, was furnace recrystallized and subsequently laser annealed in a vacuum of 2 × 10−3 Pa. The fluences of the laser pulses were 1.4 and 1.9 J cm−2, the number of pulses directed to the same irradiation site was between 1 and 300. The repetition rate of the laser pulses varied between 5 and 100 Hz. The temperature evolution in the irradiated samples in one- and multipulse regimes were numerically calculated. The samples were analysed by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy and differential scanning calorimetry. A strong increase of the laser damage level, manifested by melting and ablation with the increase of the pulse repetition rate, was observed. This effect is attributed to the heat accumulation in the sample at higher pulse repetition rate. Additional effects, such as roughening of the sample surface, increase of the optical absorption, decrease of the thermal diffusivity, and decrease of the thickness of irradiated sample can also amplify this effect.

Effect of laser repetition rate on the melting and ablation of Ni24Zr76 alloy ribbon

Effect of laser repetition rate on the melting and ablation of Ni24Zr76 alloy ribbon

Capacitive Discharge Graphite Furnace Laser-Excited Atomic Fluorescence of Thallium

For the first time, an anisotropic graphite furnace heated by capacitive discharge was used for laser-excited atomic fluorescence spectrometry. A detection limit of 5 fg for thallium was obtained with a laser repetition rate of 500 Hz and a peak integration time of 80 ms. The use of a capacitive discharge furnace allows for a shorter integration time, which in turn should allow for integration of less background noise, and improved detection limits. Theoretically, the magnitude of the shot noise should be proportional to the square root of the integration time, and inversely proportional to the square root of the laser repetition rate. Experimental data illustrated the effect of laser repetition rate, but were inconclusive with respect to integration time. The linear dynamic range of the calibration curve was six orders of magnitude, which was comparable to that normally obtained for laser-excited atomic fluorescence in modern commercial graphite furnaces. Thallium was accurately determined in NIST biological samples at levels one to two orders of magnitude below the detection limit of electrothermal atomic absorption spectrometry, with an analytical precision between 8 and 20%. The interference effects of calcium, sodium chloride, and potassium chloride on the thallium signal were investigated and shown to be similar to both laser-excited atomic fluorescence in a conventional furnace and capacitive discharge furnace atomic absorption results reported in the literature.

Biliary calculi fragmentation by a 308 nm excimer laser: a preliminary study.

The use of a 308-nm XeCl excimer laser for biliary stone fragmentation is reported. The 130-nsec laser pulses are delivered via UV grade fused silica fibers to the target stones immersed in normal saline solution and placed in direct contact with the fiber. Sixty biliary calculi, 20 cholesterol and 40 pigment, were fragmented in vitro. The energy delivered per unit mass of the stone is kept constant at 50 mJ/mg. The effect of laser repetition rate, energy fluence, and fiber core size on stone fragmentation was studied. Fragmentation thresholds for a variety of biliary calculi of known composition were measured. It was found that higher fragmentation efficiency was obtained with larger fluence, lower repetition rate, and fiber of larger core. Our study indicates that the 308-nm excimer laser may be effective as a laser lithotriptor with low threshold and good efficiency for biliary stone fragmentation.