Irradiation Pulse Number Research Articles

Direct Femtosecond Laser Processing for Generating High Spatial Frequency LIPSS (HSFL) on Borosilicate Glasses with Large-Area Coverage

Large-area nanostructuring of glasses using intense laser beams is a challenging task due to the material’s extreme non-linear absorption of laser energy. Precise optimization of the process parameters is essential for fabricating nanostructures with large-area coverage. In this study, we report the findings on creating high-spatial-frequency LIPSS (HSFL) on borosilicate glass through direct laser writing, using a femtosecond laser with a wavelength λ = 800 nm, pulse duration τ = 35 fs, and repetition frequency frep = 1 kHz. We measured the single-pulse ablation threshold and incubation factor of Borosilicate glasses to achieve high-precision control of the large-area surface structuring. Single-spot experiments indicated that, when there was higher fluence and a larger number of irradiated laser pulses, a melt formation inside the irradiated area limited the uniformity of LIPSS formation. Additionally, the orientation of the scan axis with the laser beam polarization was found to significantly influence the uniformity of LIPSS generated along the scan line, with more redeposition and melt formation when the scan axis was perpendicular to the laser beam polarization. For large-area processing, the borosilicate glass surface was scanned line-by-line by the laser beam, with a scan orientation parallel to the polarization of the laser. The optical characterization revealed that the transmittance and reflectance of the borosilicate glass decreased significantly after processing. Additionally, the surface’s wettability changed from hydrophilic to super-hydrophilic after processing. These chemical contamination-free and uniformly distributed structures have potential applications in optics, microfluidics, photovoltaics, and biomaterials.

A deep learning-based predictive simulator for the optimization of ultrashort pulse laser drilling

Ultrashort pulse laser drilling is a promising method for the fabrication of microchannels in dielectric materials. Due to the complexity of the process, there is a strong demand for numerical models (simulators) that can predict structures produced under specific processing conditions in order to rapidly find optimal processing parameters. However, the validity of conventional laser drilling simulators for dielectrics has been confined to a range of strict interpolations of the data used during the construction of the model, and thus, their usefulness is limited. Here, we demonstrate simulator-based optimization for ultrashort pulse laser drilling in dielectrics based on an iterative deep neural network which is trained to predict microchannel structure after a small number of irradiated pulses. Our approach predicts the development of hole shapes over a wide variety of conditions and allowed the discovery of 20% more energy efficient processing strategies than in the initial experimental data. More broadly, our approach can address realistic problems in laser processing considering a variety of processing parameters, and thus enabling improved performance of next-generation smart laser processing systems.

Formation of chemical compounds in the surface layers of titanium under the action of a high-power ion beam of nanosecond duration

The features of the change in the chemical state of the surface layers of commercially pure titanium under the action of a high-power ion beam (HPIB) were investigated using X-ray photoelectron spectroscopy (XPS). It has shown that HPIB irradiation leads to the interaction of titanium with carbon and nitrogen adsorbed on the surface and the formation of chemical compounds. The influence of the number of HPIB irradiation pulses on the formation of oxycarbonitride compounds in the surface layers of titanium has been established. Three possible mechanisms of the formation of titanium oxycarbonitride compounds during HPIB irradiation were considered: the interaction of molten titanium with adsorbed nitrogen and carbon-containing surface contaminants; the interaction of the evaporated titanium with the components of the residual atmosphere of the vacuum chamber, followed by the deposition of the resulting compounds on the surface and carbothermal reduction of the oxidized surface layer of titanium.

Femtosecond-laser processing incubation in Diamond-like carbon

Diamond-like carbon (DLC) has been commercialized as an advanced material for combining diamond and graphite properties with other material systems, with ease of processing and reduced costs. DLC has been broadly studied as an interesting material for applications ranging from biology to optics, passing through mechanics and electronics. Hence, for given applications, complex patterning of DLC is required, which can be accomplished by conventional lithographic approaches, as well as laser structuring. In this way, this paper focuses on studying the incubation process, the dependence of the threshold fluence with the number of irradiated pulses, on the fs-laser microstructuring of DLC films. We determined a single pulse fluence of (1.0 ± 0.1) J/cm2, a value that decreases to a plateau of (0.13 ± 0.04) J/cm2 for a large number of pulses. The slow evolution of the fluence threshold with the number of pulses indicates a low incubation parameter, such that a large number of pulses is required to damage the material. By modeling the single-pulse damage threshold fluence considering multiphoton and avalanche ionization, we were able to estimate a two-photon absorption cross-section for DLC of (1.5 ± 0.5) × 10−60 m4s1photon−1, which is consistent with the one known for diamond. Finally, Raman spectroscopy of the fs-laser processing DLC sample indicates a decrease of the sp3 fraction and a more ordered phase.

Luminescent studies of bisphenol A solutions under the action of the electron beam

The results of measuring the luminescence of ambient air and solutions of an organic compound during their irradiation with a high-current pulsed electron beam with an average energy of Ee=170 keV and a duration of 2 ns, generated by the RADAN-303 accelerator, are presented. It is shown that, under such exposure, the transformation of dissolved bisphenol A occurs, which is accompanied by an increase in the absorption coefficient of the solution in the wavelength range of more than 300 nm and, as a consequence, a decrease in the intensity of air luminescence bands with an increase in the number of irradiation pulses. The results showed the degradation of the BPA solution to complete decomposition under the action of an electron beam treatment. Keywords: electron beam, air, solutions of organic compounds, luminescence, absorption, decomposition.

Люминесценцентные исследования растворов бисфенола A под действием электронного пучка

The results of measuring the luminescence of ambient air and solutions of an organic compound during their irradiation with a high-current pulsed electron beam with an average energy of Ee=170 keV and a duration of 2 ns, generated by the RADAN-303 accelerator, are presented. It is shown that, under such exposure, the transformation of dissolved bisphenol A occurs, which is accompanied by an increase in the absorption coefficient of the solution in the wavelength range of more than 300 nm and, as a consequence, a decrease in the intensity of air luminescence bands with an increase in the number of irradiation pulses. The results showed the degradation of the BPA solution to complete decomposition under the action of an electron beam treatment.

Irradiation at Ultra-High (FLASH) Dose Rates Reduces Acute Normal Tissue Toxicity in the Mouse Gastrointestinal System

Preclinical studies using ultra-high dose rate (FLASH) irradiation have demonstrated reduced normal tissue toxicity compared with conventional dose rate (CONV) irradiation, although this finding is not universal. We investigated the effect of temporal pulse structure and average dose rate of FLASH compared with CONV irradiation on acute intestinal toxicity. Whole abdomens of C3H mice were irradiated with a single fraction to various doses, using a 6 MeV electron linear accelerator with single pulse FLASH (dose rate=2-6×106 Gy/s) or conventional (CONV; 0.25 Gy/s) irradiation. At 3.75 days postirradiation, fresh feces were collected for 16S rRNA sequencing to assess changes in the gut microbiota. A Swiss roll-based crypt assay was used to quantify acute damage to the intestinal crypts to determine how tissue toxicity was affected by the different temporal pulse structures of FLASH delivery. We found statistically significant improvements in crypt survival for mice irradiated with FLASH at doses between 7.5 and 12.5 Gy, with a dose modifying factor of 1.1 for FLASH (7.5 Gy, P < .01; 10 Gy, P < .05; 12.5 Gy, P < .01). This sparing effect was lost when the delivery time was increased, either by increasing the number of irradiation pulses or by prolonging the time between 2 successive pulses. Sparing was observed for average dose rates of ≥280 Gy/s. Fecal microbiome analysis showed that FLASH irradiation caused fewer changes to the microbiota than CONV irradiation. This study demonstrates that FLASH irradiation can spare mouse small intestinal crypts and reduce changes in gut microbiome composition compared with CONV irradiation. The higher the average dose rate, the larger the FLASH effect, which is also influenced by temporal pulse structure of the delivery.

LIPSS-covered annular ablation region induced on ZnO surface by focusing femtosecond laser beam spatially shaped by bubbles in cedar oil

In this paper, without circular scanning of laser focus, annular ablation region was reported to be induced by femtosecond laser static irradiation of ZnO surface immersed in oil, and these annular ablation regions were all covered by laser-induced periodic surface structures (LIPSS) upon irradiation of either linearly-polarized light or circularly-polarized light. This novel phenomenon was found to be related to the bubbles in-situ generated and attached at laser focus inside oil during irradiation process. With increasing irradiation pulse number, the shallow annular ablation region transited to a deep annular groove, and the unablated region surrounded by the annular ablation region was finally ablated to a LIPSS-covered spherical-crown-shaped bump. Finite-difference-time-domain (FDTD) simulation shows that the bubble at the laser focus may act as a beam shaping device to partly transform a Gaussian-intensity-profile beam to an annular-intensity-profile beam and the increased bubble size induces a decrease of the radius of the annular beam, which can roughly explain the experimentally observed morphology transformation phenomenon. Further analysis of light field transformation based on matrix optics further demonstrates that the annular intensity profile is originated from hollow beams generated after incident light passing through the bubble-type radial polarizer.

Non-equilibrium engineering of chemically grown SiO2/Si by UV nanosecond pulsed laser annealing from the viewpoint of bias temperature instability sources

The impacts of high-temperature UV nanosecond pulsed laser annealing (LA) on a chemically grown SiO2/Si system are systematically investigated as a function of the number of irradiated pulses. A progressive transition is observed for both the tetrahedral SiO4 network and the SiO2/Si interface, being assumed to play an essential role in controlling bulk SiO2 traps and interface states. The presented results open the perspective to improve bias temperature instability of low thermal budget gate stacks by UV ns pulsed LA, particularly in emerging 3D integrations.

Surface ablation of poly allyl diglycol carbonate polymer using high-repetition-rate femtosecond laser

Laser ablation (LA) has been acknowledged as a universal laser processing tool to develop new devices. Typically, LA involves removal of material with a high energy femtosecond (fs) laser, which has a relatively low-pulse repetition rate (kHz). This constricts the processing speed to avoid separation between successive pulses during the device patterning. Hence, it is reasonable to use a high-repetition-rate laser as an alternative approach to enable high-speed laser processing. We experimented on the surface ablation of poly allyl diglycol carbonate (PADC) using a high-repetition-rate (MHz) fs laser. The objective of this study is to investigate the laser-ablated pattern on PADC and the effect of the number of laser pulses on the ablation rate. For the purpose of this experiment, straight trenches were formed directly on the PADC surface with 800-nm fs-pulsed laser writing. It was found that the ablation mechanism in the high-repetition-rate regime was primarily controlled by the number of irradiated pulses. At a fixed pulse energy, the ablation rate dropped from 9.6 to 3.4 pm / pulse for 1 × 105 and 8 × 105 pulses, respectively. This outcome stemmed from the disruption of laser focusing via thermally induced surface morphology changes due to excessive energy accumulation.

Laser-Assisted Fabrication for Metal Halide Perovskite-2D Nanoconjugates: Control on the Nanocrystal Density and Morphology.

We report on a facile and rapid photo-induced process to conjugate graphene-based materials with metal-halide perovskite nanocrystals. We show that a small number of laser pulses is sufficient to decorate the 2-dimensional (2D) flakes with metal-halide nanocrystals without affecting their primary morphology. At the same time, the density of anchored nanocrystals could be finely tuned by the number of irradiation pulses. This facile and rapid room temperature method provides unique opportunities for the design and development of perovskite-2D nanoconjugates, exhibiting synergetic functionality by combining nanocrystals of different morphologies and chemical phases with various 2D materials.

The surface modification of aluminum by mechanical milling of Pb coating and high current pulsed electron beam irradiation

This paper reports the surface morphology and properties of Al substrates with Pb coating, which was deposited by mechanical milling and then treated by high current pulsed electron beam (HCPEB) irradiation. After HCPEB irradiation treatment, some previously formed defects such as micro-pits, craters, and bulged nodules were eliminated, resulting in significant reduction of surface roughness of initial Pb/Al sample, and the formation of a dense and adherent Pb/Al alloying layer. Notably, when the number of irradiation pulses reached 30, there was uniform distributions of Pb in the mixture of Pb and Al elements in the milled samples, which resulted in the formation of gray layers with thickness of 12 μm (milling for 1 h) and 8 μm (milling for 2 h). These formed layers effectively improved the wear resistance. Moreover, the hardness of Pb/Al sample also improved after HCPEB irradiation due to structure defect strengthening and ultra-fine Pb-rich particles reinforcement. The above results demonstrate that HCPEB treatment is an effective method to obtain high-quality alloy coating layer, resulting in the formation of an excellent alloy.

Multilevel accumulative switching processes in growth-dominated AgInSbTe phase change material.

Highly reproducible and precisely controlled gradual variation in optical reflectivity or electrical resistance between amorphous and crystalline phases of phase change (PC) material is a key requirement for multilevel programming. Here we report high-contrast multilevel set and reset operations through accumulative switching in growth-dominated AgInSbTe PC material using a nanosecond laser-based pump-probe technique. The precise tuning of fractions of crystallized or re-amorphized region is achieved by means of controlling the number of irradiated laser pulses enabling six stable multilevels with high-reflectivity contrast of 2% between any two states. Furthermore, Raman spectra of irradiated spots validate the structural changes involved during multilevel switching between amorphous and crystalline phases.

Development of a via-hole connection process via intense pulsed light sintering with Cu micro/Ag nano-hybrid ink for a multi-layered flexible printed circuit board

In this work, the printing and sintering characteristics of pattern area and via-hole section were investigated. To analyze the printing characteristics depending on the rheology of ink, Cu micro/Ag nano-hybrid inks (Cu/Ag hybrid inks) were prepared by varying the amount of epoxy, and the fabricated inks were printed on via-hole formed substrates. The printed Cu/Ag hybrid ink films were subsequently sintered by an intense pulsed light (IPL) sintering method. Various IPL sintering parameters including irradiation energy and pulse number were employed to enhance the sintering characteristics of pattern area and via-hole section. The sintered Cu/Ag hybrid ink films were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. The detailed sintering mechanism of Cu/Ag hybrid ink inside the via-hole section was monitored in real-time by in situ resistance monitoring. As a result, the Cu/Ag hybrid ink with the optimal ratio of epoxy content (0.56 wt%) showed good printability of pattern area and via-hole section. In addition, the proposed multi-pulse IPL sintering conditions (irradiation energy: 7 J/cm2, pulse duration: 1 ms, off-time: 9 ms, pulse number: 20) exhibited low resistivity (pattern area: 5.12 μΩ cm, via-hole section: 7.79 μΩ cm) and a 5B adhesion strength level.

Effects of high-intensity pulsed ion beam irradiation on the structural thermal stability of Fe80Si7.43B12.57 metallic glass

A high-intensity pulsed ion beam was used to irradiate Fe80Si7.43B12.57 metallic glass at different energy densities and numbers of pulses to study its structural thermal stability. A large number of round holes and a few bumps appeared on the surface of the glass after 300 pulses. No obvious damage was observed on the surface after irradiation at different energy densities. High-resolution transmission electron microscopy showed that this irradiation had a strong influence on the near-surface atomic arrangement of the glass. Irradiation induced strong migration and aggregation of atoms within the glass, causing an uneven atomic arrangement. Although there was an ordered arrangement of atoms, the metallic glass remained amorphous. The surface reflectivity of the metallic glass decreased slightly after irradiation. The number of irradiation pulses had greater impact on this Fe-based metallic glass than their energy density.

Femtosecond-laser-induced submicron grating periodic structures on As2S3 and As35Se65 glasses

Submicron periodic structures produced by femtosecond Ti:sapphire laser irradiation (1 kHz, 150 fs, 3–5 μm) on As2S3 and As35Se65 glasses at edges of damage holes are observed. Effects of changes in laser wavelength (λ), pulse number, power, and irradiation time on grating periodicity were studied. Regular groove-like ripples and distinct parallel submicron periodic gratings were seen in damaged areas. The damage grating period (Λ) is approximately λ/2n. The period increases with wavelength but is hardly influenced by irradiation time and pulse number. Samples with high refractive index and low bandgap are more prone to melting, and edge stripes are not obvious.

Multi-pulse flash light sintering of bimodal Cu nanoparticle-ink for highly conductive printed Cu electrodes

In this work, bimodal Cu nano-inks composed of two different sizes of Cu nanoparticles (NPs) (40 and 100 nm in diameter) were successfully sintered with a multi-pulse flashlight sintering technique. Bimodal Cu nano-inks were fabricated and printed with various mixing ratios and subsequently sintered by a flash light sintering method. The effects of the flashlight sintering conditions, including irradiation energy and pulse number, were investigated to optimize the sintering conditions. A detailed mechanism of the sintering of bimodal Cu nano-ink was also studied via real-time resistance measurement during the sintering process. The sintered Cu nano-ink films were characterized using x-ray photoelectron spectroscopy and scanning electron microscopy. From these results, it was found that the optimal ratio of 40–100 nm NPs was found to be 25:75 wt%, and the optimal multi-pulse flash light sintering condition (irradiation energy: 6 J cm−2, and pulse duration: 1 ms, off-time: 4 ms, and pulse number: 5) was found. The optimally sintered Cu nano-ink film exhibited the lowest resistivity of 5.68 μΩ cm and 5B adhesion level.

Controllable anisotropic wetting characteristics on silicon patterned by slit-based spatial focusing of femtosecond laser.

We propose a promising method to fabricate controllable anisotropic morphologies in which the slit-based spatial focusing of femtosecond laser is used to create an elliptical-shaped intensity distribution at focal plane, inducing elliptical-shaped morphology with micro/nano-dual-scale structures. Our study shows that 1) by increasing slit width, minor axis increases while major axis and axial ratio decrease; 2) with fixed slit width and laser fluence above the threshold, axial ratio is independent of irradiation pulse number; and 3) when polarization direction is changed from 0° to 90°, the axial ratio of anisotropic morphology declines. As a case study, large-area periodic anisotropic hierarchical structures are fabricated with the bidirectional anisotropic wetting.

Narrow titanium oxide nanowires induced by femtosecond laser pulses on a titanium surface

The evolution of the nanostructure induced on a titanium (Ti) surface with increasing irradiation pulse number by using a 400-nm femtosecond laser was examined by using scanning electron microscopy. High spatial frequency periodic structures of TiO2 parallel to the laser polarization were initially observed because of the laser-induced oxidation of the Ti surface and the larger efficacy factor of TiO2 in this direction. Periodically aligned TiO2 nanowires with featured width as small as 20nm were obtained. With increasing pulse number, however, low spatial frequency periodic structures of Ti perpendicular to the laser polarization became dominant because Ti possesses a larger efficacy factor in this direction. The competition between the high- and low-spatial frequency periodic structures is in good agreement with the prediction of the efficacy factor theory and it should also be observed in the femtosecond laser ablation of other metals which are easily oxidized in air.

Pulse number controlled laser annealing for GeSn on insulator structure with high substitutional Sn concentration

Crystalline GeSn-on-insulator structures with high Sn concentration (&amp;gt;8%), which exceeds thermal equilibrium solid-solubility (∼2%) of Sn in Ge, are essential to achieve high-speed thin film transistors and high-efficiency optical devices. We investigate non-thermal equilibrium growth of Ge1−xSnx (0 ≤ x ≤ 0.2) on quartz substrates by using pulsed laser annealing (PLA). The window of laser fluence enabling complete crystallization without film ablation is drastically expanded (∼5 times) by Sn doping above 5% into Ge. Substitutional Sn concentration in grown layers is found to be increased with decreasing irradiation pulse number. This phenomenon can be explained on the basis of significant thermal non-equilibrium growth achieved by higher cooling rate after PLA with a lower pulse number. As a result, GeSn crystals with substitutional Sn concentration of ∼12% are realized at pulse irradiation of single shot for the samples with the initial Sn concentration of 15%. Raman spectroscopy and electron microscopy measurements reveal the high quality of the grown layer. This technique will be useful to fabricate high-speed thin film transistors and high-efficiency optical devices on insulating substrates.