Ultraviolet Laser Research Articles

Process modeling of laser scanning vat-photopolymerization operating under intermittent exposure conditions

Process modeling is an important tool in additive manufacturing (AM) because it elucidates the relationship between processing parameters and material properties. Ultraviolet laser scanning vat-photopolymerization (VPP-UVL) is a common additive manufacturing technique, with a strong physical modeling based on the Jacobs work curve framework. The work curve model was developed for continuous UV exposure (CE) systems, however most VPP-UVL systems to date operate under intermittent exposure (IE) conditions due to the wide adoption of solid-state pulsed-laser sources. Despite their fundamental differences, the Jacobs work curve model is still successfully employed in IE systems under high overlap conditions between pulses, however there are few investigations regarding the limiting conditions for the applicability of the Jacobs work curve model to describe the total exposure profile in intermittent exposure (IE) setups. To address this issue, a model describing a VPP-UVL-IE process was developed. The VPP-UVL-IE model presented here has the same logarithmic dependence as the Jacobs work curve model, but with the addition of a multiplying factor carrying information on the spatial distribution of energy, that is, the overlap of individual pulses. The model is adaptable for non-gaussian beam profiles and can be solved for bidimensional analysis representing layer fabrication. To test the model, a custom-built VPP-UVL-IE additive manufacturing system was used, and structures were printed and characterized using SEM and stylus profilometry. The model`s reliability was investigated under constant overlap and exposure conditions. The model is very robust under constant overlap conditions, with good agreement between experimental data and simulation. For constant exposure, there is good qualitative agreement, but discrepancies occur due to insufficient modeling of the spatial distribution of energy. The 3D printing of complex structures is demonstrated, and the model indicates that VPP-UVL-IE can offer, theoretically, faster printing compared to VPP-UVL-CE for large beam radius (>275 μm), under the assumption of Gaussian beam profile, constant irradiance during the pulses, and dwell time longer than beam travel time. Conversely, same levels of exposure can be achieved at higher scan speed using CW laser sources instead of pulsed sources for small beam radius (<275 μm). • The work curve equation can model VPP-UVL with pulsed light sources. • Pulsed light VPP-UVL can print faster than continuous scanning. • Single pulse energy dose and spatial distribution of energy dose are independent.

Raman scattering studies of low energy Ar+ ion implanted monocrystalline silicon for synchrotron applications

The surface and subsurface of the Ar+ ion implanted monocrystalline silicon was modified due to the exchange of energy and momentum of ions. This procedure is necessary for shape homogeneity of monocrystalline silicon surface useful in synchrotron application. The implantation has modified the vibrational modes of monocrystalline silicon investigated by Raman spectroscopy using polarized and unpolarized inelastically scattered light. An additional pronounced shift of peak position and broadening of the linewidth of optical phonon mode at lower frequency was observed in Ar+ ion implanted monocrystalline silicon. The shift and broadening of such peak was originated from the amorphous and nanocrystalline silicon. The maximum depth of such transformation was equal to the depth of Ar+ ion implantation with energies of 400 eV which was about 2 nm studied by ion profiling method. This was also confirmed by maximum depth of penetration of ultraviolet laser in monocrystalline silicon.

Theoretical and experimental study of ultraviolet broadband laser amplification using Ce:LiCAF crystal

Trivalent cerium ion-doped lithium calcium aluminum fluoride (Ce:LiCAF) crystals have been demonstrated as efficient amplification gain media in the ultraviolet (UV) wavelength region. However, spectral characteristics in the multipass amplification process have not yet been studied theoretically and experimentally. This work investigates the spectral effects, including gain narrowing and spectral shifting using the chromatic Frantz–Nodvik model to simulate the amplification of broadband nanosecond UV pulses from a Ce:LiCAF crystal. From the simulation results, 5.5 mJ amplified pulses can be obtained using a 4-pass amplifier with seed pulses having 0.7 mJ pulse energy, 3 ns pulse duration and 288.5 nm peak wavelength. These values are in good agreement with experimental results. Seed pulses with peak wavelength of 290.0 nm and linewidth of 2.0 nm could be shifted to 289.8 nm peak wavelength and narrowed to 1.9 nm linewidth after 4-pass amplification. Accounting for the spectral effects will enable a more accurate prediction of the amplified pulses’ characteristics, thereby aiding the applications of high-power UV lasers.

Weak reflectivity measurements of large-mode-area FBGs by a scale composed of a fiber Fabry-Perot interferometer.

In a previous study, we proposed a measuring method for the reflectivity of weak-reflection large-mode-area fiber Bragg gratings by using scale gratings. We experimentally found that the interference between two scale fiber Bragg gratings (FBGs) is beneficial for increasing reflectivity scales, which can improve the measurement accuracy. Therefore, in this study, we designed and fabricated FBG-based Fabry-Perot cavities (FBG-FP) in single-mode fibers by two inscription methods, namely ultraviolet (UV) laser exposure and femtosecond-laser direct writing. Then, a large-mode-area double-clad (LMA-DC) FBG of weak reflectivity was measured by these two scales, and the experimental results show that the Bragg resonance reflectivity is less than 4.28% and 1.14% ∼ 2.28%, respectively. This method of measuring the weak grating reflectivity based on FBG-FP scales is convenient, efficient, and accurate. It is also worth mentioning that the method of femtosecond-laser direct writing eliminates the period limitation of the phase mask, thereby expanding the measurement wavelength range of FBGs. In the future, with the improvement of fiber grating fabrication technology, it is expected that more accurate results can be obtained.

Tunable ultraviolet laser induces high signal-to-noise ratio in intrinsic fluorescence titration for protein- ligand interaction studies

Protein intrinsic fluorescence emitted by tryptophan residues is widely used for macromolecular mechanism studies, such as the protein binding by ligands and enzyme kinetics. However, modifications in fluorescence intensity can be weak using the intrinsic fluorescence in many proteins containing few tryptophan residues, or with their weak involvement in the binding. A low signal to noise ratio is a main drawback, due to the weak intensity of Xenon lamp illumination in the spectrofluorometer at the ultraviolet (UV) range, in addition to the small intrinsic fluorescence modifications with ligand binding, in many cases. We report herein the montage and the utilization of a laser light source in a spectrofluorometer in the UV range for the titration of protein intrinsic fluorescence. A significantly stronger modification of the protein intrinsic fluorescence was obtained with the titrations represented by E. coli Arginyl tRNA synthetase and human 17β-hydroxysteroid dehydrogenase type 7 with their natural ligands using a wavelength-tunable UV laser source. Also, a higher signal to noise ratio was obtained with the laser source. Finally, a similar KD was obtained with the titration using the two light sources. These results are supported by the direct binding study of equilibrium dialysis, showing that the laser source yields a reliable binding analysis. Thus, the utilization of a laser source in the intrinsic fluorescence titration and related usage can give a significant opening for the wider applications of the spectroscopy and photo technology to many new proteins with limited intrinsic fluorescence signals, while their mechanisms are critical to understand biochemistry and will facilitate medical applications. This study includes fundamental, applied, and interdisciplinary areas of physics.

Modification of Frictional Properties of Hydrogel Surface via Laser Ablated Topographical Micro-Textures.

Hydrogels and biological cartilage tissues are highly similar in structure and composition due to their unique characteristics such as high-water content and low friction coefficients. The introduction of hydrogel cartilage can effectively reduce the friction coefficient and wear coefficient of the original bone joint and the implanted metal bone joint (generally titanium alloy or stainless steel), which is considered as a perfect replacement material for artificial articular cartilage. How to accurately regulate the local tribological characteristics of hydrogel artificial cartilage according to patient weight and bone shape is one of the important challenges in the current clinical application field of medical hydrogels. In this study, the mechanism by which micro-pits improve the surface friction properties was studied. Ultraviolet lasers were used to efficiently construct micro-pits with different shapes on a polyvinyl alcohol hydrogel in one step. It was shown that by using such a maskless laser processing, the performance of each part of the artificial cartilage can be customized flexibly and effectively. We envision that the approach demonstrated in this article will provide an important idea for the development of a high-performance, continuous and accurate method for controlling surface friction properties of artificial cartilage.

Laser-induced assembly of Au nano-polyhedron clusters as stable 3D superstructures with ultrabroadband plasmonic resonance for promoting multi-band SERS

Multi-band surface-enhanced Raman spectroscopy (SERS) via multiple excitation wavelengths operated on broadband resonant substrate can empower a high-dimensional comprehensive molecular diagnosis of complex analytes. Herein, we demonstrate an extraordinary multi-resonant SERS active substrate that is composed of three-dimensional (3D) hierarchical plasmonic Au superstructures (SS) with both horizontal and vertical close-packed nano-polyhedron clusters on fluorine-doped tin oxide-coated (FTO) support. The stable solid-state Au SS can be simplistically developed by ultraviolet laser irradiation of FTO plate in HAuCl4 solution that facilitates photoexcited reduction of Au ions and then anisotropic nucleation of Au atoms. The 3D Au SS with ultrabroadband plasmonic resonance ranging from visible light to near-infrared region (400∼2000nm) provide remarkable enhanced multi-band SERS performances under 532, 633 and 785 nm excitation wavelengths, in comparison with the reference normal Au nanoparticles (NPs). Especially, the 785 nm NIR excitation of the generated SERS substrate enables the ultra-low detection limit of crystal violet (CV) molecules to be achieved as low as 10−16 M, which is obviously better than many previous works. The ultrahigh multi-band SERS activity is highly related the strong synergetic coupling effects of these interconnected Au nano-polyhedrons with hybridized multiple plasmonic modes. Besides, the ingenious Au configuration also possesses excellent SERS spatial uniformity, long-term stability and reproducibility, having more promising potentials for practical operation. Therefore, the versatile 3D plasmonic SS may grant attractive alternative pathway toward robust multi-band SERS analyses in the near future.

Probing zinc oxide as a semiconductor photosensitizer of energetic materials to laser radiation

The development of methods for laser ignition of energetic materials opens up significant prospects for improving the safety of their application. The photochemical processes can increase the selective sensitivity of energetic material to laser radiation and make the most of its capabilities. The paper considers the application of zinc oxide as a photosensitizing additive capable of significantly reducing the laser ignition threshold of pentaerythritol tetranitrate (PETN). The dependencies of the laser initiation threshold by the fundamental frequency (1064 nm), second (532 nm), and third harmonic (355 nm) of an Nd:YAG laser on the concentration of zinc oxide were studied. The ignition threshold energy density is reduced by a factor from 2 to 5, depending on the oxide concentration and the laser radiation wavelength. The proposed initiation mechanisms based on the results of the several spectroscopy methods were discussed for each laser harmonic. Laser ignition of the PETN-ZnO composite through the ultraviolet laser radiation combined the thermal and photochemical mechanism. Visible laser harmonic (532 nm) drives a purely photochemical initiation of the PETN. The obtained results indicate that a mechanism of initiation by visible light should be caused by the charge transfer in the ZnO-PETN interface. The reported experimental result does not only serve as an example of the successful photochemical ignition of energetic material but also shows that processes, techniques, and materials of semiconductor photocatalysis are applicable to laser initiation.

Complete and selective nitration of tyrosine residue in peptides caused by ultraviolet matrix-assisted laser desorption/ionization.

Complete and highly selective nitration of tyrosine (Tyr) as a residue-specific modification in peptides was found without side reactions, using ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) with a nitroaromatic reagent 3, 5-dinitrosalicylic acid (3,5-DNSA). The tyrosine nitration supported two propositions, namely, the UV-induced. NO2 attack reaction mechanism by Long et al. and the C-NO2 homolysis as a thermal process by Wiik et al. and Furman et al. With the UV-MALDI of peptides, a residue-specific reaction was observed in glycine (Gly) residue, i.e., an oxidation of the alpha-carbon of Gly due to attack of hydroxyl radical (.OH).

Effect of Dual-Pulse Temporal Shaping on Ultraviolet Nanosecond Laser Damage of Fused Silica Surface in High Fluence Regime

We present the effect of dual-pulse temporal shaping on the ultraviolet nanosecond laser damage characteristics of a fused silica exit surface in a high fluence regime. The pre- and post-pulse have the opposite effects on the damage behavior at a pulse delay of 20 ns. The pre-pulse irradiation significantly increases the main-pulse threshold, making it much higher than that of the single-pulse threshold, while the post-pulse has little effect on the main-pulse threshold. For near-threshold damage sites, the pre-pulse reduces the average damage size and depth, making them smaller than those of the single-pulse, while the post-pulse drastically increases the average size and depth, making them much larger than those of the single-pulse. The average size of the damage site is monotonously increased from 43.6 µm to 127.9 µm with increasing post-pulse energy. For the pre-pulse with a shape factor of 0.61, the damage threshold of the main pulse increases with increasing delay and nearly stabilizes after 10 ns. The underlying mechanism of the temporal-shaping effect on laser damage is discussed based on the applied precursor modification to absorption enhancement, which could provide insights for studying ultraviolet laser damage of fused silica optics.

Removal of SiC at atomic and close-to-atomic scale by nanosecond ultraviolet laser

This paper presents an exploration on the material removal of SiC at atomic and close-to-atomic scale under ambient condition by nanosecond ultraviolet laser. The corresponding mechanisms are investigated by heat conduction model, reactive molecular dynamics simulation and single shot irradiation experiments. The threshold fluence of 2.52 J/cm2 is obtained from experimental results, and distinctive material behaviors including modification and atomic layer exfoliation at near-threshold fluence are investigated using atom force microscopy. The results show that modification in the chemical composition varies with the depth due to the change in oxygen content. In addition, both the surface modification and material removal can take place at the scale of single atomic layer, which implies a promising manufacturing approach to achieve ultimate precision for a material.

High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide: II. Nitrogen dilution from 1900 to 8200 K

This work investigates the high-temperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in nitrogen (N2) to target the NO–N2 rates relevant to high-temperature air, thereby building off the argon (Ar) experiments investigated in Part I. [J. W. Streicher et al., “High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. I. Argon dilution from 2200 to 8700 K,” Phys. Fluids 34, 116122 (2022)] Again, two continuous-wave ultraviolet laser diagnostics were used to obtain quantum-state-specific time histories of NO in high-temperature shock-tube experiments, including absorbance (α) in the ground vibrational state of NO, translational/rotational temperature (Ttr), and number density of NO (nNO). The experiments probed mixtures of 2% and 0.4% NO diluted in either pure N2 (NO/N2) or an equal parts N2/Ar mixture (NO/N2/Ar). The NO/N2 experiments spanned initial post-reflected-shock conditions from 1900–7000 K and 0.05–1.14 atm, while the NO/N2/Ar experiments spanned from 1900–8200 K and 0.11–1.52 atm. This work leveraged two vibrational relaxation times from Part I (τVTNO−Ar and τVTNO−NO) and extended measurements to include the vibrational–translational and vibrational–vibrational relaxation times with N2 (τVTNO−N2 and τVVNO−N2). Similarly, this work leveraged the four rate coefficients from Part I (kdNO−Ar, kdNO−NO, kfN2O, and kzNO−O) and extended measurements to include NO dissociation with N2 (kdNO−N2). A few studies have directly inferred these rates from experiments, and the current data differ from common model values. In particular, τVTNO−N2 differs slightly from the Millikan and White correlation, τVVNO−N2 is four times slower than Taylor et al.'s inference, and kdNO−N2 is four times slower than the Park two-temperature model. The unique experimental measurements and dilution in N2 in this study significantly improve the understanding of the vibrational relaxation and decomposition of NO in high-temperature air.

High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. I. Argon dilution from 2200 to 8700 K

This work investigates the high-temperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in argon (Ar) to target NO–Ar and NO–NO interactions and to augment the subsequent inference of rates for NO diluted in nitrogen (N2). [J. W. Streicher et al., “High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. II. Nitrogen dilution from 1900 to 8200 K,” Phys. Fluids (submitted)]. In both Part I and Part II, two continuous-wave ultraviolet laser diagnostics were used to probe quantum-state-specific time-histories of NO behind reflected shocks in high-temperature shock-tube experiments, enabling inferences of multiple vibrational relaxation times and reaction rate constants for NO decomposition reactions. These diagnostics both probed absorbance (α) in the ground vibrational state of NO but in multiple rotational states utilizing light at 224.8150 and 226.1025 nm. The absorbance was subsequently used to infer quantum-state-specific time-histories for translational/rotational temperature (Ttr) via the absorbance ratio and number density of NO (nNO) via α, Ttr, and the absorbance cross sections (σ). The experiments for Ar dilution probed mixtures of 2% NO/Ar, 1% NO/Ar, and 0.4% NO/Ar for initial post-reflected-shock conditions from 2200–8700 K and 0.12–0.97 atm. Further analysis of the absorbance, temperature, and number density time-histories yielded two vibrational relaxation times (τVTNO−Ar and τVTNO−NO) and four rate coefficients for multiple NO decomposition reactions (kdNO−Ar, kdNO−NO, kfN2O, and kzNO−O)—each of which is extended to higher temperatures than any previous study and with reduced scatter and uncertainty. Generally, these rate data are consistent with data from the literature, although τVTNO−Ar and τVTNO−NO are observed to differ strongly from both the Millikan and White correlation and Park two-temperature model.

Recent Advances in Single-Cell Metabolomics Based on Mass Spectrometry

Recent Advances in Single-Cell Metabolomics Based on Mass Spectrometry

Research on gold film-assisted ultraviolet nanosecond laser machining of diamond microgrooves

Diamond microgrooves have great application prospects in biomedicine, high-power semiconductor devices, optical imaging, chip thermal management, and cutting tools. In this work, the effect of gold film on ultraviolet nanosecond laser machining of single-crystal diamond microgrooves was investigated through simulation and experimental research. The simulation results show that the ablation width and depth gradually increase with the increase of gold film thickness, and the ablation depth reaches a maximum value when the thickness of gold film increases to 100 nm. The surface morphology and elemental analysis show that the gold film was first melted and then re-solidified around the microgrooves under ultraviolet nanosecond laser irradiation. The ablation threshold of Au-coated diamond is 4.05 J/cm2, 24 % smaller than that of uncoated diamond. The material removal rate of Au-coated microgrooves is always higher than that of uncoated microgrooves at the same laser intensity. At laser intensity of 8.5 J/cm2, it is 1.1 times higher than that of uncoated diamond microgrooves. Moreover, the physical mechanism of gold film during laser machining of diamond microgroove is analyzed. This study provides a new method for efficient machining of diamond microgrooves.

Admp regulates tail bending by controlling ventral epidermal cell polarity via phosphorylated myosin localization in Ciona.

Ventral tail bending, which is transient but pronounced, is found in many chordate embryos and constitutes an interesting model of how tissue interactions control embryo shape. Here, we identify one key upstream regulator of ventral tail bending in embryos of the ascidian Ciona. We show that during the early tailbud stages, ventral epidermal cells exhibit a boat-shaped morphology (boat cell) with a narrow apical surface where phosphorylated myosin light chain (pMLC) accumulates. We further show that interfering with the function of the BMP ligand Admp led to pMLC localizing to the basal instead of the apical side of ventral epidermal cells and a reduced number of boat cells. Finally, we show that cutting ventral epidermal midline cells at their apex using an ultraviolet laser relaxed ventral tail bending. Based on these results, we propose a previously unreported function for Admp in localizing pMLC to the apical side of ventral epidermal cells, which causes the tail to bend ventrally by resisting antero-posterior notochord extension at the ventral side of the tail.

Determination of Barbiturates by Femtosecond Ionization Mass Spectrometry.

Barbiturates are highly susceptible to dissociation in mass spectrometry (MS) because of their long side chains combined with a nonaromatic ring consisting of several carbonyl and amine groups. As a result, they exhibit extensive α-cleavage and subsequent rearrangement, making the identification of these compounds difficult. Although a library of electron ionization MS (EIMS) is available, most barbiturates have very similar fragment patterns. Accordingly, it would be desirable to develop a technique for soft ionization, providing a molecular ion and large fragment ions as well. In this study, a molecular ion was clearly observed, in addition to large fragment ions, for a variety of barbiturates based on multiphoton ionization MS (MPIMS) using a tunable ultraviolet femtosecond laser as the ionization source (fs-LIMS). This favorable result was achieved when the optimal laser wavelength for minimizing the excess energy remaining in the ionic state was used. An examination of the photofragmentation pathways suggested that an H atom in the side chain was abstracted by an oxygen atom in the carbonyl group in the ring structure thus initiating fragmentation and subsequent rearrangement. Barbiturates that are substituted with alkyl groups (amobarbital and pentobarbital) had narrower spectral regions for optimal ionization than the other barbiturates with alkyl and alkenyl groups (butalbital and secobarbital) and more with alkyl and phenyl groups (phenobarbital). All of the barbiturates studied provided unique mass spectral patterns in fs-LIMS, which was useful for the reliable identification of these compounds in practical trace analysis.

Laser Processing of KBr‐Modified SnO2 for Efficient Rigid and Flexible Ambient‐Processed Perovskite Solar Cells

A combination of KBr modification and laser processing is utilized to prepare SnO2 films for rigid and flexible perovskite solar cells (PSCs). The KBr modification effectively passivates the defects at the interface between SnO2 and perovskite as well as grain boundaries of the perovskite film. A power conversion efficiency (PCE) of 20.14% is achieved with the KBr‐modified SnO2 for the rigid PSCs fabricated under a relative humidity of around 65–75%, compared to the pristine SnO2 films with a PCE of 18.66%. Then, a picosecond ultraviolet laser is employed to process KBr‐modified SnO2 films on flexible substrates with a rapid scanning rate of 100 mm s−1. The laser process improves the PCEs and durability of the PSCs. The flexible PSCs fabricated by the laser remain over 80% of their initial PCEs after 1000 bending cycles, higher than those fabricated by the hot plate showing 40% of their initial PCEs after the same bending cycles.

A study of the manufacturing process of custom dimensional material standards using pulsed laser systems

New market requirements have led to the manufacturing of parts in the microscale. To produce this kind of parts, one of the options is to adapt the traditional manufacturing processes to work with dimensions of the order of micrometers. At this point, it is worth asking how it is possible to ensure with enough certainty that the dimensions of the parts meet the model requirements. Thus arises the need to use measuring instruments, with adequate traceability to the unit of length of the SI, capable of measuring on this scale. Among the different types, it seems that optical measuring instruments are going to be the most important in the near future. The traceability of this equipment is not currently standardized. One of the main problems is that there are no standard, cost-effective material references to give traceability to the measurement results obtained with these instruments at microscale. In this document, we aim to define the optimal process parameters for the manufacturing of custom material standards using ultraviolet laser systems. For this task, we have carried out a qualitative and quantitative analysis of the results using two different measuring instruments: an optical microscope and a stylus instrument.

Laser fabrication of graphene oxide/reduced graphene oxide papers for ethanol vapor driven actuator

This report aims to illustrate a highly effective method to obtain a kind of graphene oxide (GO)/reduced graphene oxide (RGO) paper by Ultraviolet (UV) laser direct writing method. Due to removing the oxygen-containing groups and introducing the layered micro-nano structures on one side of the GO film. The as-prepared GO/RGO papers exhibit anisotropically deformation triggered by ethanol vapor and could enable the potential applications for stimuli-response actuators.