Laser Repetition Research Articles

On the seam welding of ultra-thin commercially pure (CP) grade-2 titanium foils

ABSTRACT This communication reports a comparative study on the seam welding of CP grade-2 Ti foils with thicknesses of 10 µm, 50 µm and 100 µm using a repetitive nanosecond pulsed fibre laser and a lamp-pumped pulsed Nd-YAG laser. The welding was accomplished in a lap joint configuration. A fine acicular microstructure was observed in the welds made by the nano second fibre laser, whilst a coarse basket weave microstructure was observed in the welds made by the Nd-YAG laser. The tensile properties of the welds carried out by the nano-second fibre laser was found to be superior with respect to the welds carried out by the pulsed Nd-YAG laser. Welds made by the nano-second fibre laser could withstand ~17% more load, whereas elongation before fracture was almost double to that of the welds made by the Nd-YAG laser. This variation is comprehensible on the basis of the finer weld microstructure evolved in nanosecond laser welding due to the very low heat input and high cooling rate associated with this process. This study established with certitude that nanosecond duration pulsed fibre lasers are a better choice for welding thin titanium foils in comparison to long pulsed Nd-YAG lasers.

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1．はじめに

Photo-Switchable Nanoripples in Ti3C2Tx MXene

MXenes are two-dimensional materials with a rich set of chemical and electromagnetic properties, the latter including saturable absorption and intense surface plasmon resonances. To fully harness the functionality of MXenes for applications in optics, electronics, and sensing, it is important to understand the interaction of light with MXenes on atomic and femtosecond dimensions. Here, we use ultrafast electron diffraction and high-resolution electron microscopy to investigate the laser-induced structural dynamics of Ti3C2Tx nanosheets. We find an exceptionally fast lattice response with an electron-phonon coupling time of 230 fs. Repetitive femtosecond laser excitation transforms Ti3C2Tx through a structural transition into a metamaterial with deeply sub-wavelength nanoripples that are aligned with the laser polarization. By a further laser illumination, the material is reversibly photo-switchable between a flat and rippled morphology. The resulting nanostructured MXene metamaterial with directional nanoripples is expected to exhibit an anisotropic optical and electronic response as well as an enhanced chemical activity that can be switched on and off by light.

BSCI-08. In vivo two-photon characterization of tumor-associated macrophages and microglia (TAM/M) and CX3CR1 during different steps of brain metastasis formation from lung cancer

Abstract Background Brain metastases represent a common complication of lung cancer and dramatically limit prognosis in affected patients. The influence of tumor-associated macrophages and microglia (TAM/M) and their receptor CX3CR1 on different steps of brain metastasis formation from lung cancer is poorly characterized, but might be of therapeutic relevance. Methods We established an orthotopic cerebral metastasis model using CX3CR1-proficient (CX3CR1GFP/wt) and -deficient (CX3CR1GFP/GFP) mice with green-fluorescent TAM/M. A cranial window was prepared, and intracarotid injection of red-fluorescent Lewis Lung Carcinoma-cells (tdtLLC) was performed two weeks later. Formation of brain metastases was followed by repetitive two-photon laser scanning microscopy. Results After intracarotid injection, intravascular tumor cells extravasated into the cerebral parenchyma and eventually formed micrometastases (≤50 cells) and mature macrometastases (>50 cells). We observed phagocytosis of extravasated tumor cells by TAM/M during early steps of metastatic growth. Notably, these anti-tumor effects of TAM/M diminished during later steps of metastasis formation and were accompanied by TAM/M accumulation and activation. CX3CR1-deficiency resulted in a lower number of extravasated tumor cells, and only a small number of TAM/M were visualized during early steps of metastasis formation (extravasation, formation of micrometastases) in such mice. In contrast, progression of extravasated tumor cells into micrometastases was more frequently found in CX3CR1-deficient mice. Overall, these mechanisms resulted in a comparable number of mature macrometastases between CX3CR1-deficient and -proficient mice. Conclusion Our findings indicate that unspecific inhibition of CX3CR1 might not be a suitable therapeutic approach to prevent cerebral dissemination of lung cancer cells. Given the close interaction between TAM/M and tumour cells during metastasis formation, other therapeutic approaches targeting TAM/M function warrant evaluation. Such concepts might be evaluated in vivo using the herein established orthotopic mouse model.

Robust Ultralong Lead Halide Perovskite Microwire Lasers.

Hybrid organic-inorganic lead halide perovskite microwires are potential building blocks for realizing on-chip integrated optoelectronic devices. However, the length-controllable synthesis of one-dimensional hybrid perovskite microwires has been rarely reported, especially the ones with lengths in the millimeter scale. Herein, methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) micro and milliwires are demonstrated using single-crystal PbBr2 microwires synthesized via template-free solution-phase growth as the lattice framework. Following the PbBr2 template, the as-converted perovskite microwires possess controllable lengths ranging from tens to thousands of micrometers. In addition, Fabry-Perot (FP) lasing was realized in both MAPbBr3 and FAPbBr3 microwires, attesting to their excellent crystal quality and the efficient optical confinement of the natural cavity. These unique properties result in the first demonstration of FP perovskite microwire lasers with submillimeter lengths. More interestingly, the microwire lasers show excellent photostability under repetitive pulsed laser excitation for over 8 × 106 cycles. Such findings demonstrate that the solution-converted hybrid lead bromide microwires have excellent optoelectronic performances promising for practical applications, and the size controllability indicates that this novel fabrication process may be feasible for large-scale industrial production.

Identification of the fate and regenerative mechanism of zebrafish melanocyte progenitor cells and melanocytes after laser-induced pigment ablation.

Lasers are known to be the most effective treatment modality for pigmentary skin diseases. However, melanocytes and melanin pigment often recur or leave post-inflammatory hyperpigmentation after the laser procedure. Studies have reported on the role of progenitor cells in pigment cell regeneration, which can be constantly replenished through mitosis. However, the response of unpigmented melanocyte progenitor cells to laser treatment is poorly understood. In this study, we used adult zebrafish skin as the melanocyte regenerative system and examined the response of melanocyte progenitor cells to laser photothermolysis. The two groups of adult zebrafish were irradiated with 1064 nm wavelength laser system of Q-switched neodymium:yttrium-aluminum-garnet (Nd:YAG) laser with 0.3 or 0.7 J·cm-2 . We compared the regeneration of pigment at different energy levels by measuring new melanocyte counts and pigment area. We traced and quantitatively compared the melanocyte lineage cells by immunohistochemical staining using specific markers such as sox10, mitfa, and dct during the regeneration process. Three repetitive laser ablations were also held to test the postinflammatory hyperpigmentation. After the laser ablation of melanocytes, most of the new melanocytes appeared between Days 5 and 10. In high-energy irradiation of 0.7 J·cm-2 , the unpigmented mitfa-expressing cells showed significant decrease (p < 0.05) and showed delay in the differentiation process of melanocyte lineage cells. After repeated laser irradiation, hyperpigmentation did not appear and the final recovery ratio of the pigmented area was 87.5% and 75.3% at the 0.3 and 0.7 J·cm-2 energy levels, respectively. We suggest that laser treatment overcoming the recurrence should be planned based on the adequate energy level targeting the melanocyte progenitor cells. High-energy irradiation may induce apoptosis of progenitor cells and delay their process of differentiation. Short-term repetitive sessions of laser therapy can reduce the pigmentation in the long-term observation.

Polarisation-modulated photon-counting 3D imaging based on a negative parabolic pulse model.

Indirect methods based on intensity for time-of-flight measurement have attracted considerable research interest in recent years because they can provide high spatial resolution in 3D imaging. However, the majority of indirect methods are inapplicable when echo signals are small (e.g., less than one photon). We propose a novel polarisation-modulated photon-counting 3D imaging method based on a negative parabolic pulse model (NPPM) to solve this problem. We measure weak signals using the number of received photons after repetitive pulsed laser emission. We establish a computational method by exploring the relationship between photon flight time that corresponds to the polarisation-modulated state of photons controlled by phase shift and calculated photon rates from received photon-counting values based on Poisson negative log-likelihood function to calculate the distance. We specifically utilise the NPPM to estimate distribution of echo signals and reduce ranging error given that echo signals are constantly time-varying. We build the first experimental system for polarisation-modulated photon-counting 3D imaging for verification by integrating it with a dual-axis galvo scanning device. Experimental results demonstrate that the proposed method can achieve ranging accuracy at the millimeter-level and exhibit superior 3D imaging performance even when the average received number of echo signals per pulsed laser emission is smaller than 0.05.

Selective Direct Laser Writing of Pyrolytic Carbon Microelectrodes in Absorber-Modified SU-8.

Pyrolytic carbon microelectrodes (PCMEs) are a promising alternative to their conventional metallic counterparts for various applications. Thus, methods for the simple and inexpensive patterning of PCMEs are highly sought after. Here, we demonstrate the fabrication of PCMEs through the selective pyrolysis of SU-8 photoresist by irradiation with a low-power, 806 nm, continuous wave, semiconductor-diode laser. The SU-8 was modified by adding Pro-Jet 800NP (FujiFilm) in order to ensure absorbance in the 800 nm range. The SU-8 precursor with absorber was successfully converted into pyrolytic carbon upon laser irradiation, which was not possible without an absorber. We demonstrated that the local laser pyrolysis (LLP) process in an inert nitrogen atmosphere with higher laser power and lower scan speed resulted in higher electrical conductance. The maximum conductivity achieved for a laser-pyrolyzed line was 14.2 ± 3.3 S/cm, with a line width and thickness of 28.3 ± 2.9 µm and 6.0 ± 1.0 µm, respectively, while the narrowest conductive line was just 13.5 ± 0.4 µm wide and 4.9 ± 0.5 µm thick. The LLP process seemed to be self-limiting, as multiple repetitive laser scans did not alter the properties of the carbonized lines. The direct laser writing of adjacent lines with an insulating gap down to ≤5 µm was achieved. Finally, multiple lines were seamlessly joined and intersected, enabling the writing of more complex designs with branching electrodes and the porosity of the carbon lines could be controlled by the scan speed.

Direct-write laser-induced self-organization and metallization beyond the focal volume in tellurite glass

We report on a self-organized process initiated on a potassium-tungsten-tellurite glass surface under repetitive femtosecond laser irradiation in a regime where cumulative effects lead to a localized melting. Specifically, we show that self-organized periodic patterns consisting of parallel nanoplanes perpendicular to the laser polarization are forming and extending beyond the zone under direct laser exposure. Examination of the modified regions revealed a phase change from a glassy tellurium-oxide to a crystalline tellurium. In addition, we observe that this self-organization process, associated with elemental redistribution and deoxygenation, is triggered by the optical-field strength. We suggest that early self-organized nanostructures formed by a local-field enhancement is subsequently reinforced by a metallization event in an open-air atmosphere.

Laser-induced breakdown in liquid water: Influence of repeated laser pulses on plasma formation and emission

We have experimentally investigated the influence of repeated laser pulses on laser-induced breakdown (LIB) in water following a focused 1064 nm laser pulse, for repetition rates between 0.5 and 20 Hz and up to 100 pulses. LIB plasma image data are collected as a function of repetition rate and laser pulse number, using an intensified CCD camera with sub-nanosecond camera-laser timing. Each pulse induces multiple breakdowns, shockwaves, and macroscopic cavitation bubbles. These effects disappear by the time the next pulse arrives. Even so, we find that the water target retains effects from previous pulses that persist up to roughly a second and that modify subsequent LIB properties including plasma location, spatial extension, and total emission intensity. We quantify the effects by tracking the first moment of the plasma emission intensity, which we call the “center of intensity” (CoI). Three distinct reproducible repetition-rate-dependent phases are identified with repeated pulses. In particular, (1) with initial pulses, emission intensity decreases and the CoI broadens and propagates downstream from the laser focus reaching a maximum axial distance away from the focal point, (2) with additional pulses, the CoI narrows and travels back upstream toward the laser, and (3) with continued pulses, the plasma gains back a portion of the lost intensity and the CoI reaches a steady state position, not at the focal point. Two possible causal effects are highlighted; particle inclusion concentration changes and microbubbles in the laser path.

Thermal effects in molecular gas-filled hollow-core fibers.

Few-cycle sources with high average powers are required for applications to attosecond science. Raman-enhanced spectral broadening of Yb-doped laser amplifiers in molecular gases can yield few-cycle pulses, but thermal excitation of vibrational and rotational degrees of freedom may preclude high-power operation. Here we investigate changes in the spectral broadening associated with repetitive laser interactions in an ${{\rm{N}}_2}{\rm{O}}$-filled hollow-core fiber. By comparing experimental measurements of the spectrum associated with each laser pulse to simulations based on a density matrix model, we find that losses in a spectral bandwidth and transmission are largely dominated by thermal excitation of the gas.

Graded functionality obtained in NiTi shape memory alloy via a repetitive laser processing strategy

The introduction of graded functionality to NiTi shape memory alloy is being pursued to widen martensitic transformation intervals, enhance the controllability and further promote their widespread application as smart components. However, it is far away to obtain bulk graded NiTi alloy using current methods. In this work, a repetitive laser processing strategy was designed and employed to manufacture functionally graded NiTi alloy. The highest martensitic transformation interval was measured to be 126.7 ± 0.4 °C. The graded functionality was confirmed to display as the continuous increase in mechanical recoverable strain with regard to the applied strain. Furthermore, the graded functionality is independent upon the processing parameters and loading conditions. The microstructure gradient was characterized with the increased grain size and increased amount of B19′ phase, as well as the variation of interior defects along the gradient direction. This is exactly because of the microstructure gradient that causes the dependence of fracture mechanism on the loading condition. The graded functionality was finally analyzed to derive from swift of deformation mechanisms between the detwinning process of pre-existing B19′ variants and the stress-induced martensitic B19′ phase transformation. This work indeed provides a novel and effective approach to manufacture three-dimensional graded NiTi alloy, which could undertake complex structure design thereby catering to desired functional performance in various areas.

BSA/Silver Nanoparticle-Loaded Hydrogel Film for Local Photothermal Treatment of Skin Cancer.

To develop a hydrogel film containing bovine serum albumin (BSA)-coated silver nanoparticles (BSA/AgNP) and evaluate its applicability for topical photothermal treatment (PTT) of skin cancer. BSA/AgNP-loaded hydrogel films were prepared and their swelling, bioadhesive, mechanical, and photothermal properties were characterized in vitro and in vivo. The synthesized BSA/AgNP exhibited a narrow size distribution with good size stability and, notably, possessed great photothermal activity that could stably maintain through repetitive laser irradiation. The BSA/AgNP-loaded hydrogel films showed favorable swelling, bioadhesive, tensile, and photothermal properties. Based on these results, when tested the anti-cancer effects in B16F10 s.c. tumor-bearing mice, the PTT with the topical treatment of BSA/AgNP-loaded hydrogel films could significantly inhibit the tumor growth by a single treatment with no apparent toxicity. Overall, the results of this study demonstrated that the BSA/AgNP-loaded hydrogel films may serve as an effective but safe topical PTT agent for the treatment of skin cancer.

Observation and Control of Unidirectional Ballistic Dynamics of Nanoparticles at a Liquid-Gas Interface by 4D Electron Microscopy.

Understanding and controlling the dynamics of active Brownian objects far from equilibrium are fundamentally important for emerging technologies such as artificial micro/nanomotors for drug deliveries and noninvasive microsurgery. However, direct observation and control of unidirectional propulsion of individual nanoscale objects are technically challenging due to the required spatiotemporal resolution. Here, we report in situ visualization and manipulation of unidirectional superfast ballistic dynamics of a single-photon-activated gold nanoparticle (NP) along the liquid-gas interface by four-dimensional electron microscopy (4D EM) at nanometer and nanosecond scales. We observed that, upon repetitive femtosecond laser excitation, the NP at the liquid-gas interface exhibits a continuously superfast unidirectional translation with a linear dependence of its root mean squared velocity (νrms) on either the laser fluence or repetition rate. Under a single femtosecond pulse excitation, the NP exhibits a superfast ballistic translation at the nanosecond time scale. Combined experiment and physical modeling reveals that the superfast unidirectional, ballistic translation is driven by unidirectional random impulsive forces arising from the nanobubbles (NBs) induced by enhanced laser heating as a result of plasmonic excitation, which is controllable by tuning the laser characteristics. This directional plasmonic NB-propulsion mechanism sheds light on the design of light-controllable artificially intelligent micro/nanomotor systems.

On the cyclic fatigue of adhesively bonded aluminium: Experiments and molecular dynamics simulation

Direct bonding of metals with resin plays a critical role in the jointing of dissimilar materials. The adhesion strength is known to be dependent on strain (loading) rate due to the strain rate sensitivity of polymeric resin. This study evaluated the adhesion durability of an epoxy resin adhesive on an aluminum alloy under cyclic impact loading. First, we used a repetitive laser shock adhesion test (LaSAT), which enabled us to evaluate the impact strength of interfacial fractures (i.e., adhesion and its durability). In this method, a pulsed YAG laser was used to generate strong elastic waves, which resulted in the interfacial fracture of the adhesive/aluminum alloy. We prepared two types of specimens with different curing temperatures (20 °C and 100 °C) and found that the specimen with the higher curing temperature showed higher adhesion strength and durability. We revealed that the adhesion strength showed cyclic fatigue characteristics, and that higher curing temperature improved fatigue strength. To elucidate this mechanism on the molecular level, we conducted a molecular dynamics (MD) simulation for the epoxy resin adhesive on aluminum alloy. The model featured an all-atom model of the Al2O3/epoxy resin interface, and we performed cyclic tensile deformation until delamination. We found that the number of loading cycles until delamination increased when the applied tensile stress was lower. We also found that the 400 K curing model had greater adhesion strength than the 300 K curing model. This tendency was confirmed by the results of the LaSAT experiments. Our comprehensive study incorporating LaSAT experiments and MD simulations evaluated adhesion strength and revealed its fracture mechanism.

Treatment of small and medium retinoblastoma tumors with Iris diode laser

Differing techniques have been reported for focal laser therapy for patients with small and medium retinoblastoma. We report the technique used at our center; and report the functional and anatomical outcomes for small and medium retinoblastomas treated with focal laser therapy with or without systemic chemotherapy. A retrospective case study was conducted including pediatric patients with macular retinoblastoma treated with systemic chemotherapy and laser ablation from July 1990 to July 2015 at Washington University School of Medicine/Saint Louis Children's Hospital. Fourteen eyes (11 patients) with small and medium retinoblastoma tumors were treated with repetitive indirect laser hyperthermia and seven of those patients were treated with systemic chemotherapy as well. Using the International Retinoblastoma classification, one eye was stage A, 10 eyes were stage B, and three eyes were stage C. The mean follow-up time was 7.7 years. There were no recurrences of tumor in the patients. Final visual acuity outcomes were 20/20 to 20/50 in four eyes, 20/60 to 20/200 in four eyes, and 20/400 or less in six eyes. None of the patients developed metastatic disease. The evidence for systemic chemotherapy and diode laser therapy is limited to case series and retrospective reviews, but evidence suggests that it is an effective treatment for small and medium sized retinoblastoma tumors involving the macula with the potential for good visual outcomes.

3D time-domain beam mapping for studying nonlinear dynamics in multimode optical fibers.

Characterization of the complex spatiotemporal dynamics of optical beam propagation in nonlinear multimode fibers requires the development of advanced measurement methods, capable of capturing the real-time evolution of beam images. We present a new space-time mapping technique, permitting the direct detection, with picosecond temporal resolution, of the intensity from repetitive laser pulses over a grid of spatial samples from a magnified image of the output beam. By using this time-resolved mapping, we provide, to the best of our knowledge, the first unambiguous experimental observation of instantaneous intrapulse nonlinear coupling processes among the modes of a graded index fiber.

Evolution of a Superhydrophobic H59 Brass Surface by Using Laser Texturing via Post Thermal Annealing.

To fabricate an industrial and highly efficient super-hydrophobic brass surface, annealed H59 brass samples have here been textured by using a 1064 nm wavelength nanosecond fiber laser. The effects of different laser parameters (such as laser fluence, scanning speed, and repetition frequency), on the translation to super-hydrophobic surfaces, have been of special interest to study. As a result of these studies, hydrophobic properties, with larger water contact angles (WCA), were observed to appear faster than for samples that had not been heat-treated (after an evolution time of 4 days). This wettability transition, as well as the evolution of surface texture and nanograins, were caused by thermal annealing treatments, in combination with laser texturing. At first, the H59 brass samples were annealed in a Muffle furnace at temperatures of 350 °C, 600 °C, and 800 °C. As a result of these treatments, there were rapid formations of coarse surface morphologies, containing particles of both micro/nano-level dimensions, as well as enlarged distances between the laser-induced grooves. A large number of nanograins were formed on the brass metal surfaces, onto which an increased number of exceedingly small nanoparticles were attached. This combination of fine nanoparticles, with a scattered distribution of nanograins, created a hierarchic Lotus leaf-like morphology containing both micro-and nanostructured material (i.e., micro/nanostructured material). Furthermore, the distances between the nano-clusters and the size of nano-grains were observed, analyzed, and strongly coupled to the wettability transition time. Hence, the formation and evolution of functional groups on the brass surfaces were influenced by the micro/nanostructure formations on the surfaces. As a direct consequence, the surface energies became reduced, which affected the speed of the wettability transition—which became enhanced. The micro/nanostructures on the H59 brass surfaces were analyzed by using Field Emission Scanning Electron Microscopy (FESEM). The chemical compositions of these surfaces were characterized by using an Energy Dispersive Analysis System (EDS). In addition to the wettability, the surface energy was thereby analyzed with respect to the different surface micro/nanostructures as well as to the roughness characteristics. This study has provided a facile method (with an experimental proof thereof) by which it is possible to construct textured H59 brass surfaces with tunable wetting behaviors. It is also expected that these results will effectively extend the industrial applications of brass material.

Phase locking and homodyne detection of repetitive laser pulses

A method to realize pulse laser phase locking and homodyne detection is proposed, which can be used in lidar and continuous variable quantum key distribution (CVQKD) systems. Theoretical analysis shows that homodyne detection of pulse laser has a sensitivity advantage of more than 4 dB over heterodyne detection. An experimental verification setup was constructed to realize phase-locking and homodyne detection of pulse lasers at repetition rates from 50 kHz to 2.4 MHz. For 320 ns signal laser pulses at 300 kHz with peak power of -65 dBm, the phase error is 8.9° (mainly limited by the chirp effect in the modulation of signal laser), and the detection signal-to-noise ratio reaches 20.2 dB. When the peak power is reduced to -75 dBm, phase locking and homodyne detection can still be achieved. Homodyne detection based on phase locking could serve as a novel weak-laser-pulse receiving method with high sensitivity and anti-interference performance.

A Combined CFD, ANFIS and NSGA-II Model for Repetitive Pulse Laser Drilling Process

The article is focused on maximizing the dimensions of a hole drilled through repetitive pulsed laser drilling process by employing the artificial neuro-fuzzy inference system (ANFIS) and multi-objective Genetic Algorithm (GA) with the help of the data obtained from numerical modelling. An axisymmetric two-dimensional numerical model is developed for studying this repetitive pulse laser drilling process in a cylindrical work piece to determine the drilled hole depth and radius and the temperature at the point of laser application. The governing differential equations are discretized using the Finite volume method and the Tri-Diagonal Matrix Algorithm is used to solve the resulting algebraic equations to obtain temperature distribution inside the computational domain. The Enthalpy-Porosity method is used to track the solid-liquid interface during the melting process. The laser source considered here is Gaussian and volumetric in nature and the computations are carried out taking different laser parameters like energy, number of pulses, pulse width, duty cycle and beam radius. The artificial intelligence method ANFIS is adopted to model the process using the result data obtained from numerical investigation. Separate ANFIS models are developed for the prediction of the depth, radius, center temperature, HAZ and taperness of the simulated drilled hole. These models are used as objective functions for a multi-objective genetic algorithm program to get a set optimal solutions for maximizing the depth and radius of the drilled hole and the temperature at the point of laser application in an objective to achieve highest material removal. Another section is also presented carrying out the multi-objective optimisation of considering maximiisation of radius, depth and center temperature along with minimisation of the HAZ and the tapoerness of the hole.