Nanosecond Laser Pulses Research Articles

Experimental investigation on microstructure, mechanical and optical properties of RB-SiC ablated by nanosecond pulsed laser in nitrogen atmosphere

Reaction-bonded silicon carbide (RB-SiC) has gained significant attention owing to its exceptional physical and chemical properties. Improving the photovoltaic efficiency and mechanical properties of RB-SiC is conducive to further enhancing its applicability. Laser gas nitriding is an effective strategy to simultaneously reduce the surface reflectivity and enhance the surface hardness of materials. Accordingly, the evolution of microstructure, mechanical and optical properties of RB-SiC composite ablated by nanosecond pulsed laser in nitrogen atmosphere was comprehensively investigated. After laser gas nitriding, the surface hardness of RB-SiC composite was increased by 10.6–44.6 %. The chemical composition analysis indicated that Si-based nitride was formed on the laser-ablated surface, which was mainly responsible for the hardness enhancement. Additionally, the reflectivity of the laser-ablated surface was reduced by up to 95.2 % compared to the original RB-SiC composite. This study provides a straightforward and effective method to simultaneously improve the mechanical and optical properties of RB-SiC composites.

High average power and high repetition rate eye-safe Raman laser driven by a two-crystal Nd:YVO4 laser.

We report on a high average power and high repetition rate nanosecond pulsed eye-safe KGW Raman laser intracavity driven by an acousto-optic Q-switched 1342 nm two-crystal Nd:YVO4 laser. Taking advantages of the carefully selected two-composite-laser-crystal configuration, the thoroughly optimized gate-open time of acousto-optic modulator and the ingeniously designed U-shaped resonator, substantial power and efficiency enhancements as well as superior mode matching have been enabled. Under the injected pump power of 64.5 W, the average output powers of the first-Stokes fields at 1496 and 1527 nm can be up to 8.1 and 9.5 W with 25 kHz repetition rate and 3.2 µs gate-open time, respectively, corresponding to the optical power conversion efficiencies of 12.6% and 14.7%. Meantime, the resultant pulse widths are determined to be 4.6 and 6.3 ns with the peak powers of approximately 70 and 60 kW, respectively. The beam quality can be maintained with M2 < 1.5 across the entire output power range.

Structural correlation to linear and non-linear optical properties of Bi3+ ion doped Li2O–SrO–B2O3 glasses: Optical switching and limiting applications

A series of bismuth doped lithium strontium borate glasses are synthetized using melt-quench technique. Compositional modification resulted in the maximum refractive index (2.306) and lowest optical bandgap (3.36 eV) for 2 mol% of Bi2O3. Raman as well as FTIR investigations reveal the presence of diborate, dipentaborate and metaborate ring type structural units and structural interconversions occur between diborate and metaborate rings. These structural interconversions are in support of the trends observed in linear and non-linear optical properties. Photoluminescence spectra reveal a prominent emission peak around 572 nm corresponding to the 2P3/2 → 2P1/2 transition. Commission Internationale de I'Eclairage (CIE) and correlated colour temperature (CCT) studies indicate the glasses emit greenish-yellow light around 7172 K. Z-scan approach is utilised to examine non-linear optical properties with a nanosecond pulsed laser operating at 532 nm. Z-scan investigation revealed good optical nonlinearity in all the glasses. Open aperture normalized transmittance spectra is used to evaluate the values of non-linear absorption coefficient (ꞵ) and optical limiting. ꞵ is in the range of 0.42–3.2x10−10 mW−1 and optical limiting values are in the range of 1.642–2.078x1012 Wm−2. Z-scan studies reveal RSA and RSA to SA (W- patterns) which suggests that the investigated glasses show saturable absorption (SA) as well as reverse saturable absorption (RSA) pattern. According to these findings, the glasses under investigation are promising materials for solid state lasers, greenish-yellow LEDs, compositional tuning optical switching and limiting applications.

Nanosecond pulsed laser irradiation of titanium in acetone: The effects of coating film structures on the hydrogen absorption properties

Pulsed laser irradiation was performed on titanium in acetone and then the hydrogen storage properties were investigated under a hydrogen atmosphere. TiOxCy films were formed by the irradiation of Ti powders with a fluence of 0.36 J cm−2. The irradiation increased the hydrogen absorption temperatures. Thus, the TiOxCy films showed a specific kind of hydrogen barrier effect. Irradiation with a fluence of 0.36 J cm−2 was performed to coat the Ti plate. Although the TiOxCy films formed on the surface, the amount of hydrogen absorption increased compared with the non-irradiated plate because of the change in surface chemical bonding states and high porosity of the films due to the irradiation. The phases generated by irradiation were controlled by the laser fluence. With a low fluence of 0.17 J cm−2, TiO2 films were formed instead of TiOxCy films, and these dense films showed better hydrogen barrier effects than the non-irradiated plate.

Short-length nanosecond pulsed 1.653-μm DBR fiber Raman laser oscillator fabricated by femtosecond laser direct-writing

We demonstrate a nanosecond pulsed 1.653-μm fiber Raman laser (FRL) oscillator in a short-length distributed Bragg reflector (DBR) cavity made by femtosecond laser direct-writing technique. The DBR FRL cavity is composed of commercial ultrahigh NA germanium-doped silica fiber with a short length of 10 m and a pair of fiber Bragg gratings (FBGs). The FBGs were directly inscribed on the ends of the Raman gain fiber, using femtosecond-laser point-by-point (PbP) writing method. A home-made 1.55-μm nanosecond fiber laser source was used as the pump of the FRL. 1.653-μm nanosecond pulsed laser oscillation with maximum average output power of 125 mW (corresponding to peak power of 6.6 W), 3 dB linewidth of ≤ 0.15 nm, and slope efficiency of 48 % was obtained under 1.55-μm pump with optimized repetition rate of 10 MHz. The optical signal-to-noise-ratio (OSNR) of the 1.653-μm narrow-linewidth Raman lasing is above 70 dB. The short-length DBR FRL output exhibits decently high degree of polarization (DOP) and high degree of linear polarization (DOLP) above 80 % in 3-hour period, benefitted from the birefringent FBGs made by femtosecond laser direct-writing method. It is expected that by further reducing the cavity length to < 1 m, the 1.653-μm DBR FRL oscillator made by femtosecond laser direct-writing method is promising as single-frequency narrow-linewidth, highly linearly polarized, high-OSNR laser sources for high-sensitivity CH4 gas detection.

Photoluminescence probing of light absorption centers at silica laser damage.

We use photoluminescence (PL) imaging to study damage growth precursors within laser damage sites on the surface of silica. Damage site evolution is induced by multiple shots of UV nanosecond pulsed laser at various energy densities and monitored throughout the early stages of growth. Wide-field PL imaging rapidly locates microscopic light absorption centers within the silica damage site. Our quantitative analysis shows that damage sites with strong local PL intensity show a higher probability of growth upon subsequent laser pulses. Scanning electron microscopy (SEM) paired with a study of PL spectrum shows that the strong PL intensity appears from the subsurface fractures with high defect density, which provides a local light absorption center leading to significant damage growth. We believe that this result offers an efficient optical damage mitigation strategy by providing a rapid and non-destructive optical inspection approach.

Study of atoms and multiply charged ions features in the nanosecond laser produced Mo plasma in vacuum using optical emission spectroscopy and time-of-flight electrostatic energy analyzer

The species including atoms and multiply charged ions in the laser produced molybdenum (Mo) plasma are investigated in this work using optical emission spectroscopy and time-of-flight electrostatic energy analyzer (TOF-EEA). Nanosecond laser (5 ns, 1064 nm,) pulses were focused on the Mo target surface with a spot size of 0.4 mm2, energy of ∼150 mJ/pulse (corresponding to a power density of ∼7.5 GW cm−2) to generate the Mo plasma in vacuum environment. Time-resolved spectral analysis was carried out to investigate the temporal evolution of continuous background, atomic, and monovalent ionic spectral signals. The Saha–Boltzmann method is applied for spectral fitting, providing insight into the temporal evolution of electron temperature (T e) and electron density (n e). Over the time from 40 ns to 500 ns, the T e decreases from 3.6 eV to 0.52 eV, and the n e decreases from 2.5 × 1020 cm−3–1.0 × 1015 cm−3. Linear fitting extrapolation predicts the T e and n e could be even up to 6.3 eV and 2.5 × 1022 cm−3, respectively, at the early stage of 10 ns. This indicates the generation of multiply charged ions during the laser ablation process. The multiply charged ions up to 6 charge states were observed by the TOF-EEA and the energy distributions for the different charged ions were also obtained. It was found the ion kinetic energy is positively related to the number of charge state indicates the existence of acceleration electric field. The equivalent accelerating potential is determined as approximately 570 V at the current laser power density. This research provides a significant reference for the establishment of models for laser ablation plasmas and a profound understanding of the underlying physical processes.

Nanosecond laser-assisted fabrication of Ti6Al4V surfaces with gradient wettability and robust cross-linked microstructures

Abstract Surfaces with gradient wettability outperform singular superhydrophobic surfaces in terms of self-cleaning efficiency, anti-contamination properties, and fluid manipulation. These attributes offer extensive application potential across various industrial and scientific domains. This study introduces and employs nanosecond pulsed laser ablation to create wettability gradient surfaces on Ti6Al4V alloys, featuring robust cross-linked frame microstructures. Experimental results demonstrate that by varying the ridge width (w), associated with the liquid-solid contact fraction, we can achieve varying wettability and mechanical durability in these cross-linked frame microstructures. Wettability tests indicate static contact angles ranging from 150.7° to 105.45°. Furthermore, the sandpaper linear abrasion test illustrates a decrease in material wear rate with an increase in w. Under similar test conditions, the proposed surfaces demonstrate superior mechanical durability compared to two other prevalent wettability surface structures. The proposed surfaces, efficiently and eco-friendly produced through nanosecond laser fabrication, hold tremendous potential for diverse applications.

Can pulsed laser treatment reduce microbial adhesion on the surface of resin denture base materials?

Objectives. The present work investigates the influence of pulsed nanosecond laser patterning of two commercial heat cure poly (methyl methacrylate) denture base materials, Trevalon and DPI heat cure, on their surface characteristics such as roughness, hydrophobicity, and microbial adhesion. Methods. A Q-switched Nd:YAG solid state nanosecond pulsed laser at a wavelength of 532 nm with a fluence of 2.55 × 1010 W cm−2 having a pulse frequency of 10 Hz was used at varying translation stage speed and vertical spacing for the surface patterning of denture base materials. The surface properties of control and patterned materials were characterized by measuring the compositional changes, wettability, and roughness. Microbial adhesion was assessed by incubating the specimens in Candida albicans suspension at 37 °C for 2 h followed by estimating the number of adherent Candida cells. Results. The micro-Raman spectroscopic studies indicated no chemical changes in the materials due to laser patterning. However, laser patterning increased the average surface roughness from 0.02 ± 0.005 and 0.04 ± 0.003 μm to 3.85 ± 0.20 μm and 3.70 ± 0.12 μm respectively for Trevalon and DPI heat cure materials. In addition, a reduction in the surface wettability was evident as manifested by an increase in water contact angle from 76 ± 2° to 105 ± 1° for Trevalon and from 72 ± 1° to 97 ± 1° for DPI heat cure. Finally, the microbial adhesion studies clearly indicated that the surfaces with higher hydrophobicity and roughness following laser patterning exhibited reduced microbial adhesion. Significance. Laser patterning can be utilized to tune the surface features of denture base materials and thus offer a promising method to create microbial adhesion resistant surfaces. Clinically, such surfaces help in reducing the incidence of denture stomatitis, especially in immunocompromised patients, without altering the composition and bulk properties of denture base materials.

Study on the influence of side-blown airflow velocities on plasma and combustion wave generated from fused silica induced by combined pulse laser

This study examines the impact of variations in side-blowing airflow velocity on plasma generation, combustion wave propagation mechanisms, and surface damage in fused silica induced by a combined millisecond-nanosecond pulsed laser. The airflow rate and pulse delay are the main experimental variables. The evolution of plasma motion was recorded using ultrafast time-resolved optical shadowing. The experimental results demonstrate that the expansion velocities of the plasma and combustion wave are influenced differently by the side-blowing airflow at different airflow rates (0.2 Ma, 0.4 Ma, and 0.6 Ma). As the flow rate of the side-blow air stream increases, the initial expansion velocities of the plasma and combustion wave gradually decrease, and the side-blow air stream increasingly suppresses the plasma. It is important to note that the target vapor is always formed and ionized into plasma during the combined pulse laser action. Therefore, the side-blown airflow alone cannot completely clear the plasma. Depending on the delay conditions, the pressure of the side-blowing airflow, the influence of inverse Bremsstrahlung radiation absorption and target surface absorption mechanisms can lead to a phenomenon known as the double combustion waves when using a nanosecond pulse laser. Both simulation and experimental results are consistent, indicating the potential for further exploration of fused silica targets in the laser field.

Dual‐Stoichiometry Copper Sulphide Nanoparticles by Laser Ablation in DMSO: Synthesis and Biomedical Applications for Enhanced Photothermal Therapy and Photoacoustic Imaging

AbstractCopper sulphide nanoparticles are synthesized by laser ablation of a copper target in DMSO by a 527 nm nanosecond pulsed laser. These nanoparticles have double stoichiometry (CuS and Cu1.8S) and crystalline structures, sizes under 30 nm, and they present substantial absorbance in the second near‐infrared window and photoluminescence in the visible region. The nanoparticles are used as photothermal and photoacoustic agents at 1080 nm and 1064 nm, respectively. Utilizing as a photothermal agent, these nanoparticles rapidly exhibit local heating, photothermal stability, and a temperature change of 52.2 °C within 300 s at 1 mg mL−1 concentration and 3.23 W cm−2 laser intensity. On the other hand, while employed as a photoacoustic agent, they enhanced the contrast significantly and increased the brightness proportional to their concentrations when compared to ultrasound imaging. Additionally, the biocompatibility properties of these nanoparticles were tested with cancer cells, and they were subjected to laser ablation to assess their photothermal effects. In this article, we demonstrate that copper sulphide nanoparticles synthesized by pulsed laser ablation hold great promise for photothermal and photoacoustic applications, especially in biomedical applications.

High-Precision Photoacoustic Neural Modulation Uses a Non-Thermal Mechanism.

Neuromodulation is a powerful tool for fundamental studies in neuroscience and potential treatments of neurological disorders. Both photoacoustic (PA) and photothermal (PT) effects are harnessed for non-genetic high-precision neural stimulation. Using a fiber-based device excitable by a nanosecond pulsed laser and a continuous wave laser for PA and PT stimulation, respectively, PA and PT neuromodulation is systematically investigated at the single neuron level. These results show that to achieve the same level of neuron activation recorded by Ca2+ imaging, the laser energy needed for PA stimulation is 1/40 of that needed for PT stimulation. The threshold energy for PA stimulation is found to be further reduced in neurons overexpressing mechano-sensitive channels, indicating direct involvement of mechano-sensitive channels in PA stimulation. Electrophysiology study of single neurons upon PA and PT stimulation is performed by patch clamp recordings. Electrophysiological features induced by PA are distinct from those by PT, confirming that PA and PT stimulation operate through different mechanisms. These insights offer a foundation for the rational design of more efficient and safer non-genetic neural modulation approaches.

Discrimination of cow, goat, and sheep milk by femtosecond and nanosecond laser-induced breakdown spectroscopy

In the present work, femtosecond Laser Induced Breakdown Spectroscopy, fs-LIBS, is applied for the first time for the analysis of the inorganic content and the identification of the animal origin of milk samples (i.e., cow, goat, and sheep). The fs-LIBS spectra were analyzed by different machine learning algorithms (i.e., multivariate statistical analysis), and their performance was assessed. The obtained classification accuracies of the milk samples based on the animal origin were found to attain values up to 95 %, depending on the algorithm used. For comparison purposes, LIBS experiments were also carried out employing nanosecond laser pulses, ns-LIBS. The obtained results from both the fs- and ns-LIBS experiments were found to be in excellent agreement between them. The present findings, using fs laser excitation, demonstrate and extend the capabilities of LIBS technique, for the identification of the animal origin of milk samples, thus making fs- and/or ns-LIBS technique a powerful and useful tool for milk authentication, quality, and safety control related research.

Nanosecond laser induced periodic silicon microstructures for broadband antireflective applications

Laser-driven micro/nano-texturing is a remarkably efficient and cost-effective technique, having diverse applicability in the realm of photonics, optoelectronics, and photovoltaics. Here, we demonstrate the high-intensity nanosecond laser-matter interactions for periodic micro/nanostructures on silicon, exclusively to emphasize the effects of laser parameters for large-scale integration. A wide range of periodic structure rapid fabrication over a wafer level is systematically studied for optimised parameters using nanosecond laser pulses (1064 nm, 50 ns) at different pulse repetition rates ranging from 10 to 40 kHz. A comprehensive understanding based on physical, structural, and optical studies reveals a strong correlation between the formation of nano/microstructure morphology and the laser parameters. The laser-matter interactions result in trench-like periodic patterns, formation of nanostructure agglomerations as light-trap-centres on the silicon surface. Such nano/micro periodic structures provide a dominant reduction in reflection losses of up to 80 % over a broad UV-NIR spectral region. The studies further demonstrate that the periodic structural patterns could also be potentially utilised for large-scale optical diffraction applications. In addition, the extra layer coating of Silicon Nitride (Si3N4) on the textured silicon further reduces the reflection losses, which is an added advantage of integrated silicon technology. The present work could open a commercial pathway for the rapid fabrication of nanosecond laser-based nanotexturing and periodic grating structures, enabling significantly reduced reflection and improved light trapping in large-scale Silicon-based applications due to its speed, repeatability, and high precision.

Simultaneous improvement in surface quality and hardness of laser shock peened Zr-based metallic glass by laser polishing

Laser shock peening (LSP) is an effective method to improve the plasticity of metallic glasses (MGs) but it also deteriorates the surface quality and decreases the surface hardness, which in turn hinders the applications of MGs. Accordingly, this study was attempted to achieve the simultaneous improvement in surface quality and hardness of LSPed Zr-based MG by nanosecond pulsed laser polishing. LSP of the MG was firstly performed by using a high-energy pulsed laser to prepare the LSPed MG samples, and then laser polishing of the LSPed MG surface was conducted by using nanosecond pulsed laser. The effects of laser polishing parameters (i.e., average laser power, scanning speed, overlap rate, and number of irradiation cycle) on the surface quality and hardness of the LSPed MG were investigated in detail. The experimental results showed that after laser polishing under specific conditions, a 90% reduction in surface roughness and a 11 % increase in surface hardness could be achieved for the LSPed MG surface. This study provides a simple and effective method to simultaneously improve the surface quality and hardness of LSPed MGs, which would be of great significance for the practical engineering applications of MGs.

Non-contact targeted removal of remnant powder particles from additive manufacturing builds with nano-second laser pulsing

Post-production precision cleaning is one critical challenge in Additive Manufacturing (AM) production of high-value, critical parts/builds. A part shedding particle during its operational use could cause severe complications in critical applications such as medical devices and aerospace components. This study presents a non-contact precision particle removal procedure based on a novel Dry Laser Cleaning (DLC) framework. DLC utilizes the thermo-elastic wave generation/propagation and high-frequency surface acceleration fields to dislodge and remove remnant particles due to inertial forces from a well-defined (user-specified) cleaning zone in a non-contact manner, as opposed to cleaning the entire build surface, which could lead to undesirable re-deposition and re-location of particles as well as high cost. The removal of spherical Stainless Steel 316 L (SS316L) micro-particles commonly used in AM from polished, atomically flat Silicon (Si) wafer substrates with uniform high free surface energy using a nano-second laser pulse is experimentally and computationally investigated. In general, a rough surface has a lower average free surface energy; thus, its cleaning, compared to Si wafer, is less challenging. The data with the SS316-Si material pair offers a consistent baseline. In parallel to experiments, a fully-coupled thermo-mechanical finite element study is also conducted for transient surface acceleration simulations to predict the effectiveness of SS316L micro-particle removal from a Si wafer substrate with laser pulsation. It is observed that SS316L micro-particles with a diameter larger than 5.61 μm, 8.43 μm, and 12.24 μm can be removed entirely at 3 mm, 6 mm, and 9 mm distances from its epicenter, respectively. Accordingly, a work-of-adhesion of 990.8 mJ/m2 is selected between the SS316-Si material pair, consistent with the literature. The reported experiments and simulations demonstrate the potential of the proposed rapid precision particle removal procedure for cleaning critical powder-based AM builds, where the targeted removal of particles is essential for the safe and reliable operation of high-value builds and parts.

Solid-to-plasma transition of polystyrene induced by a nanosecond laser pulse within the context of inertial confinement fusion.

Laser direct drive (LDD) inertial confinement fusion (ICF) involves irradiating a spherical target of thermonuclear fuel coated with an ablator, usually made of polystyrene. Laser energy absorption near the target surface leads to matter ablation, hydrodynamic shocks, and ultimately capsule implosion. The conservation of spherical symmetry is crucial for implosion efficiency, yet spatial modulations in laser intensity can induce nonuniformities, causing the laser imprint phenomenon. Understanding laser imprint, especially considering the initial solid state, is essential for advancing LDD ICF. A first microscopic model of solid-to-plasma transition was built in 2019, accounting for laser absorption in the solid state with a band-structure-based ionization model. This model has been improved to include chemical fragmentation and a more accurate description of electron collision frequency in various matter states. The latest development involves assessing the model's reliability by comparing theoretical predictions with experimental observations. Despite the success of this approach, questions remain, leading to further investigations and observations under different irradiation conditions. This work presents an experiment with a nanosecond pulse, taking into account hydrodynamic effects, and measures transmission dynamics over the entire laser beam area to observe two-dimensional effects. The objective is to adapt the theoretical model, couple it with a hydrodynamic code, and observe additional effects related to the initial solid state.

Nonlinear energy absorption during optical breakdown in bulk dielectrics

Portion of energy employed by a single, tightly focused laser pulse in various transparent dielectrics was demonstrated versus the incident energy fluence or alternatively, intensity. The measurements were conducted for both the femtosecond and nanosecond laser pulses. Moreover, the energy scattered during the long-lived breakdown process was estimated within the integrating sphere. It appeared that the recorded strongly non-linear dependence of absorptance on the energy fluence (intensity) can be well approximated by the sigmoidal Hill function. The stationary points of the absorptance curve were used to determine threshold-like values characterizing breakdown development. In this way, we defined threshold for the non-linear absorption which can be considered as an empirical laser-induced breakdown threshold (LIBT). Additionally, the fitting parameters of the experimental data substantiated the absorption character for short and long laser pulses.

Control under optical, nonlinear optical and kinetic properties of two-dimensional gradient CdSe1-xSx nanocrystals

Peculiarities of photoluminescence (PL) and absorption spectra of gradient nanocrystals CdSe1-xSx colloidal solution were studied by pump and probe technique under stationary excitation of excitons by nanosecond laser pulses. The PL peaks and differential transmission peaks positions changing were revealed with the CdSe1-xSx nanocrystals composition variation. Bleaching of excitons transitions was observed for high pump intensities and explained by excitons space filling effect. The changing of the differential transmission peaks positions was explained by the nanocrystal band gap dependence on the chemical composition. Time-resolved PL measurements from the sub-picoseconds to tens of nanoseconds range demonstrated a strongly different behavior for gradient nanocrystals and pure CdSe nanocrystals. For pure nanocrystals two typical time-scales were resolved contrary to the gradient nanocrystals where (with the growth of sulfur concentration) only a single time scale was revealed. The observed modification of time-resolved PL for gradient nanocrystals is caused by the presence of traps in the gradient samples due to the chemical composition modification.

Drop-on-demand bioprinting: A redesigned laser-induced side transfer approach with continuous capillary perfusion

We present a drop-on-demand (DOD) bioprinting method based on a novel implementation of laser-induced side transfer (LIST). Our approach involves continuous bioink perfusion through a glass capillary featuring a laser-machined hole in the capillary wall, serving as a nozzle. Focused low-energy nanosecond laser pulses are employed for precise droplet ejection. This innovative design separates the control of the bioink flow rate inside the capillary from the printing rate (drop ejection), leading to an enhanced printing workflow. We assessed the impact of key printing parameters, such as laser energy and flow conditions, on printing quality. Furthermore, we utilized the redesigned LIST to bioprint human umbilical vein endothelial cells (HUVECs). Our findings indicate that the printed HUVECs exhibit no viability loss and demonstrate the ability to recruit perivascular cells, including pericytes and fibroblasts. The redesigned LIST can be utilized in tissue engineering applications requiring DOD cell printing.