Low Laser Fluence Research Articles

Preparation of silicon naphthalocyanine nanoparticles by laser ablation in liquid and their optical properties

Silicon naphthalocyanine nanoparticles were successfully prepared by laser ablation in liquid. Silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) powders in deionized water were irradiated with nanosecond-pulsed laser (Nd:YAG, SHG) to prepare nanoparticles. The prepared nanoparticles were investigated by scanning electron microscopy, dynamic light scattering and spectrophotometry. The shape was polygonal and partially spherical. The primary and secondary particle sizes were reduced with an increase in laser fluence. Absorbance was increased at low laser fluence and was constant at high laser fluence with the increase in laser fluence. The ratio of the two absorbance peaks was changed with the increase in laser fluence.

Pico- to nanosecond pulsed laser-induced forward transfer (LIFT) of silver nanoparticle inks: a comparative study

Silver nanoparticle inks are among the key functional materials used in printed electronics. Depositing it by laser-induced forward transfer remains a challenging task because the non-linear rheological nature of these inks narrows the range of the laser processing parameters. Understanding, therefore, the influence of the laser parameters on the ejection dynamics and deposition quality is of critical importance. The influence of the laser pulse duration from pico- to nanosecond-laser-induced jet dynamics was investigated using time-resolved shadowgraphy imaging. Jet speed and surface area analyses showed that in the lower laser fluence level range, picosecond pulses induce higher surface area ejections which propagate at higher velocities. As the laser fluence levels were increased, the difference in jet velocity and surface area evolutions narrows. Deposition analysis showed a similar behavior with lower transfer thresholds and larger depositions at lower fluence range when picosecond-laser pulses were used.

Laser-induced emission of TiO2 nanoparticles in flame spray synthesis

Oxide nanoparticles are widely studied because of their unique properties, including their crystalline phase, surface area, and porosity, which make them attractive for several applications. These properties are related to the increase in the surface/volume ratio when moving from the bulk to the nanoscale. For this reason, a diagnostic tool capable of monitoring the nanoparticle size and concentration during synthesis is particularly valuable. The laser-induced incandescence technique is widely used to provide such information. This study explored the applicability of this technique to TiO2 nanoparticles in flame spray synthesis. The fluorescence, flame emission, and incandescence signals were investigated. Time-resolved spectral measurements were first carried out on TiO2 nanoparticles deposited on a filter. At low laser fluences, the fluorescence signal of anatase TiO2 nanoparticles was detected. At higher fluences, the incandescence signal appeared. A fluence threshold limit that depended on the matrix effect was observed, above which breakdown phenomena occur. Then, laser-induced incandescence spectral measurements were performed on the flame spray at different heights above the burner and different acquisition delay times. The analysis showed the applicability and challenges in using this diagnostic tool in flame spray synthesis.

Evaluation of laser cleaning for defouling of marine biofilm contamination on aluminum alloys

The environmental friendliness short-pulsed laser removal of marine biofilm contamination developed on the surfaces of aluminum alloy substrates was innovatively applied. Natural biofilms were formed using a seawater immersion method. The surface state was evaluated by considering the cleaning effectiveness and the harm exerted on the aluminum alloy substrate surfaces. The cleaning quality was analyzed using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), and Fourier transform infrared spectroscopy (FT-IR). The laser-induced damage/change to the substrate surface was evaluated using laser scanning confocal microscopy (LSCM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The results indicate that a nanosecond laser can be successfully used to quickly remove marine biofilm contamination from the surface of an aluminum substrate by instantaneous thermal ablation. No clear thermal damage can be found on the laser-cleaned aluminum surface at a lower laser fluence (1.08 J/cm2). In addition to the cleaning of marine biofilm contamination, a higher laser fluence (4.14 J/cm2) can also contribute to the formation of a hierarchical micro- and nanostructured oxidized layer. It is composed of amorphous-nanocrystalline phases.

Fabrication of hierarchical surface textures using multi-pulse direct laser interference patterning with nanosecond pulses

Abstract In this study, we report on the one-step fabrication of hierarchical microstructures on stainless steel substrates using multi-pulse Direct Laser Interference Patterning. The complex structures are produced using a two-beam interference setup with nanosecond laser pulses. The effect of laser fluence and number of pulses on the formation of multi-scale microstructures is systematically investigated. The resulting surface structures are characterized by scanning electron microscopy and white light interference microscopy. It was observed that at low laser fluence, the periodic microstructures remain defined and grow in height above the initial surface, independent on the number of pulses. For higher laser fluences as well as larger number of pulses, the structures adopt a crater-like morphology including a periodic line-like structure.

Metal ablation study with a 10 picosecond laser under low and median fluence

Single shot 10 ps laser ablation on aluminium and gold at a wavelength of 1064 nm under low and median laser fluence was studied theoretically and experimentally. Simulations have been performed by using an optimized two temperature model. It is revealed that electron-phonon coupling ability of the target metal affected both the ablation depth and target metal’s morphology. Under low fluence, melting could well be the relevant ablation mechanism. With laser fluence rising, vaporization or phase explosion happens thereafter, depending on material’s electron-phonon coupling ability.

Nanosecond Pulsed Laser Ablation on Stainless Steel − Combining Finite Element Modeling and Experimental Work

In the present work, finite element simulations to investigate nanosecond pulsed laser ablation of stainless steel under low laser fluence conditions is performed. The laser has a wavelength of 1064 nm as well as a Gaussian spatial and temporal energy distribution. The utilized model of heat transfer consists of thermal conduction, thermal convection, and thermal irradiation. Temperature‐dependent material's properties including absorptivity and the instantaneous material removal by evaporation are considered. Corresponding laser ablation experiments on stainless steel are also conducted, demonstrating good agreement in the widths of the evaporated area and the heat affected zone obtained by simulation results. The 2D simulation results of single pulse laser ablation demonstrate that low laser fluences ranging from 26.53 to 46.42 J cm−2 have a significant impact on both temperature distribution and material removal profile. Particularly for the width of the evaporated area, the maximum deviation of predicted values from experimental results is smaller than 10%. The 3D simulation results of groove texturing using multiple laser pulses also shows an excellent agreement with the experimental work in terms of the resulting groove morphology.

Nanosecond pulsed laser ablation of silicon—finite element simulation and experimental validation

In the present work, we perform finite element simulations to investigate the ablated surface morphology of silicon by nanosecond pulsed laser ablation using low laser fluences ranging from 14.92 to 23.21 J cm−2. The utilized finite element model comprehensively considers the following aspects: (1) combined effects of thermal conduction, convection and radiation on heat conduction; (2) temperature-dependent material properties; (3) instantaneous update of the laser focus due to evaporation-induced surface recession for low laser fluences; and (4) spatial and temporal Gaussian energy distribution of the laser pulse. Experimental work using the same laser machining parameters compared to the conducted finite element simulations are carried out to validate the simulation results. Through the optimization of the laser machining parameters by 2D and 3D finite element simulations and the respective experimental validations for eliminating heat-affected zone and promoting forming accuracy, high accuracy aligned micro-grooves are fabricated on silicon with high anti-reflective properties in a wide range of wavelengths between 400 and 2000 nm. This is fairly comparable with the performance of similar silicon microstructures by femtosecond laser ablation. Consequently, the current work presents a way to fabricate precise surface microstructures with high anti-reflective properties on silicon at low cost by nanosecond pulsed laser ablation.

Field emission from laser-processed niobium (110) single crystals

Niobium is a technologically important material, which is typically used for superconducting radio-frequency applications. Superconductive cavities made of niobium require contamination-free and smooth surfaces to ensure the best performance of a particle accelerator. Interior surfaces of niobium cavities are usually obtained by centrifugal barrel polishing, buffered chemical polishing, and electropolishing or a combination of these methods. However, a standard inspection of the inner cavity surface after the treatment still shows the presence of sharp features limiting cavity performance. Laser polishing is a potential alternative or can be used as an additional final step for a more efficient surface treatment toward higher electric field gradients. In the present study, a chemically polished (110)-oriented single crystal niobium surface was processed with a focused pulsed laser with a pulse duration of 3.5 ns and a repetition rate of 10 Hz. The laser fluence and the number of pulses were varied in the range from 0.68 up to $4.27\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$ and from 20 up to 200, respectively. The magnitude of laser-induced surface structures and boiling traces was systematically studied by means of scanning electron microscopy, atomic force microscopy, and optical profilometry. Finally, the local field emission behavior was investigated and correlated with the observed surface modifications. Typical current-field characteristics and a field enhancement statistic of laser-processed areas are presented in the study. The surface processing with a rather low laser fluence of $0.68\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$ yielded high onset electric fields of $650--940\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ with field enhancement factors below 10. The processing with a higher laser fluence and/or a higher number of pulses resulted in boiledlike structures emitting at $70--190\text{ }\text{ }\mathrm{MV}/\mathrm{m}$.

Non-thermal lattice dynamics in α-quartz induced by femtosecond laser pulses: An ab initio study

Laser-patterning techniques are typically based on local-heating phenomena, whereas recently developed short-pulse lasers operating on the order of femtoseconds may also induce non-thermal processes in materials. As it is difficult to study non-thermal processes experimentally, ab initio simulations are crucial for understanding laser-patterning phenomena under these conditions. In this study, real-time time-dependent density functional simulations were performed to examine the kinetic energies of the atoms in a slab of α-quartz under a laser fluence per pulse of 10 J/cm2, a wavelength of 800 nm, and full width at half-maximum (FWHM) values ranging from 10 fs to 100 fs. The simulation results suggested the occurrence of thermal/non-thermal crossover at an FWHM value of approximately 50 fs. Smaller FWHM values resulted in non-thermal lattice dynamics. Even under a lower laser fluence per pulse, a shorter pulse initiated non-thermal dynamics with a duration of several tens of femtoseconds.

Enhancement of thermal conductivity of titanium dioxide nanoparticle suspensions by femtosecond laser irradiation

This work reports significant increase in the thermal conductivity (k) of aqueous titanium dioxide (TiO2) nanoparticle (NP) suspensions by ultrafast laser irradiation. At relatively low laser fluence (F ∼ several tens of mJ/cm2), a laser stabilization process occurred, which did not reduce primary particle size, but yielded increase from 1.04 to 1.12 in the thermal-conductivity ratio of the aqueous TiO2 nanoparticle suspension. At relatively high laser fluence (F ∼ several J/cm2), laser fragmentation of the NPs reduced the primary particle size, and as a consequence k ratio increased to 1.26. Theoretical models that consider the percolation effects of aggregates, Brownian motion, interfacial liquid layering, and formation of electric double layer (EDL), were used to explain the increase in k. This work shows that ultrafast laser irradiation offers an effective non-chemical means to improve the thermophysical and colloidal properties of TiO2 nanofluids.

Mechanisms and advanced photothermal modelling of laser-induced shape transformations of colloidal gold nanorods by nanosecond laser pulses.

We propose an advanced photothermal model based on a modified Takami model (MTM) to explain the mechanisms of shape changes of colloidal gold nanorods (NRs) induced by nanosecond laser pulses. This model takes into account the orientation of NRs, the radiative and convective losses, and the phase transitions of NRs. It was applied to the determination of the evolution of temperature and the shape and size transformations of NRs during the laser exposure. A series of measurements arising from the interaction between Au NRs and nanosecond laser pulses were analyzed by TEM measurements and the MTM model. We have demonstrated that the fragmentation and reshaping processes govern the nanoparticle (NP) shape. At high laser fluence, the complete fragmentation leads to a population of nearly spherical NPs, while at a moderate laser fluence, the partial fragmentation and reshaping processes generate a bimodal distribution. At low laser fluence, uncommon φ-shape NPs were produced as a result of the competition of cooling and reshaping processes. We also demonstrated that it is possible by the MTM model to determine the laser fluence required to suppress some specific NR shapes and to predict the NP size and shape distributions obtained after the laser exposure.

The nature of the ultrafast magnetic phase transition in nickel revealed by correlating EUV-MOKE and ARPES spectroscopies

By correlating time- and angle-resolved photoemission (Tr-ARPES) and time-resolved transverse- magneto-optical Kerr effect (Tr-TMOKE) measurements, both at extreme ultraviolet (EUV) wavelengths, we uncover the nature of the ultrafast photoinduced magnetic phase transition in Ni. This allows us to explain the ultrafast magnetic response of Ni at all laser fluences - from a small reduction of the magnetization at low laser fluences, to complete quenching at high laser fluences. We provide an alternative explanation to the fluence-dependent recovery timescales commonly observed in ultrafast magneto-optical spectroscopies on ferromagnets: it is due to the bulk-averaging effect and different depths of sample exhibit distinct dynamics, depending on whether a magnetic phase transition is induced. We also show evidence of two competing channels with two distinct timescales in the recovery process, that suggest the presence of coexisting phases in the material.

Effective treatment of patterned pigmented lesion with low-fluence neodymium-doped yttrium aluminum garnet laser

Effective treatment of patterned pigmented lesion with low-fluence neodymium-doped yttrium aluminum garnet laser

In-Situ and Ex-Situ Characterization of Femtosecond Laser-Induced Ablation on As₂S₃ Chalcogenide Glasses and Advanced Grating Structures Fabrication.

Femtosecond laser pulse of 800 nm wavelength and 150 fs temporal width ablation of As2S3 chalcogenide glasses is investigated by pump-probing technology. At lower laser fluence (8.26 mJ/cm2), the surface temperature dropping to the melting point is fast (about 43 ps), which results in a clean hole on the surface. As the laser fluence increases, it takes a longer time for lattice temperature to cool to the melting point at high fluence (about 200 ps for 18.58 mJ/cm2, about 400 ps for 30.98 mJ/cm2). The longer time of the surface heating temperature induces the melting pool in the center, and accelerates material diffusing and gathering surrounding the crater, resulting in the peripheral rim structure and droplet-like structure around the rim. In addition, the fabricated long periodic As2S3 glasses diffraction gratings can preserve with high diffraction efficiency by laser direct writing technology.

A comparative study of surface characterization and corrosion performance properties of laser surface modified biomedical grade nitinol

Laser melting and quick solidification of shape memory alloy surfaces promise to explore the multifunctional capabilities required for enhanced biocompatibility of different implantable medical devices. Before subsequent bio-trial, understanding of the effect of cooling rate vis-à-vis laser fluence on surface characteristics and corrosion protection performance is essential before designing a tailor-made biocompatible surface for various implantable devices. The current study systematically investigates the effect of laser fluence on physical, mechanical and chemical surface characteristics along with corrosion protection performance of the modified surface as compared to bare Nitinol surface. Various phases were prominent on the top surface, namely nickel and titanium-rich phase along with different nickel-titanium intermetallics and nano-structure of titanium oxide, based on varying melting pool recirculation time and cooling rate with laser fluence energy. At low laser fluence up to 4 J/mm2, no significant melting pool were formed and only transformation to the martensitic phase of Nitinol took place on the top surface, which seemed to be highly too much corrosion-prone under simulated body fluid. At moderate laser fluence of 6–8 J/mm2, mostly titanium-rich phases are prominent on the surfaces on account of optimum recirculation of melting pool and subsequent surfacing out of comparably light phase of titanium. Titanium-rich phases on top surface exhibit superior corrosion resistance as compared to all other samples including bare nitinol. However, titanium oxide nano-particles-reinforced martensitic structure is formed under high laser fluence due to over recirculation of molten pool. The modulus of elasticity also varied from 10 to 110 GPa based on top surface formation under different fluence levels, and thus this process can act as a tailor-made controllable pre-treatment process over the traditional coating processes.

Intrinsic five-photon non-linear absorption of two-dimensional BN and its conversion to two-photon absorption in the presence of photo-induced defects

Unlike most two-dimensional materials, h-BN nanoplatelets (BNNPs) exhibit multi-photon absorption. This was previously attributed to a two-photon absorption (2PA) process at 1.16 eV by Kumbhakar et al., Advanced Optical Materials, 3, 828, 2015. This is counter-intuitive because a 2PA process at 1.16 eV cannot excite electrons across the wide band gap of BNNPs (∼5.75 eV). Here, through systematic experimental and theoretical work, we provide compelling evidence that BNNPs exhibit five-photon absorption (5PA) at a low-laser fluence (<0.6 J/cm2). Furthermore, we show that a high laser fluence (>0.6 J/cm2) generates photo-induced defects in BNNPs that support a 2PA process. Accordingly, we assert that BNNPs inherently exhibit 5PA at a low laser fluence. In the previous study reported by Kumbhakar et al. a high laser fluence was used, which led to the observation of 2PA process.

Low-fluence Q-switched Nd:YAG laser for the treatment of melasma in Asian patients.

Melasma is a common acquired facial pigmentary disorder that is resistant to various modalities of treatment. Recently, the low-fluence 1064-nm Q-switched neodymium-doped yttrium aluminum garnet laser (QSNY) has emerged as the new gold standard of melasma treatment in Asia. To evaluate the efficacy and safety of low-fluence 1064-nm QSNY in Asian patients with melasma. Data of patients with melasma treated with low-fluence QSNY in the department of dermatology at the Korea University Anam Hospital were reviewed retrospectively. A 1064-nm QSNY with 8mm spot size and fluence of 1.2-2.0J/cm2 at 10Hz was used. The results were evaluated using the modified Melasma Area and Severity Index (mMASI) score and the physician's global assessment (PGA). A total of 40 Korean patients with Fitzpatrick skin type III to IV were included. Patients were treated for a median of 10 sessions at a median of 1-week intervals. The mean mMASI score decreased by 54.23% from a baseline of 3.19±2.64 to 1.46±1.06 after treatment. On PGA, 1 out of 40 patients (2.5%) showed excellent improvement, 14 (35%) good, 15 (37.5%) fair, 6 (15%) poor, and 4 (10%) no improvement. However, two patients showed mottled hypopigmentation and rebound hyperpigmentation. Low-fluence 1064-nm QSNY appears to be a safe and effective treatment for Asian patients with melasma.

Intensity-dependent reflectance modulation of femtosecond laser pulses in GaAs nanocylinders with magnetic resonances

We experimentally demonstrate modulation of reflectance in periodic arrays of subwavelength gallium arsenide nanocylinders with Mie-type resonances due to absorption saturation and changes in the refractive index of the semiconductor material of metasurface. The intensity-dependent reflectance modulation of up to 30% in the vicinity of the magnetic dipole resonance at a low laser fluence below 200 μJ/cm2 is shown by I-scan measurements.

Surface morphology of titanium alloy with monolayer microparticles under different single pulse laser energy

The surface morphology characteristics of titanium alloy under single pulse energy with local field enhancement effect were studied. The surface morphology of titanium alloy was observed by scanning electron microscope and local field enhancement theory was discussed by the FTDT simulation. The key difficulty of monolayer particles deposition was solved by production of a hydrophilic film and spin coating method. The formation mechanism of the surface morphology was discussed. Our results indicate the surface morphology is related to local field enhancement and the laser parameters including energy, size and distribution of laser spot. By using lower laser fluence than the ablation threshold of titanium alloy, laser cleaning can be obtained. When the microparticles-enhanced laser energy reaches the ablation threshold of titanium alloy surface, micropores whose size between 300 nm and 400 nm are easily obtained. The micropores distribution is related to the shape of laser spot. When the laser energy of laser spot exceeds the ablation threshold, microparticles help to control the shape of hole. The results show that particles can effectively influence the quality of laser processing, which is theory and practical significance on promoting laser processing technique in the application of particles on titanium alloy.