Varying Laser Fluence Research Articles

A study integrating in-process thermal signatures, microstructure, and corrosion behaviour of AISI 316L coatings on low carbon steel substrate deposited by laser-directed energy deposition (L-DED)

Surface coating through Laser Directed Energy Deposition (L-DED) of AISI 316L on low-carbon steel was carried out by varying laser fluence. The resulting melt pool characteristics, obtained through IR pyrometer, played a significant role in evolution of the microstructure. A higher heat input resulted in an equiaxed grain structure with finer grain sizes and improved deposition quality. The quantitative phase analysis revealed the presence of a dominant austenite phase and a secondary ferrite phase, along with traces of molybdenum carbide. Corrosion analysis reveals improved resistance at a higher laser fluence, attributed to increased content of corrosion-resistant alloying elements (Mo, Cr) and predominance of the austenite phase. Electrochemical Impedance Spectroscopy (EIS) confirms a higher corrosion resistance at a higher laser fluence, supported by a significantly higher polarization resistance and presence of a duplex-structured passive film. Results indicate that moderate laser fluence levels, ranging from 5 to 7.5 kJ/cm2, accompanied by medium to high thermal signatures and moderate cooling rates, yield coatings with refined microstructures, enhanced mechanical strength, and corrosion resistance. For superior corrosion resistance, a laser fluence of 10 kJ/cm2 is recommended, although careful consideration of its potential impact on mechanical properties is advised.

Unexpected Performance of a Bifunctional Sensitizer/Activator Component for Photon Energy Management via Upconversion.

We here report on the observation of upconverted photoluminescence (UC-PL) from the blue-light-emitting 9,10-diphenylanthracene (DPA) mixed with the yellow-light-absorbing bifunctional sensitizer/activator component of (3,3,7,8,12,13,17,18-octaethylporphyrin-22,24-diid-2-one) PtII (PtOEP-K). Yellow-to-blue UC-PL (0.680 eV spectral upshift) is achieved at room temperature under ultralow power continuous incoherent photoexcitation (220 μW/cm2) despite the absence of triplet energy transfer (TET) between PtOEP-K and DPA. Under selective CW-laser photoexcitation of PtOEP-K in DPA:PtOEP-K, a 2.5% UC-PL quantum yield is obtained; that is an improvement exceeding by more than 3 orders of magnitude the UC-PL quantum yield of TTA-UC material combinations wherein no TET is operative. The PL response of DPA:PtOEP-K to varying laser fluence suggests that bimolecular annihilation reactions between triplet-excited PtOEP-K facilitate the UC-PL activation in DPA. These findings pave the way toward low-complexity strategies for the reduction of transmission losses in solar energy technologies through an innovative wavelength upshifting protocol involving excitonic materials.

Study on Radiation Damage of Silicon Solar Cell Electrical Parameters by Nanosecond Pulse Laser

This experimental study investigates the damage effects of nanosecond pulse laser irradiation on silicon solar cells. It encompasses the analysis of transient pulse signal waveform characteristics at the cells’ output and changes in electrical parameters, such as I–V curves before and after laser irradiation under varying laser fluence and background light intensities, and explores the underlying action mechanisms of laser irradiation. The study reveals that as the laser fluence increases up to 4.0 J/cm2, the peak value of the transient pulse signal increases by 47.5%, while the pulse width augments by 88.2% compared to the initial transient pulse signal. Furthermore, certain parameters, such as open-circuit voltage, short-circuit current, and peak power obtained, from the measured I–V curve indicate a threshold laser fluence for functional degradation of the solar cell at approximately 1.18 ± 0.42 J/cm2. Results obtained from laser irradiation under different background light intensities underscore the significant influence of background light on laser irradiation of silicon cells, with the most severe damage occurring in the absence of light. Moreover, findings from laser irradiation at multiple locations on the silicon cell demonstrate a linear decrease in the output voltage of the silicon cell with an increase in the number of irradiation points.

Simultaneous atmospheric pressure plasma jet etching and laser irradiation for ultra-precise optical glass processing

The use of beam-based technologies to process optical elements with nanoscale precision enables the fabrication of freeform surfaces. In particular, atmospheric pressure plasma jets (APPJs) have desirable properties, e.g., depth precision < 5 nm, low surface roughness and processing at atmospheric conditions. However, the composition of optical glasses and glass ceramics, containing metal oxides, leads to the formation of non-volatile reaction products that remain on the substrate surface. These residues reduce the etching rate and cause severe roughening of the surface. Laser irradiation has already been demonstrated as a promising option for removing the residual layer and the aim of the current work is to integrate it into the APPJ system for simultaneous processing. Therefore, an excimer laser (λ = 248 nm; tPulse = 20 ns) with a maximum pulse frequency of 100 Hz was added to a plasma jet setup and experiments with varying laser fluences as well as laser frequencies were performed on N-BK7 substrates. White light interferometry was used to analyse the samples. The experiments showed an improved etching result with higher removal rates for the combined process at high laser pulse frequency (100 Hz) and fluences in the range of 0.1-0.45 J·cm-2.

Exploring ablation of GaAs at atomic and close-to-atomic scale by pulsed laser and 3D TTM–MD simulations

Pulsed laser technology offers significant advantages in atomic and closed-to-atomic scale manufacturing due to its high-resolution capabilities in non-contact fabrication as well as its high-efficiency and low-cost. Pulsed laser irradiation experiments on a GaAs surface were conducted to study ablation and atomic-level modification. A three-dimensional two-temperature model, combined with the molecular dynamics (3D TTM–MD) method, was developed to investigate the mechanisms responsible for the material removal and surface modification during pulsed laser irradiation. Results show that phase explosion is the main mechanism for material removal at high fluences, leading to explosively decomposed into a two-phase mixture of liquid and vapor. As fluence increases, the proportion of vapor component rises, while amorphization becomes dominate near the damage threshold fluences. This work reveals the atomic dynamics of GaAs under varying laser fluences, offering valuable insights for laser processing towards to the atomic and close-to-atomic scale.

Understanding non-stochiometric deposition of multi-principal elemental NiCoCr thin films

Multi-principal elemental NiCoCr thin films with compositional flexibility have potential applications as a thermal barrier, corrosion-resistant coatings, and functional energy materials. In this study, the non-stochiometric multi-principal element alloy thin film formation behavior from an equiatomic ablation target of NiCoCr using pulsed laser deposition has been reported. Here, the effect of varying laser fluence on compositions of constituent elements in the NiCoCr thin films is systematically investigated. The increase in laser fluence typically increases the likelihood of acquiring undesirable particulates in thin films. Therefore, to maintain the film quality the laser fluences are adjusted within the range of 1.3–3.3 J·cm−2, while the film thicknesses are kept constant at ∼30 nm. Results show that the constituent elements are well-distributed in the formed NiCoCr thin films, however, having average atomic percentages of Ni and Cr twice as high as Co irrespective of the equiatomic composition of individual elements in the ablation target. This stochiometric alteration in the as-deposited thin films is governed by the rate of ablation and subsequent evaporation of each element, which is facilitated by the optical properties and vapor pressure of individual constituent elements. Additionally, electron energy-loss spectroscopy and x-ray absorption spectroscopy analysis reveal that the thin films have partially oxidized, with the Cr oxidation contributing the most to the O-K edge in comparison to Ni and Co. This study presents a laser-assisted pathway for fabricating compositionally flexible NiCoCr thin films and develops an understanding of the non-equilibrium laser-material interactions and how the intrinsic properties of constituent elements affect the stoichiometric flow of materials from the target to the final film deposition.

Formation of Laser-Induced Periodic Surface Structures on Reaction-Bonded Silicon Carbide by Femtosecond Pulsed Laser Irradiation

Reaction-bonded silicon carbide (RB-SiC) is an excellent engineering material with high hardness, stiffness, and resistance to chemical wear. However, its widespread use is hindered due to the properties mentioned above, making it difficult to machine functional surface structures through mechanical and chemical methods. This study investigated the fundamental characteristics of laser-induced periodic surface structures (LIPSSs) on RB-SiC via femtosecond pulsed laser irradiation at a wavelength of 1028 nm. Low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL) formed on the surface along directions perpendicular to the laser polarization. SiC grains surrounded by a large amount of Si show a reduced threshold for LIPSS formation. By varying laser fluence and scanning speed, HSFL–LSFL hybrid structures were generated on the SiC grains. Transmission electron microscopy observations and Raman spectroscopy were carried out to understand the formation mechanism of the hybrid LIPSS. A possible mechanism based on the generation of multiple surface electromagnetic waves due to the nonlinear response of SiC was proposed to explain the hybrid structure formation. Furthermore, the direction of laser scanning with respect to laser polarization affects the uniformity of the generated LIPSS.

Microsphere femtosecond laser sub-50 nm structuring in far field via non-linear absorption

Creation of arbitrary features with high resolution is critically important in the fabrication of nano-optoelectronic devices. Here, sub-50 nm surface structuring is achieved directly on Sb₂S₃ thin films via microsphere femtosecond laser irradiation in far field. By varying laser fluence and scanning speed, nano-feature sizes can be flexibly tuned. Such small patterns are attributed to the co-effect of microsphere focusing, two-photons absorption, top threshold effect, and high-repetition-rate femtosecond laser-induced incubation effect. The minimum feature size can be reduced down to ~30 nm (<italic>λ</italic>/26) by manipulating film thickness. The fitting analysis between the ablation width and depth predicts that the feature size can be down to ~15 nm at the film thickness of ~10 nm. A nano-grating is fabricated, which demonstrates desirable beam diffraction performance. This nano-scale resolution would be highly attractive for next-generation laser nano-lithography in far field and in ambient air.

Effect of laser irradiation on the tribological properties of RF-sputtered nickel oxide (NiO) thin films

The present work aims at investigating the effect of laser irradiation on the tribological properties of RF-sputtered NiO thin films deposited on industrial grade aluminum substrate. A semiconductor laser based on Nd:YAG operating at its 4th harmonic wavelength, λ = 266 nm with varying laser fluence and spot size of about 5 μm is irradiated on the NiO film. The localized heating allows for smoothening of the NiO film along with contributions to the changes in the stoichiometry of NiO (reduction of excess oxygen). In particular, the effects of tuning laser fluence and the subsequent tribology tests pertaining to the coefficient of friction variations for tribological tests are discussed.

Multiscale modeling of ultrafast melting phenomena

Ultraviolet Nanosecond Laser Annealing (LA) is a powerful tool for both fundamental investigations of ultrafast, nonequilibrium phase-change phenomena and technological applications (e.g., the processing of 3D sequentially integrated nano-electronic devices) where strongly confined heating and melting is desirable. Optimizing the LA process along with the experimental design is challenging, especially when involving complex 3D-nanostructured systems with various shapes and phases. To this purpose, it is essential to model critical nanoscale physical LA-induced phenomena, such as shape changes or formation and evolution of point and extended defects. To date, LA simulators are based on continuum models, which cannot fully capture the microscopic kinetics of a solid–liquid interface. In this work a fully atomistic LA simulation methodology is presented, based on the parallel coupling of a continuum, finite elements, μm-scale electromagnetic-thermal solver with a super-lattice Kinetic Monte Carlo atomistic model for melting. Benchmarks against phase-field models and experimental data validate the approach. LA of a Si(001) surface is studied varying laser fluence and pulse shape, assuming both homogeneous and inhomogeneous nucleation, revealing how liquid Si nuclei generate, deform and coalesce during irradiation. The proposed methodology is applicable to any system where the atom kinetics is determined by a strongly space- and time-dependent field, such as temperature or strain.

Wavelength and decay time tuning of intraband transitions in low spatial frequency laser-induced periodic surface structures

We present the study of the alteration of wavelength and decay time of intraband transitions in the laser-induced periodic surface structures (LIPSS) of silver formed by varying the incident laser peak fluence. The studies are performed using a femtosecond laser (λ0 = 800 nm) based ultrafast transient absorption spectroscopy (UFTS)setup. Using the same laser, an array (~2 mm size) of laser-induced periodic surface structures were obtained in a single-step fabrication process in the open air without translating the sample. Each unit cell of the array comprises low spatial frequency LIPSS (LSFL) near the center with nanoparticles/clusters around the edges. As obtained from 2D Fast Fourier Transform, the overall periodicity(Λ) of LSFL is within 0.714λ0–0.723λ0 range. The nanoparticles/clusters show a random size distribution that varies from 50 nm to 300 nm. In UFTS measurement, the LIPSS was excited by a 3.3 eV laser. The SPP/SPR dip in the data is an indication of intraband transition or hot electron generation. The effect of change in structuring due to varying laser fluence on these intraband transitions is also analyzed. It is observed that there is a non-linear redshift (~16 nm) in the SPR peak wavelength with a significant enhancement in the relaxation times of the hot electrons due to localized annealing of shallow defects. Therefore, a large tunability in wavelength of SPP/SPR and lifetime of hot electrons was achieved by varying the laser fluence. The study highlights the suitability of LIPSS in applications requiring efficient energy transfer, enhanced photocatalytic action, and tunable sensors.

A characterization of laser cleaning painting layer from steel surface based on thermodynamic model

In this study, the environmentally friendly nanosecond ultraviolet (UV) laser is an innovatively employed laser cleaning to remove the painting layer from the AH36 steel substrate. The feasibility of UV laser cleaning the painting layer is innovatively proposed and it has been calculated by the model theoretically, followed by elaborating the prominent interaction mechanism of UV laser exactly. The initial cleaning threshold and completely cleaning threshold are 2 J/cm2 and 5 J/cm2, respectively. Afterwards, the UV laser-cleaned surface quality is evaluated by the scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical microscopy (OM), and optical profiler (OP), respectively. The mechanical properties have enhanced dramatically after laser cleaning and characterized by the Vickers hardness tester and universal testing machine. By varying laser fluences (2, 5, 7 J/cm2) during laser cleaning, microstructures registering various sizes of corrugated shaped, craters, and ring shaped could be acquired. In addition, the mechanical properties analysis including rapid melting, quenching, and dislocation density effects illustrates that laser cleaning could effectively increase surface microhardness, tensile strength, and bending strength. Thus, laser cleaning method has emerged as a favorable means to strip painting layer in lieu of traditional methods for marine industry as well as this study could promote the development of laser cleaning in the field of marine engineering.

Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing

Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.

Numerical simulation of the period of surface micro-protrusions generated on titanium and stainless steel targets by femtosecond laser irradiation

This article reports our simulation results on a period of surface micro-protrusions, which are generated on titanium and stainless steel 304 target surfaces by femtosecond pulsed laser irradiation. The period of the generated micro-protrusions for varying laser fluence level has been estimated using an approach derived from the linear hydrodynamic Kuramoto Sivashinsky model. Some of the parameters, needed for calculating the period of surface micro-protrusions, have been estimated by numerically solving one-dimensional heat equations. Temperature evolution inside the target upon irradiation with a femtosecond laser pulse has been simulated using the two temperature model until the time electron and lattice subsystems attain thermal equilibrium. Thereafter, temperature evolution in the target has been simulated by defining a single temperature of the target at every position and time. We have validated our theoretical model by comparing simulated variation of period of surface micro-protrusions with incident laser fluence, ablation depth per pulse, and time required for thermalization between electrons and lattice subsystems with the reported experimental data for titanium target. Subsequently, the validated model has been used to simulate the period of surface micro-protrusions which are generated on the stainless steel 304 target via femtosecond laser irradiation.

Prediction of Optimum Process Parameters Fabricated by Direct Laser Interference Patterning Based on Central Composite Design.

In this study, we report on the optimization of the direct laser interference patterning process by applying the design of experiments approach. The periodic line-like microstructures of a 8.50 µm spatial period were fabricated by a two-beam interference setup with nanosecond laser pulses, varying laser fluence, pulse overlap, and hatch distance. Central composite design with three factors and five levels was implemented to optimize the required number of experiments. The experimental and numerical results show the impact of various structuring process parameters on surface uniformity. The responses measured are the structure height, height error, and waviness of the pattern. An analysis of the microstructures on the patterned surface was conducted by confocal microscopy and scanning electron microscopy. A 3D-characterization method based on morphological filtering, which allows a holistic view of the surface properties, was applied, and a new qualification scheme for surface microstructures was introduced. Empirical models were also developed and validated for establishing relationships between process parameters and performance criteria. Multi-objective optimization was performed to achieve a minimal value of structure height errors and waviness.

Investigation on the effects of laser parameters on the plasma profile of copper using picosecond laser induced plasma spectroscopy

Laser induced plasma generation and characterization, affected by laser parameters and sample physical properties, represent important phenomena in many fields of applications. In this work, we present new studies on the effects of an appropriate combination of laser wavelengths and pulse energies on the generated plasma characterization using a single shot picosecond Nd:YAG laser. The plasma plume of a pure copper sample has been generated by laser induced plasma spectroscopy (LIPS) using a single shot 170 ps laser pulse with wavelengths (266, 355, 532 and 1064 nm) and varying laser fluence (10–41 J/cm2). The spectral intensities of Cu I 324.75, 327.39, 515.32 and 521.82 nm have been observed. The plasma electron temperature and density have been determined from the Boltzmann plots and Stark-broadening profiles of the plasma spectral lines, respectively, assuming the local thermodynamic equilibrium (LTE) condition. It has been found that the electron temperature and electron density values increase from around 10,000 to 20,000 K and around 2 × 1017 to 1.5 × 1018 cm−3, respectively, with the increase in the laser wavelength and pulse fluence gradually. These observations can be understood due to the variations of mass-ablation rates, inverse-Bremsstrahlung, and photo-ionization with the studied pulse wavelength and pulse energy. The obtained results explore the opportunity to control specific generated plasma parameters by applying proper picosecond pulse parameters which can be considered in many fields of material science spectroscopic analysis and control the plasma interaction dynamics.

Stretchable and Skin-Conformable Conductors Based on Polyurethane/Laser-Induced Graphene.

The conversion of various polymer substrates into laser-induced graphene (LIG) with a CO2 laser in ambient condition is recently emerging as a simple method for obtaining patterned porous graphene conductors, with a myriad of applications in sensing, actuation, and energy. In this paper, a method is presented for embedding porous LIG (LIG-P) or LIG fibers (LIG-F) into a thin (about 50 μm) and soft medical grade polyurethane (MPU) providing excellent conformal adhesion on skin, stretchability, and maximum breathability to boost the development of various unperceivable monitoring systems on skin. The effect of varying laser fluence and geometry of the laser scribing on the LIG micro–nanostructure morphology and on the electrical and electromechanical properties of LIG/MPU composites is investigated. A peculiar and distinct behavior is observed for either LIG-P or LIG-F. Excellent stretchability without permanent impairment of conductive properties is revealed up to 100% strain and retained after hundreds of cycles of stretching tests. A distinct piezoresistive behavior, with an average gauge factor of 40, opens the way to various potential strain/pressure sensing applications. A novel method based on laser scribing is then introduced for providing vertical interconnect access (VIA) into LIG/MPU conformable epidermal sensors. Such VIA enables stable connections to an external measurement device, as this represents a typical weakness of many epidermal devices so far. Three examples of minimally invasive LIG/MPU epidermal sensing proof of concepts are presented: as electrodes for electromyographic recording on limb and as piezoresistive sensors for touch and respiration detection on skin. Long-term wearability and functioning up to several days and under repeated stretching tests is demonstrated.

Evolution of femtosecond laser damage in a hafnia-silica multi-layer dielectric coating.

To optimize optical coating materials, designs, and technologies for high damage resistance, understanding the growth of laser damage is of paramount importance. In this Letter, we show the evolution of femtosecond laser damage in a hafnia-silica (HfO2/SiO2) multilayer dielectric mirror coating. Depending on various spatial features of damaged sites, we identified several regimes of the laser-material interaction with varying laser fluence and incident number of pulses. A change in surface roughness has been observed only for a small number of pulses, and interestingly, a threshold number of pulses is found for nanocrack formation. We report the polarization-dependent orientation of nanocracks and their growth with an increasing number of pulses. The presented results demonstrate that the laser damage originates from the nanobumps and surface roughening, which then leads to the formation of nanocracks. The presented experimental results acknowledge the existing theoretical models in bulk dielectrics to explain the formation of nanostructures by interference of the incident laser with the scattering radiation from laser-induced inhomogeneities and growth of the field enhancement due to nanoplasma.

Pulsed photoacoustic and photothermal response of gold nanoparticles

Gold nanospheres and nanorods are promising agents for photoacoustic imaging and photothermal therapy. In this work, seed-mediated methods were optimized for the synthesis of gold nanosphere and nanorod collides of different sizes. Nanosecond pulse photoacoustic and photothermal analysis of these nanoparticles was carried out and compared with finite element simulation. The simulations were performed to quantify the size dependent photoacoustic signal enhancement for nanospheres and nanorods. The non-sphericity contribution of nanospheres was found to enhance the photoacoustic signal. Nanosecond pulse photoacoustic studies of nanorods of different aspect ratio were carried out. Nanorods of aspect ratio ∼4.8 were found to be the most efficient photoacoustic signal generators. Photoacoustic studies of nanorods at varying laser fluence were performed and threshold fluence of 5 mJ cm−2 was observed. Nanorods exposed to nanosecond laser pulses underwent size and shape variations which were confirmed by optical absorbance and transmission electron microscopy measurements. Simulations of nanorods of different aspect ratio and diameter were performed to investigate the photoacoustic signal enhancement and photothermal stability. The miniature size nanorod with a diameter of 10 nm and aspect ratio of 5 was found to be most appropriate in terms of photoacoustic signal generation and photothermal stability.

Laser-based decontamination of metal surfaces

This study presents the results of surface decontamination using a pulsed 150 W Nd:YAG laser (pulse width 105–230 ns, wave length 1064 nm) for the removal of epoxy paint surfaces with varying laser fluence and pulse duration. The ablation thresholds for metal and paint were determined to optimize process selectivity and the influence of laser fluence on removal rates was investigated. Modifications of the surface and grain structure of the steel substrates were characterized using a laser scanning microscope and SEM. Increased laser fluence leads to faster removal rates, while short pulses minimize changes of the metals grain structure. To simulate surface contamination, important radionuclides for decommissioning, such as Co-60, Cs-137 and Sr-90, were substituted using their stable isotopes. The decontamination effect was investigated using XRF-analysis before and after laser treatment. The results indicate that optimized laser processing parameters lead to a high decontamination factor.