Laser Ablation Threshold Research Articles

Femtosecond green laser processing of magnesium alloy

In this paper, a femtosecond green laser with wavelength of 515 nm was used to process the AZ31 magnesium alloy. The laser ablation threshold and ablation rate of Mg alloy were calculated. The mechanism of femtosecond green laser process was determined. The effects of surface microstructures on corrosion rate of AZ31 magnesium alloy was compared and analyzed. The results show that the laser ablation threshold of AZ31 magnesium alloy is 1.46 J/cm2, the ablation rate is 0.68 μm/pulse in the laser fluence of 8.36 J/cm2, the ablation rate increases with the laser fluence increasing. The high-quality holes can be fabricated with the laser fluence of 8.36 J/cm2 and the pulse number of 1000. In terms of the corrosion rate of magnesium alloy, the groove structure is less than that of the columnar structure and less than that of the smooth surface, among which the corrosion rate on the microstructural surface is about 1/3~1/2 of that on the smooth surface in 24 hours.

Investigation of ancient wall paintings in Mogao Grottoes at Dunhuang using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is utilized to analyze the ancient wall paintings in the Dunhuang Mogao Grottoes. First, based on a combination of the signal-to-noise ratio (SNR) of the LIBS spectra and the sizes of the ablation craters on simulated samples, a laser ablation threshold is suggested to preventing excessive damage on the ancient wall paintings. Second, coupling with the principal component analysis (PCA) method, LIBS is used to differentiate atacamite and malachite green pigments on simulated sample, the classifiable model is successfully used to differentiate and classify the three unknown green pigments with different tones on a real mural piece, which dates back to the Yuan Dynasty. Third, in combination with a cross-sectional analysis performed via optical microscopy, LIBS is employed to depth profile analysis of green painted layers, a fitting relationship of the ablation depth and the number of laser ablation pulses is obtained. Finally, the thickness of the green painted layers on the mural piece is calculated by means of the relationship, which is close agreement with the result of the cross-sectional analysis.

FOTOACÚSTICA PULSADA APLICADA A LA DETERMINACIÓN DE UMBRALES DE ABLACIÓN LÁSER EN METALES

Se determinaron los umbrales de ablación láser de los metales cobre, oro y plata para la radiación láser de 1064 nm de un láser Nd:YAG de nanosegundos utilizándose la técnica fotoacústica pulsada para registrar las señales fotoacústicas producidas debido a la interacción del metal con los pulsos láser. Dichas señales se analizaron mediante correlación estándar y los cambios en la amplitud fotoacústica conforme aumento la fluencia láser. Los resultados indican que las señales fotoacústicas son muy sensibles a cambios físicos que experimenta una muestra solida debido a la interacción con pulsos láser, pudiendo diferenciarse claramente el cambio del régimen termoelástico al de ablación y por consiguiente el umbral de ablación. Los valores obtenidos para la fluencia umbral fueron: (1.71±0.08) J/cm2 para el cobre, (1.65±0.09) J/cm2 para el oro y (1.15±0.05) J/cm2 para la plata.

Colossal laser ablation threshold of Ge crystal due to formation of GeO2 nanolayer: “Lid Effect” - Subsurface boiling mechanism

Colossally increased laser ablation threshold intensity Ith = 637.0 MW/cm2 of mechanically polished Ge crystal was found. It exceeds by factor of 100 that of chemically etched Ge single crystal (1 0 0), which is about 6.0 MW/cm2. This phenomenon is related to the formation of thin GeO2 layer during the action of laser pulse (wavelength λ = 1064 nm, pulse duration 6 ns, energy in a pulse W = 0.12 J, repetition rate of 10 Hz and beam diameter 2 mm) on the mechanically polished amorphous α-Ge. The increased threshold intensity is explained by the “lid effect”- a non-monotonous temperature distribution with maximum in the bulk of the crystal, where melting and even boiling of material takes place under a solid GeO2 nanolayer, preventing the ablation at a much lower intensity. Applying SEM and optical microscope methods, appearance of bubbles at the irradiated surface was observed. In experiments, the fundamental frequency of pulsed Nd:YAG laser in the regime of multi-pulse irradiation of the same surface spot was used. The thickness of GeO2 layer, estimated from SEM images and by the use of Casino simulation software, is up to 50 nm. According to our calculations of temperature field in GeO2/Ge structure, the maximum of temperature is located near the interface and Ge boiling temperature is reached under solid GeO2 layer at a sub-threshold laser intensity. The theoretical and experimental results are thus explained by the subsurface boiling mechanism of laser ablation in GeO2/Ge structure in the presence of solid GeO2 layer, acting as a “lid”. The possible applications of the investigation results are proposed.

Effect of surface roughness on the ultrashort pulsed laser ablation fluence threshold of zinc and steel

Abstract The single and multiple pulse laser ablation threshold of zinc and steel at picosecond laser pulse duration is studied as a function of initial surface roughness at laser wavelengths of 515 and 1030 nm. The initial surface topographies and the resulting crater morphologies are analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Reflectivity measurements of the initial surfaces show increased absorptivity with increasing surface roughness. It was found that the single pulse ablation threshold increases with increasing effective surface area; the latter resulting from surface roughness. Rougher surfaces tend to have a higher degree of incubation as well. From the experimental and simulation results, it appears that the absorbed energy contributes more to residual heat than to material ablation when effective surface area increases.

Surface roughening of Al2O3/Al2O3-ceramic matrix composites by nanosecond laser ablation prior to thermal spraying

Al2O3/Al2O3 ceramic matrix composites are candidate materials for high-temperature applications such as gas turbines. As water vapor corrosion of Al2O3/Al2O3-CMC (ceramic matrix composite) is a major issue, the application of suitable environmental barrier coatings is inevitable. An important factor for coating adhesion, especially in thermal spraying, is mechanical interlocking. Therefore, a rough substrate surface is needed. Although it has been proven that laser ablation is a suitable method for surface preparation of metallic substrates, no studies on Al2O3/Al2O3-CMCs are available. Therefore, the suitability of surface preparation of an Al2O3/Al2O3-CMC by laser ablation for use prior to atmospheric plasma spraying was examined. The laser ablation threshold fluence for Al2O3/Al2O3-CMC was determined. The effects of different processing parameters on the surface were studied. Various surface morphologies were obtained, such as cauliflower and honeycomb structures. The samples were characterized by white light interferometry, laser microscopy, and scanning electron microscopy. The obtained surface structures were coated with Gd2Zr2O7. It was found that the adhesion strength of coatings on laser treated samples was drastically increased.

Effect of fs/ps laser pulsewidth on ablation of metals and silicon in air and liquids, and on their nanoparticle yields

The effect of laser pulsewidth, tuned in the range τ = 0.3–10 ps, on infrared (IR) laser ablation (LA) thresholds of bulk gold, silver and silicon in air and in liquid environments (deionized water, isopropyl alcohol (IPA)) was studied in this work. The influence of laser pulsewidth on the morphology and yield of the ejected nanoparticles (NPs), as well as its impact on the efficiency of NP generation, was studied by UV–vis spectral measurement of the extinction coefficient spectra of NP colloids, fabricated by multi-pass ablation of bulk targets in both solvents at different pulse duration values, by analytical disc centrifuge and by high-resolution scanning electron microscopy. The ablation thresholds are raising with the increase of laser pulsewidth, exhibiting sublinear dependences. The extinction coefficient dependence on τ exhibited a non-monotonous character in case of water medium for all materials, and was gradually descending in case of LA in IPA. Additional MD-TTM (molecular dynamics- two-temperature model) modeling was performed for single-shot laser exposition of bulk Au target in air with τ = 0.3 ps and 4 ps, which allowed distinguishing the main mechanisms of the material removal, the formation of NPs, and the observed crater topography. The performed simulations supported the trend on the ablation threshold increase with the increase of laser pulse duration.

High repetition rate femtosecond laser heat accumulation and ablation thresholds in cobalt-binder and binderless tungsten carbides

Femtosecond (fs) laser ablation has been studied for the potential of fast, high precision machining of difficult-to-machine materials like binderless tungsten carbide. Obstacles that have limited its efficiency include melting from heat accumulation (HA), particle shielding, and plasma shielding. To address HA without shielding effects, high repetition rate (57.4 MHz), ultra-low fluence fs laser irradiation is performed to study the incubation effect and subsequent HA-ablation threshold of fine-grained tungsten carbides. Exposure times on the order of 100 ms were conducted in air with fluences (1.82 to 9.09 mJ/cm2) two orders of magnitude below the single fs pulse ablation thresholds reported in literature (0.4 J/cm2). Heat accumulation at high repetition rate explains the ultra-low fluence melt threshold behavior resulting in melt crowns around ablated holes and grooves. The results of this study aid in predicting heat buildup in high repetition rate laser irradiation for applications that wish to achieve high ablation rates of difficult-to-machine, ultrahard materials and help enable shaping of binderless tungsten carbide for use in applications too extreme for bindered tungsten carbide.

Laser ablation of block copolymers with hydrogen-bonded azobenzene derivatives

Supramolecular assemblies (PS-b-P4VP (AzoR)) are fabricated by hydrogen-bonding azobenzene derivatives (AzoR) to poly(4-vinyl pyridine) blocks of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP). PS-b-P4VP(AzoR) forms phase separated nanostructures with a period of ∼75–105 nm. A second length scale structure with a period of 2 μm is fabricated on phase separated PS-b-P4VP(AzoR) by laser interference ablation. Both the concentration and the substituent of AzoR in PS-b-P4VP(AzoR) affect the laser ablation process. The laser ablation threshold of PS-b-P4VP(AzoR) decreases as the concentration of AzoR increases. In PS-b-P4VP(AzoR) with different substituents (R = CN, H, and CH3), ablation thresholds follow the trend: PS-b-P4VP(AzoCN)<PS-b-P4VP(AzoCH3)<PS-b-P4VP(AzoH). This result indicates that the electron donor group (CH3) and the electron acceptor group (CN) can lower the ablation threshold of PS-b-P4VP(AzoR).

Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing

The research explores the use of femtosecond laser to clean the surface of Ti-6Al-4V alloy. The laser ablation threshold was determined from either depths or diameters of laser-induced craters. Both methods are expected to give the same results; however, it was found that ablation threshold can be determined more reliably from the diameters than from the depths. This can be understood from the study of the surface morphology. As the laser fluence increased, the ablated surface became increasingly rougher, making it difficult to measure the depths accurately. In contrast, diameters of laser-induced craters could be measured with much better certainty and thus enabled the ablation threshold to be determined more accurately. The ablation threshold was found to be 0.142 ± 0.010 J/cm2 for 100 laser pulses at 130 fs pulse duration and 790 nm wavelength and it tends to decrease with increase in number of laser pulses. The single-pulse ablation threshold was determined to be 0.272 ± 0.021 J/cm2.

The influence of the ionization regime on femtosecond laser beam machining mono-crystalline diamond

Diamond is widely used in a series of industrial and scientific applications, including the micro-electro-mechanical systems, optical elements, ultra-precision machining tools, etc. Femtosecond laser beam machining is promising for high precision and high accuracy machining diamond material for its famous non-thermal machining regime. Initially, the laser ablation threshold was determined for the low and high laser fluence stage, respectively. Then, the ionization regime of femtosecond pulsed laser beam machining mono-crystalline diamond with different laser power was discussed. The influence of the ionization regime on laser machining also was analyzed basing on Raman analysis and laser machined surface topography. The laser ablation threshold was 0.29 J/cm2 for the low laser fluence stage and 2.35 J/cm2 for the high laser fluence stage, respectively. The calculation of the Keldysh parameter indicated that the multi-photon ionization prevailed in the low laser fluence stage while the tunneling ionization became dominant in the high laser fluence stage. The transition of the ionization regime from the multi-photon ionization to the tunneling ionization occurred around the laser fluence 3.2 J/cm2. Graphitization of diamond material was discovered on laser machined diamond surface by means of Raman analysis. Graphitization would be enhanced by the tunneling ionization regime in the high laser fluence stage. A strong surface cracking tendency was also observed in laser machining in the high fluence stage.

Characterization, optical properties and laser ablation behavior of epoxy resin coatings reinforced with high reflectivity ceramic particles

Thermal damage induced by high power energy, especially high power laser, significantly affects the lifetime and performance of equipment. High-reflectance coating/film has attracted considerable attention due to its good performance in the damage protection. Preparing a high-reflectance coating with high reaction endothermal enthalpy will effectively consume a large amount of incident energy and in turn protect the substrate from thermal damage. In this study, a low temperature process was used to prepare coatings onto substrate with complex shape and avoid thermal effect during molding. An advanced high reflection ceramic powder, La1−xSrxTiO3+δ, was added in the epoxy adhesive matrix to improve the reflectivity of coating. The optical properties and laser ablation behaviors of coatings with different ceramic additive ratio of La1−xSrxTiO3+δ and modified epoxy-La1−xSrxTiO3+δ with ammonium polyphosphate coatings were investigated, respectively. We found that the reflectivity of coatings is extremely high due to mixed high-reflection La1−xSrxTiO3+δ particles, up to 96% at 1070 nm, which can significantly improve the laser resistance. In addition, the ammonium polyphosphate modifies the residual carbon structure of epoxy resin from discontinuous fine particles structure to continuous and porous structure, which greatly enhances the thermal-insulation property of coating. Furthermore, the laser ablation threshold is improved obviously, which is from 800 W cm−2 to 1000 W cm−2.

The influence of the focus position on laser machining and laser micro-structuring monocrystalline diamond surface

Micro-structured surface on diamond is widely used in microelectronics, optical elements, MEMS and NEMS components, ultra-precision machining tools, etc. The efficient micro-structuring of diamond material is still a challenging task. In this article, the influence of the focus position on laser machining and laser micro-structuring monocrystalline diamond surface were researched. At the beginning, the ablation threshold and its incubation effect of monocrystalline diamond were determined and discussed. As the accumulated laser pulses ranged from 40 to 5000, the laser ablation threshold decreased from 1.48 J/cm2 to 0.97 J/cm2. Subsequently, the variation of the ablation width and ablation depth in laser machining were studied. With enough pulse energy, the ablation width mainly depended on the laser propagation attributes while the ablation depth was a complex function of the focus position. Raman analysis was used to detect the variation of the laser machined diamond surface after the laser machining experiments. Graphite formation was discovered on the machined diamond surface and graphitization was enhanced after the defocusing quantity exceeded 45 µm. At last, several micro-structured surfaces were successfully fabricated on diamond surface with the defined micro-structure patterns and structuring ratios just by adjusting the defocusing quantity. The experimental structuring ratio was consistent with the theoretical analysis.

Correlation among Structure, Water Peak Absorption, and Femtosecond Laser Ablation Properties of Ge–Sb–Se Chalcogenide Glasses

A series of Ge11.5SbxSe88.5–x (x = 5, 10, 15, 20, 25, 30) chalcogenide glasses were fabricated aiming at investigating the role of structure and water peak absorption in determining femtosecond laser ablation thresholds (Fth). The results indicate that the optical band gap decreases and that the Vickers hardness increases with increasing Sb content. Meanwhile, Raman spectra were measured before the femtosecond laser ablation experiment, suggesting a rise in average bond energy in the six glass samples with increasing Sb concentration. Ablation of the sample disks in air were performed with high repetition rate ultrashort laser pulses (150 fs, 1 kHz) at different wavelengths (2.86 and 4 μm) to investigate how the water peak absorption coefficient affects ablation thresholds, which reveals the dominating role of the multiphoton ionization (MPI) progress on the ablation threshold fluence. The results will be useful for photonic devices based on Ge–Sb–Se glasses applied in high-power laser operations to preve...

皮秒激光微制造As 2 Se 3 玻璃红外增透性表面

采用紫外皮秒激光在As2Se3玻璃表面以线扫描形式快速制备大面积周期性点阵式增透微结构，获得了红外透光性能提高的硫系玻璃样品。研究确定了As2Se3玻璃的激光刻蚀阈值，并研究设计了合适线扫描工艺方法。所制样品相对于原样在波长11.0 μm~12.4 μm范围内，透过率平均提高10.0 %；波长13.0 μm~14.2 μm范围内，透过率平均提高5.2 %。激光扫描制备方法没有破坏样品表面原有的浸润性，整个制备过程均在空气开放环境下进行，成本低，工艺可控性强，效率高，制备8 mm×8 mm的表面微结构，仅用时3.65 s，且表面微结构单元尺寸及间距可按材料应用需求调控。分析表明，当激光能量较低时，对该硫系玻璃的去除以“冷加工”为主，不会有明显的热效应，得到微结构的硫系玻璃表面元素组成未发生改变；激光能量较高时，会存在一定的热效应，使得刻蚀点出现熔融态，在微坑边缘出现凸起或翻边。

Stoichiometry control in Cu2ZnSnS4 thin films grown by pulsed laser deposition

We synthesized Cu2ZnSnS4 (CZTS) thin films on soda lime glass substrates using pulsed laser deposition. Deposition parameters and sputtering target compositions were varied in order to optimize the compositions and morphologies of the CZTS films. At room temperature, films deposited using a sputtering target composition of Cu1.83Zn1.16Sn0.95S4.05, consisting of the nominally desired metal ratios and excess sulfur, were found to be heavily Sn-rich and S-deficient. By reducing the Sn concentrations in the sputtering targets, film compositions could be adjusted to the desired stoichiometries. Laser fluence was found to strongly influence the Cu:Zn ratios in the films, with a clear laser ablation threshold visible at ∼2.5 J/cm2, above which the film stoichiometry did not significantly change. Increasing the pulse repetition rate slightly increased the relative Zn and S concentrations in the films but also introduced micron-sized cracks. Substrate temperature was shown to have a significant effect on the composition and quality of the deposited films. While smoother films with larger-width grains were achieved at temperatures of 400 and 500 °C, significant losses in Zn and Sn were observed. Recommendations on the optimized deposition parameters and sputtering target compositions are presented.

Nano-ablation of silica by plasmonic surface wave at low fluence.

Formation of ripples by ablation of surfaces of laser-irradiated materials is an example of ultrafast energy delivery. Herein, we report on fs-laser optical imprinting of periodic nano-grooves on silica substrate at only 25% of the laser ablation threshold via an interface plasmonic light localization at the ZnS film (top) interface with silica (bottom) by plasmonic surface wave. The nano-grooves were formed throughout ZnS with the same period and orientation imprinted onto the underlying silica. Based on a detailed account of the multi-photon and avalanche ionization using the Drude model, laser-induced plasmonic ablation describes quantitatively the energy deposition from the top ZnS to the substrate of silica.

OLED Luminaires: Device Arrays with 99.6% Geometric Fill Factor Structured by Femtosecond Laser Ablation.

The plethora of design opportunities renders organic light emitting diodes (OLEDs) ideal luminaires for general lighting applications. Progressing from lab-scale device concepts to large-area applications calls for smart device designs that are scalable and, at the same time, unsusceptible to resistive losses within the electrodes. By employing direct pulsed femtosecond laser structuring, we fabricate OLED luminaires comprising monolithically interconnected OLED arrays. We determine the laser ablation thresholds and the optimized process parameters for all functional layers. The clean laser cuts with precise ablation depths show no detectable damage to adjacent layers or any ridges, hence avoiding unwanted short-cuts or device isolation. All processes are scalable. The 3-fold structuring of the OLED luminaire is confined within 45 μm and hence below the resolution limit of the human eye, yielding a geometric fill factor beyond 99% and therefore a very homogeneous device perception.

Simulation of femtosecond laser absorption by metallic targets and their thermal evolution

AbstractThe interaction of femtosecond laser with initially cold solid metallic targets (Al, Au, Cu, Mo, Ni) was investigated in a wide range of laser intensity with focus on the laser energy absorption efficiency. Our developed simulation code (FEMTO-2D) is based on two-temperature model in two-dimensional configuration, where the temperature-dependent optical and thermodynamic properties of the target material were considered. The role of the collisional processes in the ultrashort pulse laser–matter interaction has been carefully analyzed throughout the process of material transition from the cold solid state into the dense plasma state during the pulse. We have compared the simulation predictions of the laser pulse absorption with temperature-dependent reflectivity and optical penetration depth to the case of constant optical parameters. By considering the effect of the temporal and spatial (radial) distribution of the laser intensity on the light absorption efficiency, we obtained a good agreement between the simulated results and available experimental data. The appropriate model for temperature-dependent optical parameters defining the laser absorption efficiency will allow more accurate simulation of the target thermal response in the applications where it is critical, such as prediction of the material damage threshold, laser ablation threshold, and the ablation profile.

Optical and electrical properties of indium tin oxide films near their laser damage threshold

In this study, we investigated whether the optical and electrical properties of indium tin oxide (ITO) films are degraded under laser irradiation below their laser ablation threshold. While performing multi-pulse laser damage experiments on a single ITO film (4.7 ns, 1064 nm, 10 Hz), we examined the optical and electrical properties in situ. A decrease in reflectance was observed prior to laser damage initiation. However, under sub-damage threshold irradiation, conductivity and reflectance of the film were maintained without measurable degradation. This indicates that ITO films in optoelectronic devices may be operated below their lifetime laser damage threshold without noticeable performance degradation.