Femtosecond Laser Ablation Process Research Articles

The Shape of The Exfoliated Surface during Femtosecond Laser Ablation

Femtosecond laser ablation processes on metals are systematically observed using a Ti:sapphire laser pump and the soft x-ray laser probe reflective imaging system. In addition to the dark disk at the femtosecond laser-irradiated spot, an elliptical shadow that expands in the horizontal direction in the nanosecond time domain was observed. We show that the shadow is caused by the exfoliated surface during femtosecond laser ablation. By analyzing the shadow, we can determine the evolution of the exfoliated surface over time. The shape of the film is the result of the relationship between the spatial distribution of the laser beam profile and the ablation threshold of the metal.

Creating Hierarchical Topographies on Fibrous Platforms Using Femtosecond Laser Ablation for Directing Myoblasts Behavior.

Developing an artificial extracellular matrix that closely mimics the native tissue microenvironment is important for use as both a cell culture platform for controlling cell fate and an in vitro model system for investigating the role of the cellular microenvironment. Electrospinning, one of the methods for fabricating structures that mimic the native ECM, is a promising technique for creating fibrous platforms. It is well-known that align or randomly distributed electrospun fibers provide cellular contact guidance in a single pattern. However, native tissues have hierarchical structures, i.e., topographies on the micro- and nanoscales, rather than a single structure. Thus, we fabricated randomly distributed nanofibrous (720 ± 80 nm in diameter) platforms via a conventional electrospinning process, and then we generated microscale grooves using a femtosecond laser ablation process to develop engineered fibrous platforms with patterned hierarchical topographies. The engineered fibrous platforms can regulate cellular adhesive morphology, proliferation, and distinct distribution of focal adhesion proteins. Furthermore, confluent myoblasts cultured on the engineered fibrous platforms revealed that the direction of myotube assembly can be controlled. These results indicate that our engineered fibrous platforms may be useful tools in investigating the roles of nano- and microscale topographies in the communication between cells and ECM.

Superhydrophobic laser-ablated stainless steel substrates exhibiting Cassie–Baxter stable state

In order to produce long-lasting, non-wettable surfaces, femtosecond laser ablation process was used to produce micro/nano structures on metallic surfaces. The effects of femtosecond laser irradiation process parameters (fluence, scanning speed and laser beam overlap) on the hydrophobicity of the resulting micro/nano-patterned morphologies on stainless steel were studied in detail. Seven distinctly different micro/nano patterns, namely nano-rippled, paraboloidal, sinusoidal, triple roughness, cauliflowered, tulip and scaly patterns, were fabricated. The latter pattern (scaly) was found to be identical to that of the butterfly wing structure. All patterns were classified according to laser intensity and scanning speed. Consequently, the fabricated substrates were chemically treated by silanization (Young contact angle (CA) of about 105°) in order to examine the additional effects of nanopatterning on wettability. The analysis of wettability revealed enhanced superhydrophobicity for most of these structures, particularly for the cauliflowered and scaly patterns with CAs in excess of 160° and contact angle hysteresis of less than 10°. Finally, the cauliflowered pattern was found to possess a stable composite state, which is important for certain applications such as friction (Cassie–Baxter).

Ozone flexible sensors fabricated by photolithography and laser ablation processes based on ZnO nanoparticles

Ozone gas sensors on flexible substrate with Ti/Pt interdigitated electrodes have been fabricated by standard photolithography and femtosecond laser ablation processes under the same conditions. ZnO nanoparticles deposited by drop coating method have been used as sensitive thin film material with 280nm thickness due to their excellent characteristics on gas detection. The physical properties obtained by both processes have been studied and their critical effects on the gas sensitive response are discussed. It was found that although both samples present small dimensional variations, the samples fabricated by laser ablation shows a bigger sensitive response to ozone at 200°C. However, both samples have been used as ozone gas sensor and present excellent sensing characteristics, giving good stability, fast response/recovery and excellent range of detection from 5ppb to 300ppb.

Influence factors of precision of femtosecond laser ablating 0Cr18Ni9 stainless steel

The 0Cr18Ni9 stainless steel sheet was cut and drilled by a femtosecond laser, and the influence factors of machining accuracy of femtosecond laser ablation were studied. The morphology of stainless steel ablation zone and metallographic structure of cutting seam were examined by means of Nikon SMZ800 optical microscope and 4XC optical metallographic microscope. Based on the laser speckle on the surface of stainless steel observed by a coaxial CCD in the process of stainless steel ablation, the laser beam divergent propagation behavior was simulated using COMSOL Multiphysic numerical simulation software, and the beam divergence angle was calculated. Experimental and simulation results demonstrate that when the laser repetition rate is 5 kHz, the edge of cutting seam and hole are obviously blackened; metallographic phase shows that grain size significantly gets larger at cutting seam and heat affected zone cant be avoided in the femtosecond laser ablation process; metal-air hybrid plasma induced by ultra-high power density femtosecond laser makes the laser beam scatter along the direction of propagation in the process of machining, the divergence angle is between 6 to 10. The existence of heat affected zone and the behavior of the metal-air hybrid plasma are the main factors influencing the precision of femtosecond laser ablation.

Time-Resolved Soft X-Ray Imaging of Femtosecond Laser Ablation Processes on Metals

Femtosecond laser ablation processes on platinum, gold, and tungsten were observed by the single shot pump and probe reflective imaging using a soft x-ray laser probe. To avoid the timing error due to the jitter, we adopted a posteriori correction technique by simultaneous measurement of timing between the pump and probe pulses for every single shot, using a soft x-ray streak camera. A clear difference was found in the temporal behavior of the dynamical response of the soft x-ray reflectivity depending on the irradiated laser fluence in these three materials. On the other hand, the narrow dark rings were found in Pt and W, while an additional bright ring was found outside the dark disk in Au. Our result gives the experimental data comparable with various numerical simulations.

Ammonia sensing properties of ZnO nanoparticles on flexible substrate

Abstract We report the ammonia gas sensing properties of ZnO nanoparticles on flexible substrates. The flexible platform contains Ti/Pt interdigitated electrodes for gas detection and a micro heater device. The Ti/Pt film has been deposited by Magnetron Sputtering with thickness of 5nm and 100 nm respectively. The circuit patterning has been fabricated by standard photolithography and femtosecond laser ablation processes. ZnO thin film has been deposited by drop coating process with a thickness of 275 nm and the gas sensing properties towards ammonia have been studied. The physical properties obtained by both processes have been observed and theirs effects on the gas sensitivity have been discussed. Despite there are small variations in the physical characteristics between the samples fabricated by different patterning processes, both devices present high sensitivity and reproducibility to different ammonia concentrations from 5 ppm to 100 ppm at 300 °C.

Experimental verification of femtosecond laser ablation schemes by time-resolved soft x-ray reflective imaging

Pump and probe reflective imaging using a soft x-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. In addition, the point where the reflectivity dip was observed at t = 10 ps and t = 40 ps, coincides with the position of the edge of reflectivity drop at t = 160 ps. These results give the critical information about the femtosecond laser ablation.

Resonant effects in nonlinear photon absorption during femtosecond laser ablation of Nd-doped silicate glass

This study investigates the resonant effects in nonlinear photon absorption in femtosecond laser ablation of Nd-doped silicate glass (Nd:glass). During the femtosecond laser ablation process, the resonant ablation threshold fluence is decreased by up to 40% compared with that of ordinary ablation. However, it is found that the resonant effect is closely related with laser intensity, and lower laser intensities are required to achieve a significant enhancement. When the intensity is lower than 2.28×10(14) W/cm(2) at which multiphoton ionization dominates, resonant effect is enhanced by a factor of 1.4 to 4.4. When the intensity is higher than 2.28×10(14) W/cm(2), at which intensity tunnel ionization dominates, the resonant effect becomes weak and gradually fades away. It is shown that the resonant effect is still important for multiphoton ionization yet insignificant for tunnel ionization.

Single Femtosecond Laser Pulse Irradiation of Silicon on Different Crystallographic Facet Planes

Experiment on ablation of silicon wafer on different crystallographic facet planes by single laser pulse irradiation was carried out with a femtosecond pulsed laser operating at a wavelength of 780 nm and a pulse width of 160 fs. The quality and morphology of the laser ablated silicon surface were evaluated by atomic force microscopy. The ablation threshold fluences on different crystallographic facet planes were obtained through the relationship between the squared diameter of the craters and pulse energy. The effects of different crystallographic facet planes of silicon wafer on the process of femtosecond laser ablation of silicon wafer were studied.

The Influence of DOS Effects on Ablation Properties in High Energy Femtosecond Laser Ablation Process

The numerical simulation of high-energy femtosecond laser ablation on metal target is studied in this paper. Based on the two-temperature model (TTM), a new model considering the effects of the electron density of states (DOS) on electronic heat capacity, electron-phonon coupling coefficient and electronic thermal conductivity is established. As an example of gold target, the relationship between the melting threshold and the thickness of gold films is numerically calculated. Our result is more consistent with the experimental datum in contrast to the results without considering the DOS effects. This shows that the revised TTM of high-energy femtosecond laser ablation (i.e. DOS-TTM) is more reasonable compared with general used TTM.

High-fluence femtosecond laser ablation of silica glass: effects of laser-induced pressure

A pump-probe technique is employed to investigate the dynamic process of femtosecond laser ablation of silica glass, and three stress waves are observed inside the silica glass in the time-resolved shadowgraphs at a fluence of 40 J cm−2. It is believed that the first stress wave is a thermoelastic wave generated by thermal expansion, while the second and the third are generated subsequently by the mechanical expansions as a result of the extremely high pressure induced by the laser. By employing digital holography, the ablation crater is investigated to identify the ablation results, and high-density silica glass is found beneath the ablation crater, which is also the result of the laser-induced pressure. Furthermore, it is found that more stress waves will be generated at a higher fluence, and the thresholds for the generation of the first, second and third stress wave are measured to be about 2 J cm−2, 6 J cm−2 and 31 J cm−2, respectively. The theoretical calculation result shows that the laser-induced pressure increases with the increase in fluence and can be on the scale of TPa, and we believe a higher pressure will induce more mechanical expansions and therefore more stress waves.

Synthesis of nanocrystalline cubic zirconia using femtosecond laser ablation

We report on the synthesis of nanocrys- talline zirconia in liquid using femtosecond laser ablation. Nanocrystalline cubic zirconia has been prepared by femtosecond laser ablation of zirconium in ammonia, while nanocrystalline tetragonal and monoclinic zirconia was synthesized in water. The physical and chemical mechanisms of the formation of nanocrystalline metastable zirconia are discussed. The intrinsic properties of femtosecond laser ablation in liquid and OH -1 may be responsible for the synthesis of cubic zirconia. It is suggested that the femtosecond laser pulse can create higher tempera- ture and pressure conditions at a localized area in the liquid than the nanosecond laser pulse and the cooling is also faster in the femtosecond laser ablation process, which determined the difference between the products synthesized with femtosecond and nanosecond-pulsed laser ablation.

Femtosecond laser ablation of sapphire on different crystallographic facet planes by single and multiple laser pulses irradiation

Ablation of sapphire on different crystallographic facet planes by single and multiple laser pulses irradiation was carried out with a femtosecond pulsed laser operating at a wavelength of 780 nm and a pulse width of 164 fs. The quality and morphology of the laser ablated sapphire surface were evaluated by scanning electron microscopy and atomic force microscopy. For single laser pulse irradiation, two ablation phases were observed, which have a strong dependency on the pulse energy. The volume of the ablated craters kept an approximately linear relationship with the pulse energy. The threshold fluences of the two ablation phases on different crystallographic facet planes were calculated from the relationship between the squared diameter of the craters and pulse energy. With multiple laser pulses irradiation, craters free of cracks were obtained in the ‘gentle’ ablation phase. The threshold fluence for N laser pulses was calculated and found to decrease inversely to the number of laser pulses irradiating on the substrate surface due to incubation effect. The depth of the craters increased with the number of laser pulses until reaching a saturation value. The mechanism of femtosecond laser ablation of sapphire in two ablation phases was discussed and identified as either phase explosion, Coulomb explosion or particle vaporization. The choice of crystallographic facet plane has little effect on the process of femtosecond laser ablation of sapphire when compared with the parameters of the femtosecond laser pulses, such as pulse energy and number of laser pulses. In the ‘gentle’ ablation phase, laser-induced periodic surface structures (LIPSS) with a spatial period of 340 nm were obtained and the mechanism of the LIPSS formation is discussed. There is a potential application of the femtosecond laser ablation to the fabrication of sapphire-based devices.

A novel microelectrode array combining screen-printing and femtosecond laser ablation technologies: Development, characterization and application to cadmium detection

The elaboration of a new kind of microelectrode array based on femtosecond laser ablation and screen-printing process was reported. The electrochemical behavior of this sensor, constituted by a square array of 8 × 8 screen-printed carbon-based microelectrodes, was characterized by cyclic voltammetry (CV) and compared with macroelectrodes. With an interelectrode distance-to-electrode diameter ratio of 21, this screen-printed microelectrode array (SPμEA) has shown a superior diffusion behavior and a greater mass-transport. The analytical performances of this mercury modified SPμEA were evaluated for cadmium detection. Promising results were obtained with synthetic cadmium solutions in acetate buffer by square-wave anodic stripping voltammetry (SWASV) technique with a detection limit of 1.3 μg L −1 and were confirmed with application to a river sample.

Optical extinction for determining the size distribution of gold nanoparticles fabricated by ultra-short pulsed laser ablation

This work presents a method, based on measurements of the optical extinction spectra, to determine the size of spherical gold nanoparticles produced using the femtosecond laser ablation process in deionized water. By using an improved theoretical model that modifies the contribution of the free electrons to the dielectric function introducing a size-dependent term, it is possible to fit the full experimental extinction spectrum considering a certain size distribution. Additionally, in order to obtain complementary measurements of the size distribution, TEM analysis was performed. The results obtained showed that the predominant nanoparticle size distribution ranges from 1 to 11 nm in terms of radii. An optical extinction measurement together with an appropriate theoretical model based on Mie’s theory represents a simple, low-cost, fast and easy method to describe a multimodal size distribution of spherical gold nanoparticles.

Preparation of porous microstructures on NiTi alloy surface with femtosecond laser pulses

Porous microstructures on Nickel-Titanium (NiTi) alloy surfaces were prepared by linearly polarized femtosecond lasers with moving focal point at a certain speed. It was found that various novel microstructures from feather-like ripples to cluster-like porous textures could be formed with increasing laser energy. Particularly, when the laser energy was 400 μJ, a periodic porous metal surface was generated. Measurement of X-ray diffraction showed that the grains on the sample surface were refined through femtosecond laser ablation processes, but the crystal structures still kept their original states. Analysis by X-ray photoelectron spectroscopy revealed that Ni/Ti on the sample surface was changed with an evident oxidization of titanium element under different laser energies. This investigation provides a new approach to improve the biocompatibility of NiTi-based implant devices.

An analysis of the dependence on photon energy of the process of nanoparticle generationby femtosecond laser ablation in a vacuum

The dependence on laser wavelength of the process of nanoparticle generation by ultrashortlaser ablation of solid matter in a vacuum has been investigated both experimentally andtheoretically. The study has been carried out for a Ni target by using laser pulses of≈300 fs duration at two different laser wavelengths: in the visible(λ = 527 nm) andultraviolet (λ = 263 nm), respectively. The size distribution of the nanoparticles, which is quite broad in thecase of visible light, becomes significantly narrower and slightly shifts towards smaller sizesfor ultraviolet light. Molecular dynamics simulations confirm the dependence of theprocess on the laser wavelength by showing that the laser photon energy affects thematerial relaxation and, thus, the nanoparticle generation process. This, in turn,indicates that the photon energy can be used as an effective parameter to controlthe nanoparticle size distribution in femtosecond laser ablation of solid matter.

A study of substrate temperature distribution during ultrashort laser ablation of bulk copper

With the aid of an infrared thermograph technique, we directly observed the temperature variation across a bulk copper specimen as it was being ablated by multiple femtosecond laser pulses. Combining the experimental results with simulations, we quantified the deposited thermal power into the copper specimen during the femtosecond laser ablation process. A substantial amount of thermal power (more than 50%) was deposited in the copper specimen, implying that thermal effect can be significant in femtosecond laser materials processing in spite of its ultrashort pulse duration.

Molecular dynamic study of volumetric phase change induced by a femtosecond laser pulse

During the last decade, femtosecond laser has been used for micro-machining of various materials. In this work, the phase change phenomena during femtosecond laser ablation are investigated using molecular dynamics (MD) simulation. The process of femtosecond laser ablation of nickel is calculated. The temperature and stress history, and the generation and growth of gas bubbles are traced. For different laser pulse fluences, the material ablation process is analysed to reveal the effect of temperature and stress.