Ultrashort Pulsed Laser Ablation Research Articles

Silver–silver oxide core–shell nanoparticles by femtosecond laser ablation: core and shell sizing by extinction spectroscopy

The generation of small silver metal nanoparticles (Nps) by ultrashort pulsed laser ablation has been an active area of research in recent years due to their interest in several fields of applied research such as biotechnology and material research, in particular those with sizes smaller than 10 nm. In general, laser ablation tends to produce environmentally clean metal Nps compared with wet chemical methods. However, since silver may be oxidized in the presence of water or ethanol, core–shell silver–silver oxide (Ag–Ag2O) Nps can be formed, whose size and thickness must be determined and characterized for functionalization related to future applications. This work analyses the size characteristics of core–shell Ag–Ag2O colloid nanostructures (smaller than 10 nm) obtained by femtosecond laser ablation of solid silver targets in different liquid media (water or ethanol) through the study of their optical extinction spectra. A fit of full experimental spectrum using Mie theory allows the determination of core size and shell thickness distributions as a function of fluence. The red-shift of the plasmon peak wavelength with respect to the bare-core peak wavelength at 400 nm, produced by the oxide shell, may be easily measured even for very small thicknesses. It was found that the dominant Ag2O effective thickness is inversely proportional to the fluence, reaching a maximum of 0.2 nm for a fluence of 60 J cm−2 and a minimum of 0.04 nm for a fluence of 1000 J cm−2.

Spectral dependences of conical emission in gases: Minimization of scattering for ultra-short pulsed laser ablation

Parameters of conical emission (CE), at focusing of femtosecond and short picosecond pulses in ambient gases, are characterized in a wide range of experimental conditions. Scattered and absorbed energy, the beam profiles and spectra are compared for different duration and wavelengths of laser radiation (harmonics of Ti:Sa and Yb:YAG laser). Shorter wavelengths were found to be most beneficial for elimination of CE. Nearly no scattering of short picosecond pulses was observed at 515 nm in the energy density range up to 300 J/cm2. The results are analyzed in terms of the contributing ultra-fast phenomena effecting refraction index of gases (the Kerr effect, different mechanisms of ionization). Advantages of the visible pulses are illustrated in deep drilling experiments.

Hybrid machining of microstructures using a combination of electrical discharge machining milling and laser ablation

Using laser beam machining with ultrashort laser pulses, microstructures in the micrometer region can be manufactured in virtually all materials, regardless of their mechanical properties. However, machining times are slow if good surface qualities are required. Micro electrical discharge machining (EDM) is also capable of removing material nearly without process forces, but with a higher processing speed if structure sizes exceed a certain value. With the combination of both processes, specific advantages can be realized while disadvantages can be partially eliminated. Therefore, a hybrid machine tool was developed that combines the two processes of ultrashort pulsed laser ablation and EDM milling. No reclamping is necessary. In this study, the limitations and challenges of the single processes are investigated. After that, the novel hybrid machine tool is presented, along with strategies for the efficient and economic manufacturing of microstructures in materials that are difficult to machine by conventional processes.

Numerical modeling of ultrashort-pulse laser ablation of silicon

Silicon ablation by a single ultrashort laser pulse is simulated through a computer model. The agreement between results obtained through the model and experimental data found in the literature supports the hypothesis made by the authors in considering thermal evaporation as the dominant ablation mechanism in silicon. Two distinctive thresholds are defined for the ablation procedure leading to a better interpretation of experimental data. The dependence of ablation fluence thresholds on both wavelength and pulse width is discussed. An approximate analytical model describing the crater formation process is proposed and indicative results are presented.

In-Situ Conjugation – Tailored Nanoparticle-Conjugates by Laser Ablation in Liquids

The generation and characterisation of nanoparticulate carrier systems is important for drug delivery, biosensing and in vivo or in vitro diagnostics. Conventionally, nanoparticle generation is based on chemical synthesis methods requiring time intensive reaction and additive design for each material. Successive purification and surface functionalisation is often required after the nanoparticle generation to achieve pure nanoparticle-conjugates. Ultra short pulsed laser ablation in liquids has evolved as a powerful tool for nanoparticle generation showing advantages in the nearly unlimited material variety and purity. We perform the ablation in solutions containing bioactive reagents, realizing an in-situ conjugation of nanoparticles. This novel method enables the generation of diverse nanoparticle conjugates, demonstrated at the example of dodecanthiol stabilized gold nanoparticles in hexane. A first bioconjugation with oligonucleotides was carried out additionally. We studied the influence on productivity of nanoparticle generation and evaluated the integrity of biomolecules after laser ablation. A quenching of nanoparticle growth and narrowing of the surface plasmon resonance, which both increases in media with higher conjugative agent concentrations is observed. Since laser generated gold nanoparticles show electron accepting properties due to a partial oxidation, the presented in-situ conjugations can be exploited to other application, e.g. the coupling to polymeric matrices, as far as the conjunctive molecules bear electron donor moieties.

A simplified predictive model for high-fluence ultra-short pulsed laser ablation of semiconductors and dielectrics

Ultra-short pulsed laser ablation is a very complicated process and a predictive model is very desirable for process design and optimization in practical applications. However, the molecular dynamics or hydrodynamic models, although they are powerful and necessary tools for the study of the fundamental physics, are time-consuming and difficult to apply for practical applications. In this paper, a predictive, simplified and easy to apply model has been developed for high-fluence ultra-short laser ablation of semiconductors and dielectrics. Unlike many other simplified models, this model does not involve any free adjustable variables. The model predictions agree well with experimental measurements for femtosecond laser ablation, while the model is not very applicable for pulse durations more than ∼10 ps.

Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm 2, which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm 2. Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.

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.

Applications of ultra-short pulsed laser ablation: thin films deposition and fs/ns dual-pulse laser-induced breakdown spectroscopy

In this paper, we report a survey of two of the large number of possible practical applications of the laser ablation performed by an ultra-short pulse laser, namely pulsed laser deposition (PLD) and fs/ns dual-pulse laser-induced breakdown spectroscopy (DP-LIBS). These applications differ from those using just longer pulsed lasers as a consequence of the distinctive characteristics of the plasma produced by ultra-short laser beams. The most important feature of this plasma is the large presence of particles with nanometric size which plays a fundamental role in both applications.

Nanocomposite manufacturing using ultrashort-pulsed laser ablation in solvents and monomers

On the basic of literature data on ablation was characterized as a method of nanoparticles preparation. The own research concerned the direct ultrafast laser ablation of solids used to produce the engineering nanoparticles with narrow size distributions. Without using any chemical precursors, stoichiometric nanoparticles are generated from different materials (silver, gold, NiTi alloy) and directly dispersed in a liquid, e.g. acrylic acid, acetone, water, or a monomer of polymeric material. The produced nanoparticle solutions may be used to fabricate nano-modified plastic materials with specially designed properties by polymerization directly subsequent to particles' production, without the necessity to re-disperse the particle aggregates. In order to control the size distribution and related properties, nanomaterial characterization using on-line measurements of the particle size is applied. The developed processes give access to new homogeneous nanomaterials such as multi-component nanosystems.

Molecular dynamics study of the role of material properties on nanoparticles formed by rapid expansion of a heated target

Rapid expansion of a heated target and its decomposition into fragments is investigated by using molecular dynamics simulations. Particular attention is focused on the void formation and nucleation that governs the target disintegration. The cluster formation process is investigated as a function of material properties (initial temperature, interaction potential and composition). Calculation results demonstrate the influence of these properties on void nucleation and growth and on the characteristic parameters of nanoparticles to be formed. In particular, larger initial temperature and expansion rate lead to the formation of smaller fragments. These effects are found to be similar for three different materials (silicon, nickel and metal alloy). In addition, the stoichiometrical cluster composition obtained in the expansion of a binary alloy is found to be fairly well preserved. The calculation results can be used for the interpretation of the experimental findings showing the formation of nanoparticles by short and ultra-short pulse laser ablation of both simple and more complex materials.

Multiphoton imaging of ultrashort pulse laser ablation in the intracellular parasite Theileria

Theileria annulata is an intracellular parasite that infects and transforms bovine leukocytes, inducing continuous proliferation of its host cell both in vivo and in vitro. Theileria-infected cells can easily be propagated in the laboratory and serve as a good model for laser ablation studies. Using single pulses from an amplified ultrashort pulse laser system, we developed a technique to introduce submicrometer holes in the plasma membrane of the intracellular schizont stage of Theileria annulata. This was achieved without compromising either the viability of the organisms or that of the host cell that harbors the parasite in its cytoplasm. Multiphoton microscopy was used to generate image stacks of the parasite before and after ablation. The high axial resolution allowed precise selection of the region of the membrane that was ablated. It also allowed observation of the size of the holes generated (in fixed, stained cells) and determination of the structural changes in the parasite resulting from the laser pulses (in living cells in vitro). This technique opens a new possibility for the transfection of Theileria or delivery of molecules to the schizont that may prove useful in the study of this special host-parasite relationship.

Laser milling: Pulse duration effects on surface integrity

Laser milling of engineering materials is a viable alternative to conventional methods for machining complex microcomponents. The laser source employed to perform such microstructuring has a direct impact on achievable surface integrity. At the same time, the trade-offs between high removal rates and the resulting surface integrity should be taken into account when selecting the most appropriate ablation regime for performing laser milling. In this paper the effects of pulse duration on surface quality and material microstructure are investigated when ablating a material commonly used for manufacturing microtooling inserts. For both micro- and nanosecond laser regimes, it was estimated that the heat-affected zone on the processed surface is within 50 μm. When performing ultra-short pulsed laser ablation, the effects of heat transfer are not as evident as they are after processing with longer laser pulse durations. Although some heat is dissipated into the bulk when working in pico- and femtosecond regimes it is not sufficient to trigger significant structural changes.

Generation of nanoparticle colloids by picosecond and femtosecond laser ablations in liquid flow

Fabrication of silver nanoparticle colloids using ultrashort pulse laser ablation in water is studied. Ablation in liquid flow improves the reproducibility and increases the nanoparticle productivity by 380% compared to stationary liquid. Femtosecond laser ablation in water is 20% more efficient than picosecond laser ablation, but due to higher picosecond laser power (higher repetition rate), the nanoparticle productivity at the same pulse fluence is three times higher for picosecond laser ablation. With picosecond laser pulses, the maximum productivity of 8.6μg∕s is achieved at a pulse energy of 110μJ and repetition rate of 50kHz.

Theoretical studies of ultrafast ablation of metal targets dominated by phase explosion

Ultrashort pulse laser ablation of metallic targets is investigated theoretically through establishing a modified two-temperature model that takes into account both the temperature dependent electron–lattice coupling and the electron–electron-collision dominated electron diffusion processes for higher electron temperature regime. The electron–lattice energy coupling rate is found to reduce only slowly with increasing pulse duration, but grow rapidly with laser fluence, implying that the melting time of metallic materials decreases as the laser intensity increases. By taking phase explosion as the primary ablation mechanism, the predicted dependences of ablation rates on laser energy fluences for different laser pulse widths match very well with the experimental data. It is also found that during phase explosion the ablation rate is almost independent of the pulse width, whereas the ablation threshold fluence increases with the pulse duration even for femtosecond pulses. These theoretical results should be useful in having proper understanding of the ablation physics of ultrafast micromachining of metal targets.

Simulation of the thermionic emission during ultrashort pulse laser ablation of metals

The characteristics of thermionic emission of metal films during ultrashort pulse laser ablation are investigated by numerical simulation. The two-temperature model is used to calculate the electron and lattice temperatures while thermionic emission is incorporated into the model as a surface phenomenon. The Richardson-Dushman equation is employed to estimate the rate of thermionic electron emission. The influence of laser irradiance and the film thickness on the emission rate is examined. The change of quantum efficiency is predicted and compared with reported experimental data. The evolution of electron and lattice temperatures and the variation of relaxation time are also investigated in accordance with thermionic emission.

Plasma modeling for ultrashort pulse laser ablation of dielectrics

In ultrashort pulse (<10ps) laser ablation of dielectrics, affected materials are first transformed into absorbing plasma with metallic properties and, then, the subsequent laser-plasma interaction causes material removals. For ultrashort-pulse laser ablation of dielectrics, this study proposes a model using the Fokker-Planck equation for electron density distribution, a plasma model for the optical properties of ionized dielectrics, and quantum treatments for electron heating and relaxation time. The free electron density distribution of the plasma within the pulse duration is then used to determine the ablation crater shape. The predicted threshold fluences and ablation depths for barium aluminum borosilicate and fused silica are in agreement with published experimental data. It is found that the significantly varying optical properties in time and space are the key factors determining the ablation crater shape. The effects of fluence and pulse duration are also studied.

Laser photoablation: a new biomedical tool

Most diseases and biological processes can only be studied in a living animal in which all the complex regulatory processes are occurring. New techniques using ultrashort laser pulses are increasingly allowing researchers to manipulate cells and tissue within living organisms to study normal and diseased functionality. The study of microstrokes may, in particular, benefit from these advances. These small strokes are thought to be caused by clots or bleeding in small blood vessels within the brain that lead to the death of neurons. Clinical studies have shown that such microstrokes are relatively common in the elderly and may play a major role in dementia and diseases such as Alzheimer’s. However, no treatment is presently available, partly because reproducing microstroke lesions in animals has proved elusive. Ultrashort pulsed laser ablation provides a novel tool to produce such lesions, finally enabling the study of this disease. The technique has recently emerged as an excellent tool for the precision machining of solid state materials and has been used for the fabrication of waveguides inside transparent materials without affecting the surface. The extremely high light intensities achieved when focusing ultrashort laser pulses enable the nonlinear optical absorption of the laser light. The resulting ionized volume can be as small as the size of a diffraction-limited focus, typically ∼1μm3 , and can be confined deep below the surface. This same technique can be applied to manipulating biological structures deep within a living organism. To study the effect of microstrokes in the living brain, windows are made in the skulls of anesthetized rats and the blood vessels are labeled with an intravenous injection of fluorescent dye. We use another nonlinear optical technique, two-photon laser scanning microscopy, to image blood vessels and measure blood flow in the brain with microscopic resolution. To perturb microvessels located as deep as 500μm below the brain surface, Figure 1. Diagram of the femtosecond laser-induced rupture of a microvessel followed by hemorrhage (top, left). The other panels show the time series of five images showing the rupture of a vessel 140μm below the surface of a rat cortex. Each image is labelled with the time relative to irradiation onset. The images were recorded during two-photon excited fluorescence microscopy. The targeted vessel first explodes, leaving a dark central volume filled with red blood cells and fluorescentlylabeled blood plasma penetrating the brain tissue. The illustrations are adapted from Nishimura et al. and used with kind permission of the Nature Publishing Group.

Dynamics of confined plumes during short and ultrashort pulsed laser ablation of graphite

The optical emission from electronically excited C species in the ablation plume following the short (ns) and ultrashort (fs) UV pulsed laser ablation of graphite is studied. Wavelength, time and spatially resolved imaging of the plume, in background pressures of inert gases such as argon and helium, is performed. Analysis of images of optical emission from ${\mathrm{C}}^{+*}$ ions and ${\mathrm{C}}_{2}^{*}$ radicals, yielded estimates of the apparent velocity of emitting species, which appear to arise both from the initial ablation event and, in the presence of background gas, mainly from impact excitation. At elevated background pressures of argon $({P}_{\mathrm{Ar}})$, the formation and propagation of a shock wave is observed for ns pulses, whereas for fs pulses, the propagation of two shock waves is observed. During fs ablation, the first shock wave we associate with an initial burst of highly energetic/electronically excited ablated components, indicative of an enhanced fraction of non-thermal ejection mechanisms when compared with ns ablation. The second shock wave we associate with subsequently ejected, slower moving, material. Concurrent with the plume dynamics investigations, nanostructured amorphous carbon materials were deposited by collecting the ablated material. By varying ${P}_{\mathrm{Ar}}$ from 5 to 340 mTorr, the film morphology could be changed from mirror smooth, through a rough nanostructured phase and, at the highest background pressures for ns pulses, to a low density cluster-assembled material. The evident correlations between the film structure, the mean velocities of the emitting C species, and their respective dependences upon ${P}_{\mathrm{Ar}}$ are discussed for both pulse durations. In addition, we comment on the effect of observed initial plume dynamics on the subsequent C cluster formation in the expanding plume.

Production of nanoparticles of different materials by means of ultrashort laser pulses

Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy.