Ultrashort Laser Ablation Research Articles

飞秒激光与宽禁带物质相互作用过程中光子-电子-声子之间的微能量传导: I：光子吸收过程

飞秒激光与宽禁带物质相互作用过程中光子-电子-声子之间的微能量传导: I：光子吸收过程

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.

Ultrashort laser ablation of PMMA and intraocular lenses

The use of intraocular lenses (IOLs) is the most promising method to restore vision after cataract surgery. Several new materials, techniques, and patterns have been studied for forming and etching IOLs to improve their optical properties and reduce diffractive aberrations. This study is aimed at investigating the use of ultrashort laser pulses to ablate the surface of PMMA and intraocular lenses, and thus provide an alternative to conventional techniques. Ablation experiments were conducted using various polymer substrates (PMMA samples, hydrophobic acrylic IOL, yellow azo dye doped IOL, and hydrophilic acrylic IOL consist of 25% H2O). The irradiation was performed using 100 fs pulses of 800 nm radiation from a regeneratively amplified Ti:sapphire laser system. We investigated the ablation efficiency and the phenomenology of the ablated patterns by probing the ablation depth using a profilometer. The surface modification was examined using a high resolution optical microscope (IOLs) or atomic force microscope—AFM (PMMA samples). It was found that different polymers exhibited different ablation characteristics, a result that we attribute to the differing optical properties of the materials. In particular, it was observed that the topography of the ablation tracks created on the hydrophilic intraocular lenses was smoother in comparison to those created on the PMMA and hydrophobic lens. The yellow doped hydrophobic intraocular lenses show higher ablation efficiency than undoped hydrophobic acrylic lenses.

Study of ultrashort laser ablation of metals by molecular dynamics simulation and experimental method

The dynamics of fs-laser ablation of monocrystalline copper has been investigated by molecular dynamics simulation and experimental method. A simulation model is developed by taking the laser energy absorption, the thermal transport of free electrons as well as the energy exchange between electrons and lattice into account. The propagation of laser-induced stress wave in the simulation model is studied. The velocity of stress wave is predicted to be nearly equal to sound velocity of copper 4400 m/s. The mechanisms of ablation responsible for individual atoms and clusters are analyzed as the thermal fluctuations in the kinetic energy and tensile stresses, respectively. The rates of ablation at different fluences obtained from molecular dynamics calculations support the experimental observations of two different ablation regimes.

Nanoparticle formation by femtosecond laser ablation

The main features of ultra-short laser ablation of various materials (metals, semiconductors or insulators) have been studied through the complementary analyses of the plasma plume induced by laser irradiation, and of the deposited films. The generation of nanoparticles (in the 10–100 nm range) was observed and these findings were investigated in order to obtain information on the relevant parameters governing the formation of these nanoparticles. By the use of polyatomic targets, the non-congruent formation of nanoparticles was evidenced, demonstrating the existence of complex phenomena (phase separation, chemical reactions and interaction with the residual gas) during the laser–matter interaction and plasma expansion. An unexpected finding was the influence of the laser beam spot size on nanoparticle emission during femtosecond laser ablation. The comparison of these results with the currently admitted explanations of ultra-short laser–matter interactions clearly indicates that the existing models and simulations do not describe or explain the overall phenomena taking place during femtosecond laser ablation, and another approach has to be envisaged including the influence of the parameters evidenced in this work.

Laser-induced modification of the size distribution of nanoparticles produced during ultrashort laser ablation of solid targets in vacuum

We demonstrate that the average size and the size distribution of nanoparticles produced by ultrashort laser ablation in vacuum can be modified by irradiation of the nanoparticles' plume during its vacuum expansion with a nanosecond, UV laser pulse. The experimental results are interpreted by means of a simple analytical model of the laser–particle interaction, heating and vaporization. This technique thus shows to be a simple and general way to control, in vacuum, the average size and distribution of nanoparticles of different elements.

Mechanisms of small clusters production by short and ultra-short laser ablation

The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD–DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings.

Multi-material two-temperature model for simulation of ultra-short laser ablation

We investigate the interaction of 100 fs laser pulses with metal targets at moderate intensities (10 12 to 5 × 10 13 W/cm 2). To take into account effects of laser energy absorption and relaxation we develop a multi-material two-temperature model based on a combination of different approaches. The backbone of the numerical model is a high-order multi-material Godunov method in a purely Eulerian form. This formulation includes an interface-tracking algorithm and treats spallation at high strain rates and negative pressures. The model consistently describes the hydrodynamic motion of a two-temperature plasma and accounts for laser energy absorption, electron–phonon/ions coupling and electron heat conductivity. In particular, phase transitions are accurately taken into account by means of a wide-range two-temperature multi-phase equation of state in a tabular form. The dynamics of the phase transitions and the evolution of the heat-affected zone are modeled and analyzed. We have found that a careful treatment of the transport coefficients, as well as consideration of phase transitions is of a great importance in obtaining reliable numerical results. Calculation results are furthermore compared for two metals with different electron–phonon coupling parameters (Au and Al). We have found that the main part of ablated material results from fragmentation of melted phase caused by tensile stresses. A homogeneous nucleation mechanism alone does not explain experimentally observed ablation depth.

Femtosecond laser ablation of nickel in vacuum

We present an experimental characterization and a theoretical analysis of ultrashort laser ablation of a nickel target, which highlights the more general and peculiar features of femtosecond (fs) laser ablation of metals. The study has been carried out by using visible (527 nm) laser pulses of ≈ 300 fs duration. The vacuum expansion dynamics of the ablated species has been investigated by using fast photography and optical emission spectroscopy, while the fs laser pulse–metal interaction has been studied theoretically by means of molecular dynamics simulations. Special attention has been given to the study of the dependence of ablation depth on laser fluence, which has been carried out by comparing the SEM analysis of micro-holes drilled into the nickel samples with the predictions of the theoretical model. The main outcomes of our investigation, which are very satisfactorily reproduced and accounted for by the theoretical model, are (i) the nonlinear dependence of the ablation yield on the laser fluence, and its reliance to the electron heat diffusion, in the process of redistribution of the absorbed energy, (ii) the splitting of the material blow-off into two main classes of species, atoms and nanoparticles, characterized by different expansion dynamics, and (iii) the different degrees of heating induced by the laser pulse at different depths into the material, which causes the simultaneous occurrence of various ablation mechanisms, eventually leading to atoms and nanoparticles ejection.

Ultra-short laser ablation of metals and semiconductors: evidence of ultra-fast Coulomb explosion

We have studied ultra-fast laser ablation of Si and a metal via the neutral and ion yield, the energy distribution of emitted neutrals and the charge distribution as a function of the laser pulse width. Two processes, one leading to the ejection of fast (3–7 eV), the other to slow thermal particles, can be identified. The origin of the first process can be correlated with laser pulse widths (or pump–probe delays) and processes on a time scale below 100 fs. Results for Si confirm recent findings for Coulomb explosion (CE) and we show for the first time that CE exists as a mechanism of material removal from metals for ultra-short laser pulses.

Infrared femtosecond laser ablation of graphite in high vacuum probed by optical emission spectroscopy

We report infrared, ultra-short (780 nm, 120 fs) laser ablation of graphite in high vacuum. The plume characteristics are analyzed by wavelength-, time-, and spatially resolved optical emission spectroscopy. A multi-component structure of the plume is observed as a function of time, space, and laser fluence: (i) line emission from electronically excited carbon neutrals and ions; (ii) luminescence from excited C3 radicals; (iii) broadband visible radiation, ascribed to black-body-like emission from larger carbon clusters. The analysis of the graphite plume optical emission indicates the existence of two different ablation regimes, with the emission of large graphite fragments at low fluences and carbon radicals and atoms at larger fluences, in agreement with the theoretical description of ultra-fast ablation of graphite.

XRD studies on the femtosecond laser ablated single-crystal germanium in air

Ultrashort laser ablation of single-crystal germanium has been performed in air with femtosecond laser pulses (150 fs, 1 kHz) of 810 nm in the laser fluence range of 0.7–35.4 J/cm 2. Ablation depth dependence on the laser fluence shows that there are two different processes, which are explained in terms of electronic heating process and the optical penetration one. Structure of ablated region is characterized by means of two different XRD techniques. With increasing the laser fluence higher than 10.2 J/cm 2, the laser-processed region of germanium exhibits poly-crystalline diffraction peaks in a wide-angle ( θ / 2 θ ) scan and a split of diffraction peak of (4 0 0) plane in the rocking curve, which are absent in the lower laser fluence. These observations could be explained in terms of structural changes induced by ultrashort laser irradiation at the higher laser fluence.

Mechanism of ultrashort laser ablation of metals: molecular dynamics simulation

A theoretical model is developed and a molecular dynamics simulation technique is applied for the description of ultrashort laser ablation of metals. The ablation of Al, Ni, and Fe using 0.1, 0.5 and 5 ps laser pulses at wavelengths of 248 and 800 nm is studied. The process is investigated at fluences up to 0.5 J/cm 2. The analysis based on the temporal evolution of the ablation, the temperature, and the pressure distributions into the material reveals that a thermo-mechanical mechanism (spallation) takes place near the threshold. However, phase explosion is found to be the dominant mechanism of material removal at fluences higher than several hundreds of mJ/cm 2. The influence of the laser parameters (wavelength and pulse duration) is obtained and discussed. The ablation depth as a function of the laser fluence and the ablation threshold value are evaluated and compared with the experimental data available. Good agreement between the theory and experiments is observed.

Molecular dynamics simulation using pair and many body interatomic potentials: ultrashort laser ablation of Fe

The influence of the pair Morse potential and the embedded-atom method (EAM) based potential on the description of the properties and the ultrashort laser ablation process of Fe by molecular dynamics (MD) simulation technique is studied. The accuracy of both potentials in evaluation of the melting temperature, the linear thermal expansion coefficient and the compression behavior of Fe is estimated. The EAM potential is found to give more precise description of the properties of Fe than the Morse one. The melting temperature is overestimated only 2% in the case of EAM and 24% for the Morse potential, respectively, compared to the experimental value. A discrepancy in the presentation of the elastic properties is also observed, as it is more pronounced in the case of Morse potential.The MD simulation of laser ablation of iron is performed for ultrashort laser pulses (λ=800nm) with duration of 100fs and at fluences below the ablation threshold up to 1J/cm2. Analysis of the mechanism of ablation, evolution of the process, the temperature and the pressure distributions in the material is made.

Dynamics of the ejected material in ultra-short laser ablation of metals

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm2. The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking.

Numerical study of ultra-short laser ablation of metals and of laser plume dynamics

Numerical modeling is used to investigate the physical mechanisms of the interaction of ultra-short (subpicosecond) laser pulses with metallic targets. The laser-target interaction is modeled by using a one-dimensional hydrodynamic code that includes the absorption of laser radiation, the electronic heat conduction, the electron-phonon or electron-ion energy exchange, as well as a realistic equation of state. Laser fluences typical for micromachining are considered. The results of the ID modeling are then used as the initial conditions for a 2D plasma expansion model. The dynamics of laser plume expansion in femtosecond regime is investigated. Calculations show that the plasma plume is strongly forward directed. In addition, a two-peaked axial density profile is obtained for 400 nm laser wavelength. The calculation results agree with the experimental observations.

Modelling of ultrashort laser ablation of gold films in vacuum

Based on the concept of phase explosion, an enthalpy form of the two-step heating equations is proposed to model the superheating and material ablation of metals caused by ultrashort-pulse lasers. Numerical calculation of the electron and lattice temperatures is performed for gold films heated by a sub-picosecond UV laser in vacuum. When the lattice temperature at a material point equals 0.9Ttc (Ttc denoting the thermodynamic equilibrium critical temperature), phase explosion is assumed and that material point is removed. The calculated ablation depth matches well with experimental data.

Emission spectroscopy of aluminum nitride plasma plume induced by ultra-short pulsed laser ablation

The plasma plume expansion process of an AlN target ablated by an ultra-short Nd:glass pulsed laser (200 fs, 1.2 ps) has been surveyed by emission spectroscopy and fast imaging. The data have been collected with spatial and temporal resolution in order to find out the species speed and their angular distribution. Furthermore, it has been possible to discriminate different regimes involved during the plasma expansion by comparing the data so obtained with previous studies performed with ns laser sources. These results could also justify the cluster formation, detected by a quadrupole mass spectrometer, observed during the ultra-short laser ablation regime, but not for the ns one.

Ultrashort laser ablation of metals: pump–probe experiments, the role of ballistic electrons and the two-temperature model

The dynamics of laser ablation from metallic surfaces (Ag, Al, Fe and Ni) induced by the combined effect of two 30 fs sub-threshold laser pulses has been examined. In a pump–probe setup the yield of the emitted secondary ions and neutrals has been determined as a function of the delay between the two laser pulses. The instantaneous generation of highly excited (ballistic) electrons by the laser pulse and the thermal properties of the metal, which have been modified to be valid into the regime of high electron temperatures have been found to be determining factors for the ablation process. Unexpectedly, two distinct maxima for particle emission have been observed as a function of the time separation of the pump and the probe pulse. The energy relaxation is discussed within the frame of the two-temperature model (TTM) and it is shown that the measured behavior (in the time domain) of ablated particles can only be explained by taking into account a general expression for the thermal conductivity, valid for a wide range of electron temperatures and in addition a substantial role of hot, ballistic electrons.