Two-dimensional imaging of atomic and nanoparticle components in copper plasma plume produced by ultrafast laser ablation

We report on the spatial and temporal evolution of the plume generated during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. Time-gated imaging was used to record 2-dimensional images of plume populations. The temporal evolution of neutral (Cu*), and ionic (Cu⁺) components of the plume are separately imaged by exploiting bandpass interference filters, while nanoparticles are investigated by collecting their characteristic broadband emission. The ionic component of the plume moves two to three times faster than the neutral component, with a velocity which is almost independent of laser fluence. Plume emission intensity variations at different fluences and delay times are studied for both atomic and nanoparticle components.     

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. PACS 52.50.Jm; 61.80.Az; 72.15.Cz; 79.20.Ap; 79.20.Ds     

Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation

We report an experimental analysis of the plasma plume produced during ultrafast laser ablation of a copper target, in high vacuum. The plasma plume optical emission is studied by using a hybrid time-gated imaging technique which allows obtaining simultaneous information on the spectral and spatial characteristics of the emitting species. We used both single and double pulse ablation scheme, observing their influence on the characteristics of the ablated atomic species.     

Angular distributions of emitted particles by laser ablation of silver at 355 nm

2 to 8 J/cm². The distribution is strongly peaked in the forward direction corresponding to cos^pθ, where p varies from 5 to 12 for the largest beam spot, but is less peaked for the smallest beam spots. The total collected yield of ablated atoms is about 2×10¹⁵ Ag atoms per pulse for the highest pulse energies. Received: 28 April 1997/Accepted: 8 September 1997     

Ion dynamics in ultrafast laser ablation of copper target

We investigate the angular distribution and average kinetic energy of ions produced during ultrafast laser ablation （ULA） of a copper target in high vacuum. Laser produced plasma （LPP） is induced by irradiating the target with Ti：Sapphire laser pulses of -50 fs and 800 nm at an angle of incidence of 45°. An ion probe is moved along a circular path around the ablation spot, thereby allowing characterization of the time-of-flight （TOF） of ions at different angles relative to the normal target. The angular distribution of the ion flux is well-described by an adiabatic and isentropic expansion model of a plume produced by solid-target laser ablation （LA）. The angular width of the ion flux becomes narrower with increasing laser fluence. Moreover, the ion average kinetic energy is forward-peaked and shows a stronger dependence on the laser pulse fluence than on the ion flux. Such results can be ascribed to space charge effects that occur during the early stages of LPP formation.     

Nanoparticle plume dynamics in femtosecond laser ablation of gold

This paper describes some recent results on femtosecond laser ablation of gold. We have studied both the fast vapour/plasma and slow nanoparticle plumes using Langmuir probe, time-resolved ICCD imaging and time-resolved optical absorption measurements. The nanoparticle plume dynamics was analysed by comparing the optical emission absorption measurements with an adiabatic isentropic model of ablation plume expansion, leading to an estimate of the amount of material in the nanoparticle plume.     

Plasma plume induced during laser ablation of graphite

The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×10⁴ and 3.8×10⁴ m s⁻¹, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×10²³ m⁻³ 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×10²² m⁻³ at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.     

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利用时空分辨的测量技术,测定了XeCl紫外激光烧蚀金属Cu消融粒子的发射光谱随时间与空间的强度分布。通过在不同的氢气压强下拍摄到的XeCl308nm紫外激光烧蚀金属Cu消融粒子的发光羽照片,发现激光消融粒子发光羽的颜色在不同区域有不同的颜色,不同区域的发光羽颜色随环境气压的改变而变化。随环境气压的增大,发光羽不仅逐渐变小,而且逐渐变淡。对激光消融粒子发光羽的机理进行了探讨,激光消融粒子发光羽的发光动力学模式在不同区域有不同的发光模式,且随环境气压的变化而改变。定性地解释了所观察的实验现象。     

Analysis of plume deflection in the silicon laser ablation process

Changes in target surface morphology and ablation plume direction have been experimentally observed during the initial stages of the silicon laser ablation process. A relationship between both phenomena can be observed upon analysing the temperature field induced by the laser beam in a rough surface material. Theoretical studies on the deflection of the ablation plume are presented. These analyses are based on the hypothesis that particles that reach evaporation temperature will exit normally to the target surface with a velocity that is proportional to the surface temperature and the amount of the ablated material. Numerical solutions and experimental results of laser ablation process of silicon targets are found to agree with theoretical studies. PACS 42.25.Lc; 79.20.Dc; 02.70.Dc     

Effect of ZnO plasma plume dynamics on laser ablation

The dynamic behaviors and optical properties of a ZnO plasma plume produced by pulsed laser ablation using a Nd:YAG laser (wavelength: 532 nm, pulse width: 3 ns) were studied by fast photography using a commercial gated charge coupled device (CCD) camera linked with a delay circuit and by optical emission spectroscopy at various ambient oxygen pressures. Fast photography was conducted with a resolving power of 0.25 μs and the expansion behaviors of the laser ablation plume were observed. Plasma plume expansion velocity decreased with oxygen partial pressure. The flow of the plasma plume in the early stage of expansion of up to 3 ms agreed well with the drag model.     

Measurement of Er:YAG laser ablation plume dynamics

The dynamics of tissue ablation using an Er:YAG laser were studied using flash photography and optical pump-probe techniques. Both normal-spiking-mode and Q-switched Er:YAG laser radiation were used to study the ablation of skin and bone. Time-resolved photographs of the ablation plume were obtained using a microscope-mounted camera together with pulsed illumination from an excimer-pumped dye laser. The velocity of the plume front, obtained from the photographs, was approximately 1400 m/s. The same velocity was also measured using an optical pump-probe technique. Both techniques indicate that material removal occurred after the end of the 90-ns-long Q-switched laser pulse and that each micropulse in the normal-spiking-mode pulse train was capable of ablating and rapidly ejecting tissue.This work was supported in part by the SDIO-MFEL Program under contract # N00014-86-K-0117 and by the Arthur O. and Gullan M. Wellman Foundation     

Modeling of plume dynamics with shielding in laser ablation of carbon

The process of laser ablation of carbon in presence of background gas is simulated numerically. The plume dynamics in laser ablation is important to study for many reasons including temperature of plume particles and shielding of target by previously ablated plumes. Shielding leads directly to the change in energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. Carbon ablation is studied for single and multiple laser hits typical for synthesis of nanotubes. Two models of correction of ablated velocity and pressure resulting from shielding effect are proposed and investigated. Numerical modeling of this plume dynamics and its integral effect of shielding is challenging due to inherent high nonlinearity of the problem. Some of available numerical techniques handles nonlinearity but are dissipative, e.g. Godunov type schemes. Other techniques are less dissipative but fail to account for strong nonlinearity typical for initial stages of ablation, e.g. the ENO-Roe. To effectively model this highly nonlinear plume dynamics a combination of two of above mentioned schemes is developed so as the numerical evaluation of fluxes is close to their physical values and the scheme has minimum dissipation. The non-monotonic behavior of ablated mass as a function of time duration between two laser pulses is studied.     

Dynamics and spectroscopy of the laser plume from solid targets

The dynamics and the spectral kinetic characteristics of the plume emerging in the vicinity of graphite targets, pressed pellets consisting of zirconium oxide powder stabilized with yttrium (YSZ) and yttrium-aluminum oxides with neodymium (YAO:Nd), and single-crystal YAG:Cr are studied. The targets are irradiated in air at room temperature using a repetitively pulsed CO₂ laser with a wavelength of 10.6 μm, a peak power of up to 9 kW, a pulse energy of 1.69 J, and a pulse duration of 330 μs at a level of 0.1. The plume propagates normally to the target surface at an angle of 45° relative to the laser radiation. The spectral kinetic characteristics of the plume luminescence are discretely measured along the entire length. It is demonstrated that the plumes of all targets (except for the single-crystal YAG:Cr) represent the flows of a weakly nonequilibrium gas plasma with a temperature of 10 kK (graphite) and 3.1–4.7 kK (YSZ and YAO:Nd pressed pellets). The plume size is determined by the peak power of the laser pulse. The luminescence of the two-atom radicals (C₂ in graphite; ZrO and YO in YSZ; and YO, AlO, and NdO in YAO:Nd) dominates in all of the plumes. A single radical (YO) and the spectral lines of atoms and atomic ions are observed in the YAG:Cr plume. A relatively high temperature of the graphite plume is maintained owing to the energy of the exothermic reaction involving the association of carbon atoms and the energy of the vibrationally excited molecules resulting from this reaction. Original Text ? Astro, Ltd., 2006.     

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 (sub-picosecond) 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 1D 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. PACS 52.38.Mf; 02.60.Cb     

Angular distributions of fast electrons, ions, and Bremsstrahlung x/gamma-rays in intense laser interaction with solid targets

We study the angular distributions of fast electrons, ions, and bremsstrahlung x/ gamma-rays generated during the interaction of an ultrashort intense laser pulse with solid targets. A relation is found on the angular directions for fast electrons and ions as a function of the particle's kinetic energy, experienced Coulomb potential changes, and the incident angle of the laser pulse. It is valid independent of the acceleration mechanisms and the polarization of the laser pulse, as confirmed by particle-in-cell simulations. The angular distribution of bremsstrahlung x/gamma-rays is presented to show explicitly its correlation with the corresponding angular distributions of electrons.     

Molecular dynamics simulation of ultrafast laser ablation of fused silica film

Ultrafast laser ablation of fused silica is studied using molecular dynamics simulations. Ionization and generation of free electrons, absorption of the laser energy by free electrons and energy coupling between free electrons and ions are considered. The BKS potential is applied and modified to describe molecular interactions and the effect of free electrons. Smooth particle mesh of the Ewald method (SPME) is adopted to calculate the Coulomb force. It is found that the electrostatic Coulomb force, which is caused by the ionization, plays an important role in the laser ablation process.     

Studies of craters’ dimension for long-pulse laser ablation of metal targets at various experimental conditions

Long pulse laser shots of the PALS iodine laser in Prague have been used to obtain metal target ablation at various experimental conditions. Attention is paid mainly to the dependencies of the crater diameter on the position of minimum laser-focus spot with regard to the target surface, by using different laser wavelengths and laser energies. Not only a single one, but two minima, independently of the wavelength, of the target irradiation angle and of the target material, were recorded. Significant asymmetries, ascribed to the non-linear effects of intense laser beam with pre-formed plasma, were found, too. Estimations of ejected mass per laser pulse are reported and used to calculate the efficiency of laser-driven loading. Results on metal target ablation and crater formation at high intensities (from 2 × 10¹³ to 3 × 10¹⁶ W/cm²) are presented and compared. Crater depth, crater diameter and etching yield are reported versus the laser energy, in order to evaluate the ablation threshold fluence.     

Probe studies of laser erosion plume arising at silicon ablation in vacuum

An erosion plume arising at the ablation of silicon by a solid-state laser (λ = 1.06 μm) is studied with a Langmuir probe. The time-of-flight curves of the probe ion current are obtained for a plasma beam formed by intersecting plumes from two targets and for an erosion plume from one silicon target. The probe-target distance is varied in the range 40–157 mm. The time-of-flight curves for the ions of the erosion plume are sums of the velocity one-dimensional Maxwell distributions for four groups of ions. It is found that a plasma beam formed by intersecting plumes from two targets does not contain all groups of ions present in initial plumes.