Negative-ion emission during laser ablation of multicomponent materials

The time of flight technique coupled with an electrostatic energy filter has been used for composition and energy-distribution analysis of the ion species emitted during laser ablation of multicomponent materials Y-Ba-Cu-O, Pb-Sn-Te. The negative-ion output and kinetic-energy distribution as a function of the laser wavelength and the laser fluence on the target were measured. A high output of the negative ions of matrix elements comparable with the positive-ion emission was detected. The barium negative-ion formation was observed in spite of the negative electron affinity of alkaline earth elements in the ground electronic state. The mechanism of negative-ion formation based on the ternary collisional recombination in the laser plasma is analyzed.     

Energy distribution of ions produced by excimer-laser ablation of solid and molten targets

2 . The measurements reveal components with different charge-to-mass ratio and distinct components with the same charge/mass ratio. The most probable kinetic energy has values of several tens of eV for singly charged ions, and is larger by a factor exceeding 2 for doubly charged ions. The role of target material and state, solid or liquid, laser photon energy and fluence has been investigated and is discussed in comparison to the findings of previous investigations. An estimate of the electrostatic plasma potential in PLA, based on electron loss rate arguments, is presented to account for the high ion energies observed. Received: 9 March 1998 / Accepted: 27 November 1998 / Published online: 24 February 1999     

Direct observation of the temperature field during ablation of materials by multiple femtosecond laser pulses

We report a direct observation of the temperature field on a steel specimen during ablation by multiple femtosecond laser pulses using an infrared thermography technique. From the experimental results and simulation study of the temperature field, we quantified the deposited thermal power into the specimen during the ablation process. We found that more than two thirds of the incident laser power was deposited in the steel specimen when ablated by multiple femtosecond laser pulses. This result provides further understanding of the heating effect in materials processing by ultrashort laser pulses.     

Observation of magnetic-field-enhanced excitation and ionization in the plume of KrF-laser-ablated magnesium

The influence of a magnetic field on the plume produced by KrF-laser ablation of magnesium in vacuum has been investigated using time-integrated photography, streak photography, spectroscopy and charge probes. Line emission spectra in the 200–600 nm interval and effective stream velocities for the plume obtained from the spatiotemporal emission from specific neutral and ion lines are reported. Time of flight velocities are also deduced from measurements using simple charge-collector probes. Changes in the plume structure and dynamics, and enhanced emission and ionization are observed in the presence of the field. A qualitative explanation of the results is given in terms of a magnetohydrodynamic model.     

Pattern transfer and photoluminescence damage assessment of deep-submicrometer features etched by photon-induced cryoetching

We present a study of pattern-transfer and etch-induced damage in photon-induced cryoetching. Features with effective radii as small as 100 nm have been formed in both bulk and layered GaAs/AlGaAs materials. A measurement of the photoluminescence of etch-defined deep- submicrometer structures material suggests that this form of etching results in minimal process-induced damage. Modeling of the luminescence vs feature size for these features shows that the luminescence is limited only by carrier diffusion and non-radiative surface recombination.     

UV-laser ablation of ductile and brittle metal films

Single-shot laser damage of Ni and Cr films on fused silica substrates has been studied as a function of film thickness, utilizing 248 nm/14 ns pulses and detection by probe beam deflection. Threshold fluences for visible damage and vaporization are compared to predictions of the heat diffusion model. The model fits thresholds for visible damage well and identifies their origin, which is melting for Ni films and brittle-to-ductile phase transition for Cr films. When predicting thresholds for vaporization, the diffusion model is of limited success in case of Ni films but fails completely for Cr films, indicating that transient thermal properties of the material should be taken into account. Microscopic inspection shows that Cr films rupture at low fluences before entering the common sequence of melting and vaporizing with increasing fluence. 17 December 1996/Accepted: 17 December 1996     

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

Characteristics of thermionic electron emission during femtosecond laser ablation of gold film are studied numerically. For the rigorous calculation of electron and lattice temperatures, an enhanced two-temperature model with transient thermal and optical properties is developed and it is demonstrated that the model predicts the damage threshold fluences closely matching experimental data. From the calculated electron emission characteristics, quantum efficiency and electron current density are estimated.     

Exploration of pulse timing for multiple laser hits within a combined heat transfer, phase change, and gas dynamics model for laser ablation

Laser ablation involves heat transfer, phase changes and/or chemical reactions, and gas dynamics. All three of these processes are tightly coupled with each other. A model has previously been developed to simulate the nanosecond scale laser ablation of carbon. This model has been extended to accommodate longer term simulations and multiple laser pulses. The effects of varying the timing of a second laser pulse by tens of nanoseconds are explored. It is shown that by changing this interval one can control the total mass ablated and the mass transfer rate.     

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.     

Ultrashort pulse laser ablation of silicon: an MD simulation study

Received: 30 July 1997/Accepted: 5 August 1997     

Pulsed laser ablation of solids: transition from normal vaporization to phase explosion

We present experimental data on mass removal during 1064-nm pulsed laser ablation of graphite, niobium and YBa₂Cu₃O_7-δ superconductor. Evidence for the transition from normal vaporization to phase explosion has been obtained for these materials, showing a dramatic increase in the ablation rate at the threshold fluences of 22, 15 and 17.5 J/cm², respectively. A numerical model is used to evaluate the ablation rate and temperature distribution within the targets under near-threshold ablation conditions. The results are analyzed from the viewpoint of the vaporized matter approaching the critical point with increasing laser fluence. A possible means of the estimating the thermodynamic critical temperature from the data for nanosecond laser ablation is discussed. It is suggested that the critical temperature of refractory metals is higher than that estimated with the traditional methods due to plasma effects. An analogy with the boiling crisis (the transition from nucleate to film boiling) is drawn to explain the formation of ablation craters with spallated edges. Received: 18 May 2000 / Accepted: 14 July 2000 / Published online: 22 November 2000     

Porphyrin-sensitized laser swelling and ablation of polymer films

Laser-induced morphological changes of poly(methyl methacrylate), poly(N-vinylcarbazole), and gelatin films doped with porphyrins have been studied by etch depth measurement and scanning electron microscopy. An irreversible swelling of the irradiated surface was observed for all films in the case of low laser fluence. The swelling was replaced by ablation when the fluence was increased. The etch depth depends on the irradiation fluence and the dye concentration in the polymer. The observation of the irradiated surfaces suggests that the thermal effect is predominant both for swelling and ablation. The surface temperature at which swelling or ablation is initiated was estimated, assuming that these morphological changes take place at a certain temperature for any dye concentration in each polymer film.     

Fabrication of Mg-doped ZnO thin films by laser ablation of Zn:Mg target

Mg-doped ZnO thin films were fabricated by laser ablation of Zn:Mg targets consisting of Mg metallic strips and Zn disk in oxygen atmosphere with a goal to facilitate convenient control of Mg contents in the films. The characteristics of the deposited films were examined by analyzing their photoluminescence (PL), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) spectra. Mg contents as analyzed by XPS indicate that the target composition is fairly transferred to the deposited films. The wurtzite structure of ZnO was conserved even for the highly doped ZnO films and there was no Mg- or MgO-related XRD peaks. With increase in the Mg content, the bandgap and PL peak energy shifted to blue and the Stokes shift became larger.     

Depth-profile investigations of triterpenoid varnishes by KrF excimer laser ablation and laser-induced breakdown spectroscopy

The ablation properties of aged triterpenoid dammar and mastic films were investigated using a Krypton Fluoride excimer laser (248 nm, 25 ns). Ablation rate variations between surface and bulk layers indicated changes of the ablation mechanisms across the depth profiles of the films. In particular, after removal of the uppermost surface varnish layers there was a reduction of the ablation step in the bulk that was in line with a significant reduction of carbon dimer emission beneath the surface layers as detected by laser-induced breakdown spectroscopy. The results are explicable by the generation of condensation, cross-linking and oxidative gradients across the depth profile of triterpenoid varnish films during the aging degradation process, which were recently quantified and established on the molecular level.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

Detection of neutral products formed during excimer laser ablation of polyimide by UV and VUV laser photoionization/mass spectrometry

Some of the neutral species which are produced in the laser ablation of polyimide have been characterized using multiphoton ionization/time of flight mass spectrometry. Three different wavelengths (193 nm, 157 nm, and 118 nm) have been used in an attempt to effect soft ionization of the products formed during or after the initial laser ablation of the polymer. Neutral photo-ablation products detected using this scheme range from atomic to high molecular weight species which, depending on the probe wavelength, include pure carbon clusters as well as a broad distribution of heteroatom containing clusters. However, there is virtually no overlap in the mass spectra recorded at each probe wavelength. When probing with 193 nm, marked changes are observed in the mass spectra as a function of the probe flux used. At moderate fluxes, pure carbon clusters (fullerenes) are observed. The identification of a large distribution of species other than pure carbon clusters is in dramatic contrast to the recent observation [W.R. Creasy, J.T. Brenna: Chem. Phys. 126, 453 (1988)] of the positively charged ionic species produced, which are solely carbon clusters. These results suggest that the neutral and ionic products observed after ablation of the polymer are due to both condensation of the atomic and molecular fragments which form during the ablation laser pulse and nascent polymer fragments. Various implications of this result for the unambiguous determination of the true ablation product distribution are discussed.     

Statistical properties of the laser emission in a low-Q cavity

We investigate theoretically the stationary statistical properties of the laser radiation in a low-Q cavity with field, polarization, and population fluctuations. Eliminating adiabatically the electric field from the Maxwell-Bloch equations, coupled Langevin equations with bothadditive andmultiplicative noises are derived and are transformed into the multivariable Fokker-Planck equation of a probability density of the light intensity and the population difference. It is solved by the expansion into orthonormal sets, and a vector recurrence equation of motion of the expansion coefficients is given whose stationary solutions are analytically obtained in theMatrix continued-fraction. The stationary distribution function of the radiation intensity are calculated with several values of control parameters. We discuss the variance of the intensity distribution, the photon-counting coefficient, and the cross-correlation between intensity and population as a function of the pump parameter, and reveal the novel and characteristic features of the bad-cavity laser system. The comparison with the good-cavity (high-Q cavity) case is also made.     

Observations and analysis of resonant laser ablation of GaAs

Laser ablation of solid GaAs samples has been studied using one tunable pulsed dye laser. At relatively low laser power, enhancements of up to several hundred times have been observed in the yield of resonantly ionised Ga using laser wavelengths corresponding to the atomic transition 4² P _1/2-4² D _3/2. The influences of laser power and target geometry, on the ion yield and spectral profile, are discussed. It is argued that the resonant excitation and ionisation processes occur in the gas phase of the atoms ablated from the sample surface, and the observed asymmetric spectral profile results from laser-induced collisional processes, e.g., optical collisions, under conditions of relatively high atomic density in the interaction region. Potential applications are foreseen for resonant laser ablation in trace analysis.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.