Dynamics of ions produced by laser ablation of ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al at 193 nm

We study the dynamics of ions produced upon ablation of Al and ceramic Al₂O₃ targets using nanosecond laser pulses at 193 nm (6.4 eV) as a function of the laser fluence from threshold up to 12 J cm⁻². An electrical (Langmuir) probe located at 40 mm from the target surface has been used for determining the ion yield and calculating the kinetic energy distributions. The results for both targets show the existence of a significant amount of ions having kinetic energies >200 eV (≈20% around threshold fluence), and kinetic energies are up to >1.5 keV. The results are related with the existence of direct photonionization processes caused by the photon energy of the laser being higher than the ionization potential of Al (5.98 eV). Comparison of the ion yield when ablating the two types of targets for fluences above threshold to data reported in the literature suggests that the magnitude of the yield and its threshold are parameters depending on the thermal properties of the target rather than on the laser wavelength. Around threshold, the different behavior of ion yield when ablating Al and Al₂O₃ targets suggests that the threshold for neutral aluminium and ion species in the case of ablation of the Al₂O₃ target must be similar.     

Damage/ablation morphology of laser conditioned sapphire under 1064 nm laser irradiation

The damage/ablation morphologies and laser induced damage threshold (LIDT) of three different sapphire samples: original, 1064 nm laser conditioned and 10.6 μm CO₂ laser polished substrates are investigated with ns pulses laser irradiation. The results indicate that the damage resistance capability cannot be enhanced by 1064 nm laser conditioning or CO₂ laser polishing. The damage/ablation morphology of 1064 nm-laser conditioned samples is same as that of the original sapphire. But it is different from the damage/ablation morphology of the CO₂ laser polished sapphire. The “gentle and strong” ablation phases are observed in this work and several phenomena are observed in the two ablation phases. Ripple is observed in the “gentle” ablation processes, while convex spots and raised rims are observed in the “strong” ablation processes. Meanwhile, stripe damage and pin-points are observed in the CO₂ laser conditioned sapphire after ns laser irradiation. The formation mechanisms of the phenomena are also discussed for the explanation of related damage/ablation morphology. The results may be helpful for the damage/ablation investigation of sapphire in high power laser systems.     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

UV laser-induced grating formation in PDMS thin films

Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxane (PDMS) thin films is demonstrated. Ellipsometric measurements provide values of the optical constants of the films as well as their thicknesses, which are below 1 m. At fluences above 160 mJ/cm² two pulses of UV light induce gratings with at minimum 1-m periods and crossed gratings with 4-m periods. The structure heights are between 10 nm and 20 nm with ridge widths of several hundred nanometres. The ablation occurs after a single incubation pulse with a threshold that increases logarithmically with the ablation wavelength increasing from 157 nm to 1064 nm. At 193 nm the ablation rate for 2 J/cm² is 127 nm/pulse. PACS 79.20.La; 34.50.Dy; 68.55.Jk     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².     

Picosecond laser ablation of nickel-based superalloy C263

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F _th(D1)¹=0.68±0.02 J/cm² and F _th(D2)¹=2.64±0.27 J/cm² from data of the crater diameters D _1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low.     

Dynamics of phase transitions induced by femtosecond laser pulse irradiation of indium phosphide

The structural transformation dynamics of single-crystalline indium phosphide (InP) irradiated with 150 fs laser pulses at 800 nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150 fs up to 500 ns. The results obtained show that for fluences above a threshold of 0.16 J/cm² thermal melting of the material occurs on the timescale of 1–2 ps. The evolution of the reflectivity on a longer timescale reveals the reflectivity of the liquid phase and shows resolidification times typically around 10–30 ns after which an amorphous layer several tens of nanometers thick is formed on the surface. This amorphous layer significantly alters the optical properties of the surface and finally leads to a reduced ablation threshold for subsequent laser pulses. Single-pulse ablation at higher fluences (>0.23 J/cm²) is preceded by an ultrafast phase transition (non-thermal melting) occurring within 400 fs after the arrival of the pulse to the surface. PACS 79.20.Ds; 78.47.+p; 64.70.-p     

Ablation of silicon suboxide thin layers

We investigate the ablation of SiO _x thin films on fused silica substrates using single-pulse exposures at 193 nm and 248 nm. Two ablation modes are considered: front side (the surface of a film is irradiated from above) and rear side (a film is irradiated through its supporting substrate). Fluence is varied from below 200 mJ/cm² to above 3 J/cm². SiO _x films of thickness 200 nm, 400 nm, and 600 nm are ablated. In the case of rear-side illumination, at moderate fluences (around 0.5 mJ/cm²) the ablation depth corresponds roughly to the film thickness, above 1 J/cm² part of the substrate is ablated as well. In the case of front-side ablation the single-pulse ablation depth is limited for all film thicknesses to less than 200 nm even at fluences up to 4 J/cm². Experimental results are discussed in relation to film thickness, fluence, and ablation mode. Simple numerical calculations are performed to clarify the influence of heat transport on the ablation process.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

All-optical ion generation for ion trap loading

We have investigated the all-optical generation of ions by photo-ionisation of atoms generated by pulsed laser ablation. A direct comparison between a resistively heated oven source and pulsed laser ablation is reported. Pulsed laser ablation with 10 ns Nd:YAG laser pulses is shown to produce large calcium flux, corresponding to atomic beams produced with oven temperatures greater than 650 K. For an equivalent atomic flux, pulsed laser ablation is shown to produce a thermal load more than one order of magnitude smaller than the oven source. The atomic beam distributions obey Maxwell–Boltzmann statistics with most probable speeds corresponding to temperatures greater than 2200 K. Below a threshold pulse fluence between 280 mJ/cm² and 330 mJ/cm², the atomic beam is composed exclusively of ground-state atoms. For higher fluences ions and excited atoms are generated.     

KrF pulsed laser ablation of polyimide

In the present paper, polyimide surfaces were processed with pulsed KrF laser radiation at fluences near the ablation threshold. The morphology of the processed surfaces was studied by scanning electron microscopy and chemical analyses performed by electron dispersive spectroscopy. The formation of conical structures was observed for radiation fluences lower than 0.5 J/cm². The areal density of cones increases with the number of pulses and decreases with the radiation fluence. At low fluences (<150 J/cm²), cones are formed due to shadowing by calcium phosphate impurities while for higher fluences the main mechanism of cones formation is believed to be radiation hardening.     

Ablation of polymers by focused EUV radiation from a table-top laser-produced plasma source

We have investigated ablation of polymers with radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. A Schwarzschild objective with Mo/Si multilayer coatings was adapted to the source, generating an EUV spot of 5 μm diameter with a maximum energy density of ∼1.3 J/cm². In combination with a Zirconium transmission filter, radiation of high spectral purity (2% bandwidth) can be provided on the irradiated spot. Ablation experiments were performed on PMMA, PTFE and PC. Ablation rates were determined for varying fluences using atomic force microscopy and white light interferometry. The slopes of these curves are discussed with respect to the chemical structure of the polymers. Additionally, the ablation behavior in terms of effective penetration depths, threshold fluences and incubation effects is compared to literature data for higher UV wavelength.     

Femtosecond laser ablation of cemented carbides: properties and tribological applications

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800 nm, 100 fs). Small and moderate fluences (2, 5, 10 J/cm²) and up to 5×10⁴ pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2 J/cm², whereas for 5 and 10 J/cm² irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2 J/cm² and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviour. PACS 42.65.Cs; 79.60.Ds     

Incubation in the UV irradiation of condensed CHCl<Subscript>3</Subscript> solids

Matrix-Assisted-Pulsed-Laser-Evaporation (MAPLE) has emerged as a very promising technique for the deposition of polymers and biopolymers in intact and functional form. However, our understanding of the mechanism of the procedure is still limited. Here, we examine laser-induced (248 nm) desorption from condensed CHCl₃ solid, which has been employed as a potential matrix in MAPLE. We find that the absorption of the condensed halocarbon increases significantly with successive laser pulses, as a result of the formation and accumulation of strongly absorbing products. This results in a significant increase of the ejection efficiency in the irradiation with successive laser pulses. Thus, in studies employing multi-pulse irradiation protocols, the attained laser-induced temperatures are considerably higher than what is estimated on the basis of the absorption coefficient of CHCl₃. Thus, contrary to previous suggestions, ablation of CHCl₃ frozen solid at 248 nm may be due to explosive boiling. A number of additional implications are also discussed.     

Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pump-probe microscope. It allows recording images of the laser-excited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within ≈1.5 ps, while the normalized transmission decreases within ≈400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma is ≈50 ps, and its maximum electron density amounts to 5.5×10²² cm⁻³.     

Femtosecond laser written stress-induced Nd:Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (Nd:YAG) channel waveguide laser

Single tracks and pairs of tracks were written into undoped and Nd-doped YAG crystals using a commercial femtosecond laser system delivering pulses with pulse duration of 140 fs and pulse energies up to 10 μJ. The pulses were focused by a 50× microscope objective below the surface of the crystals. Due to the elasto-optical effect, stress-induced birefringence was observed in domains surrounding the single tracks and between the pairs of tracks. Waveguiding was demonstrated in certain channels in these domains. To investigate the underlying guiding mechanism highly selective chemical etching of the modified material was performed with etching rates up to 5 μm/h. Pumped at 808 nm, laser operation at a wavelength of 1064 nm was achieved. The maximum output power was 25.5 mW at 261 mW of launched pump power with a slope efficiency of 23%.     

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20?J/cm²). Experiments with picosecond pulses at 355?nm are carried out for all five metals and in addition at 532?nm, and 1064?nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355?nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.     

Pulsed laser-induced ablation of absorbing liquids and acoustic-transient generation

2 CrO₄ are irradiated by a KrF excimer laser (λ=248 nm, FWHM=24 ns) with moderate energy density (up to 100 MW/cm²) below the plasma-formation threshold. The ablation process, including the vapor-cavity formation and the acoustic-wave propagation is visualized by laser-flash photography. The ablation thresholds are determined by measuring the generated pressure transients and vapor-phase kinetics using a broadband piezoelectric pressure transducer and a simultaneous optical-transmission probe, respectively. The mechanisms of liquid ablation and acoustic-pulse generation are investigated based on the thermoelastic behavior of the liquid medium and the evaporation dynamics. A numerical model is proposed to describe the explosive-vaporization process at high laser fluences. The computation results are compared with the experiment. In short-pulse heating, ablation can be initiated at low laser fluences by the tensile component of the thermoelastic stress without a significant increase in the liquid temperature. On the other hand, if the heating rate is rapid enough to achieve a high degree of superheating of the liquid, the abrupt increase of the homogeneous-bubble-nucleation rate leads to explosive vaporization, which then plays the major role in the ablation dynamics. The pressure transient in the liquid is generated thermoelastically at low laser fluences, but the contribution of the vapor-phase expansion and/or the recoil momentum exerted by the ablation plume becomes significant at high laser fluences. Shock waves are formed in the ambient air in the case of explosive vaporization. The propagation of these wave fronts is in good agreement with the numerical-computation results. Received: 8 February 1998/Accepted: 10 February 1998