Dynamics of plume and crater formation after action of femtosecond laser pulse

Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluence F_inc of incident radiation exceeds a threshold of “evaporation” (F_inc)_ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold (F_inc)_abl. Thus the spallation exists within the limits (F_inc)_abl<F_inc<(F_inc)_ev. A laser beam has a maximum fluence (F_inc)_c on the axis of the beam. The threshold F_ev separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if F_abl<F_c<F_ev, and (2) with the shell having an aperture in the center (like the volcano muzzle) if F_c>F_ev.     

Ablated matter expansion and crater formation under the action of ultrashort laser pulse

The action of a subpicosecond laser pulse on a target made of an absorbing condensed substance is considered. The propagation of an electron heat conduction wave and the crystal lattice heating prior to the hydrodynamic expansion of the target are analyzed. In these initial interaction stages, a heated layer with a thickness of d _T is formed at the target surface. The dependence of d _T on the absorbed laser energy density F[J/cm²] is evaluated. The motion of ablated matter in the expansion stage is described using a numerical solution of the equations of gasdynamics and the results of molecular dynamics (MD) simulations. The MD simulations are performed using a large number (～10³) of parallel processors, which allows the number of model atoms to be increased up to a level (about 3.5 × 10⁷) close to that encountered under real experimental conditions.     

Nanoparticle formation during laser ablation of solids in liquids

The experimental data on the generation of metal and semiconductor nanoparticles during their laser ablation in liquids is reviewed. The dependence of the morphology of noble metal nanoparticles on the liquid type and laser parameters is discussed. The data on the kinetics of the formation of alloyed Au-Ag nanoparticles under laser irradiation of a mixture of colloid solutions of individual nanoparticles are presented. The effect of femtosecond laser beam self-action during metal ablation in liquids via the second harmonic generation at Ag nanoclusters is discussed. The data on the generation of core-shell nanoparticles during laser ablation of alloys and in the presence of the chemical interaction of formed nanoparticles with surrounding liquid are presented. It was shown that laser ablation of CdS and ZnSe crystals leads to the formation of quantum dots of these semiconductors in solution. The parameters controlling the properties of nanoparticles during ablation in liquids and possible applications of the method are discussed.     

A short pulse mode-locked Nd-glass laser

A mode-locked Nd-glass laser with a ring cavity configuration has been made. With this laser, using a resonator made with prisms instead of mirrors, a stable TEM₀₀ mode is obtained. Two photon measurements performed on the whole pulse train consistently yield a pulsewidth of 3.5 ps, with the proper contrast ratio 3:1.     

Thresholds of plasma formation in silicon identified by optimizing the ablation laser pulse form

Using an evolutionary algorithm combined with pulse shaping, we have identified that rapid plasma formation in Silicon can occur already at a fluence of about 150 mJ/cm2 if a substantial part of the laser energy is deposited efficiently around 200 fs after an exciting laser pulse. Nonthermal solid-to-liquid phase transition leads to the increase of the deposited energy in the material. Highly charged ions have been observed in the mass spectrum. While the pulse optimization procedure allowed us to identify the plasma formation, further experiments where the influence of the laser pulse width on the ablation yield was studied and Two-Pulse-Correlation experiments provided additional proof for the appearance of rapid plasma formation.     

Ripple formation in the chromium thin film during laser ablation

The beam of a nanosecond pulse laser tightly focused to a line was applied for the back-side ablation of the chromium thin film on a glass substrate. The stripe ablated with a single laser pulse had sharp edges on both sides and ridges of the melted metal around it. The partially overlapping pulses formed a wide cleaned area with a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were developed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the line ablated with a single laser pulse. The ripples were located periodically (∼4 μm) and were orientated perpendicularly to the long axis of the beam spot. Their orientation did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor-liquid-solid contact line during evaporation of thin liquid films appears to be the most probable process responsible for the observed phenomena. Formation of regular gratings with the unlimited line length was experimentally implemented by using the above-mentioned technique.     

Simulation of elastic displacement of surface during laser short pulse heating of gold

The laser short pulse heating initiates nonequilibrium heating of the substrate material, which in turn results in the thermal stresses developing in the region below the surface. The surface temperature can be measured possibly through the monitoring of the resulting surface displacement. This requires in detail investigation into the surface displacement and surface temperature rises across the heated spot during the laser short pulse heating process. In the present study, the laser short pulse heating of gold surface is considered and the temperature rise at the surface and elastic displacement of the surface are investigated. The spatial and temporal distributions of surface displacement and surface temperature are predicted and the elastic response of the substrate material due to temperature rise is explored. It is found that the temporal and spatial distributions of the surface displacement do not follow the temperature rise at the surface. Consequently, care should be taken when measuring the temperature rise at the surface by means of monitoring the surface displacement during a laser short pulse heating process.     

Pulse shaper assisted short laser pulse characterization

We demonstrate that a pulse shaper is able to simultaneously act as an optical waveform generator and a short pulse characterization device when combined with an appropriate nonlinear element. We present autocorrelation measurements and their frequency resolved counterparts. We show that control over the carrier envelope phase allows continuous tuning between an intensity-like and an interferometric autocorrelation. By changing the transfer function other measurement techniques, for example STRUT, are easily realized without any modification of the optical setup. PACS 42.65.Re; 42.30.Lr; 42.30.Rx     

KrF laser ablation of a polyethersulfone film: Effect of pulse duration on structure formation

Polyethersulfone (PES) films were processed with KrF laser irradiation of different pulse durations (τ). Scanning electron microscopy (SEM) and Raman spectroscopy were employed for the examination of the morphology and chemical composition of the irradiated surfaces, respectively. During ablation with 500 fs and 5 ps pulses, localized deformations (beads), micro-ripple and conical structures were observed on the surface depending on the irradiation fluence (F) and the number of pulses (N). In addition, the number density of the structures is affected by the irradiation parameters (τ, F, N). Furthermore, at longer pulse durations (τ = 30 ns), conical structures appear at lower laser fluence values, which are converted into columnar structures upon irradiation at higher fluences. The Raman spectra collected from the top of the structures following irradiation at different pulse durations revealed graphitization of the ns laser treated areas, in contrast to those processed with ultra-short laser pulses.     

Dynamics of formation and decay of supercritical fluid silver colloid under pulse laser ablation conditions

The effect of the density of supercritical carbon dioxide (SC-CO₂) on the dynamics of formation of supercritical fluid (SCF) silver colloids during pulse laser ablation and their post-pulse degradation was studied by in situ UV/vis absorption spectroscopy. Laser irradiation of a silver target in SC-CO₂ caused ablative formation of Ag nanoparticles of different shapes and sizes: quasi-spherical particles (~4 nm) and larger Ag nanoparticles (hundreds of nanometers). A change in the colloid density from 0.24 to 0.82 g/cm³ caused significant changes in the dynamics of ablative formation of large and small particles, the rate of aggregation of small Ag particles into large particles, and the rate of gravitation-induced sedimentation of nanoparticles in the SCF colloid.     

Surface ripple changes during Cr film ablation with a double ultrashort laser pulse

Surface modification is investigated experimentally by varying the time separation of double femtosecond laser radiation and surface ripples by varying the time separation and polarization direction of double pulses train. Nanometer-sized particles are formed during resolidification of the molten region when the second pulse arrives within 10 ps and the molten material is ejected much after 10 ps. The ripple in the outer region remains oblique to the sum of the vector direction of the two pulses when the time delay is zero. With time delay ranging from 0.5 to 10 ps and different polarization directions of the laser radiation, the ripple generally aligned perpendicular to the polarization direction of the electric field with multiple pulses in the vicinity of ablation threshold is effectively eliminated without fragments at the edge. Furthermore, remnant ripples on irradiated area at higher energies with the same polarization direction are removed by irradiation at a lower energy with each different polarization direction of double pulse. Based on morphological observations for different time delays, possible mechanisms of ripple formations and eliminations are suggested.     

Monitoring nanoparticle formation during laser ablation of graphite in an atmospheric-pressure ambient

Excimer laser ablation of highly oriented pyrolytic graphite (HOPG) was performed at atmospheric pressure in an N₂ and in an air ambient. During the ablation, nanoparticles condensed from the material ejecta, and their size distribution was monitored in the gas phase by a Differential Mobility Analyzer (DMA) in combination with a Condensation Particle Counter (CPC). Size distributions obtained at different laser repetition rates revealed that the interaction between subsequent laser pulses and formed particles became significant above 15 Hz. This interaction resulted in laser heating, leading to considerable evaporation and a decrease in the size of the particles. X-ray photoelectron spectroscopy revealed that approximately 8% nitrogen was incorporated into the CN_x particles generated in the N₂ ambient, and that the nitrogen was mostly bonded to sp³-hybridized carbon. Monodisperse particles were also deposited and were analyzed by means of Raman spectroscopy to monitor size-induced effects. PACS 81.07.-b; 61.46.+w; 79.70.+q     

Mechanisms of ultra-short laser pulse ablation from ionic crystals

In ultra-short laser pulse ablation from dielectric crystals two different ablation regimes are observed: 1) At laser intensities well below the single shot damage threshold, Coulomb explosion upon multiphoton surface ionization is the dominant process, yielding electrons and fast positive ions. 2) At higher laser intensities, ablation exhibits signs of hyper-thermal emission (phase explosion) as a result of rapid hot electron thermalization. This regime is characterized by massive etching, mostly of neutral particles. The transition from regime one to two is associated with the appearance of an additional, slower group of positive ions, saturation in the positive ion yield, and the occurrence of negative ions. PACS 61.80.Ba; 61.82.Ms; 79.20.Ds     

Pulsed laser ablation of GaAs using nano pulse length

Ablation using very short pulses has shown a great promise in facilitating the growth of complex multi-element films with stoichiometries matching those of their parent materials. GaAs is an important material in the electronic and opto-electronic industries and due to its compound structure it is an intriguing candidate for pulsed laser deposition. This work investigates the effect of nanosecond laser pulse lengths on the ablation of GaAs in an inert atmosphere. The number of pulses was varied in order to find the optimal condition for nano particles formation in our setup. The deposited structures were studied by grazing incidence small angle X-ray scattering and atomic force microscopy. It is shown that the GaAs nanoparticle sizes and size distributions can be controlled partly by the number of laser pulses applied in their production.     

Measurement of crater geometries after laser ablation of bone tissue with optical coherence tomography

The feasibility of measuring crater geometries by use of optical coherence tomography (OCT) is examined. Bovine shank bone on a motorized translation stage with a motion velocity of 3 mm/s is ablated with a pulsed CO2 laser in vitro. The laser pulse repetition rate is 60 Hz and the spot size on the tissue surface is 0.5 mm. Crater geometries are evaluated immediately by both OCT and histology methods after laser irradiation. The results reveal that OCT is capable of measuring crater geometries rapidly and noninvasively as compared to histology. There are good correlation and agreement between crater depth estimates obtained by two techniques, whereas there exists distinct difference between crater width estimates when the carbonization at the sides of craters is not removed.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Ultra short pulse laser annealing of FeB metallic glasses

We present the results of picosecond laser annealing of as-quenched Fe₈₅B₁₅ and Fe₈₂B₁₈ metallic glasses. The influence of the laser radiation on the surface and bulk properties are studied using CrK X-ray diffraction and transmission Mössbauer spectroscopy. The X-ray data show that the amorphous nature of the surface of the samples can be improved with laser treatment. The mat (cooling) surfaces of the ribbons appear to be more affected by the laser treatment, and show a higher stability. The Mössbauer data reveal that laser annealing of ribbon surfaces also affects the bulk properties of these materials due to induced stresses from the surface layer. The magnetic properties of these materials can be modified by laser annealing.     

Sub-micron ablation of metallic thin film by femtosecond pulse laser

The ability to machine very small features in a material has a wide range of applications in industry. We ablated holes into thin film of 100 nm thickness made from various metals by femtosecond pulsed laser ablation. Using a Ti:Sapphire laser which supplies a laser pulse of 150 fs duration at central spectrum wavelength of 400 nm, we have produced a series sub-micron holes, whose diameters are less than 200 nm with a focused laser spot of 1.7 μm. We found that the material damage threshold has a great influence on the quality of the produced features. Experimental results shows that the heat-affected zone and the degree of being affected reduce with the increase of threshold value.