Analysis of the nonlinear absorption mechanisms in ablation of transparent materials by high-intensity and ultrashort laser pulses

The mechanisms of nonlinear absorption in transparent materials under irradiation with ultrashort laser pulses are considered theoretically. Nitride semiconductor, sapphire and others transparent dielectrics were investigated. The ablation threshold for these materials is within multi-TW/cm² range. The model was used based on the tunneling absorption under the irradiation by high-intensity ultrashort pulses in terms of the theory of ionization of solid in a field of strong electromagnetic wave. The effect of the energy gap of material on the threshold of laser ablation was adequately explained.     

Surface modulation of silicon surface by excimer laser at laser fluence below ablation threshold

Controlled single step fabrication of silicon conical surface modulations on [311] silicon surface is reported utilizing KrF excimer laser [λ=248 nm] at laser fluence below ablation threshold laser fluence. When laser fluence was increased gradually from 0 to 0.2 J/cm² for fixed 200 numbers of shots; first nanopores are observed to form at 0.1 J/cm², then very shallow nanocones evolve as a function of laser fluence. At 0.2 J/cm², nanoparticles are observed to form. Up to 0.15 J/cm² the very shallow nanocone volume is smaller but increases at a fast rate with laser fluence thereafter. It is observed that the net material volume before and after the laser irradiation remains the same, a sign of the melting and resolidification without any ablation.     

Fabrication of compositionally graded thin films by gravity-assisted pulsed laser ablation

A compositionally graded thin film of FeSi₂ was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure was successfully produced under a gravity field of 5400 G. We demonstrate that the atomic fraction of Fe, the heavier component of the thin film measured by scanning electron microscope/energy dispersive X-ray (SEM-EDX), showed increasing spatial distribution with the direction of gravity. We found that optimal laser fluence exists to give a thin film having the largest possible spatial compositional gradient. We found that surface energy density on the substrate surface is the key parameter to control the composition distribution. Furthermore, the ratio of Fe/Si of the film did not match that of the target. This result shows that the Si component is selectively etched during the film-forming process. Relatively high laser fluence as well as a very narrow space between the target and the substrate are essential to etch the film once it is deposited, in order to re-ionize and etch Si selectively while gravity accelerates both Fe and Si particles to the direction of gravity. We hypothesize that this process accounts for both the change in the stoichiometry and the formation of composition distribution.     

Preparation of enamel coating by laser irradiation

Enamel coatings is produced on steel substrates using CW CO_2 laser irradiation from the same raw material as used for industrial enamel coating. The formed coating fairly well join to the substrate and has a bumpy glossy surface. Its microstructure is uniform and glassy with a liitle of small pores. It contains less Na and K than the initial composition. Fe element has been transfered into the coating, the content is high in the interface zone and low in the main body. This distribution may be at a time establish the good chemical stability of coating and its strong adhension to substrate. The present investigation also makes comparison of the laser irradiated coating with the ordinarily enamelled one.     

Deposition of nanoparticles during laser ablation of nanoparticle-containing targets

We analyze the morphology of ablated nanoparticles after their laser-induced deposition on various substrates. We show that, at moderate laser intensity of the 210 ps pulses on the surface of nanoparticle-containing materials (<5×10⁹ W?cm^?2), the deposited material remains approximately the same as the initial nanoparticles. We compare these deposited nanoparticles with the debris obtained by the laser ablation of bulk material of the same origin as nanoparticles at different intensities of laser radiation. The presence of nanoparticles in laser plumes allowed for analyzing high-order nonlinear optical properties of nanoparticles. The efficient high-order harmonic generation was achieved during propagation of femtosecond pulses through such plasmas.     

VUV 157 nm laser ablation of composite structures

We report on the laser ablation of composite prismatic structures using a vacuum ultraviolet (VUV) 157 nm F₂ laser. Polycarbonate and CR-39 substrates have been intentionally seeded with silver wires and silicon carbide whiskers respectively. The seed particles remain attached to the underlying substrate after laser ablation, forming composite silver-polycarbonate and silicon carbide-CR-39 interfaces. Strong optical absorption at 157 nm in the polymeric substrates allows precise control over the depth between the base of the substrate and composite interface. The surface roughness of the as-received seed particles has a significant effect on the final surface quality of the ablated structures. The textured surface on the silicon carbide whiskers is resolved on the walls of the ablated structures. This is in contrast to the composite structures formed using silver wires, which have a comparatively smoother surface.     

Synthesis of crystalline TiN and Si particles by laser ablation in liquid nitrogen

Laser ablation of titanium and silicon targets immersed in liquid nitrogen was carried out using a YAG laser at 1.06 μm. Synthesized particles were collected and were characterized by TEM, SEM, EDS, XRD, and XPS. In the case of a titanium target, the synthesized particles had an atomic ratio of N/Ti=0.4 and a polycrystalline structure with many XRD peaks of TiN. This result indicates the usefulness of laser ablation in liquid nitrogen for synthesizing nitrides. On the other hand, in the case of a silicon target, the nitridation of the synthesized particles was negligible, and the synthesized particles had a polycrystalline structure of pure cubic silicon. This means that the oxygen-free environment realized by liquid nitrogen is useful for synthesizing particles with negligible oxidation.     

Temporal pulsewidth and the wavelength dependences of the product ions obtained by laser ablation of solid C<Subscript>60</Subscript>

By ablating solid C₆₀ with a laser pulse, we observe various processes such as the prompt- and the delayed-ionization of C₆₀, the fragmentation into molecular ions and the formation of cluster ions. We found these processes show distinct dependences on the temporal pulse width, the power and the wavelength of the ablation laser. From the observations, we could confirm efficient coupling of laser energy to C₆₀ through the molecular absorption even with a laser pulse width less than the electron-phonon coupling time of the C₆₀ molecule.     

Submicrometer grating fabrication in fused silica by interferometric laser-induced backside wet etching technique

Submicrometer period fused silica gratings were produced by two-beam interferometric laser-induced backside wet etching technique (TWIN LIBWE). The fourth harmonic of a Nd:YAG laser beam was spatially filtered in two steps, and the smoothened beam was split into two parts and interfered at incident angles of 60°, 30°, 14°, and 7.7°, respectively, on the backsides of fused silica plates that were in contact with a liquid absorber. The periods of the produced fused silica gratings were, respectively, 154 nm, 266 nm, 550 nm, and 990 nm. In the next step, TWIN-LIBWE setup was completed by using a coupling rectangular prism in order to reach immersion setup, which made possible to fabricate 104 nm period fused silica grating. This is the smallest laser-generated grating constant in fused silica at present. The morphology of the etched gratings was characterized by atomic force microscope. Important parameters (modulation depth, low-pass filtered waviness, quality factor) of the produced gratings were determined. Evolution of the grating parameters was also studied in the 990 nm case: the dependence of modulation depth, waviness, and quality factor on the number of laser pulses was investigated.     

Characteristics of γ-Al2O3 nanoparticles generated by continuous-wave laser ablation in liquid

Laser ablation in liquid is one of the most widely investigated methods for generating various nanoparticles (NPs) that are difficult to produce using other means. In this paper, we report the generation of Al-oxide NPs by continuous-wave (CW) fibre laser ablation of corundum (α-Al₂O₃) target submerged in deionised water. The effects of CW fibre laser power and radiation time have been investigated. Characterisation of the NPs generated, in terms of size, size distribution, shape, chemical composition, and phase structure, was carried out by means of high-resolution transmission electron microscopy (HR-TEM), high angle annular dark field (HAADF) in scanning-transmission (STEM) mode, energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The results show that the average size of Al-oxide NPs, in the range of 17 to 29 nm, increased with increasing the laser power and laser exposure time, and the NPs are dominated by stoichiometric γ-Al₂O₃ with a minor phase of α-Al₂O₃. The mechanism involved in the CWLAL is also discussed.     

From polymer films to organic nanoparticles suspensions by means of excimer laser ablation in water

This study highlights the preparation of organic nanoparticles (NP) by laser ablation (LA) of polymeric materials in water. Experiments focused on poly(ethylene terephtalate) (PET) were carried out with the KrF laser pulse (248 nm). Size distribution and concentration of nanoparticles were deduced from suspensions turbidity measurements with the aid of Mie model, by Atomic Force Microscopy (AFM) on the basis of a statistical study and Scanning Electron Microscopy (SEM). The obtained results show that assemblies of spherical NP with a mean diameter 50 nm were synthesised. Composition and surface chemistry of NP were investigated using the Confocal Micro-Raman Spectroscopy (CMRS) and X-ray Photoelectron Spectroscopy (XPS). It indicates that NP are graphitic carbon rich and have a polymeric structure like polyacetylene. The possible mechanisms responsible of NP synthesis by under water LA of polymers was briefly discussed by investigating other polymers targets.     

Thresholds for front-side ablation and rear-side spallation of metal foil irradiated by femtosecond laser pulse

The mechanical action of laser exposure on a foil may result in the ablation of irradiated front layer and the rear-side spallation. The dynamics of an Al foil is studied by means of two-temperature (2T) hydrodynamics and molecular dynamics (MD). It is found that the rear-side spallation threshold F _s exceeds the front-side ablation threshold F _a. We propose to extend the common approach in laser-matter experiments by pump–probe measuring of the rear-side displacement.     

Discharge instability induced by external laser preionization of a XeF discharge laser

External-laser-induced preionization of excimer lasers was investigated. A discharge XeF laser was preionized by two different UV lasers [a KrF laser (λ=249 nm) and an ArF laser (λ=193 nm)], and the improvements in performance of the XeF laser were compared. The XeF laser beam profiles were measured by an intensified CCD (ICCD) camera with temporal resolution of 10 ns. Striated XeF laser profiles were obtained with 249 nm laser preionization, whereas there was no striation in the profiles for 193 nm laser preionization. These striations originated from discharge in the XeF laser induced by laser preionization. The influence of excited rare-gas atoms on the discharge instability was examined.     

Formation of cavitation-induced pits on target surface in liquid-phase laser ablation

It is well known that a crater is formed on the target surface by the irradiation of intense laser pulses in laser ablation. In this work, we report that additional pits are formed on the bottom surface of the ablation crater due to the collapse of a cavitation bubble in liquid-phase laser ablation. We observed the formation of several cavitation-induced pits when the fluence of the laser pulse used for ablation was approximately 5 J/cm². The number of cavitation-induced pits decreased with the laser fluence, and we observed one or two cavitation-induced pits when the laser fluence was higher than 10 J/cm². In addition, we examined the influence of the liquid temperature on the formation of cavitation-induced pits. The collapse of the cavitation bubble was not observed when the liquid temperature was close to the boiling temperature, and in this case, we found no cavitation-induced pits on the bottom surface of the ablation crater. This experimental result was discussed by considering the cavitation parameter.     

In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation

In-situ functionalization of gold nanoparticles with fluorophore-tagged oligonucleotides is studied by comparing femtosecond laser ablation in stationary liquid and in biomolecule flow. Femtosecond laser pulses induce significant degradation to sensitive biomolecules when ablating gold in a stationary solution of oligonucleotides. Contrary, in-situ conjugation of nanoparticles in biomolecule flow considerably reduces the degree of degradation studied by gel electrophoresis and UV–Vis spectrometry. Ablating gold with 100 μJ femtosecond laser pulses DNA sequence does not degrade, while the degree of fluorophore tag degradation was 84% in stationary solution compared to 5% for 1 mL/min liquid flow. It is concluded that femtosecond laser-induced degradation of biomolecules is triggered by absorption of nanoparticle conjugates suspended in the colloid and not by ablation of the target. Quenching of nanoparticle size appears from 0.5 μM biomolecule concentration for 0.3 μg/s nanoparticle productivity indicating the successful surface functionalization. Finally, increasing the liquid flow rate from stationary to 450 mL/min enhances nanoparticle productivity from 0.2 μg/s to 1.5 μg/s, as increasing liquid flow allows removal of light absorbing nanoparticles from the ablation zone, avoiding attenuation of subsequent laser photons.     

Fabrication of two types of one-dimensional Si–C nanostructures by laser ablation

Two types of one-dimensional (1D) nanostructures—amorphous silicon carbide (SiC) nanowires, 5–30 nm thick and 0.5–2 μm long, and carbon nanotubes (CNTs) filled completely with crystalline SiC nanowires, 10–60 nm thick and 2–20 μm long—were synthesized by the laser ablation of carbon-silicon targets in the presence of high-pressure Ar gas up to 0.9 MPa. All the CNTs checked by transmission electron microscopy contained SiC, and no unfilled CNTs were produced. We discuss the growth of the two nanostructures based on the formation of molten Si–C composite particles and their instabilities leading to the precipitation of Si and C.     

Comparison of plume expansion in femtosecond laser ablation on oxidized and non-oxidized Sm surfaces

Time development of Sm⁺ and Sm ablation plume produced by the femtosecond laser irradiation has been investigated. The two-dimensional spatial profiles of Sm and Sm⁺ emitted from oxidized and non-oxidized Sm surface were visualized using a planar laser-induced fluorescence method. It was observed that the flow velocity of Sm⁺ is much faster than that of Sm plume in both surfaces. The plumes from the oxidized Sm surface show higher velocity than that from non-oxidized surface, which is originated by the small electric conductivity at the surface. Expansion property observed for Sm⁺ and Sm plume in the oxidized Sm surface ablation implies the formation of the Knundsen layer nearby the surface. Meanwhile, continuous emission of Sm indicates the large contribution of heating effect to emission process at the non-oxidized surface. We conclude that the fsLA process strongly depends on the electric property of the ablated surface and the heating effect contributes to the particle emission process on the conductive material surface.     

Mechanisms of femtosecond laser ablation of dielectrics revealed by double pump–probe experiment

We study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectrics. A femtosecond pump–probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. To get more information in the complex process of excitation and relaxation mechanisms involved during and after the interaction, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a controlled density of carriers, and a second one at 800 nm with variable pulse duration, from 50 fs to 10 ps, reaches an excited solid. In Al₂O₃, we show that the total density of carriers never exceeds the sum of the densities excited by the two pulses sent independently. This means that the second pulse deposits further energy in the material by heating the previously excited carriers, and that no electronic multiplication occurs. On the other hand, in SiO₂, it is possible, under specific conditions, to observe an increase of carrier density due to impact ionization. All these results demonstrate that the avalanche process, which is often invoked in the laser breakdown literature, does not play a dominant role in optical breakdown induced by short pulses.