Modelling chemical kinetics of small clusters after nanosecond laser ablation

Nanoclusters of various materials have recently been obtained by laser ablation. Strong evaporation of a condensed phase caused by laser irradiation is well known to generate an overcooled vapour. Further expansion thereof increases the oversaturation degree and facilitates homogeneous nucleation and cluster growth. To investigate homogeneous nucleation at very high expansion rates attained at nanosecond laser ablation, kinetic equations are applied describing all the possible gas-phase chemical reactions of dissociation and coalescing between small clusters. Additional cooling due to thermal emission by clusters is taken into account. Twenty smallest carbon molecules are considered. The model is applied to nanosecond laser ablation of graphite in vacuum. The resulted vapour molecular composition is characterised by dominating molecules C₃ and C₅ and an exponential drop of heavier clusters concentrations with their mass. The growth of heavier clusters is controlled by the balance between liberating the latent heat of their formation and the energy losses by expansion and thermal emission.     

Plasma density scale length determination in a deuterium,laser generated plasma

In a single-beam laser pellet interaction experiment, photographs of the plasma for two laser frequency harmonic radiations (2ω₀ and 3ω₀/2) are obtained; they provide information on plasma density scale length and hydrodynamic expansion.     

An analytical continuum-based model of time-of-flight distributions for pulsed laser ablation

An analytical model for time-of-flight (TOF) distributions of particles produced by pulsed laser ablation in vacuum has been proposed. The model takes into account the hydrodynamic expansion stage of the ablation plume and is based on a ‘sudden freeze’ model developed previously for steady-state supersonic jets. It is assumed that a continuum-like expansion of the plume takes place until a freezing time moment t _free (or, alternatively, until a ‘freezing distance’ x _free) whereupon the collisionless expansion begins. The proposed model is applied for analysis of experimental data on graphite ablation with nanosecond laser pulses. For verification, the analytical distributions are compared with calculated results obtained using a hybrid model combining a thermal model of laser-induced material heating with calculations of the plume dynamics by the direct simulation Monte Carlo (DSMC) method. It is shown that the proposed model can accurately estimate the surface temperature for conditions when the common approach fails.     

Time resolved X-ray photography of the expansion of nanosecond CO2 laser produced plasmas

Time resolved X-ray photography of intense nanosecond CO₂ laser interaction with a number of different targets illustrates the complex expansion of the critical density region from the target, and the strong influence that superthermal electrons have on the plasma dynamics and the radial expansion of the superthermal coronal about the target.     

Thermal properties and continuous-wave laser performance of Yb:LuVO<Subscript>4</Subscript> crystal

A laser crystal of Yb:LuVO₄ with high optical quality was grown by the Czochralski technique. Its thermal properties including specific heat, thermal expansion coefficients, and thermal conductivities along the a- and c-axis have been measured for the first time. Continuous-wave laser output up to 3.5 W at 1031 nm was obtained at room temperature through end-pumping by a high-power diode laser. The corresponding optical conversion efficiency was 43% and the slope efficiency was 72%. PACS 42.55.Xi; 66.60.+a     

Laser glazing of lanthanum magnesium hexaaluminate

Lanthanum magnesium hexaalumminate (LMA) is an important candidate for thermal barrier coatings due to its thermal stability and low thermal conductivity. On the other hand, laser glazing method can potentially make thermal barrier coatings impermeable, resistant to corrosion on the surface and porous at bulk. LMA powder was synthesized at 1600 °C by solid-state reaction, pressed into tablet and laser glazed with a 5-kW continuous wave CO₂ laser. Dendritic structures were observed on the surface of the laser-glazed specimen. The thicker the tablet, the easier the sample cracks. Cracking during laser glazing is attributed to the low thermal expansion coefficient and large thickness of the sample.     

Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm² Si, SiO and Si₂ have comparable intensity and Si_n (n≤7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, u_T²∝T_S, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Si_n clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Si_n provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered. PACS 82.30.Nr; 81.05.Gc; 78.70.-g     

Enhanced laser cleaning via direct line beam irradiation

A fluence advantage was achieved in dry/damp laser cleaning by reduction of the laser beam dimensions. 0.1 m Al₂O₃ particles were removed from glass slides using a KrF excimer laser (248 nm). As the width of the rectangular beam was reduced, a decrease in the threshold fluence required for particle removal was observed. Modelling based on the simplified thermal–mechanical response of the substrate and particle to the laser pulse does not describe the experimental results presented here. The case of dynamic expansion must be further considered, but it is believed that other mechanisms are involved in a full explanation. PACS 42.62.Cf     

Photoexpansion and photobleaching effects in oxysulfide thin films of the GeS2+Ga2O3 system

Oxysulfide systems undergo structural transformations upon illumination with laser light of near bandgap energy, as well as chalcogenide materials (glasses and films). In this paper, photoinduced effects such as photoexpansion and photobleaching were observed in GeS₂+Ga₂O₃ (GGSO) films synthesized by electron beam evaporation. A surface expansion of the thin films and a shift to shorter wavelengths of the optical absorption edge were observed as a result of UV laser irradiation (wavelength of 351 nm) and they are dependent on laser power density, exposure time and film composition. These parameters were varied to evaluate and enhance the observed effects. In addition, the irradiated GGSO samples exhibited a decrease in refractive index, measured with a prism-coupling technique, which makes these films suitable candidates for applications as gratings and waveguides in integrated optics.     

MATFESA: strain and refractive index field estimation after femtosecond laser interaction with transparent material

In this paper we present the development of an open code (“MATFESA”) based on the Finite Element Method (FEM) which can be used to estimate the strain and refractive index fields after femtosecond laser writing process by means of an iterative analysis. The fs-laser pulse residual stress control is the key to obtain high performance guiding structures for photonics. The whole complex physical problem consists in almost three steps inside the material during/after femtosecond laser interaction which cannot be analyzed using thermodynamic equilibrium equations. These are: ionization, expansion and re-solidification. In the numerical model solved, a mechanical expansion is introduced in the focal plane to simulate laser interaction at intensities above the optical breakdown threshold. Numerical results were compared to experimental measurements of optical guided modes in LiNbO₃ fs-waveguides. The MATFESA model was compared with ABAQUS commercial software in order to verify the strain field results and also to test the 2D, plane strain approximation.     

Relaxation phenomena in electronic and lattice subsystems on iron surface during its ablation by ultrashort laser pulses

Single-shot ablative spallation and fragmentation thresholds, as well as corresponding ablative crater depths, were measured on the surface of iron using optical interferometry, for different ultrashort laser pulse widths in the range τ_las = 0.3–3.6 ps. The nonmonotonic dependence of these thresholds on τlas with the minimum near 1.2 ps (the characteristic electron-phonon relaxation time τ _ep ) represents transport and emission relaxation phenomena for nonthermalized and thermalized carriers, generated by sub- and picosecond laser pulses, respectively. Compared to rather slow spallative ablation, much faster—picosecond—fragmentation ablation of the iron surface through hydrodynamic expansion of its supercritical fluid ceased for τ_las > τ _ep as a result of evaporative cooling.     

Self‐focusing of a cosh‐Gaussian laser beam in magnetized plasma under relativistic‐ponderomotive regime

The combined effect of relativistic and ponderomotive nonlinearities on the self‐focusing of an intense cosh‐Gaussian laser beam (CGLB) in magnetized plasma have been investigated. Higher‐order paraxial‐ray approximation has been used to set up the self‐focusing equations, where higher‐order terms in the expansion of the dielectric function and the eikonal are taken into account. The effects of various lasers and plasma parameters viz. laser intensity (a₀), decentred parameter (b), and magnetic field (ω_c) on the self‐focusing of CGLB have been explored. The results are compared with the Gaussian profile of laser beams and relativistic nonlinearity. Self‐focusing can be enhanced by optimizing and selecting the appropriate laser‐plasma parameters. It is observed that the focusing of CGLB is fast in a nonparaxial region in comparison with that of a Gaussian laser beam and in a paraxial region in magnetized plasma. In addition, strong self‐focusing of CGLB is observed at higher values of a₀, b, and ω_c. Numerical results show that CGLB can produce ultrahigh laser irradiance over distances much greater than the Rayleigh length, which can be used for various applications.     

Efficiency of optical-to-acoustic energy conversion upon the interaction of a pulsed laser radiation with a liquid: I. Calculation of the efficiency upon acoustooptic interaction

Relationships for the efficiency of conversion of laser radiation energy to acoustic energy for the acoustooptic (thermal) mechanism of interaction are derived. The cases of short and long laser pulses interacting with the rigid and free boundaries of a heavily absorbing liquid are considered. The efficiency is numerically calculated for the situation when the radiation of a transverse-excitation atmospheric (TEA) CO₂ laser interacts with water with regard for the temperature variation of the volume thermal expansion coefficient of the latter.     

Determination of the rotational temperature and the molecular density in an expanding NH3 jet by infrared absorption

Rotational temperatures are determined by measuring the absorption of infrared laser radiation. The possibilities of this method are critically examined and tested. As a result the molecular density in the expansion could be determined, too. Color-center laser radiation has been absorbed by a molecular jet of NH₃. An anomalous line shape has been observed, related to a Doppler shift from molecules moving along the various streamlines. No deviations from a thermal rotational distribution have been observed.Work supported by Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)     

Dynamics of a pulsed laser generated tin plasma expanding in an oxygen atmosphere

Semiconducting tin oxide can be successfully deposited by means of the laser ablation technique. In particular by ablating metallic tin in a controlled oxygen atmosphere, thin films of SnO _x have been deposited. The partial oxygen pressure at which the films are deposited strongly influences both the stoichiometry and the structural properties of the films. In this work, we present a study of the expansion dynamics of the plasma generated by ablating a tin target by means of a pulsed laser using time and space resolved optical emission spectroscopy and fast photography imaging of the expanding plasma. Both Sn I and Sn II optical emission lines have been observed from the time-integrated spectroscopy. Time resolved-measurements revealed the dynamics of the expanding plasma in the ambient oxygen atmosphere. Stoichiometry of the films has been determined by means of X-ray photoelectron spectroscopy and correlated to the expansion dynamics of the plasma.     

Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Pattern structures fabricated on ZnS–SiO2/AgO x /ZnS–SiO2 thin film structure by laser direct writing technology

In this work, we used the multilayered ZnS–SiO₂/AgO _x /ZnS–SiO₂ films as the laser direct writing materials, and pattern structures with different shapes and sizes were directly written with green laser (λ=488 nm). Compared with traditional photoresist materials, the pattern structures can be directly formed in this film structures without developing and etching procedures and also can be directly written by very low laser power. By tuning the laser parameters precisely, pattern structures with different sizes and shapes could be obtained as well. The analysis indicates that the formation mechanism of the pattern structure is mainly due to the volume expansion caused by AgO _x decomposition into silver particles and oxygen. The oxygen applies pressure to the ZnS–SiO₂ layer and makes a hollow shell under the film. The aspect ratios of the patterns rapidly increase from the minimum of 0.012 in laser power of 3.0 mW to the maximum of 0.201 in laser power of 5.0 mW. The thermal stability of the patterns was also qualitatively studied.