Near-threshold Ablation Research Articles

Laser based surface treatment of bioactive glass: Dependence on pulsed laser ablation

Experimental results and theoretical simulation of material ablation on nanosecond Nd:YAG laser irradiation of bioactive glass targets, is presented. The process of pulsed laser ablation critically affects both, laser based surface modification and pulsed laser deposition (PLD) of bioactive glass (BG). A thermal model based theoretical simulation has been carried out describing heat-transport, melting and vaporization of a laser irradiated BG target under near-threshold ablation conditions. Calculated mass ablation rate per laser pulse has been compared with our experimental observations over the average laser fluence of 0.5J/cm2 to 6J/cm2. With increasing laser fluence, possibility of material ablation approaching phase explosion has been discussed by comparing the calculated maximum temperature reached by the laser irradiated target and the estimated thermodynamic critical temperature for BG. Our investigations indicate that with average laser fluence restricted to ∼5 J/cm2 material ablation occurs largely via normal vaporization ensuring deposition of an uniform, homogeneous BG coating through PLD. Target surface temperatures estimated from our calculations also suggest that laser based surface modification of BG can be reproducibly carried out without causing target crystallization and surface damage through crater formation for irradiating laser fluence in the region of ∼ 5J/cm2.

Amplification and regulation of periodic nanostructures in multipulse ultrashort laser-induced surface evolution by electromagnetic-hydrodynamic simulations

The formation of periodic structures in ultrafast laser-irradiated surfaces implies dynamic coupling between the incoming light and the light-driven material. To capture the mutual influence and feedback between light and evolving surface topographies, we investigate numerically the evolution of metal surfaces irradiated by multiple femtosecond laser pulses of sub-, near-, and slightly above-threshold ablation fluence. The multiphysical model combines Maxwell equations and thermohydrodynamic approach based on electron-ion heat transfer and compressible Navier-Stokes equations and allows us to account for interpulse feedback on the resulting surface topographies. First pulses of the subthreshold energy lead to material swelling, nanocavitation few tens of nanometers below the surface, and, as a result, nanoroughness formation on the initially flat surface. Further pulses contribute to the development of periodic surface structures, enhanced absorption, and increased removal rate. Cavitation in the tips of ripples is found to play a crucial role in modification and regulation of surface topography for sub- and near-threshold ablation fluences. At higher laser pulse energy, thermal ablation is mostly involved in surface modification, and the ablation rate per power reaches its maximum at three to five times the ablation threshold fluence, resulting from the optimal heat penetration depth for laser ablation. The numerical results offer a better understanding of the surface topography modifications upon multipulse femtosecond laser irradiation.

Parameters of a multicomponent laser-induced plasma

We report on the results of analysis of parameters of a laser-induced plasma, which were obtained using emission diagnostics of a laser torch plasma from a CuInSe2 polycrystalline target. The density and temperature of electrons in a laser torch with a complex composition are determined. The laser-induced plasma was formed in the near-threshold ablation conditions with a polycrystalline target used for obtaining thin films based on CuInSe2.

Heating and ablation of tokamak graphite by pulsed nanosecond Nd-YAG lasers

The results on laser heating and ablation of graphite tiles of thermonuclear tokamaks are presented. Two pulsed Nd-YAG lasers (20 Hz repetition rate, 5 ns pulse duration and 10 kHz repetition rate, 100 ns pulse duration) were applied for ablation measurements. The ablation thresholds (1.0±0.5 J/cm2 for 5 ns and 2.5±0.5 J/cm2 for 100 ns laser pulses) were determined for the Tore Supra tokamak graphite tiles (backside) nonexposed to plasma. The high repetition rate Nd-YAG laser (10 kHz, 100 ns pulse duration) and the developed pyrometer system were applied for graphite heating measurements. Some unexpected features of laser heating of the graphite surface were observed. They were explained by the presence of a thin surface layer with the properties different from those of the bulk graphite. The theoretical models of laser heating and near-threshold ablation of graphite with imperfectly adhered layer were developed to interpret the experimental results.

Timescales in the response of materials to femtosecond laser excitation

The interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser–matter interaction for longer pulse durations. For femtosecond laser excitation the fundamental physical processes such as energy deposition, melting, and ablation are separated in time. By choosing proper time windows, the various processes can be investigated separately. We present selected examples of theoretical studies of free electron excitation in metals, timescales of different melting processes, and peculiarities of near-threshold ablation. Depending on the timescales and intensity ranges, the discussed processes are combined in an overall picture of possible pathways of the material from excitation to ablation.

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 YBa2Cu3O7-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.

Enhanced ablation and photoacoustic excitation in near-threshold laser ablation of liquid-coated surfaces

The effect of an artificially deposited liquid film on a laser-ablated metal surface at near-threshold laser fluences is presented. Experiments utilizing a Q-switched Nd:YAG laser (wavelength=1064 nm, full width at half maximum=6 ns) reveal that deposition of a liquid film results in substantial reduction of ablation threshold and enhancement of ablation yield. In addition, enlarged photoacoustic emission is detected in the ablation involving the liquid film. The augmentation of ablation efficiency at high laser fluences is known to be induced by confinement of laser-generated plasma in the liquid layer. In contrast, the enhancement in the near-threshold regime, as in the present study, is primarily due to the photomechanical effect associate with the explosive liquid vaporization in the strongly superheated liquid layer. The effect of liquid-film thickness on near-threshold ablation are also examined for two different liquids, water and water/isopropanol mixture.

Near-threshold ablation of target material irradiated with pulsed ion beams

The process of target ablation induced by pulsed ion beams is investigated. The beam power density of 0.01–0.1 GW/cm2 is around the sublimation energy of the target material. The thickness of the layer to be removed from the target is estimated from the temperature distribution calculated numerically with a diffusion equation. The emitted particles are collected with C collectors, which are later subjected to RBS and SEM/EPMA analyses. It is found that ablated particles forming a film on the collector amount to only 10% of the target atoms within the maximum range of the ions, while a significant amount of the ablated material is observed as macroscopic particles with equivalent diameters up to several tens of micrometers. TOF measurements of the ablation plume together with RBS measurements of thin-film targets after irradiation suggest that the formation of the film is predominated by vaporization from the surface of the ablated mass, which is recondensed on the collector before completing the vaporization.