Sylvie Noël

Reducing nanoparticles in metal ablation plumes produced by two delayed short laser pulses

Using fast imaging and atomic force microscopy, we demonstrate that the fraction of nanoparticles in ablation plumes produced by short pulse laser irradiation of metals is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for gold and copper, it is shown that a significant nanoparticle reduction is only observed if the delay between both laser pulses exceeds the characteristic time of electron-lattice thermalization. We propose the reduced electronic heat transport at large lattice temperature as the dominant mechanisms for the observed nanoparticle reduction.

Impact of ultraviolet light during rapid thermal diffusion

Rapid thermal processing for junction formation is emerging as a low cost technique for solar cell as well as for other semiconductor device production. Compared to conventional furnace processing, process differences are not only in very high heating and cooling rates, but also in the incoherent emitted radiation spectrum, which can act on dopant diffusion. The photons emitted from tungsten halogen lamps go from far ultraviolet, over visible to infrared light. In this work additional mercury ultraviolet lamps are used during rapid thermal annealing to analyze the influence of high energetic photons on diffusion mechanisms. The diffusion results are discussed in terms of radiation spectrum, involving analysis of diffusion profiles and sheet resistances.

Mechanisms of nanoparticle formation by short laser pulses

Numerical modeling is performed to study cluster formation by laser ablation. The developed model allows us to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking, evaporation and coalescence during the ablation plume expansion. Both of these mechanisms are found to affect the final cluster size distribution. Plume cluster composition is correlated with plume dynamics. The results of the calculations demonstrate that cluster precursors are formed during material ablation through both thermal and mechanical target decomposition processes. Then, clusters react in collisions within the plume. In vacuum, rapid plume expansion and cooling take place leading to the overall decrease in the reaction rates. In the presence of a gas, additional collisions with background gas species affect the cluster size distribution. Growth of larger clusters can be observed at this stage. Calculation results explain several recent experimental observations.

Influence of irradiation conditions on plume expansion induced by femtosecond laser ablation of gold and copper

The influence of the irradiation conditions on plume expansion induced by femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, two main components were observed by fast imaging. They were identified by optical emission spectroscopy: the fast component is composed of atoms and ions whereas the slow one mainly contains nanoparticles. Experiments show that the velocities of plasma species depend weakly on laser fluence. Deposition of nanoparticles on a substrate and analyses by atomic force microscopy show that the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component, and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.

Formation of nanoparticles by short and ultra-short laser pulses

The main objective of this study is to explain the experimental observations. To simulate material ablation, plume formation and its evolution, we developed a combined molecular dynamics (MD) and direct simulation Monte Carlo (DSMC) computational study of laser ablation plume evolution. The first process of the material ablation is described by the MD method. The expansion of the ejected plume is modelled by the DSMC method. To better understand the formation and the evolution of nanoparticles present in the plume, we first used separate MD simulations to analyse the evolution of a cluster in the presence of background gas with different properties (density, temperature). In particular, we examine evaporation and growth reactions of a cluster with different size and initial temperature. As a result of MD calculations, we determinate the influence of the background gas parameters on the nanoparticles. The reactions rates such as evaporation/condensation, which are obtained by MD simulations, are directly transferred to the DSMC part of our combined model. Finally, several calculations performed by using MD-DSMC model demonstrate both plume dynamics and longer-time cluster evolution. Calculations results are compared with experimental findings.