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Intensity modulated radiation therapy (IMRT) is an advanced technique used to concentrate the prescribed dose in the tumour while minimizing exposure to healthy tissues. Success in IMRT is greatly dependent upon the localization of the target volume and normal tissue, thus accurate contouring is crucial. In this paper, we describe an automated atlas-based image contouring system and our approach for improving the system by performing a full-scale optimization of registration parameters using high-performance computing. To achieve this, we use manually pre-contoured CT images of ten head and neck patients. For any parameter set, each patient data is registered with the remaining patients. Accuracy of the resulting contours is determined automatically by comparing their overlap with manually defined targets using Dice's similarity coefficient (DSC). This allows us to compare all permutations of the image registration parameter sets and input data to investigate their impact on final contour accuracy. Investigating the parameter space required 27,000 image registrations and 216,000 DSC computations. To perform these registrations we introduced a large cluster of high-performance computers and developed a parallel testing harness. The metrics collected from the tests show a wide range of performance, indicating that parameter selection is crucial in our contouring system. By selecting an optimized parameter set, we increased the mean overlap of the automatically contoured regions of interest by 50% and reduced registration time by 50% compared to the original parameters. Our findings illustrate that full-scale optimization is an effective method for improving the performance of the automated image contouring system.

Classical molecular dynamics simulation technique is applied for investigation of the iron ablation by ultrashort laser pulses at conditions of deep hole for the first time. Laser pulse duration of 0.1 ps at wavelength of 800 nm is considered. The evolution of the ablated material in deep hole geometry differs completely from the free expansion regime as two major mechanisms are important for the final hole shape. The first one is the deposition of the ablated material on the walls, which narrows the hole at a certain height above its bottom. The second mechanism is related to ablation of the material from the walls (secondary ablation) caused by its interaction with the primary ablated particles. Properties of the secondary ablated particles in terms of the velocity and the angular distribution are obtained. The material removal efficiency is estimated for vacuum or in Ar environment conditions. In the latter case, the existence of well-defined vapor cloud having low center of the mass velocity is found. The processes observed affect significantly the material expulsion and can explain the decrease of the drilling rate with the hole depth increase, an effect observed experimentally.

A theoretical model is developed and a molecular dynamics simulation technique is applied for the description of ultrashort laser ablation of metals. The ablation of Al, Ni, and Fe using 0.1, 0.5 and 5 ps laser pulses at wavelengths of 248 and 800 nm is studied. The process is investigated at fluences up to 0.5 J/cm2. The analysis based on the temporal evolution of the ablation, the temperature, and the pressure distributions into the material reveals that a thermo-mechanical mechanism (spallation) takes place near the threshold. However, phase explosion is found to be the dominant mechanism of material removal at fluences higher than several hundreds of mJ/cm2. The influence of the laser parameters (wavelength and pulse duration) is obtained and discussed. The ablation depth as a function of the laser fluence and the ablation threshold value are evaluated and compared with the experimental data available. Good agreement between the theory and experiments is observed.

The influence of the pair Morse potential and the embedded-atom method (EAM) based potential on the description of the properties and the ultrashort laser ablation process of Fe by molecular dynamics (MD) simulation technique is studied. The accuracy of both potentials in evaluation of the melting temperature, the linear thermal expansion coefficient and the compression behavior of Fe is estimated. The EAM potential is found to give more precise description of the properties of Fe than the Morse one. The melting temperature is overestimated only 2% in the case of EAM and 24% for the Morse potential, respectively, compared to the experimental value. A discrepancy in the presentation of the elastic properties is also observed, as it is more pronounced in the case of Morse potential. The MD simulation of laser ablation of iron is performed for ultrashort laser pulses ( λ = 800 nm) with duration of 100 fs and at fluences below the ablation threshold up to 1 J/cm 2 . Analysis of the mechanism of ablation, evolution of the process, the temperature and the pressure distributions in the material is made.

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm2. The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking.

Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally. The laser fluence varied from the ablation threshold up to 100 J cm−2. Above 1 J cm−2, the ablation rate depended on the laser pulse duration, with the shortest pulse producing the highest value. A change in the ablation rate as the laser fluence increased was also observed. These results were analysed using molecular dynamics simulations. We show that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed. Furthermore, due to the electron heat diffusion, the overheated volume increases and involves material located deeper than the skin depth. An increase in the pulse duration results in a decrease in the degree of overheating.

We have investigated ultrashort laser micromachining of metals, both from the point of view of the basic physical processes, and the technological implications. The process of hole drilling of Ni with approximate to 300 fs SHG (X = 527 nm) Nd-glass and Al samples with 100 fs Ti:sapphire (), = 800 nm) laser pulses, respectively, has been experimentally addressed by using time-gated optical emission spectroscopy of the ablated material and SEM analysis of the targets. The ablation process has also been analyzed by classical, molecular dynamics (MD) simulations, by using a Morse potential to describe the interaction between the atoms, and taking into account the electron heat diffusion contribution. The dependence of the ablation depth on laser fluence, as measured by SEM analysis, is in good agreement with the numerical simulations and is also well correlated with the optical emission yield of the expanding plume. (c) 2007 Elsevier B.V. All rights reserved.

We present an experimental characterization and a theoretical analysis of ultrashort laser ablation of a nickel target, which highlights the more general and peculiar features of femtosecond (fs) laser ablation of metals. The study has been carried out by using visible (527 nm) laser pulses of approximate to 300 fs duration. The vacuum expansion dynamics of the ablated species has been investigated by using fast photography and optical emission spectroscopy, while the fs laser pulse-metal interaction has been studied theoretically by means of molecular dynamics simulations. Special attention has been given to the study of the dependence of ablation depth on laser fluence, which has been carried out by comparing the SEM analysis of micro-holes drilled into the nickel samples with the predictions of the theoretical model. The main outcomes of our investigation, which are very satisfactorily reproduced and accounted for by the theoretical model, are (i) the nonlinear dependence of the ablation yield on the laser fluence, and its reliance to the electron heat diffusion, in the process of redistribution of the absorbed energy, (ii) the splitting of the material blow-off into two main classes of species, atoms and nanoparticles, characterized by different expansion dynamics, and (iii) the different degrees of heating induced by the laser pulse at different depths into the material, which causes the simultaneous occurrence of various ablation mechanisms, eventually leading to atoms and nanoparticles ejection.

Laser ablation of Al, Ni, Fe, and Cu was investigated for pu lse durations of 0.1, 1 and 5 ps at fluences up to 100 J.cm -2 . The ablation depth per pulse was measured and the ablation thresholds were estimated. The increase of the pulse duration results in a decrease of the ablation rate. The presence of a molten phase is clearly expressed at fluences above 10 J.cm -2 . Molecular Dynamics simulation was used to model the laser ablation process in the case of Fe, Al and Ni. Keywords: Laser ablation, ultrashort pulses, metals, experimental, molecular dynamics 1. INTRODUCTION The growing application of the laser systems generating ultrashort laser pulses in the material processing is based on the some advantages that they offer compared with nanosecond pulses. The experiments on machining a wide range of materials by ultrashort pulses [1-4] showed that they ensure negligible heat diffusion into the material and absence of plasma during the laser pulse. As a consequence, the threshold for ablation decreases (by an order of magnitude in some cases [4]) compared to nanosecond pulses with precise sub-micron machining becoming possible. These features of the ultrashort laser pulses are also proven to be very useful in treatment of materials with high heat conductivity, like metals and semiconductors. Recent investigations [5-14] showed that the process of interaction of ultrashort laser pulses with matter is very complicated and strongly influenced by the parameters of laser radiation, properties of the material, processing conditions and environment. Experimental work on laser ablation of metals [5-7] indicated that in contrast to the case with nanosecond pulses, electron thermal diffusion can play an important role in the process of dissipation of the absorbed energy, while the properties of the electron - phonon coupling are important for the femtosecond laser damage. It was also shown that the process of ultrashort laser ablation is accompanied by fast nonlinear phase transitions, such as melting and amorphisation [11], and formation of 3-D structures within the ablation region [9,10,13] which affects the ablation rate. Drilling with high laser intensities (10

Ablation of Fe, Al, Ni, and Cu by laser pulses at durations of 0.1, 1, and 5 ps is investigated experimentally. The laser fluence used vaires from below the ablation threshold up to 100 J/cm 2 . The ablation rate depends on the laser pulse duration at laser fluences above several J/cm 2 as the shorter pulse produces higher ablation rate. A change of the ablation regime with the laser fluence increase is also observed. The presence of molten material is clearly expressed at fluences above 10 J/cm 2 for all pulse durations used. These effects can be referred to the contribution of the electron heat diffusion in the distribution of the absorbed energy. The traces of solidified molten material suggest for realizations of melt ejection mechanism of ablation.

We used time-gated optical emission spectroscopy to investigate the characteristics of aluminum plumes and their vacuum expansion after femtosecond laser ablation at different fluences. The prominent feature is the presence of two main classes of species in the plume: very fast Al atoms and ions preceding the plume bulk essentially constituted of much slower Al nanoparticles expanding with a ten times smaller average velocity. Atomic force microscopy of deposited Al nanoparticles evidenced an average size of about 10 nm with a pretty narrow size distribution. These results and the peculiar feature of nanoparticle formation during femtosecond laser irradiation of matter were very satisfactorily interpreted and reproduced by molecular-dynamics simulation of the process. Finally, the analysis of the dependence on laser fluence of the ablation process showed an initial logarithmic increase of ablation yield, up to about 500 mJ/cm2, followed by a sudden and very steep increase at higher fluences. According to our numerical calculations, this latter feature can be ascribed to the increase of the overheated material volume due to electron heat diffusion.

Laser ablation of alumina (Al2O3), aluminum nitride (AIN) and silicon nitride (Si3N4) ceramics by 6 ns Nd:YAG laser operating at wavelengths of 1064 nm, 532 nm and 355 nm is studied. For all materials the maximum ablation rate is observed at wavelength 1064 nm, and the smallest one for irradiation at 532 nm in the case of Si3N4 and AIN. The numerical model, based on the heat-transfer equation describes the temperature distribution in the material and the evaporation process. The calculated depths of the drilled holes are in agreement with experiment.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser ablation of iron by femtosecond and picosecond pulses is investigated using molecular dynamics (MD). The electron heat conduction is taken into account in the simulations. Several stages of the ejection of the particles is determined. The compression and the subsequent relaxation of the pressure into the bulk lead to removal of large amount of the material. These processes are also accompanied with the formation of voids in the material. Two different regimes of the ablation are observed depending on the laser fluence. The estimated threshold of ablation is in good agreement with the experimental data found in the literature. The dynamics of the shock wave and the temperature distribution in the material are also obtained.

The dependence on laser wavelength of the process of nanoparticle generation by ultrashort laser ablation of solid matter in a vacuum has been investigated both experimentally and theoretically. The study has been carried out for a Ni target by using laser pulses of approximate to 300 fs duration at two different laser wavelengths: in the visible (lambda = 527 nm) and ultraviolet (lambda = 263 nm), respectively. The size distribution of the nanoparticles, which is quite broad in the case of visible light, becomes significantly narrower and slightly shifts towards smaller sizes for ultraviolet light. Molecular dynamics simulations confirm the dependence of the process on the laser wavelength by showing that the laser photon energy affects the material relaxation and, thus, the nanoparticle generation process. This, in turn, indicates that the photon energy can be used as an effective parameter to control the nanoparticle size distribution in femtosecond laser ablation of solid matter.