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In spite of the fact that more than five decades have passed since the invention of laser, some topics of laser-matter interaction still remain incompletely studied. One of such topics is plasma impact on the overall phenomenon of the interaction and its particular features, including influence of the laser-excited plasma re-radiation, back flux of energetic plasma species, and massive material redeposition, on the surface quality and processing efficiency. In this paper, we analyze different plasma aspects, which go beyond a simple consideration of the well-known effect of plasma shielding of laser radiation. The following effects are considered: ambient gas ionization above the target on material processing with formation of a “plasma pipe”, back heating of the target by both laser-driven ambient and ablation plasmas through conductive and radiative heat transfer, plasma chemical effects on surface processing including microstructure growth on liquid metals, complicated dynamics of the ablation plasma flow interacting with an ambient gas that can result in substantial redeposition of material around the ablation spot. Together with a review summarizing our main to-date achievements and outlining research directions, we present new results underlining importance of laser plasma dynamics and photoionization of the gas environment upon laser processing of materials.

International audience; A comparative experimental study of pulsed laser ablation in vacuum of two binary semiconductors, zinc oxide and indium phosphide, has been performed using IR-and visible laser pulses with particular attention to cluster generation. Neutral and cationic Zn_n O_m and In_n P_m particles of various stoichiometry have been produced and investigated by time-of-flight mass spectrometry. At ZnO ablation, large cationic (n>9) and all neutral clusters are mainly stoichiometric in the ablation plume. In contrast, indium phosphide clusters are strongly indium-rich with In_4 P being a magic cluster. Analysis of the plume composition upon laser exposure has revealed congruent vaporization of ZnO and a disproportionate loss of phosphorus by the irradiated InP surface. Plume expansion conditions under ZnO ablation are shown to be favourable for stoichiometric cluster formation. A delayed vaporization of phosphorus under InP ablation has been observed that results in generation of off-stoichiometric clusters.

Neutral and cationic Zn n O m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with $\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+}$ and $\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+}$ being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric $\mathrm{Zn}_{n}\mathrm{O}_{n}^{+}$ clusters (n

This work presents hybrid bulk heterojunction solar cells based on dye-sensitized zinc oxide (ZnO) nanorods blended with poly(3-hexylthiophene) (P3HT). Tetra(4-carboxyphenyl)porphyrin (TCPP) molecules were grafted onto the surface of ZnO nanorods to enlarge the absorption spectrum of the blend. We demonstrate that additional bands in the external quantum efficiency (EQE) spectra corresponding to Soret and Q-band absorption can already been observed at very low dye concentration at the ZnO surface. Therefore, direct grafting of TCPP onto ZnO nanorods leads to very efficient electron injection process into the ZnO nanorods after the light absorption of the dye. However, the overall photocurrent of the devices decreases gradually with TCPP concentration at the ZnO nanorod surface. The recombination dynamics of the photogenerated charges at the P3HT:ZnO interface are investigated by transient absorption spectroscopy on micro- to millisecond time scales. We observe that the lifetime of the P3HT polarons is red...

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.

Pulsed laser deposited nanocrystalline V 2 O 5 thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and optical spectroscopy. The films were deposited on amorphous glass substrates, keeping the O 2 partial pressure at 13.33 Pa and the substrate temperature at 220 °C. The characteristics of the films were changed by varying the laser fluence and repetition rate. XRD revealed that films are nanocrystalline with an orthorhombic structure. XPS shows the sub-stoichiometry of the films, that generally relies on the fact that during the formation process of V 2 O 5 films, lower valence oxides are also created. From the HRTEM images, we observed the size evolution and distribution characteristics of the clusters in the function of the laser fluence. From the spectral transmittance we determined the absorption edge using the Tauc plot. Calculation of the Bohr radius for V 2 O 5 is also reported.

Hybrid nanomaterials were synthesized by adopting femtosecond laser ablation of a ZnO target in pure ethanol. Dye molecules were grafted onto the ZnO nanoparticles by mixing colloidal ZnO–ethanol solution prepared by laser ablation to Tetramethylrhodamine B isothiocyante, or to Rhodamine B solutions. Strongly facetted nanohybrid particles were observed with an average size of 15 nm, by HRTEM measurements. From photoluminescence spectroscopy of the nanohybrids after single and multiphoton excitations, we observed energy transfer from the ZnO nanoparticles towards the attached dye molecules. Moreover, IR excitation spectra of the hybrid nanomaterials reveal the emission of the grafted dye via two photon absorption of the ZnO.

ZnO nanoparticles were synthesized by means of femtosecond laser ablation of a ZnO target in different pure liquids such as deionized water and ethanol, and in solutions of doand octa-decanethiol. Samples produced in water at low laser fluence contained nanoparticles whose radius is less than the Bohr radius as revealed by photoluminescence measurements that illustrate explicitly the effect of quantum confinement directly linked to the presence of nanoparticles. In fact, particles of about 1 nm in diameter were identified by AFM and TEM observations, which also show the increase in ablated particle size when increasing the fluence. Processing in ethanol and at low fluence led to the formation of ZnO particles of a few nanometers in diameter. When ablating in thiol solutions, slow cluster-growth promotes the formation of facetted particles.

The photoluminescent properties of a neutral surfactant templated highly ordered mesoporous silica CMI-1 have been studied and compared with those synthesized with cationic surfactant. The absence of the emission at 1.9 eV is for the first time observed and is quite important for opto-electronic nanocomposite preparation using CMI-1 as matrix. Also for the first time, the possible origin of the emission band at 3.89 eV is identified.

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions have also a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and harge-carrier transport.

International audience; We study the steel surface modifications induced by irradiation of several lasers in air.The surface modifications are found to depend on the laser properties (Krf, 248 nm, 26 ns; XeCl, 308 nm, 25 ns; Nd:YAG, 1064 or 532 nm, 5 or 10 ns).The treated surface features (morphology, chemical composition and crystalline structure) are analysed by complementary techniques, such as X-ray photoelectron spectroscopy, nuclear reactions, scanning electron microscopy and grazing incidence X-ray diffraction.First of all, it is shown that the carbon coming from the machining oils is successfully removed under all the studied conditions.The incorporation of oxygen and nitrogen (beyond 1 mm in depth) to the surface is also shown.In addition to the original ferrite phase, new crystalline compounds are detected after treatment such as austenite, oxides (FeO or Fe2O3, Fe3O4) and nitride (εFe2-3 N).Depending on the laser characteristics, the main formed crystalline phases are different (oxide or nitride).A discussion of the expected surface modification mechanisms is presented. Under some experimental conditions, the formation of a backdeposition layer composed of oxides and hydroxides is observed

Rich populations of clusters have been observed after femtosecond laser ablation of bulk silicon in vacuum. Size and velocity distributions of the clusters as well as their charge states have been analyzed by reflectron time-of-flight mass spectrometry. An efficient emission of both neutral silicon clusters Si n (up to n =6) and their cations Si n + (up to n =10) has been observed. The clusters are formed even at very low laser fluences, below the ablation threshold, and their relative yield increases with fluence. We show the dependencies of the cluster yield as well as the expansion dynamics on both laser wavelength and laser fluence. The mechanisms of the cluster formation are discussed.

L'ablation de metaux par laser impulsionnel ultrabref (femtoseconde) est etudiee en analysant les crateres par microscopie optique. La profondeur du cratere a ete mesuree en fonction du nombre d'impulsions et de la fluence. Les resultats sont compares a un modele analytique decrivant l'interaction laser-matiere en regime femtoseconde.

The condensation of zinc oxide nano clusters produced in gas phase has been evidenced in the ablation of a ZnO solid target by a pulsed ArF laser. We compare the spatio-temporal evolution of the shape of ZnO ablated plasma plume, both in vacuum and in controlled gas environments (Helium and/or Oxygen) from monitoring with CCD camera and from spectroscopic studies. The expansion of the plasma plume and the growth of the nanoclusters depend strongly upon the collisions between the ablated particles and surrounding gas molecules, as well as on the chemical reactions in the case of oxygen. The spectra of the observed plasma emittion are mainly due to atomic transitions in the neutral Zn. We have also observed the photoluminescence of the nanoclusters suspended in the gas and their decomposition in the beam of ArF laser., Comment: In French

Ces travaux de recherche ont ete realises dans le cadre du projet de developpement du Laser MegaJoule (LMJ). Parmi les experiences d'interaction laser - matiere a tres haut flux, certaines conduiront a un depot de particules d'or sur les elements internes de la chambre d'interaction. Pour nettoyer ce depot de particules, la possibilite d'utiliser un procede automatise base sur l'ablation laser a ete etudiee. Un modele numerique simplifie a ete developpe et une etude experimentale realisee sur des echantillons d'acier inoxydable ou de B 4 C recouverts d'un film d'or d'une epaisseur de ∼ 20 nm deposes par PVD. Ces travaux montrent que plus de 95 % du film d'or peuvent etre enleves avec quelques tirs d'un laser XeCl des que la densite d'energie depasse 3 J/cm 2 .

Motivee par des necessites economiques et environnementales, l'industrie est amenee a developper de nouveaux procedes propres, notamment pour les applications concernant la preparation de surface (nettoyage, decapage...). Dans ce domaine, l'utilisation du laser excimere presente de nombreux avantages. En effet, en raison d'un temps d'interaction de courte duree (ns), suivi d'un echauffement et d'un refroidissement rapide de la zone irradiee sur une profondeur tres fine (μm), il est possible de modifier localement les proprietes physico-chimiques d'une surface sans changer les proprietes intrinseques du materiau. Le but de nos travaux a consiste en l'etude des effets d'un traitement par laser excimere (XeCl) sur les proprietes chimiques (XPS et AES), structurales (XRD et GIXRD) et morphologiques (MEB) de surface d'acier. Outre le nettoyage de la surface, les analyses chimiques realisees sur de l'acier ont mis en evidence pour des densites d'energie importantes la formation d'une couche de Fe 2 O 3 au detriment de FeOOH. Les observations MEB ont revele la creation de structures spheriques nanometriques. Ce traitement permet notamment d'ameliorer la tenue a la corrosion de la surface.

Investigations on the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters (for two sorts of steel and a hard metal substrate) are reported in this work. Experiments were performed in air with a Ti:sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5 and 10 J/cm 2 . After the laser irradiation, cuts were made through the processed samples and pore cross-sections were obtained. Through their metallographical analysis it was possible to evidence crystalline structure changes in the immediate vicinity of the laser induced microholes; the extent of modified material zones increased for higher laser fluences. In such zones, a chemical composition quasi similar to that of the raw material was found and the performed nanohardness tests revealed hardness increases.

International audience; This study focuses on the surface modification of a low alloy steel by XeCl laser under ambient conditions. These modifications were investigated employing two complementary methods. Auger electron spectroscopy was employed for chemical characterisation and depth profile and Scanning electron microscopy for the surface morphology after laser irradiation. It is shown that laser treatment induce the formation of Fe(III) oxide, mainly Fe2O3. This oxide can be described as a multilayer structure. One part, e.g. the upper layer, is formed with Fe2O3 aggregates due to the coalescence of small particles of Fe2O3 formed in the plasma by reaction of matter ejected from the surface with O2 gas in the atmosphere. Under this layer deposited on the surface, we find a Fe2O3 structured layer corresponding to the oxidation of the melt layer.

The response of cemented tungsten carbide and of titanium carbonitride was investigated with respect to damage and ablation properties, under interaction with ultrashort laser pulses. These carbide materials present high microhardness and are of significant interest for tribological applications. The experiments were carried out in air with a commercial Ti:sapphire laser at energy densities on the target up to 6.5 J/cm2. The irradiated target surfaces were analyzed with optical, SEM and AFM techniques and the damage and ablation threshold values were determined using the measured spot diameters and the calculated incident energy density distributions.

Structural and optical characterization of crystalline Cu 2 O thin films obtained by thermal oxidation of Cu films at two different temperatures 800 °C and 900 °C are investigated in this work. X-ray diffraction measurements indicate that synthesized films consist of single Cu 2 O phase without any interstitial phase and show a nano-grain structure. Scanning Electron Microscopy observations indicate that the Cu 2 O films have a micro-scale roughness whereas High Resolution Transmission Electron Microscopy highlights that the nanocrystalline structure is formed by superposition of nearly spherical nanocrystals smaller than 30 nm. Photoluminescence spectra of these films exhibit at room temperature two well-resolved emission peaks at 1.34 eV due to defects energy levels and at 1.97 eV due to phonon-assisted recombination of the 1s orthoexciton in both film series. Emission characteristics depending on the laser power is deeply investigated to determine the origin of recorded emissions. Time-integrated spectra of the 1s orthoexciton emission reveals the presence of oxygen defects below the conduction band edge under non-resonant two-photon excitation using a wide range of excitations wavelengths. Optical absorption coefficients at room temperature are obtained from an accurate analysis of their transmission and reflection spectra, whereas the optical band gap energy is estimated at about 2.11 eV. Results obtained are of high relevance especially for potential applications in semiconductor devices such as solar cells, optical sources and detectors.

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm2 fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated.

Experimental results on the gas-phase formation of neutral and cationic phosphorus clusters are presented. The clusters were synthesized by visible (532 nm) or UV (193 nm) laser ablation of crystalline red phosphorus under high vacuum conditions and were analyzed using TOF mass spectrometry. Neutral Pn clusters produced by 532-nm ablation are found to be even-numbered while P n + cations are mainly odd-numbered, with P 7 + and P 21 + being the most abundant ions. For UV laser ablation, stable compound clusters, neutral P7H3 and P23H5, and cations P23H 6 + were synthesized for the first time. The formation of Pn clusters by thermal vaporization of red phosphorus into a cold He gas was also investigated and only small clusters (n

Nous montrons que l'utilisation d'un laser UV impulsionnel (ArF * , λ = 193 nm) est tres benefique pour la synthese de nano-agregats de Si dans le regime d'ablation. L'ejection ionique de monomeres Si + et de petits agregats Si x + (x = 2-11) augmente fortement le taux de condensation dans la plume induite par laser UV. De plus, l'importante production d'ions non thermiques assure la stabilisation de la temperature de la surface irradiee permettant de controler la taille des nano-agregats (1-5 nm) a l'aide d'un seul parametre: la fluence laser. On peut alors changer la position spectrale de la bande de photoluminescence des nano-agregats de Si du proche IR au proche UV par la selection de la taille des nano-agregats. Ces resultats sont en accord avec l'effet du confinement quantique.

In French; International audience; The studies of interaction of the UV photons with bulk and nanocristalline silicon by time-of-flight (TOF) mass-spectrometry allowed to reveal two populations of Si ion monomers. The first rapid population ejected even at low laser fluences is attributed to Colombian repulsion between the surface charges which are induced by emission of the photoelectrons. When the laser fluence increases, the second slow ion population appears. This population originates from the thermal processes in the irradiated material. The original result of this work is the observation of the non-thermal ion population which represents a considerable part of the laser-induced plume.; L'étude de l'interaction entre photons UV et le silicium massif et nanocristallin par spectrométrie de masse à temps de vol permet de distinguer deux populations de monomères ioniques Si+. Une population rapide est éjectée dès les faibles fluences laser, dont l'origine est attribuée aux répulsions Coulombiennes entre les charges de surface créées par l'émission de photo-électrons. Alors que la fluence d'irradiation est augmentée, une population plus lente apparaît issue de phénomènes thermiques initiés dans le matériau irradié. La mise en évidence d'une population ionique non thermique qui constitue une part importante des ions présents dans la plume laser est un résultat original de cette étude.

Cet article presente une etude effectuee sur le nettoyage par ablation laser UV de contaminants fixes a la surface de materiaux et sur le developpement de systemes laser permettant le traitement automatise de grandes surfaces. Les conditions d'irradiation permettant la suppression des oxydes par ablation laser UV et garantissant l'innocuite du procede ont ete definies. Le dispositif de nettoyage est constitue d'un laser XeCl, d'un systeme de transport par fibres optiques, d'une buse d'aspiration et d'un systeme permettant le controle en temps reel de la qualite du nettoyage.

The physical phenomena involved during three-dimensional axisymmetric laser-induced plasma expansion into background gas are numerically studied. For this purpose, a multispecies hydrodynamic model is developed which considers the effects of mass and ambipolar diffusions, thermal conduction, viscosity, and nonequilibrium conditions for ionization. This model is applied to describe quantitatively the Si plasma plume expansion into Ar or He gases. It is shown that the mechanism of plasma expansion depends critically on both the pressure and mass of the background gas. The shock front expansion is found to be strongly correlated with ion dynamics. A pronounced difference between heavy-particle and electron temperatures indicates a persistent lack of equilibrium between the heavy particle and the electron in the plasma plume expansion. The Si atoms of the rarefied plume are essentially driven by the backward-moving background gas as a result of a mass diffusion process. It is also noted that the diffusion processes are only important in the last expansion stage, and are less significant in the first stage. Therefore, it is shown that a computation which does not include diffusion effects (Euler equations) can adequately describe only the earliest stage of plasma expansion into background gas. The ability of the Navier-Stokes hydrodynamic multispecies model to predict the key role of the background gas type (Ar, He) and pressure is demonstrated.

We report a photoluminescence study of silicon nanoclusters produced by laser ablation. It was found that by varying the preparation parameters it was possible to change the mean cluster size in the range 1–5 nm. Within this size variation, the photoluminescence band shifts in a wide spectral region from near ultraviolet to near infrared. This size-dependent photoluminescence of Si nanoclusters is consistent with a quantum confinement effect. The observed influence of cluster oxidation on the luminescence properties also supports the quantum confinement interpretation. We proposed a discrete size model which supposes that the spectral position of the luminescence band is essentially determined by the volume of clusters with a complete outer atomic layer. In the framework of this model, we were able to deconvolute the observed luminescence bands into a set of fixed Gaussian bands. The model is supported by the observation of a size selective doping of Si nanoclusters whose effect was well explained by Auger recombination. Finally, our model allowed us to obtain a dependence of the optical gap on the cluster size which is in good agreement with existing calculations of Si nanocrystal electronic structure.

We describe the basic principles of nanoparticle synthesis by conventional pulsed laser ablation. The generalization of the Zeldovich and Raizer theory of condensation has been performed for inhomogeneous laser-induced plume where the rates of nucleation as well as the condensation times are different for different parts of the plume. The theoretical development and analysis of the experimental results are given for condensation, expansion and properties of silicon nanoclusters.

The condensation of Si clusters, produced by ns-laser ablation, is completed typically after a few μs at sufficiently low pressure of background gas. The photoluminescence from free gas suspended Si-cluster was experimentally observed after a long delay time of the order of a few hundred μs. We show that this delay time is related to long cooling and crystallization processes necessary to reach the state with pronounced particle photoluminescence.

We show that conventional pulsed laser ablation (PLA) of Si and Ge targets in inert buffer gases is an efficient method of nanocluster synthesis. From a photoluminescence study of Si and Ge nanoclusters produced by PLA we have demonstrated the possibility of tuning the luminescence band from the near infrared to the near ultraviolet regions. The stabilization of the properties of Si nanoclusters by reactive (H2 gas) PLA synthesis was proved by photoluminescence measurements. Finally, we report a photoluminescence study of gas-suspended Ge nanoclusters during their preparation. They exhibit a broad luminescence spectrum extended from UV to the blue-green region and modulated by a molecule-like structure. We propose an interpretation of the vibronic structure involving Ge-O-Ge vibrations at the surface of photo-excited clusters. To the best of our knowledge, we report here the first observation of vibrational effects from gas-suspended Ge nanoclusters.

International audience; The results of numerical simulation of pulsed laser ablation both in vacuum and into a background gas are presented. The influences of different processes, such as time evolution of the surface temperature, interspecies interactions (elastic collisions, recombination-dissociation reaction), interaction with an ambient gas, and excitations-relaxation processes on time-of-flight (TOF) distributions are examined. Experimentally obtained time-of-flight distributions are further analyzed, based on the results of numerical simulation. It is found that with the aid of numerical results one can explain not only the shape of the TOF distribution, but also the distance dependency of its maximum position (mean delay time). In addition, the mechanisms leading to the appearance of bimodal time-of-flight distribution are revealed. The study presents particular interest for the analysis of experimental results obtained during pulsed laser ablation.

L'etude presentee porte sur la simulation 2-D de l'expansion d'un plasma induit par ablation laser UV sous atmosphere neutre L'ecoulement bidimensionnel. axisymetrique, instationnaire, dissipatif et ionise est etudie par la resolution des equations de Navier-Stokes. L'ablation laser etudiee est celle d'un materiau simple, le silicium (Si) sous atmosphere d'argon ou d'helium. L'ecoulement est constitue de quatre especes monoatomiques Si, Si+, electrons et Ar/He. L'ionisation est consideree hors equilibre et caracterisee par des processus tels que l'ionisation par impact electronique, la recombinaison a trois corps et la recombinaison radiative.

L'etude de l'interaction laser matiere presente des interets multiples tant sur le plan theorique que sur le plan technique. D'un point de vue theorique, elle permet de mieux decrire les proprietes des plasmas hors equilibres. Il est donc essentiel de bien comprendre tous les processus intervenant lors de cette interaction. La modelisation numerique permet de prendre en compte la brievete des phenomenes etudies. Cette modelisation est basee sur une approche monodimensionnelle instationnaire qui comprend, l'equation de diffusion des paires electrons trous, l'equation de la chaleur et enfin l'equation de conservation de l'energie electronique Le systeme d'equations obtenu est resolu a l'aide d'un schema aux differences finies. Le code de calcul est en bon accord avec des resultats numeriques deja existants sur des cas tests picosecondes. La difference entre les longueurs d'ondes UV et IR a ete mise en evidence dans le cas des impulsions nanosecondes.

We report a photoluminescence study of silicon nanoclusters deposited by laser ablation. This technique allows to obtain clusters with a reduced size dispersion. Taking advantage of that as well as of the possibility to tune selectively the cluster size relatively to the preparation parameters, we are able to make a fine correlation between the emission band and the corresponding emitting size. The photoluminescence band can be tuned in a wide spectral region spreading from the near ultraviolet to the near infrared depending on preparation conditions. The correlation with atomic force microscopy measurements provides a size dependence of the luminescence which is fully consistent with the quantum confinement interpretation. To our knowledge, we present here the first observation of a wide spectral range tuning of the emission bands of nanocrystalline silicon by using a dry technique compatible with clean vacuum processing.

International audience; A three-dimensional Monte Carlo simulation of the laser desorbed flow of particles was performed. In addition to the gas-phase collisions, the chemical processes, i.e., recombination and dissociation are considered. The width of the angular distributions as a function of the radius of the evaporated area is analyzed. The comparison between the results obtained in the reactionless flow and in the flow with chemical reactions shows that in the presence of chemical processes the focusing effect of collisions is partly concealed and more wide angular distributions can be observed.

International audience; Laser ablation from one- and two-component targets into a diluted gas background is simulated by combined direct simulation-test particle Monte Carlo method. The spatial and velocity distributions of particles deposited at the plane substrate are calculated. The approach developed has allowed us to consider the influence of the collisions both among the ablated particles and between the ablated and ambient gas particles on the uniformity of film stoichiometry. It is found that the increase of the background gas pressure results in the more uniform distribution of the stoichiometrical ratio of the deposited particles with different masses.

Monte Carlo simulation is used to study the role of chemical reactions in the gas flow of particles laser desorbed from the target into a vacuum. The influence of recombination and dissociation processes on the properties of the gas flow is considered. It was found that chemical reactions have a significant effect on the composition of the desorption jet and on the angular and mean energy distributions of the desorbed particles. The study of these phenomena is of a particular interest for the understanding of the process of thin film deposition by pulsed laser ablation.

Experimental results on the etching of SiC ceramics induced by a copper vapor laser in various environments (air, (CH3)2SO (DMSO), N2H4, H2O) are presented. Strongly non-equilibrium conditions of the etching process result in the decomposition of the SiC ceramics surface. XPS and X-ray diffractometry reveal the following layered structure of SiC ceramics etched in air: an amorphous oxide layer SiOx, a several μm thick layer of Si clusters with an average size of 300 A, and a partially amorphous underlying layer of SiC. The areas of SiC ceramics etched in liquid environment do not contain any debris. SiC ceramics etched in air and in N2H4 promotes the electroless deposition of both Ni and Cu from corresponding electroless solutions resulting in the selective metallization of irradiated areas. Unoxidized Si species are the catalytic centers responsible for the initiation of metal deposition. The effects of the surrounding medium on the morphology of the etched area observed by scanning electron microscopy and atomic force microscopy are discussed.

International audience; Using various band-gap materials and tightly focused femtosecond laser pulses with wavelengths in the range 1200-2200 nm, we show that nonlinear absorption is independent of the wavelength except for narrow gap semiconductor materials. This observation corresponds to a transition between multiphoton ionization and tunnel ionization for an adiabaticity parameter of about 3, which compares favorably with Keldysh predictions. Our results indicate that long wavelengths must open up an alternative to pulse shortening for ultraprecision optical breakdown in dielectrics.

Spring Meeting of the European-Materials-Research-Society (E-MRS) / Symposium N / Symposium O / Symposium V on Laser Materials Processing for Micro and Nano Applications, Strasbourg, FRANCE, MAY 14-18, 2012; International audience; We investigate experimentally local non-linear absorption when 1.3 mu m wavelength femtosecond pulses are tightly focused inside n-type doped silicon. We show that 130 fs pulses with only few nanojoule energy is enough to initiate free-carrier generation. Our results also demonstrate that the laser energy deposition is independent on the doping concentration for substrates with free-carrier densities up to 10(18) cm(-3). For deep focusing experiments, the energy deposition can remain confined in micron-scale focal regions provided we perform the experiments with focused beam corrected for spherical aberration. The high degree of control observed in the experiments and the independence on doping are major assets for future 3D-micromachining technology developments based on this approach. (C) 2012 Elsevier B. V. All rights reserved.

Summary form only given. In the past few decades, 3D direct writing with focused femtosecond laser pulses has emerged as a technique for microfabrication inside transparent dielectrics like fused silica. It has been demonstrated that the control and the confinement of the non-linear interaction can lead to a wide variety of modifications: from a small uniform change of the refractive index to structural modifications opening the way to applications like optical memory encoding, optical waveguide and/or microfluidic channel fabrication [1-3]. The purpose of our study is to extend these 3D fabrication capabilities to semiconductors, a domain that has still not been exploited. We specially concentrate on silicon (Band Gap=1.1 eV) which remains the basis material for microelectronics and telecommunication applications. For comparison, we perform also the experiments on fused silica (Band Gap=9 eV) and various other materials with band gap varying from 1.1 to 13.6 eV.

17th International School on Quantum Electronics - Laser Physics and Applications (ISQE), Nessebar, BULGARIA, SEP 24-28, 2012; International audience; We investigate the non-linear absorption of 1.3 mu m femtosecond laser pulses strongly focused inside silicon and fused silica. Through transmission diagnostics, multiphoton initiated energy deposition is clearly observed inside these two materials with nanojoules laser pulse energy when using 0.3 numerical aperture objective. For silicon, the non-linear interaction is strongly dependent on the focusing depth due to the presence of spherical aberration contrarily to fused silica. Below the surface, we find a difference of three orders of magnitude between the intensity thresholds for nonlinear absorption at 1.3 mu m wavelength inside the two tested materials due to the difference of number of photons required for non-linear absorption. By measuring the transmission of the ionizing pulses during multiple pulse irradiation, irreversible modifications of the material are monitored inside fused silica in accordance with previous studies at 800 nm. For similar laser energy deposition, the response of bulk silicon remains unchanged over more than twenty thousands pulses suggesting no irreversible modifications are initiated.

An axisymetric two-dimensional modeling of laser-induced plume dynamics near the plasma ignition threshold in the ambient gas is carried out to provide a better understanding of the pulsed laser deposition process. In our modeling the ionization process is absent and the plume consists of neutral gas. The numerical investigations are performed with initial conditions close to an irradiance of about 100 MW/cm 2 . This model takes into account different initial conditions in the plume just after the ablation pulse and follows the hydrodynamic behaviour of the subsequent plume expansion. The hydrodynamics induced by the plume expansion are govemed by the two-dimensional unsteady Euler equation which are solved with the numerical method, based on a finite difference scheme associated with a flux corrected transport algorithm LCPFCT. The modeling was able to account for many important features of the gas plume dynamics. In particular, it is shown that the hydrodynamic behaviour and thermal relaxation of the gas plume are strongly dependent on the initial gradient of pressure and density between gas plume and background gas. Reversed flow of particles moving toward the target at certain distances along the R axis is clearly observed at a late expansion stage ( > 2 μs).

Pulsed laser ablation of a NbTe2 target was performed by an excimer laser in helium atmosphere at different fluences. Ablation plume characterization was realized by optical Time-of-Flight spectroscopy. Velocities and kinetic energies of the ablated particles were determined as a function of the fluence and of the background pressure. We observed an increase of the velocity of Te2 molecules when increasing the helium pressure. We show that this effect is due to three bodies associative reactions between monoatomic Te species and background gas particles.

Ablation of Si into He, Ar and O2 atmosphere by excimer laser has been investigated to understand the formation of molecules and clusters. Temperature is an obvious parameter controlling the stability of diatomic molecules. In this paper we report the measurements, by laser induced fluorescence (LIF), of the temperature of Si atoms and rotational temperature of SiO in laser induced plasma. Thermal relaxation processes of atomic Si in He and Ar atmospheres have been studied by measuring the Doppler profile of Si line using the 3d 1P0−3p2 1S transition centered at 263.128 nm. The thermalization of Si atoms is slower in Ar atmosphere than in He. The rotational temperature of SiO was measured by pumping the A1φ−X1Σ transition. In 5–400 μs delay after ablation pulse, the SiO temperature decreases from 2000 to 400 K. No evidence of nonequilibrium of the rotational and vibrational modes was observed. LIF measurements indicate that the formation of SiO molecules is better visualized as a gas phase equilibrium reaction rather than a ballistic collision in the temperature range 400–2000 K.

Experimental results are presented on the ablation of SiC ceramics in air under irradiation with XeCl and ArF excimer laser beam. X-Ray photoelectron spectroscopy analysis reveals the enrichment of the ablated areas in Si under laser beam energy densities of 1–3.5 J cm −2 . At higher energy densities the ablated areas contain Si oxide. The non-stoichiometric ablation of SiC allows one to activate selectively the SiC surface for metal deposition from electroless plating solution. Both Ni and Cu deposits show excellent adherence to both SiC ceramics and single-crystal SiC.

The synthesis and spectroscopic properties of a series of new organic electron donors derived from dibenzo[2,3:5,6]pyrrolizino-[1,7-bc]indolo[1,2,3-lm]carbazole are described. These compounds can be considered as planar hybrids of carbazole and p-phenylenediamine and possess high thermal stability. The electron donating properties and one- and two-photon spectroscopic features of methoxy derivatives are very sensitive to the number and positions of the methoxy groups. The X-ray structure of the 2,11-dimethoxy derivatives features slipped π-stacks and short interplanar distance between the molecules in the stacks similar to those found in the unsubstituted derivative.

Of a series of 4-substituted 1,8-naphthalimides, fluorescent 4-(6-piperidinyl-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)benzoic acid (4) was found to be a sensitive molecular probe for ZnO nanoparticles. We investigated in detail one- and two-photon absorption properties of this fluorophore. In nonpolar solvents, the acid 4 absorbs at about 400 nm and fluoresces at 500 nm with a fluorescence lifetime of about 7 ns, similar to the ester 6 and typical of the lifetimes of other derivatives of this type. Although the anionic form of this acid is not fluorescent, partial ionization of 4 in polar solvents, such as ethanol and acetonitrile, is not only accompanied by the expected decrease in the fluorescence quantum yield, but also gives rise to bathochromic shifts of both absorption and fluorescence and dual fluorescence with lifetimes of 0.2–0.3 ns and 6 ns ascribed to the formation of anionic complexes. The interaction with the ZnO surface brings about further considerable changes in the fluorescence patterns.

Conference on Frontiers in Ultrafast Optics - Biomedical, Scientific, and Industrial Applications XIII, San Francisco, CA, FEB 03-05, 2013; International audience; 3D laser microfabrication inside narrow band gap solids like semiconductors will require the use of long wavelength intense pulses. We perform an experimental study of the multiphoton-avalanche absorption yields and thresholds with tightly focused femtosecond laser beams at wavelengths: 1.3 mu m and 2.2 mu m. For comparisons, we perform the experiments in two very different materials: silicon (semiconductor, similar to 1.1 eV indirect bandgap) and fused silica (dielectric, similar to 9 eV direct bandgap). For both materials, we find only moderate differences while the number of photons required to cross the band gap changes from 2 to 3 in silicon and from 10 to 16 in fused silica.