W. Marine

Photoluminescence of silicon nanoclusters with reduced size dispersion produced by laser ablation

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 Auge...

Strategy of nanocluster and nanostructure synthesis by conventional pulsed laser ablation

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.

MONTE CARLO SIMULATION OF PULSED LASER ABLATION FROM TWO-COMPONENT TARGET INTO DILUTED AMBIENT GAS

Laser ablation from a binary target into a diluted gas background is studied by means of a Monte Carlo simulation. The influence of the ambient gas on the spatial and mean energy distribution of particles deposited at the distant detector is considered. Thermalization of the particles, the random scattering effect and the backscattering of particles were observed. Considerable modification of the deposited film thickness profiles due to collisions of the ablated particles with the ambient gas is shown. The increase of the ambient gas pressure was found to affect the stoichiometry distribution of deposited and backscattered particles. The study is of a particular interest for the development of the thin film growing technique known as pulsed laser deposition.

Efficient third-harmonic generation in a thin nanocrystalline film of ZnO

Nonlinear optical conversion is studied in thin films of wide-bandgap materials. Very high conversion efficiency to the third-harmonic radiation is achieved for an unamplified femtosecond Cr4+:forsterite laser in a submicron-thick film of a nanocrystalline ZnO pulsed-laser-deposited on a fused silica substrate.

Two-photon pumped random laser in nanocrystalline ZnO

Thin film of ZnO nanoparticles with dimension of about 5–10nm were fabricated by the pulsed laser ablation method. By using a femtosecond laser beam at 700nm to pump micrometer-thick films the authors observed two-photon-induced lasing at 385nm. Experimentally obtained dependence of the threshold on the excitation spot radius r0 is closer to (1∕r02) than to (1∕r0), thus suggesting efficiency of the feedback by scattering-random lasing. The experimental data on nonlinear transmission of the film at the wavelength of pumping are presented.

LOW-TEMPERATURE SYNTHESIS OF SILICON OXIDE, OXYNITRIDE, AND NITRIDE FILMS BY PULSED EXCIMER LASER ABLATION

Silicon oxide, oxynitride, and nitride films are deposited, at low temperature (≤450 °C) by pulsed ArF excimer laser ablation from silicon, silicon monoxide, fused silica, and silicon nitride targets, performed under vacuum and in an oxygen atmosphere. The specific influence of laser fluence, target materials, substrate temperature, and oxygen pressure on the composition and final properties of SiOxNy grown layers is investigated using various complementary experiments such as infrared optical absorption, Rutherford backscattering, Auger electron spectroscopy, ellipsometry, and scanning electron microscopy. The process conditions are optimized in order to deposit good quality silicon oxide and silicon nitride thin films.

Dry excimer laser cleaning applied to nuclear decontamination

Abstract Excimer laser ablation is a very powerful tool of dry cleaning. This technique allows the removal or oxide or painting deposited on a material without any modifications of the chemical and physical properties of its surface. This method has been effectively used in many areas. In nuclear industry, there is a great interest to develop in developing an efficient dry decontamination process. A review of the main laser decontamination experiments performed in the world is presented. Our laser cleaning prototype based on excimer laser ablation process is described. This prototype has been tested in nuclear facilities. It is mainly composed of a XeCl laser, a bundle of fibers for beam transmission, optical systems, collection cell with filter for ablated particle recovery, computer control of cleaning efficiency and beam displacement. Different kinds of materials, which are representative of contamination usually found in nuclear field, have been irradiated. Decontamination factors (initial activity/residual activity) higher than 15 for fixed contamination and up to 100 for unfixed contamination have been obtained. These performances demonstrate that the laser-based technique is the most efficient one for dry and fast decontamination.

Synthesis and laser processing of ZnO nanocrystalline thin films

Abstract We present the results of experiments on synthesis of ZnO nanoclusters by reactive pulsed laser deposition (PLD). The nanoclusters were formed and crystallised in the gas phase and deposited on SiO2 substrates. The nanostructured films were characterised by conventional photoluminescence (PL). The PL spectra consist of a narrow UV excitonic band and a broad visible band related to defects in the film. The film preparation conditions such as the substrate temperature, ambient gas nature and pressure, were optimised in order to increase the intensity of excitonic emission and prevent the formation of defects. A post-growth annealing by UV laser radiation improved the optical quality of the deposited films. The photoluminescence intensity was found to be dependent significantly on the laser fluence and on the number of shots per site. The nature of the defects responsible for the observed luminescence in a visible range is discussed.

Analytical thermal model of ultraviolet laser ablation with single‐photon absorption in the plume

An analytical thermal model of ultraviolet (UV) laser ablation has been proposed. The approximation used to describe the absorption of the incident beam in excimer laser induced ablation plasma, for the case of single‐photon absorption σeff(t)=const. (where σeff is the effective absorption cross section of ejected species) gives analytical results on UV laser ablation kinetics. In particular, the changing (increasing or decreasing) slope of the curve of etch depth, z, versus logarithmic fluence, ln E, is explained.

Enhancement of exciton emission from ZnO nanocrystalline films by pulsed laser annealing

Pulsed ArF laser annealing in air and in hydrogen atmosphere improves the optical properties of ZnO nanostructured films. Independently on the ambient atmosphere, laser annealing produces two major effects on the photoluminescence (PL) spectra: first, the efficiency of the exciton PL increases due to the decrease of the number of non-radiative recombination centers; second, the intensity of the defect-related orange band decreases because of the removing of excessive oxygen trapped into the films during deposition. However, annealing in the ambient air also increases the intensity of the green band related to oxygen vacancies. We show that the combination of laser annealing and passivation of oxygen vacancies by hydrogen results in films free of defect-related emission and keeps intact their nanostructural character.