Marc Sentis

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.

Material decomposition mechanisms in femtosecond laser interactions with metals

A numerical hydrodynamic study of femtosecond laser ablation is presented. A detailed analysis of material decomposition is performed using a thermodynamically complete equation of state with separate stable and metastable phase states and phase boundaries. The lifetime of the metastable liquid state is estimated based on the classical theory of homogeneous nucleation. In addition, mechanical fragmentation of the target material is controlled based on available criteria. As a result, several ablation mechanisms are observed. A major fraction of the ablated material, however, is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas-mixture or mechanically at high strain rate and negative pressure into liquid droplets and chunks. The calculation results explain available experimental findings.

Possible surface plasmon polariton excitation under femtosecond laser irradiation of silicon

The mechanisms of ripple formation on silicon surface by femtosecond laser pulses are investigated. We demonstrate the transient evolution of the density of the excited free-carriers. As a result, the experimental conditions required for the excitation of surface plasmon polaritons are revealed. The periods of the resulting structures are then investigated as a function of laser parameters, such as the angle of incidence, laser fluence, and polarization. The obtained dependencies provide a way of better control over the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material.

Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy.

Offering mild, non-invasive and deep cancer therapy modality, radio frequency (RF) radiation-induced hyperthermia lacks for efficient biodegradable RF sensitizers to selectively target cancer cells and thus avoid side effects. Here, we assess crystalline silicon (Si) based nanomaterials as sensitizers for the RF-induced therapy. Using nanoparticles produced by mechanical grinding of porous silicon and ultraclean laser-ablative synthesis, we report efficient RF-induced heating of aqueous suspensions of the nanoparticles to temperatures above 45-50°C under relatively low nanoparticle concentrations (<1 mg/mL) and RF radiation intensities (1–5 W/cm2). For both types of nanoparticles the heating rate was linearly dependent on nanoparticle concentration, while laser-ablated nanoparticles demonstrated a remarkably higher heating rate than porous silicon-based ones for the whole range of the used concentrations from 0.01 to 0.4 mg/mL. The observed effect is explained by the Joule heating due to the generation of electrical currents at the nanoparticle/water interface. Profiting from the nanoparticle-based hyperthermia, we demonstrate an efficient treatment of Lewis lung carcinoma in vivo. Combined with the possibility of involvement of parallel imaging and treatment channels based on unique optical properties of Si-based nanomaterials, the proposed method promises a new landmark in the development of new modalities for mild cancer therapy.

Combined continuous–microscopic modeling of laser plume expansion

A hybrid model is developed to study the dynamics of laser plume expansion in vacuum or into a background gas. The method takes advantages of both continuous and microscopic simulation approaches. As a result of the combination of two different numerical methods, such as, large particles and direct Monte Carlo simulation, the model describes high-rate laser ablation for a wide range of background pressures (from zero to hundreds Pa). The model is used to investigate laser plume interaction with background gases. Particularly, the plume–gas mixing and energy exchange are taken into account. The dynamics of the laser plume expansion is investigated. Snowplow effect is observed at sufficiently high pressures. At smaller pressures, strong plume–gas mixing takes place near the contact surface. The simulation results explain experimentally obtained spatial maps of the reaction products formed during the plume expansion into a reactive background gas.

Size-controllable synthesis of bare gold nanoparticles by femtosecond laser fragmentation in water

We report a size-controllable synthesis of stable aqueous solutions of ultrapure low-size-dispersed Au nanoparticles by methods of femtosecond laser fragmentation from preliminary formed colloids. Such approach makes possible the tuning of mean nanoparticle size between a few nm and several tens of nm under the size dispersion lower than 70% by varying the fluence of pumping radiation during the fragmentation procedure. The efficient size control is explained by 3D geometry of laser fragmentation by femtosecond laser-induced white light super-continuum and plasma-related phenomena. Despite the absence of any protective ligands, the nanoparticle solutions demonstrate exceptional stability due to electric repulsion effect associated with strong negative charging of formed nanoparticles. Stable aqueous solutions of bare gold nanoparticles present a unique object with a variety of potential applications in catalysis, surface-enhanced Raman spectroscopy, photovoltaics, biosensing and biomedicine.

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.