O. Utéza

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.

Toward determinism in surface damaging of dielectrics using few-cycle laser pulses

We introduce a quantitative measurement of the determinism of laser-induced damaging at the surface of a dielectric material, e.g., fused silica. Using laser pulses ranging from 7 to 300 fs, we demonstrate that laser damage occurrence tends to be dramatically deterministic at 7 fs, which is attributed to the increasing importance of tunneling ionization as the major channel for the generation of free-carriers in the conduction band.

Long range nanostructuring of silicon surfaces by photonic nanojets from microsphere Langmuir films

Large arrays of sub-micrometre blind holes and with a filling ratio up to 60% on areas of millimetre square are realized on silicon. The structuration ensues from combining both Langmuir–Blodgett deposition technique and ultraviolet nanosecond laser-assisted photonic nanojet ablation through C18 functionalized silica microspheres. Different laser fluence ranges and numbers of laser shots are studied to understand the tradeoff between size, quality of the craters and surface morphology after laser irradiation. In particular, tuning the irradiation fluence yields selectivity of the characteristic lateral dimension of the imprinted craters on the substrate and laser operation in multishot mode allows obtaining high quality and regularity of the surface morphology of the resulting millimetre square arrays of holes. This simple, fast, long-range and low-cost near-field nanolithography technique is of interest for fabricating devices with new functionalities and finds applications in many fields in nanoscience and nanoengineering.

Control over a phase state of the laser plume ablated by femtosecond laser: Spatial pulse shaping

The conditions for the formation of a fully atomized laser-ablated plume using subpicosecond laser pulses have been studied theoretically and implemented experimentally. It is shown that the low-intensity wings, which generally exist in the spatial distribution of laser intensity in the focal plane and can contain a substantial part of the incident laser energy, are responsible for low-threshold phase transformations of the target and this is the major source of particulates in the ablated plume. By truncating these wings with a simple aperture and positioning the target in the image plane of that aperture, particulates can be eliminated from the plume. This is demonstrated in experiments on deposition of Si films using the truncated beam in combination with the proper choice of laser fluence. This results in an almost totally atomized plume and consequently in droplet-free deposition of thin films.

Radioactive oxide removal by XeCl laser

We present the results on cleaning, achieved in nuclear facilities, of metallic surfaces polluted by radioactive oxides. Experimental results of excimer laser decontamination have been obtained for different radionuclides (Cs, Co, Eu, etc.) deposited under various conditions (fixed or unfixed contamination). Our laser decontamination prototype is 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. We show that the use of the excimer laser ablation technique for decontamination of nuclear facilities has many advantages such as the possibility of remote control, dry process, and, especially, the absence of secondary wastes (clean process). Decontamination factors (DF: initial activity/residual activity) higher than 15 for fixed contamination and up to 100 for unfixed contamination are obtained.

Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting

The reversible phase transition induced by femtosecond laser excitation of Gallium has been studied by measuring the dielectric function at

Dynamics of femtosecond laser absorption of fused silica in the ablation regime

775\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

Self-limited underdense microplasmas in bulk silicon induced by ultrashort laser pulses

with

Absorption of a single 500 fs laser pulse at the surface of fused silica: Energy balance and ablation efficiency

\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{fs}

Complete characterization of damage threshold in titanium doped sapphire crystals with nanosecond, picosecond and femtosecond laser pulses

temporal resolution. The real and imaginary parts of the transient dielectric function were calculated using the Fresnel formulae from the absolute reflectivity of a Gallium layer measured at two different angles of incidence. The time-dependent effective electron-phonon collision frequency; the heat conduction coefficient; and the volume fraction of the new phase were recovered directly from the experimental data. The time and space dependent electron and lattice temperatures in the layer undergoing the phase transition were reconstructed without ad hoc assumptions. We converted the temporal dependence of the electron-phonon collision rate into its temperature dependence, and demonstrated that the electron-phonon collision rate has a nonlinear character. This temperature dependence converges to the known equilibrium function during the cooling stage. The maximum fraction of the new phase in the laser-excited Gallium layer reached only 60% even when the deposited energy was twice the equilibrium enthalpy of melting. We demonstrate that the rate at which the phase transition proceeds and a fraction of the material transformed into the new phase depends strongly on the temperature of the laser-excited Gallium layer, and is restricted by the thickness of this layer which is only several tens of nanometers for the whole range of the pump laser fluences. The kinetics of the phase transformation after the laser excitation can be understood on the basis of the classical theory of the first-order phase transition.