P. Delaporte

Optical and morphological investigation of backward-deposited layer induced by laser ablation of steel in ambient air

The irradiation of a steel surface in atmospheric pressure ambient was performed to study the surface nanostructuring resulting from the formation of a backward-deposited layer. The dynamics of the plume expansion and of the nanoparticle deposition process were investigated by in situ time-resolved optical analysis. Scanning electron microscopy and atomic force microscopy were employed to investigate the morphological characteristics of the backward deposited layer. The observations revealed a particular shape of plasma expansion. The latter is characterized by the formation of two vortices at the plasma plume periphery, where a high density of condensed nanoparticles are generated. It is shown that the surface nanostructuring is mainly due to a backward nanoparticles flux which leads to a deposition process during several tens of microseconds. The effects of laser wavelength on nanoparticle formation and surface nanostructuring are presented and discussed.

Non-linear absorption of 1.3-μm wavelength femtosecond laser pulses focused inside semiconductors: Finite difference time domain-two temperature model combined computational study

We present a theoretical model, which describes local energy deposition inside IR-transparent silicon and gallium arsenide with focused 1.3-μm wavelength femtosecond laser pulses. Our work relies on the ionization rate equation and two temperature model (TTM), as we simulate the non-linear propagation of focused femtosecond light pulses by using a 3D finite difference time domain method. We find a strong absorption dependence on the initial free electron density (doping concentration) that evidences the role of avalanche ionization. Despite an influence of Kerr-type self-focusing at intensity required for non-linear absorption, we show the laser energy deposition remains confined when the focus position is moved down to 1-mm below the surface. Our simulation results are in agreement with the degree of control observed in a simple model experiment.

Characterisation of a steel surface after XeCl laser irradiation

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.

Efficient accoustic-wave damping in a high-pulse-repetition-rate XeCl laser

Acoustic phenomena may play a leading role in high average power excimer laser as soon as a high pulse repetition rate is achieved. In this paper we demonstrate both experimentally and theoretically that acoustic wave generation and evolution are strongly dependent on electric arcs appearing at the end of the discharge. These acoustic waves induce a strong decrease of laser power throughout an amplification phenomenon of pressure fluctuations at high PRF. Acoustic dampers allow us to reduce this phenomenon and to increase the laser output power up to 50%.

Vacuum ultraviolet emissions from the ionic excimer molecules (KrCs)/sup +/ and (HeAr)/sup +/ by low-energy electron-beam excitation

Vacuum ultraviolet fluorescence emissions from the ionic excimer molecules (KrCs)/sup +/ and (HeAr)/sup +/ were studied in detail by low-energy electron-beam excitation, measurements were performed as a function of gas composition, gas pressure, and deposited energy in order to investigate the kinetic mechanisms of these molecules. The rate constants of the formation and competitive reactions were determined from the observed fluorescence signal decay. Estimations of the total fluorescence yield and of the gain coefficient for these molecules are given. >

Optical fiber transmission of a high-average-power excimer laser

Transmission of XeCl excimer laser pulses through commercial silica optical fibers with high OH content have been investigated. The Ultraviolet irradiation of these optical fibers includes color center generation and two photon absorption in the fiber core. These absorbing centers disappear at room temperature. A spectroscopic study of fiber core fluorescence permitted to clearly identify the Non Bridging Oxygen Hole Centers as main type of defects created. The influence of laser parameters on transmission evolution of the optical fiber observed during irradiation are discussed. The use of a bundle of fibers to transmit high average power excimer laser is presented. The results obtained in these experiments are promising for the use of optical fibers in a wide range of excimer laser industrial applications.

Investigations on the VUV emissions of Ne-Xe-Cs gas mixtures excited by an electrical discharge

UV emissions of Ne-Xe-Cs mixtures excited by a pulsed electrical discharge were investigated experimentally as a function of gas pressure and gas composition. For the studied discharge plasmas no fluorescence emissions originating to the radiative decay of the ionic excimer molecule (XeCs)/sup +/ at 160 nm were observed. Discharge excitation seems to be unsuitable for the generation of this ionic excimer. Previously reported fluorescence emissions due to the radiative transitions in the discharge-excited (XeCs)/sup +/ molecule could not be confirmed in our experimental conditions. An upper Limit for the unknown rate constant of the formation reaction Xe*+Cs/sup +/+Ne/spl rarr/Xe/sup +/Cs+Ne was given. >

Efficient long-pulse excimer lasers at 222 and 308 nm

Improvement in efficiency and specific output energy on an XeCl long pulse laser ((lambda) equals 308 nm) and the first achievement of long pulse (150 ns FWHM) laser emission from KrCl (B - > X) at (lambda) equals 222 nm are reported. The system, which includes x- ray preionization and double discharge (pulser/sustainer) excitation, allows potentially high laser efficiency from a small volume active medium with relatively low pressure Ne/Xe/HCl, Ne/Xe/BCl3, Ne/Kr/HCl and Ne/Kr/BCl3 working mixtures.

First results on the development of an energetic (sub)nanosecond XeCl laser system

First steps of development of an energetic (sub)nanosecond XeCl laser source are reported. For that purpose, a non- linear pulse shortening technique based on Stimulated Brillouin Scattering (SBS) is used for the temporal reshaping of an XeCl laser pump pulse. A compression factor of 10 has been demonstrated leading to the generation of a few nanosecond FWHM pulse duration pulses from a 65 mJ - 40 ns (FWHM) XeCl laser source. Further works are planned to improve the energy of the nanosecond Stokes pulse.

Laser beam quality of a high-PRF XeCl laser

A theoretical analysis is made to describe the influence of an active disturbing medium on the far-field diffraction pattern of an excimer laser beam. Results are in good agreement with experiences. The gas density perturbations induce amplitude and phase aberrations which are responsible for the degradation of the beam characteristics in the far-field.