J. Etchepare

Comparison between ZnO films grown by femtosecond and nanosecond laser ablation

We have studied the structural properties of ZnO thin films grown on Al2O3 (00.1) single-crystal substrates by pulsed-laser deposition using either a femtosecond or a nanosecond laser. Although hexagonal ZnO films deposited on sapphire substrate were epitaxially grown in both cases, the crystalline quality was found to be very different: ZnO films grown with the femtosecond laser are characterized by a higher mosaicity, a smaller crystallite size, a larger content of defects but also smaller residual stresses than ZnO films obtained by nanosecond laser ablation. These differences can be explained according to the kinetic energy of the species evolved during laser ablation as deduced from plasma characterization with a charged-coupled device camera: close to 1 KeV in the femtosecond regime for the population species emitted from the target with the highest velocity, versus a few hundreds of eV in the case of nanosecond pulses. The high energy species irradiation associated with a femtosecond laser is likel...

Polarization selectivity in time-resolved transient phase grating

Abstract A selective polarization approach to transient phase grating experiments is presented. The high potentiality of such a technique lies in the fact that nonlinear signals show not only a temporal behaviour but also symmetry properties associated to each physical process involved, through the polarization states of the four coming beams. Subpicosecond resolution measurements of the kinetics of liquid CS 2 transient anisotropy have been performed in this way. We demonstrate that even using ultra short pulses, polarization analysis alone allows full discrimination between electronic and molecular processes, both of which intervene in the induced nonlinearity. Futhermore, a temporal delay in the onset of the molecular processes is evidenced for the first time. It is accounted for by a non instantaneous orientation of the molecules along the actinic optical field.

Generation of high-peak-power tunable infrared femtosecond pulses in an organic crystal: application to time resolution of weak infrared signals

High-intensity femtosecond pulses tunable in the 0.8–1.6-μm range have been generated by parametric amplification of a continuum white light in a new organic crystal, N-(4-nitrophenyl)-L-prolinol (NPP). The traditional concept of noncritical phase matching was revised in view of requirements linked to the observation of ultrafast subpicosecond nonlinear phenomena. The notions of θ (noncritical) and λ (noncritical) phase matching are introduced together with their applications. An experimental determination of phase-matching curves for both second-harmonic generation and three-wave mixing has been carried out. A θ noncritical phase-matching configuration for second-harmonic generation at 1.15 μm and a quasi-λ noncritical phase-matching configuration in the near IR for three-wave mixing were evidenced. Frequency and pump-intensity dependences of the gain have also been studied. Parametric emission at degeneracy was observed, with the emitted bandwidth extending from 1.0 to 1.4 μm. Time resolution of the amplified signal has been carried out by cross correlating the pump with the incoming signal, evidencing a reduced time broadening of the interacting pulses; a new spectroscopic method with subpicosecond time resolution is derived from the previous nonlinear optical characterization experiments by replacing the IR continuum from the water cell by any sample emitting in the same frequency range. This method, termed parametric amplification and sampling spectroscopy, was used for temporal analysis of amplified and emitted infrared signals generated in an NPP crystal.

Femtosecond spectroscopy with high-power tunable optical pulses

Femtosecond techniques permitting the generation of intense optical pulses tunable from the near UV to the near IR are presented. Implications for chemistry, biology, and solid-state physics are discussed. Specific cases are developed for applications such as the comparison of time-resolved polarization and absorption studies in photoexcited GaAs or malachite green in water.

Growth of heteroepitaxial ZnO thin films by femtosecond pulsed-laser deposition

ZnO thin films have been grown on various substrates by femtosecond pulsed-laser deposition. According to optical microscopy and atomic force microscopy analyses, the production of droplets is not significant using femtosecond pulses. Smooth, dense, stoichiometric, crystalline, and textured hexagonal ZnO films are epitaxially grown on (0001) sapphire at 700 °C with an in-plane epitaxial relationship corresponding to a 30° rotation of the ZnO basal plane with respect to the sapphire. Nevertheless, channeling experiments and rocking curve measurements show that the crystalline quality is not as good as that obtained with nanosecond pulses.

Interaction of a laser-produced plume with a second time delayed femtosecond pulse

Time resolved emission spectroscopy coupled with a secondary time-delayed femtosecond pulse technique has been used to study laser–matter interaction that occurs within ablation processes from a solid target, in the 1012–1014W∕cm2 energy range. It allows an examination of the emitted optical signals that characterize the species escaping from the target, namely ions, neutrals, and nanoparticles. Size distributions of nanoparticles are deduced from an analysis of the deposition substrate. The newest result concerns the huge drop of emission signal from the nanoparticles, which occurs at a delay (0.8

Generation of high peak power subpicosecond pulses in the 1.0–1.6 μm range by parametric amplification in an organic crystal

High peak power subpicosecond optical pulses have been generated by parametric amplification of a white‐light femtosecond continuum in a new organic crystal, N‐(4‐nitrophenyl)‐L‐prolinol or (NPP), with an emission spectrum extending in the 1.0–1.6 μm range.

Electronic nonlinear optical susceptibilities of silicate glasses.

Femtosecond techniques have been used to measure the electronic contribution to the nonlinear susceptibility of several standard glasses. The relation between their nonlinear efficiency and the content of network-modifying cations has been determined.

Third-order electronic susceptibilities of liquids measured by femtosecond kinetics of optical Kerr effect

Intense femtosecond optical pulses are used to discriminate temporally among the different processes involved in the third-order-induced polarization. The tensor elements deduced from the instantaneous measured components are compared with previous frequency-domain experimental results.

Subpicosecond time‐resolved luminescence spectroscopy of highly excited CuCl

An apparatus for the detection of luminescence spectra with subpicosecond resolution is described. It is applied to the measurement of the formation time of excitonic particles in CuCl.