Potoconductive properties and relaxation of free charges in high pure natural and CVD diamond were investigated and compared in the spectral range 1 .5-30 eV using a variety of pulsed laser sources including harmonics of radiation of 50 fs Ti:Sa laser. The resulting spectra were compared to experimental data in surface photoemission and speculated in terms of free carriers multiplication, effect of the surface layer, saturation of absorption, trapping of carriers and the following charge transfer between deep electron centers.
We present an exact treatment of wave propagation in some inhomogeneous thin films with highly space-dependent dielectric constant. It is based on a space transformation which replaces the physical space by the optical path. In the new space, the dispersion equation is that of a normal progressive wave. We will show that the dispersion properties of such films are plasma- or waveguide-like, the characteristic frequency being determined by the spatial characteristics of the dielectric constant's variations only. The theory is scalable, so that it can be applied in any wavelength range: optical, IR, radiofrequency, etc. depending only on the characteristic space scales. Several applications will be presented, concerning the reflection properties of such films (broadband anti-reflection, or dichroic coatings) or to the propagation and transmission through the film. We will show that depending on the type of space dependence, an incident wave can either propagate or tunnel through such films. We will investigate the behaviour of the light group-velocity and tunneling time inside or through such films. Though we can reproduce the phase-time saturation corresponding to the Hartman effect, analysis of the group velocity in the tunneling case shows no sign of superluminal propagation. A strong frequency dependence can be obtained in some situations, which allows to anticipate a strong reshaping of brodband laser pulses.
With the increasing use of ultrashort laser pulses and nanoscale-materials, one is regularly confronted to situations in which the properties of the media supporting propagation are not varying slowly with time (or space). Hence, the usual WKB-type approximations fail, and one has to resort to numerical treatments of the problems, with a considerable loss in our insight into the physics of laser-matter interaction. We will present a new approach which allows a fully analytical solution of such problems, based on a transformation of the propagation equations into a new space where phase accumulation is linear with either time or space, which greatly simplifies their treatment. Though this method is restricted to some special models of the time or space varying dielectric constant, those are however general enough to encompass practically all experimental situations. It allows to introduce the concept of “non-stationarity induced” (or “inhomogeneity induced”) dispersion. We will analyse the problem of reflection and propagation in two types of media whose dielectric constant vary rapidly at either the laser period or the laser wavelength scale. Extension of such techniques to the case of arbitrarily high non linearities will be considered too.
Periodical variations of the optical properties of solids are interesting for applications to optoelectronic materials. One
path to the realisation of such structures could be the spatially dependent photoburning of optically active lattice defects. In this experiment, specially prepared radiation defects in A1203 are submitted to illumination by two orthogonally polarised copropagating femtosecond pulses (polarised along the ordinary and extraordinary directions). Due to their different light velocities, the two pulses overpass each other inside the material, and the local behavior of the resulting polarisation produces a spatially periodic excitation of the defects with a limited spatial extension. We measure the defect luminescence which exhibits this spatially localised periodical structure. We studied the influence of the pulse duration on the width of this periodical structure and found that it has no effect when the pulse duration is varied using the chirp induced by group velocity dispersion. On the contrary, if the pulse duration is changed by manipulating the spectrum of the radiation, one observes a linear dependence of the spatial width of the modulation with the pulse duration. Hence, it is not the pulse duration that matters, but the coherence time.
Laser ablation of Cu, Al, Fe, Zn, Ni, Pb, and Mo by short pulse laser (800nm wavelength, 70fs pulse duration, 0.01-28 J/cm2 fluence range) in air was studied. Three different ablation thresholds were distinguished in all metals. The lowest ablation threshold was of one order of magnitude lower than the one observed previously. In the fluence range of 0.018-0.18 J/cm2 the ablation rate was ≈0.01 nm/pulse. A dependence of the threshold on the pulse duration was demonstrated in the range of 70 fs- 5 ps for cupper. As the laser pulse duration increased, the ablation threshold had the tendency to be higher. A periodic structure was observed at the bottom of the crater in all metals. The spacing d of the patterned structure was determined to be d=300±40 nm for 0.07 J/cm2 and d=600±40 nm for 0.22 J/cm2. The spacing depended on the laser fluence rather than on laser wavelength.
Crater shapes and plasma plume expansion in the interaction of femtosecond, picosecond and nanosecond laser pulses with various pure metal in air and noble gases at atmospheric pressure were studied. The craters formed at the surfaces were measured by an optical microscope profilometer with 0.01 micrometers depth and 0.5 micrometers lateral resolutions. The measurements of laser plasma expansion were carried out with ICCD camera with 3 micrometers spatial and 1 ns temporal resolutions. These measurements were made in 0-100 ns time delay range and at different wavelengths in 200-850 nm optical spectral range. Laser ablation efficiencies, crater profiles, plasma plume shapes at different time delays, rates of plasma expansion in both longitudinal and transversal directions to the laser beam were obtained. Experimental results were analyzed from the point of view of different theoretical models of laser beam interaction with plasma and metals. The laser pulse duration range used in our study was of particular interest, as it includes the characteristic time of electron-phonon relaxation in solids, that is, of the order off one picosecond. Thus, we could study the different regimes of laser ablation without and with laser beam/plasma plume interaction. It was found that for nanosecond pluses the laser beam absorption, as well as its scattering and reflection in plasma, were the limiting factors for efficient laser ablation and precise material processing with sharply focused laser beams.
The laser ablation threshold experiments were performed on pure metals with the fs Gaussian laser beam focused to 41.5 mm spot diameter onto metal surfaces. Three different ablation thresholds were distinguished. The multi-shot ablation threshold for Cu with 70 fs pulse was found to be 0.018 J/cm2 and of one order to magnitude lower than that one observed previously. In the fluence range of 0.018- 0.2 J/cm2 the ablation rate was approximately equal to 0.01 nm/pulse. The threshold dependence on the pulse duration was demonstrated in the range of 70 fs-5 ps for Cu. As the laser pulse width increased, the ablation threshold had the tendency to be higher. The ablation rate dependence on laser fluence for the other metals under study in our experiments with 70 fs was similar to that of CU.
Crater shapes and plasma plume expansion in the interaction of sharply focused laser beams with metals in air at atmospheric pressure were studied. Laser ablation efficiencies and rates of plasma expansion were obtained. The best ablation efficiency was observed with femtosecond laser pulses. It was found that for nanosecond pulses the laser beam absorption, its scattering and reflection in plasma were the limiting factors for efficient laser ablation and precise material sampling with sharply focused laser beams. The experimental results obtained were analyzed with relation to different theoretical models of laser ablation.
Plasma plume expansion and crater formation in the interaction of Nd-YAG laser beam (TEMoo-mode, 532 nm, 6 ns pulse duration and 10 micrometers waist at FWHM) with various metal targets in air were investigated. The craters formed at the surfaces were measured with 0.1 micrometers longitudinal and 0.5 micrometers transversal resolutions. Laser plasma expansion during laser pulse/surface interaction was measured by ICCD camera with 3 micrometers spatial and 1 ns temporal resolutions. These measurements were performed with different optical filters in the following spectral ranges (250< (lambda) <400 nm, 300<(lambda) <600 nm, 600< (lambda) <800 nm).
In this paper, we present measurements of the excited carrier density in various wide band gap oxides irradiated by short laser pulses, at intensities below and above breakdown threshold. This is achieved with the help of time resolved interferometry in the frequency domain, a technique which was successfully used to study the dynamics of photoexcited carriers in insulators. The result obtained in different experimental conditions, distance from the surface, pump intensities and duration, during or after the pump pulse, are discussed and compared to the models recently developed to explain optical breakdown.
Modern short pulse lasers are efficient tools for production of high levels of electronic excitation in solids under irradiation, a state which mimics that of the same materials after the passage of any particle which deposits its energy under the form of electronic excitation. Because they can also be used in a number of optical experiments of charge carriers and defect detection, they offer the unique opportunity of unraveling the ultrafast kinetic aspects of atomic processes induced by the electronic excitation, whose final state is the only aspect accessible in the case of other irradiations. After mentioning a few orders of magnitudes concerning the energy deposition, we will show some examples of recent experiments concerning the mechanisms of irradiation defect creation in insulators. The perspectives opened by recent developments of light sources in a wide range of wavelengths will be finally presented.
Laser ablation of pure metals by femtosecond, picosecond and nanosecond pulses is studied experimentally in air at atmospheric pressure. Craters created by interaction of visible and UV laser pulses with the targets are investigated. The dependence of the ablation efficiency in terms of ablated volume per unit of energy on the pulse duration and wavelength is discussed.
Laser ablation of pure metals by femtosecond, picosecond and nanosecond pulses is studied in air at atmospheric pressure. Craters created after interaction of visible and UV laser pulses with the targets are investigated. The dependence of the ablation efficiency in terms of ablated volume per unit of energy on the pulse duration is discussed.
We discuss the study of the kinetic aspects of charge trapping and defect creation which result from the intense electronic excitation caused in wide bandgap dielectrics by an intense laser irradiation. Because of the ultrashort time constants of such processes, they can only be studied using intense subpicosecond laser sources. More precisely, we present the results obtained in a number of optical material using a special interferometric measurement of the instantaneous refractive index, which allows to determine whether the photoinjected carriers are still in the conduction band or trapped in the deep defect states. Different types of materials (oxides and alkali halides) supporting excitonic charge trapping are studied, and a number of effects of the experimental conditions (in particular: excitation density and charge trapping impurity content) are described.
We present the results of recent experiments concerning the interaction of high intensity-short laser pulses with dielectric materials. The laser pulse creates a high density of free carriers, and two complementary experiments have been carried out to investigate the relaxation mechanisms in the solid, during and just after the excitation pulse. The first is a photoemission experiment, in which we measure the kinetic energy distribution of photoelectrons emitted by the surface of quartz ((alpha) -SiO2) samples irradiated by femtosecond laser pulses. We observe a high energy tail in the photoelectron spectra, which extend far beyond the photon energy. This shows that electrons photoexcited in the conduction band can absorb multiple photon and that this laser heating process is efficient even with sub-picosecond pulses. In the second experiment, we observe interferences in the frequency domain between two probe pulses. This technique allows us to measure the density of electrons photoexcited by a high intensity pump pulse in the conduction band within the sample, with a temporal resolution of 120 fs. The lifetime of the carriers is found to be 50 ps in MgO, 100 ps in Al2O3, and 150 fs in SiO2. We explain such a contrasted behavior in different oxides by the ultrafast formation of intrinsic defects in SiO2.
Huge dynamic Stark shifts of atomic energy levels in xenon induced by high-intensity laser light have
been mesured by a new method based upon multiphoton ionization by a tunable femtosecond laser source.
Stark-induced multiphoton resonances appear as identifiable structures in the photoelectron energy spectra1. The
change in the structures positions, when the photon energy is tuned, is directly related to the energy change of
the atomic levels as a function of intensity. Shifts of the order of the electron-volt have thus been measured in
xenon and found to vary linearly with intensity.