Thermography methods have found their applications in different fields of human activity. The non-destructive feature of
these methods along with the additional advantage by automated remote control and tests of nuclear installations without
personnel attendance in the contaminated zone are of particular interest. Laser active pyrometry and laser lock-in
thermography for in situ non-destructive characterization of micrometric layers on graphite substrates from European
tokamaks were under extensive experimental and theoretical studies in CEA (France). The studies were aimed to obtain
layer characterization with cross-checking the layer thermal contact coefficients determined by active laser pyrometry
and lock-in thermography. The experimental installation comprised a Nd-YAG pulsed repetition rate laser (1 Hz - 10
kHz repetition rate frequency, homogeneous spot) and a home-made pyrometer system based on two pyrometers for the
temperature measurements in 500 - 2600 K range. For both methods, the layer characterization was provided by the best
fit of the experimental results and simulations. The layer thermal contact coefficients determined by both methods were
quite comparable. Though there was no gain in the measurements accuracy, lock-in measurements have proved their
advantage as being much more rapid. The obtained experimental and theoretical results are presented. Some practical
applications and possible improvements of the methods are discussed.
At the bottom of ablation craters produced in many materials, e.g. dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the incident wavelength. Their orientation depends strongly on the laser polarization but not on any intrinsic crystalline parameters. An increasing number of shots results in higher contrast, better developed structures, indicating a positive feedback. The region around the impact is shown, by micro Raman spectroscopy, to undergo phase transformations like under high pressure. The structure spacing appears to depend crucially on the depth of the perturbed volume, i.e. the incident (and absorbed) energy. All observations suggest that the structures form by self-organization from instabilities induced in the material by the laser input. A general picture suggests that the irradiation results in a rapid, non-equilibrium destabilization of the crystal structure, which should not be confused with melting as a classical thermodynamic process (i.e. temperatures defined as equilibrium properties). Relaxation from this instability results in the self-assembly of the observed structures. Theoretical simulations demonstrate the feasibility of this model, which also is corroborated by comparison to other unstable situations.
Polycrystalline SiGe is attracting more and more attention in micro and optoelectronics devices both at industrial and university level. Research on both devices and material growth techniques continues at a very rapid pace in the scientific world. Low cost production techniques, capable to produce such alloys with uniform and controlled grain size, becomes of particular attention. Excimer laser crystallization has proved to be a valuable how thermal budget technique for amorphous silicon crystallization. Its main advantages are the high process quality and reproducibility joint to the possibility of tailoring the grain sizes both in small selected regions and in large areas. This technique is here applied for producing poly-SiGe alloys from amorphous SiGe films deposited on glass.
Laser ablation of Cu, Al, Fe, Zn, Ni, Pb, and Mo by short pulse laser (800nm wavelength, 70fs pulse duration, 0.01-28 J/cm<sup>2</sup> 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/cm<sup>2</sup> 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 <i>d</i> of the patterned structure was determined to be <i>d</i>=300±40 nm for 0.07 J/cm<sup>2</sup> and <i>d</i>=600±40 nm for 0.22 J/cm<sup>2</sup>. The spacing depended on the laser fluence rather than on laser wavelength.
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/cm<SUP>2</SUP> and of one order to magnitude lower than that one observed previously. In the fluence range of 0.018- 0.2 J/cm<SUP>2</SUP> 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 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.
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 (TEM<SUB>oo</SUB>-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).
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.
Method of relative deviation plot of line intensities for LTE presence verification and excitation temperature determination is suggested and analyzed. This method seems to be attractive as it does not require the values of the Einstein constants A<SUB>ji</SUB> of spontaneous emission.
We have investigated the craters created after high intensity visible and UV-laser pulse interaction with the metal surfaces in air. The experiments were performed on different pure metal samples in the intensity range typical for laser plasma analytical application experiments with nanosecond laser pulses. The craters were characterized by their depth, diameter and volume. The analysis and discussion of the obtained results are presented.
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.
Particular features of cw-laser radiation resonant interaction with ions of collisionless plasma in homogeneous strong magnetic field (order of some Tesla) are investigated theoretically. It is found that the particle rotation movement changes dramatically the resonant interaction features and the properties of laser-induced fluorescence signal that was proved experimentally. The possibility of 'ion-cyclotron photon echo' observation from coherently excited ions in homogeneous strong magnetic field is discussed.