T. Desai

Surface modification of titanium nitride film by a picosecond Nd:YAG laser

The interaction of a picosecond Nd:YAG laser (wavelength 532 nm, pulse duration 40 ps) with a polycrystalline titanium nitride (TiN) film was studied. The TiN thin film was deposited by physical vapour deposition on a silicon substrate. The titanium nitride/silicon system was modified with an energy fluence from 0.2 to 5.9 J cm−2. Multi-pulse irradiation was performed in air by a focused laser beam. Surface modifications were analysed after 1–100 successive laser pulses. Depending on the laser pulse energy and pulse count, the following phenomena were observed: (i) increased surface roughness, (ii) titanium nitride film cracking, (iii) silicon substrate modification, (iv) film exfoliation and (v) laser-induced periodical surface structures on nano- (NPSS) and micro-dimensions (MPSS).

High-intensity ultrashort laser-induced ablation of stainless steel foil targets in the presence of ambient gas

Ablation depths of stainless steel targets irradiated by 80-fs laser pulses at a flux F ≤ 40 J/cm 2 (intensity ≤ 5 X 10 14 W/cm 2 ) in the presence of air at atmospheric pressure are experimentally measured. These values are lower than the theoretical predictions for metal targets in vacuum. Results are analyzed on the basis of the role of the ambient gas and of crater formation on the behavior of the ablated material.

Shock pressure induced by 0.44 [mu]m laser radiation on aluminum targets

Shock pressure generated in aluminum targets due to the interaction of 0.44 μm (3 ω of iodine laser) laser radiation has been studied. The laser intensity profile was smoothed using phase zone plates. Aluminum step targets were irradiated at an intensity I ≈ 10 14 W/cm 2 . Shock velocity in the aluminum target was estimated by detecting the shock luminosity from the target rear using a streak camera to infer the shock pressure. Experimental results show a good agreement with the theoretical model based on the delocalized laser absorption approximation. In the present report, we explicitly discuss the importance of target thickness on the shock pressure scaling.

Radiation effects on shock propagation in Al target relevant to equation of state measurements

We present one-dimensional simulations performed using the multi group radiation hydro code MULTI with the goal of analyzing the target preheating effect under conditions similar to those of recent experiments aimed at studying the Equation of State (EOS) of various materials. In such experiments, aluminum is often used as reference material; therefore its behavior under strong shock compression and high-intensity laser irradiation (10 13 –10 14 W/cm 2 ) should be studied in detail. Our results reveal that at high laser irradiance, the laser energy available to induce shock pressure is reduced due to high X-rays generation. Simultaneously X-rays preheat the bulk of the reference material causing significant heating prior to shock propagation. Such effects induce deviations in shock propagation with respect to cold aluminum.

Surface morphology modifications of human teeth induced by a picosecond Nd:YAG laser operating at 532 nm

The interaction of an Nd:YAG laser, operating at 532 nm with 40 ps pulse duration, with human teeth was studied. The results show that teeth were significantly modified at an energy fluence of about 11 J/cm 2 . Various surface morphologies of enamel and dentine were recorded. Features on enamel include crater (conical form) in the central part and cauliflower morphology at the periphery, whereas on dentine the crater looks like a stretched dome between sharp edges. The behavior of the enamel-dentine junction area showed different morphology with respect to both tooth enamel and dentine alone. Finally, the junction channel showed a removal of collagen fibers and the formation of a needle-like bottom structure. Generally, this investigation showed that the picosecond Nd:YAG laser can ablate a tooth surface practically instantaneously, implying that large tooth surfaces can be processed in short time.

Surface Modification of Titanium by High Intensity Ultra-Short Nd:YAG Laser

The effects of an Nd:YAG laser interaction with titanium target using laser radiation at wavelengths 1.064 or 0.532 μm (40 picoseconds pulse duration) were studied. Modification of target surfaces at laser energy densities of 2.4 and 10.3 J/cm2 (λ1 laser= 1.064 μm) and 1.1 J/cm2 (λ2 laser= 0.532 μm) are reported in this article. Qualitatively, the titanium surface modification can be summarized as follows: (i) ablation of the titanium surface in the central zone of the irradiated area for both laser wavelengths; (ii) appearance of a hydrodynamic feature like resolidified droplets of the material (λ1 laser= 1.064 μm), as well as formation of the wave-like microstructures (λ2 laser= 0.532 μm); and (iii) appearance of plasma, in front of the target, with both laser wavelengths.

Laser-induced ablation and crater formation at high laser flux

Laser ablation and crater formation have been studied on a copper target using a 10 Hz Nd:YAG laser system delivering pulses up to 100 mJ in 40 ps with a flux on target F ≤ 5000 J/cm2. Crater dimensions were measured using optical microscope or scanning electron microscope. In order to understand the process of crater formation, we considered various theoretical models present in the literature and revised them taking into account the occurrence of plasma phenomena, which are important at the intensities used in this experiment. We also compared our experimental results with other results obtained at the PALS laboratory, using a 0.44 μm wavelength laser and much higher laser intensities. Finally, we explore the possibility of extending the information derived from laser-produced craters to other types of craters.

Superficial changes on the Inconel 600 superalloy by picosecond Nd:YAG laser operating at 1064, 532, and 266 nm: Comparative study

AbstractA comparative study of superficial changes on the superalloy Inconel 600, induced by a picosecond Nd:YAG laseroperating at 1064, 532, and 266 nm, is presented. All of the laser wavelengths, as well as the used fluences of 2.5(1064 nm), 4.3 (532 nm), and 0.6 J/cm 2 (266 nm) were found to be adequate for inducing surface variations. Quitedifferent surface features were produced depending on the laser wavelength used. The measured surface damagethresholds were 0.25, 0.13 and 0.10 J/cm 2 for 1064, 532, and 266 nm, respectively. Drastic differences, in function ofthe wavelength used, were recorded for the crater depths, as well the appearance of hydrodynamic effects and periodicsurface structures. Differences in crater depths were explained via an easier propagation of the first harmonic laserradiation (1064 nm) through the ejected material and plasma compared to a radiation at 532 and 266 nm. Finally,changes in the surface oxygen content caused by ultrashort laser pulses were considered.Keywords: Laser ablation; Periodic surface structures; superalloy Inconel; Surface morphology

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm2 were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.

Modification of multilayered TiAlN/TiN coating by nanosecond and picosecond laser pulses

A multilayered TiAlN/TiN coating deposited on H11 work-steel was irradiated by a TEA CO2 laser (ns pulses) and a Nd:YAG laser (ps pulses), and the effects compared. The coating was 2.17 µm thick and consisted of 45 layers. The laser-induced modifications showed dependence on laser pulse duration, pulse count and laser wavelength. The conditions for coating ablation in both cases were determined. The experiment has revealed laser-induced periodic surface structures (LPSS) on nanometre and micrometre scales, depending on the laser wavelength used. Sample surfaces were characterized before and after laser irradiation by an optical microscope, scanning electron microscope (SEM), focused ion beam (FIB) microscope and profilometry.