We demonstrate the use of supercontinuum radiation to provide enhanced guiding distances of microscopic particles
compared to the standard continuous wave or femtosecond lasers. Our technique relies on the chromatic aberration of the
lens used to form an elongated focal region within which guiding takes place. The resulting beam profile has been
modelled and shows that for a Gaussian input beam, the intensity profile after the lens can be considered as a sum of
Gaussians, one for each wavelength but with varying focal position due to dispersion. Our experimental investigations
compare radiation from continuous wave (bandwidth <1nm) and femtosecond pulsed (bandwidth > 100nm) lasers as
well as supercontinuum radiation (bandwidth > 450nm) and show good agreement with theory.
The generative process of selective laser sintering of powders such as Titanium, Platinum alloys and steel can in comparison to cw radiation significantly be improved by using pulsed radiation. With an appropriate energy deposition in the metallic powder layer, the material properties of the selective laser sintered parts can locally be tailored to the requirements of the finished work piece. By adapting the laser parameters of a Q-switched Nd:YAG laser, notably pulse duration and local intensity, the degree of porosity, density and even the crystalline microstructure can be controlled. Pulsed interaction allows minimizing the average power needed for consolidation of the metallic powder, and leads to less residual thermal stresses. With laser post processing, the surface can achieve bulk-like density. Furthermore, we present the possibility of forming metallic glass components by sintering amorphous metallic powders.
Using pulsed near infrared laser radiation for selective laser sintering bears several advantages compared to cw sintering such as low requried average power, less residual heat and improved lateral precision. By adapting the pulse length (and thus the heat diffusion length during the pulse) to the grain size of the used metal powder, the laser pulse energy can mainly by deposited in the skin of the powder particles where heating and melting is obtained, whereas the centers of the grains remain at much lower temperature and act as heat sinks after consolidation. The model described here was numerically implemented and experimentally tested with a pulsed Nd:YAG laser on titanium powder. The results of the model predictions and the performed experiments are in good agreement.
Laser-induced structure transformations in diamond single crystals implanted with light ions (H+, D+, He+) are studied by monitoring changes in the material density and optical transmission in dependence on UV laser pulsed irradiation parameters and ion implantation conditions. Characteristic features of the processes of laser annealing, graphitization, low-rate etching and explosive ablation of ion-implanted diamond layers are discussed.
Results are reported on the study of phase transformations in diamonds induced by nano- and picosecond pulses of KrF excimer laser (λ=248 nm) and second harmonic of a YAP:Nd laser (λ=539 nm). Main attention in the research was paid to i) laser-induced graphitization of high-quality CVD diamond plates and ii) laser-induced structure transitions in ion-implanted diamond single crystals. For CVD diamond, the thickness of the laser-graphitized surface layers was measured and the accumulation period for graphitization to occur was found to be longer for lower laser fluences. In the experiments with ion-implanted diamonds, multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, i.e., in an increase of the optical transmission and surface contraction. Under certain low-intensity irradiation conditions, it was also found that, competing with the annealing process, laser etching of the ion-implanted diamond occurred at extremely low rates of 10-4-10-3 nm/pulse. A correlation between the defect concentration distribution and graphitization thresholds in partially annealed ion-implanted diamonds is discussed.
The densification processes occurring in metallic powders upon interaction with pulsed laser radiation have been studied experimentally and compared with results obtained from a numerical simulation model. The analysis of the sintered samples shows consolidation features, which are in very good agreement with the model predictions. The limited amount of molten material due to pulsed interaction and the recoil pressure exerted on the powder by the plasma and the ablation plume allow to achieve a predetermined density (or porosity) from less than 60% to more than 90% of the bulk density, depending on the laser parameters and processing steps. A wider range of porosity could be achieved by mixture of the metallic powder with later removable spacers of the desired size.
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