Volume 93, Issue 11, 01 June 2003
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
93(2003); http://dx.doi.org/10.1063/1.1570951View Description Hide Description
Plasmachargecurrent distribution during deep penetration laser welding was analyzed theoretically and experimentally. The laser-induced plasma above the workpiece surface expands up to the nozzle, driven by the particle concentration gradient, forming an electric potential between the workpiece and the nozzle due to the large difference between the diffusion velocities of the ions and the electrons. The plasma-induced current obtained by electrically connecting the nozzle and the workpiece can be increased by adding a negative external voltage. For a fixed set of welding conditions, the plasmachargecurrent increases with the external voltage to a saturation value. The plasmachargecurrent decreases as the nozzle-to-workpiece distance increases. Therefore, closed-loop control of the nozzle-to-workpiece distance for laser welding can be based on the linear relationship between the plasmachargecurrent and the distance. In addition, the amount of plasma above the keyhole can be reduced by a transverse magnetic field, which reduces the attenuation of the incident laser power by the plasma so as to increase the laser welding thermal efficiency.
93(2003); http://dx.doi.org/10.1063/1.1569996View Description Hide Description
To investigate the photonicband structure of a drilled alternating-layer photonic crystal (DALPC), light transmittances were measured.Electron beam lithography was used to prepare DALPCs that had four periods in the direction with air holes penetrating the whole structure. The transmittances of TE/TM polarized lights were measured for three directions There was a common transmittance attenuated band at a wavelength of around 1.4 μm in the transmission spectra for all measured directions and polarizations. This agreed well with the photonic band gap(PBG) predicted by calculation. The results supported the good potential of Si-based DALPCs as full PBG materials.
Second harmonic generation tuning curves in quasiphase-matched potassium titanyl phosphate with narrow, high-intensity beams93(2003); http://dx.doi.org/10.1063/1.1563302View Description Hide Description
The second harmonic generation(SHG) tuning curves with temperature and angle of incidence were measured in periodically poled for narrow fundamental beams at intensities typical of quadratic soliton generation. Mutual self-focusing of the fundamental and harmonic, cascading, and walk-off for light incidence away from the poling axis drastically distorted the SHG response curves, in good agreement with theory.
Laser cleaning of alumina particles on glass and silica substrates: Experiment and quasistatic model93(2003); http://dx.doi.org/10.1063/1.1572541View Description Hide Description
Silica and microscope slides that had been pretreated with a variety of wet chemical, ultrasonic and laser irradiationsurface preparation methods were contaminated with alumina particles of around 3 μm. The samples were prepared and processed without delay so as to avoid capillary condensation in a high humidity environment and thus ensuring that the process is dry laser cleaning. They were then irradiated with a pulse of 248 nm radiation from a KrF excimer laser, over a range of fluences. The laser cleaning results show that, in this case, the surface preparation method appears to have little effect on the laser cleaning efficiency. They also demonstrate that the threshold fluence for particle removal of silica is approximately eight times higher than that for microscope slides, when so prepared. In contrast, application of Arnold et al.’s thermal expansion model of laser cleaning in the quasistatic limit to this material system predicts that the threshold fluence for silica will be about 15 times greater than that for glass. One possible explanation for this discrepancy may be that the particle’s absorptivity (0.16) is inaccurate. A value of 0.35 gives good agreement between theory and experiment.