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- ItemAnalysis of C2 SWAN bands in ablation-dominated arcs in CO2 atmosphere(Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2019) Methling, R.; Franke, St.; Götte, N.; Wetzeler, S.; Uhrlandt, D.A model circuit breaker in a high-pressure chamber filled with CO2 atmosphere is used to operate a wall-stabilized arc of several kilo-amperes between tungsten-copper electrodes surrounded by polytetrafluoroethylene nozzles. Optical emission spectroscopy (OES) is carried out via quartz plates inserted into the nozzles using a combination of an imaging spectrometer either with a high-speed video camera or with an ICCD camera. Depending on the nozzle geometry and the current, continuum from C2 Swan bands was detected as absorption as well as emission pattern. After current zero, optical absorption spectroscopy (OAS) using a xenon flashlamp as broadband background radiator was applied. An absorption around 493 nm was detected and attributed to CuF molecules. The study proofs the existence of C2 in the active phase and the formation of CuF near to current zero.
- ItemInvestigation of laser irradiated areas with electron backscatter diffraction(Amsterdam [u.a.] : Elsevier, 2012) Heinrich, G.; Höger, I.; Bähr, M.; Stolberg, K.; Wütherich, T.; Leonhardt, M.; Lawerenz, A.; Gobsch, G.In this work, two silicon nitride (SiNx) layers with two different refraction indices, deposited on polished or damageetched silicon wafers were locally irradiated by laser pulses. The focus was set on the investigation of the ablation mechanisms. Thereby, an ultra-short laser source (pulse duration 10 ps, wavelength 532 nm, Gaussian profile) was used. The irradiated areas were investigated by electron backscatter diffraction (EBSD) in order to analyze the nearsurface crystallographic orientation and crystallinity. In this work an indirect ablation was observed for SiN x (n = 1.9). Further, a change from an indirect ablation to a partial lift-off for SiNx (n = 2.1) was determined to be fluence dependent. At low fluences, the SiNx was completely removed. However, at higher fluences, SiNx was not completely removed, due to direct ablation. The two-photonabsorption coefficient of SiNx (n = 2.1) was estimated to be 2·105 cm/TW.
- ItemChirp-control of resonant high-order harmonic generation in indium ablation plumes driven by intense few-cycle laser pulses(Washington, DC : Optical Society of America, OSA, 2018) Abdelrahman, Z.; Khokhlova, M.A.; Walke, D.J.; Witting, T.; Zair, A.; Strelkov, V.V.; Marangos, J.P.; Tisch, J.W.G.We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.