Showing papers by "Arkadi Rosenfeld published in 2001"

On the physics of material processing with femtosecond lasers

Abstract: Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a “gentle” phase with low ablation rates and a “strong” etch phase with higher ablation rates, but with limitation in the structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals, and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with charge-scaled momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (“gentle” etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron-mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.

Processing of dielectric materials with shaped, ultrashort pulses and THz pulse trains

Abstract: Summary form only given. Ultrashort pulsed laser excitation of dielectrics has evidenced a material dependent response to radiation, reflected in individualized times necessary to deposit the laser energy into the lattice. Electronic mechanisms are observed in the ultrafast ablation of dielectrics, dependent on either electrostatic energy accumulation in the upper layers following intense photoelectron emission and surface break-up by Coulomb explosion, or excitation-induced lattice displacements associated with defect formation and electron trapping. These non-thermal channels for energy deposition compete with the thermal channels based on electron-phonon coupling with result in local heating. A presentation of the time scales involved in laser coupling with dielectric targets is given for a sapphire (Al/sub 2/O/sub 3/) sample.

Verfahren zur direkten Mikrostrukturierung von Materialien Method for direct microstructure of materials

Abstract: Bei einem Verfahren zur direkten Mikrostrukturierung von Materialien mittels mindestens eines ultrakurzen Einzelpulses oder einer Pulsfolge mit definiertem Energieeintrag in das Material werden erfindungsgemas zur Vermeidung von Mikrorissen und Spannungen nacheinander mindestens zwei zeitlich geformte Laserpulse oder Pulszuge auf die Oberflache des zu bearbeitenden Materials gerichtet und wird der Abstand zweier aufeinander folgender Pulse oder Pulszuge kleiner oder gleich Pikosekunden eingestellt, sodass der folgende Puls noch in die bewirkte Anderung des ersten Pulses im zu bearbeitenden Material trifft, und Energie und Dauer des Pulses werden in Abhangigkeit vom zu bearbeitenden Material eingestellt. In a method for the direct microstructuring of materials by means of at least one ultrashort individual pulse or a pulse sequence with a defined energy input into the material according to the invention directed at the surface of the material to be machined in order to avoid of micro-cracks and stresses in succession at least two temporally shaped laser pulses or pulse trains, and the distance two successive pulses or pulse trains is less than or equal to set picoseconds, so the following pulse still meets in the induced change of the first pulse in the material to be processed, and energy and duration of the pulse can be adjusted depending on the material to be processed.