Physical Processes in Solids Exposed to Supershort Laser Pulses (5-500 fs) Near the Damage Threshold

In the last few years we have witnessed a tremendous progress in the development of femtosecond lasers. Nowadays compact reliable and stable ultrashort lasers are commercially available. This has opened the way to use femtosecond lasers for material processing. Ultrashort lasers are of great importance where micrometer sized structures have to be manufactured with a steep edge and a minimized heat affected zone. Triggered by the technological interest several research groups started research on the interaction of intense femtosecond laser pulses with matter. There are basically two different tasks which could be tackled simultaneously: On one hand our research concentrated on technical aspects such as optimizing the parameters to minimize spot size and make the holes cleaner. On the other hand we tried to understand the underlying physical processes which causes damage. The most important parameter for material processing with ultrashort laser pulses is their high peak intensity at a simultaneously low pulse energy. Nontransparent materials such as metals or semiconductors absorbs laser light directly. In transparent materials such as glass, diamond or quartz (all of these materials are dielectrica) we have to rely on nonlinear absorption. Nonlinear absorption could be provided by multi-photon-absorption, i.e. light can only be absorbed if a certain number of photons are present at the same time. Consequently multi-photon absorption scales with the intensity of the laser pulses. Absorption could be also provided by "defects". "Defect" absorption is in most cases linear, but the number of "defects" depends nonlinearly on the intensity. After absorption, the stored laser energy heats the solid resulting in melting, further in boiling which leads to a removal of material. In this project we mainly concentrated us on the study of the primary absorption process in dielectrics and semiconductors. We tried to answer the following questions: Which of the above described nonlinear absorption processes is the dominant one for different pulse durations? Has the absorption process an influence on the quality of the manufactured structures? For which material is it beneficial to use the shortest available pulses for material processing? Using ultrashort laser pulses we noticed a dramatic change of the ablation characteristics with pulse duration. For glasses we observed a higher precision and reproducibility of the ablation characteristics by reducing the pulse duration and energy fluence at once. These findings have been confirmed in several measurements conducted on different materials. Based on our experimental results and further theoretical studies we identified multiphoton absorption as the dominant nonlinear absorption process in the sub-100fs regime. For applications of ultrashort laser pulse for ablation the most important feature is the manufacturing of very small structures. Due to the very well defined ablation characteristic caused by the nonlinear absorption, if the shortest available pulses are used, it is possible to make structures with a diameter smaller than the laser spot size and no heat affected zone. Due to this unique properties there are several applications in industry and medicine. Possible industrial applications include direct writing of DVDs or laser trimming of integrated circuits. In medicine ultrashort pulses could be beneficial in ophthalmology and dentistry.