Shaped Femtosecond Laser Pulse Control Of The Ablation Processes Of Condensed Materials

The control of the reaction between laser and solids(laser ablation) has been concerned in recent years. There's more and more recearch on how to control the laser ablation in order to study the physical mechanism and to expand the ablation's commercial using.In this thesis, we mainly studied the mechanism and morphology of the ablation of Si using shaping pulse. As the simplest shaping pulse double pulse produced by a beam splitter is first used to ablate the sample. When Si surface is at different distance to the laser focus, the changing of the fluorescence intensity is monitoreded as the delay time between double pulse is changed , we found that the fluorescence intensity is the lowest when the two pulse is overlapping and increases with the delay time increasing, and the increasing will stop at the certain delay time and then decline again. We compared the result of different laser intensity and explained the reason of the above-mentioned phenomenon.Pulse shaping technology modulates the shape of a single transform limited pulse and is used in many fields. My second work was to use the spacial light modulator which is computer-controled to modulate the 90fs,800nm laser pulse, then use the shaped pulse to ablate Si and optimaze the fluorescence signal. An optimization procedure is based on the feedback of the fluorescence signal in a learning genetic algorithm that iteratively improves the phase of the applied femtosecond laser pulse. We found that the fluorescence signal is increasing as the generation going on and will reach saturation when the optimum phase is found. This proves that genetic algorithm optimization can be used in the field of ablation dynamics control.The third part of my work was to drill copper sheet using various shaped pulses. We drilled the copper sheet both in air and in vocuum. The main difference of the morphology is the aggradations of the ejected maerial. In air, most ejected particle will deposite around the ablation hole, foming a rim around the hole. In vacuum, the surface of the sample is clean.