The research objective of this award focuses on the interaction between ultra-short laser pulses and materials and seeks to test the hypothesis that the target material?s electrons are accelerated into the material during ultra-short laser pulse irradiation resulting in an energy deposition thickness that is dependent on the laser pulse duration and energy. For an ultra-short laser pulse with large energy, it is postulated that this depth exceeds the optical absorption depth resulting in an observably thick melt layer and fast drilling rate that significantly exceed predictions of the current models. Further, if necessary, modifications to the electron acceleration mechanism may be proposed in order to achieve higher accuracy from numerical simulations. The numerical model will be verified by performing drilling with laser pulses ranging from 100 femptoseconds to 10 nanoseconds. The drilling depth and melt thickness will be measured and compared to results from simulations that use ponderomotive force as the electron acceleration mechanism. The drilling depths will be measured using surface profilometer methods and the melt thicknesses will be measured using electron backscatter diffraction and the transmission electron microscopy.
The results of this research will benefit industrial laser applications and advance the science of laser-matter interaction. If successful, a theoretical model will provide accurate predictions of drilling depth and melt thickness across a broad range of interaction parameters extending from ultra-short pulse lasers to long-duration pulse and continuous wave lasers using fundamental principles of physics. In the long-term, the results of this research will benefit areas of laser system manufacturing, industrial laser applications, and contribute to education on electromagnetic wave interaction with matter.
The researchers in this project expanded the understanding of the effects of ultrashort pulse laser interactions with materials. Specifically, the investigators were looking at the heating effects of a focused laser beam on a metal and the resultant damage. To be a useful tool, lasers need to cut objects without leaving much residual damage to the surrounding material. Ultrashort pulse lasers are a new tool for manufacturers, and the understanding of the material interaction properties is incomplete. These sub-nanosecond pulsed lasers are best used for micromachining, they cut slowly but cleaner than long pulse lasers. This project had three parts: modeling, experimental, and analysis, with an underlying focus on education. The modeling effort created a mathematical guide for laser users to better understand the thermal impact based on laser parameters. The experimental effort produced numerous samples using advanced picosecond and femtosecond laser processing methods. These samples were examined using optical microscopes and then studied with advanced analysis using a scanning electron microscope and focused ion beam method to expose the heat affected zone. The analysis showed grain structure of the metals and changes to that structure. A small melt zone could be observed and no evidence of micro-cracking was seen at this resolution. Several students were trained in the course of this project. A graduate student learned how to simulate laser interactions and wrote a thesis on his model. Five undergraduate students earned internship credit, which they needed for graduation. The projects they worked on ranged from running a summer camp to learning how to conduct laser testing and microscope analysis. Two other undergraduates and two graduate students were trained on how to use a scanning electron microscope and focused ion beam method. The program was successful in producing a new model of laser interactions and experimentally showing the physical results of the laser processing.
