Degree Type

Dissertation

Date of Award

1999

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

First Advisor

Palaniappa Molian

Abstract

Ultrashort pulsed lasers create a fundamentally different ablation mechanism than conventional pulsed lasers because of the ultrashort laser pulse's extreme intensity (> 1012W/cm2) and time duration (< 10--12s), which is shorter than the electron-lattice transfer time (>10--12S). Consequently, a thermally excited plasma is generated in a cool lattice. Assumptions used in conventional pulsed laser ablation are invalid for ultrashort pulses. In this work, computer modeling of ultrashort pulsed ablation was performed for diamond. A two-step model was developed in which a heat transfer, finite-difference model was formulated and tight-binding molecular dynamics simulations were performed to evaluate the dynamics of ablation events. The heat transfer model incorporated absorption of the laser light by the electrons and predicted the thermal profiles within the electrons from the start of a laser pulse to 1 ps. The tight-binding molecular dynamics predicted the threshold electron temperatures (room temperature lattice) and overall equilibrium temperature (electron and lattice at the same temperature) required for changes in structure and ablation to occur in the material. The results of both simulations were then used to predict ablation threshold, ablation volume, and the size of the heat-affected zone within the material;Ultrashort pulsed laser ablation experiments were performed on chemical vapor deposited and on single crystal diamonds, as well as on highly-oriented pyrolytic graphite, in order to verify the model predictions. Scanning electron microscopy, atomic force microscopy, profilometry, and micro-Raman spectroscopy were employed to characterize the ablated surfaces. Results showed that ultrashort pulses, compared with nanosecond laser pulses, yield lower threshold fluences, higher material removal rates, and much more precise ablation, all of which are attributed to the increased absorption coefficient and improved energy coupling. The most significant observation is that the surfaces of diamond and graphite did not undergo phase transformation, demonstrating that chemical cleanliness is increased with use of ultrashort pulses rather than nanosecond or longer pulses. In addition, thermal damage and the associated debris and recast layer formation were non-existant with ultrashort pulses. The investigation further showed that ultrashort pulsed lasers significantly reduced the feature size and improved the feature resolution, leading to sub-micron machining, which is not achievable in nanosecond or longer pulsed lasers. These experimental observations are consistent with predictions base on the finite difference and molecular dynamics models.

DOI

https://doi.org/10.31274/rtd-180813-13751

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Michael D. Shirk

Language

en

Proquest ID

AAI9950119

File Format

application/pdf

File Size

162 pages

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