Date of Award

Spring 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Weizhong Dai

Abstract

Ultrashort-pulsed lasers have been attracting worldwide interest in science and engineering because the lasers with pulse durations on the order of sub-picoseconds to femtoseconds possess capabilities in limiting the undesirable spread of the thermal process zone in a heated sample during material processing at the microscale. Prevention of thermal damage is an important factor for success of ultrashort-pulsed lasers in real applications. The thermal damage induced by ultrashort pulses is intrinsically different from that induced by long-pulse or continuous lasers. It occurs after the heating pulse is over and involves the shattering of thin metal layers (without a clear signature of thermal damage by excessive temperature) rather than the melt damage caused by high temperatures. In this dissertation, by replacing the displacement components in the dynamic equations of motion using the velocity components, and employing a staggered grid, we develop a finite difference method for studying thermal deformation in two-dimensional films exposed to ultrashort-pulsed lasers, where the thin films are a single-layered thin film and a double-layered thin film with perfectly interfacial thermal contact and imperfectly interfacial thermal contact, respectively. The method is obtained based on the parabolic two-step heat transport equations. It accounts for the coupling effect between lattice temperature and strain rate, as well as for the hot electron blast effect in momentum transfer. The developed methodology allows us to avoid non-physical oscillations in the solution.

Such oscillations have been an intrinsic feature of most numerical method proposed so far in the context of problem of interest. The development of physical-based, numerical-oscillation-free methods for thermal analysis of thin metal films subjected to heating of ultrashort-pulsed lasers represents challenging tools at the forefront of this practically important area of research.

This method is tested for its applicability by investigating the temperature rise and deformation in (1) a single-layered gold thin film, (2) a double-layered gold and chromium thin film with perfect thermal contact at the interface, and (3) a double-layered gold and chromium thin film with imperfect thermal contact at the interface. Results show that there are no non-physical oscillations in the solutions, and the method is promising.

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