Understanding Femtosecond-Pulse Laser Damage through Fundamental Physics Simulations DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Robert A. Mitchell III, B.S. Graduate Program in Physics The Ohio State University 2015 Dissertation Committee: Professor Douglass Schumacher, Advisor Professor Eric Braaten Professor Gregory Lafyatis Professor Robert Perry Copyright by Robert A. Mitchell III 2015 Abstract It did not take long after the invention of the laser for the field of laser damage to appear. For several decades researchers have been studying how lasers damage materials, both for the basic scientific understanding of highly nonequilibrium processes as well as for industrial applications. Femtosecond pulse lasers create little collateral damage and a readily reproducible damage pattern. They are easily tailored to desired specifications and are particularly powerful and versatile tools, contributing even more industrial interest in the field. As with most long-standing fields of research, many theoretical tools have been developed to model the laser damage process, covering a wide range of complexities and regimes of applicability. However, most of the modeling methods developed are either too limited in spatial extent to model the full morphology of the damage crater, or incorporate only a small subset of the important physics and require numerous fitting parameters and assumptions in order to match values interpolated from experimental data. Demonstrated in this work is the first simulation method capable of fundamentally modeling the full laser damage process, from the laser interaction all the way through to the resolidification of the target, on a large enough scale that can capture the full morphology of the laser damage crater so as to be compared directly to experimental measurements instead of extrapolated values, and all without any fitting parameters. ii The design, implementation, and testing of this simulation technique, based on a modified version of the particle-in-cell (PIC) method, is presented. For a 60 fs, 1 μm wavelength laser pulse with fluences of 0.5 J/cm2, 1.0 J/cm2, and 2.0 J/cm2 the resulting laser damage craters in copper are shown and, using the same technique applied to experimental crater morphologies, a laser damage fluence threshold is calculated of 0.15 J/cm2, consistent with current experiments performed under conditions similar to those in the simulation. Lastly, this method is applied to the phenomenon known as LIPSS, or Laser-Induced Periodic Surface Structures; a problem of fundamental importance that is also of great interest for industrial applications. While LIPSS have been observed for decades in laser damage experiments, the exact physical mechanisms leading to the periodic corrugation on the surface of a target have been highly debated, with no general consensus. Applying this technique to a situation known to create LIPSS in a single shot, the generation of this periodicity is observed, the wavelength of the damage is consistent with experimental measures and, due to the fundamental nature of the simulation method, the physical mechanisms behind LIPSS are examined. The mechanism behind LIPSS formation in the studied regime is shown to be the formation of and interference with an evanescent surface electromagnetic wave known as a surface plasmon-polariton. This shows that not only can this simulation technique model a basic laser damage situation, but it is also flexible and powerful enough to be applied to complex areas of research, allowing for new physical insight in regimes that are difficult to probe experimentally. iii To my family, who stood by me and supported me through more than a decade of higher education. iv Acknowledgments I am extremely grateful to my advisor Douglass Schumacher, for his guidance, encouragement, and support. I have much to thank him for, starting with picking up a “high-risk” graduate student such as myself after I was put in the position of looking for a new research advisor in my third year of graduate school. Working with Doug was an honor and provided a great opportunity to learn, as discussions with him were always enlightening. His recommendation to pursue this risky but rewarding topic provided me with immeasurable opportunities, and without his unique combination of supervision and freedom I would not have succeeded as I did. I would like to thank Eric Braaten, Gregory Lafyatis, and Robert Perry for serving on my thesis committee. I would also like to thank Enam Chowdhury, Kyle Kafka, Vladimir Ovchinnikov, Chris Orban, Frank King, Matt McMahon, Ginevra Cochran, and all my colleagues for many helpful discussions. Lastly, I would like to thank the entire High Energy Density Physics group at The Ohio State University, for sitting through numerous talks and practice talks on a completely foreign topic well outside their regime, and yet always providing constructive feedback and advice. v Vita May 2004 .......................................................Newark High School May 2009 .......................................................B.S. Physics, University of Delaware September 2009 to June 2012 ........................Graduate Teaching Associate, Department of Physics, The Ohio State University June 2012 to present ......................................Graduate Research Associate, Department of Physics, The Ohio State University Publications “Fundamental simulation of laser-induced periodic surface structure,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, to be submitted to Phys. Rev. Lett. (2015). “Modeling crater formation in femtosecond-pulse laser damage from basic principles,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, Optics Letters 40, 2189-2192 (2015). “Using Particle-In-Cell simulations to model femtosecond pulse laser damage,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, Proc. SPIE 9237, 92370X (2014). “Modeling femtosecond pulse laser damage using Particle-In-Cell simulations,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, Optical Engineering 53, 122507 (2014). “Modeling femtosecond pulse laser damage on conductors using Particle-In-Cell simulations,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, Proc. SPIE 8885, 88851U (2013). vi Presentations “First principles simulations of laser-induced periodic surface structure using the particle- in-cell method,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, SPIE Laser Damage 2015. “Single-shot femtosecond laser ablation of copper: experiment versus simulation,” E. A. Chowdhury, K. Kafka, R. Mitchell, H. Li, A. Yi, and D. W. Schumacher, SPIE Laser Damage 2015. “Adapting particle-in-cell simulations to the study of short pulse laser damage,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, APS Division of Plasma Physics 2014. “Using particle-in-cell simulations to model femtosecond pulse laser damage of metals and dielectrics,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, SPIE Laser Damage 2014. “Adapting particle-in-cell simulations to the study of short pulse laser damage,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, Ohio Supercomputing Center Statewide Users Group Meeting June 2014. “A new approach to laser damage using the particle-in-cell method,” D. W. Schumacher, R. Mitchell, E. A. Chowdhury, AFOSR Program Review May 2014. “Modeling femtosecond pulse laser damage on conductors using particle-in-cell simulations,” R. Mitchell, D. W. Schumacher, and E. A. Chowdhury, SPIE Laser Damage 2013. “A new technique for fast characterization of intense laser-plasma simulations,” R. Mitchell, C. Orban, V. Ovchinnikov, D. W. Schumacher, and R. Freeman, Ohio Region APS October 2011. Fields of Study Major Field: Physics vii Table of Contents Abstract ............................................................................................................................... ii Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi List of Figures .................................................................................................................... xi Chapter 1: Introduction ...................................................................................................... 1 1.1 Motivation ............................................................................................................ 1 1.2 Brief History of Laser Damage ............................................................................ 6 1.3 Laser Induced Periodic Surface Structure .......................................................... 10 Chapter 2: Theory ............................................................................................................. 16 2.1 Laser-Matter Interaction ..................................................................................... 16 2.1.1 Electron in a Plane Wave ............................................................................ 18 2.1.2 Ponderomotive Force .................................................................................. 20 2.1.3 Energy Transfer Mechanisms ..................................................................... 23 2.1.4 Ionization Processes .................................................................................... 27 2.2