All-Optical Electron Acceleration with Ultrafast THz Pulses by Wenqian Ronny Huang B.S., Cornell University (2009) S.M., Massachusetts Institute of Technology (2014) Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering and Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2017 c Massachusetts Institute of Technology 2017. All rights reserved. Author.............................................................. Department of Electrical Engineering and Computer Science September 1, 2017 Certified by. Franz X. K¨artner Professor of Electrical Engineering Thesis Supervisor Certified by. Erich P. Ippen Elihu Thomson Professor of Electrical Engineering Thesis Supervisor Accepted by . Leslie A. Kolodziejski Chair of the Committee on Graduate Students 2 All-Optical Electron Acceleration with Ultrafast THz Pulses by Wenqian Ronny Huang Submitted to the Department of Electrical Engineering and Computer Science on September 1, 2017, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering and Computer Science Abstract This thesis discusses a series of advances toward{and resulting in{the demonstra- tion of the first ultrafast THz-driven electron gun, a technology with the potential to deliver unprecedented electron beam quality to scientists studying matter at the ultrafast and ultrasmall scale via electron diffraction or x-ray imaging. In Part 1, we discuss various advances in generation of high energy pulsed THz radiation, a spectral regime uniquely effective at accelerating electrons but histori- cally lacking in efficient sources. In particular, through various improvements to the grating-based tilted pulse front (TPF) technique, we demonstrate a record conver- sion efficiency of 1%. We also implement echelon-based TPF, achieving 3.5x higher efficiency than grating-based TPF for short (∼100 fs) pulses. Finally, we reuse the residual pump to obtain a recycled efficiency around half to a quarter that of the original. This reduced efficiency can be linked to spatio-spectral distortions in the residual pump, and we characterize these distortions to better understand the asym- metric dynamics of the THz generation process. In Part 2, we discuss the design, testing, and commissioning of an electron gun driven exclusively by THz radiation. The accelerating structure, capable of broad- band, dispersionless THz propagation and sub-wavelength confinement, is analyzed through electromagnetic simulations and experimental tests. We also characterize the accelerated electrons in absolute charge and spectrum as a function of emission phase and THz energy, while showing that the behavior matches well with theory and simulation. Our first-version THz gun delivers near 1 keV electrons accelerated by field strengths surpassing that of the best operational RF guns. The gun also de- livers narrowband electron spectra which can already be used for low-energy electron diffraction. Thesis Supervisor: Franz X. K¨artner Title: Professor of Electrical Engineering Thesis Supervisor: Erich P. Ippen Title: Elihu Thomson Professor of Electrical Engineering 3 4 Acknowledgments First, I would like to thank my advisor, Prof. Franz K¨artner,whom I first met more than seven years ago through his two courses at MIT when I was a part-time student. His rigorous, yet intuitive teaching of photonics and ultrafast optics inspired my in- terest in the field so much so that joining his group was a no-brainer when I became a graduate student. As a scientist, Franz has shown me how to balance between intellectual free-thinking and persistence on a singular goal. As an advisor, he has consistently guided my research by giving detailed feedback and genuine encourage- ment through our weekly conferences and regular one-on-ones. Despite running two large research groups over two continents, Franz remained intimately aware of the details of my project and often showed excitement toward its progress, a contagious excitement that infected me and galvanized my efforts throughout the grind. I would like to also thank my second advisor, Prof. Keith Nelson, who pioneered much of the experimental work on THz pulse generation and whose path I was des- tined to cross the day I started at MIT working on THz. My gratitude goes out to him for readily taking me into his group when I asked him for ideas and guidance in summer 2014 during a crossroads in my research career. The treasury of ideas in Keith's free-spirited mind is amazing, and the echelon project has been a perfect complement to my work with Franz and overall thesis. His unfeigned passion for high quality scholarly work is a virtue I hope to emulate myself throughout my career. My appreciation goes out to my co-advisor Prof. Erich Ippen, who has been an invaluable mentor throughout my time at MIT. He asks thorough, critical questions of my research in a no-nonsense manner which has strengthened it and taught me how to check my own work. The scientific and career advice he offers has also matured me and helped me figure out what I want to do. Finally, the belief and hope that Prof. Ippen imbues in me, and indeed in all the students of our OQE group, provides an ever-present sense of importance and magnitude to our daily work. I'm grateful for my academic advisor, Prof. Cardinal Warde, who provided helpful feedback and career advice every semester that we met up. 5 I'd like to thank the K¨artnergroup folks at MIT, who have been my friends and colleagues over the past 5 years. Shu-wei Huang and Eduardo Granados were my first mentors who \showed me the ropes" of doing ultrafast laser experiments and poured their optics expertise into me like water into a sponge. Koustuban Ravi (\Ravi") has not only been a theoretical powerhouse in the THz project, but also my closest companion in the many onerous hardships and exciting adventures of graduate school research, which included our stints in Hamburg in 2014-16. I've been influenced by his vision for enhancing our group culture, as well as pursuing research topics that have real societal impact. Further, I will miss our well-rounded, truth-seeking bantering about diverse topics in technology, society, and life. As the group's senior research scientist, Kyunghan Hong's presence has been a source of stability for me throughout the many uncertain times of funding cuts and personnel changes. Kyunghan has also endured the many times I would barge into his office with only half-baked ideas or data, after which he would patiently help me process through them. Peter Krogen embodies the can-do attitude of MIT when it comes to building/fixing things, and I've learned a lot from him about tinkering that I will never find in textbooks or papers. Without Emilio Nanni, I would not have ventured as far as I did in electron accelerators. His first-rate guidance and advice in those topics, as well as in writing my second publication, were a goldmine in my scientific journey. I've appreciated Donnie Keathley's knack for teaching me photoemission physics, as well as his thoughtful leadership in group affairs. Houkun Liang was a great companion and friend during the late nights in the office and I'm lucky to have visited his lab in Singapore twice. Liang Jie Wong is a brilliant theoretician who gave me initial understanding of how to simulate a particle bunch. Hungwen Chen was a kind confidant in my early years in the K¨artnergroup who explained to me some of the ins-and-outs of research life. Guoqing \Noah" Chang has given me much insight into the rising economy and academic environment of China. Others whom I've had the fortune to learn from and interact significantly with include Patrick Callahan, Ming Xin, Billy Putnam, Michael Peng, Greg Stein, Hua Lin, and Hongyu Yang. I'd like to thank the folks in DESY at Hamburg, Germany, where I spent 50% 6 of my time from late 2014 to early 2016. Stuart Hayes was a Godsend during my time there: a great friend, fantastic travel buddy, generous roommate, and electron gun expert who provided me many great ideas. The time I spent with him and others in the IBC community in Hamburg will always be remembered as my first European life experience. Xiaojun Wu was extremely instrumental in smoothing the obstacles to do the THz gun experiments, and her kindness and hard work ethic were admirable. Anne-Laure Calendron and Huseyin Cankaya were stalwart laser jocks who maintained the laser lab to the most professional standards. Arya Fallahi was an inventor and electromagnetics expert from whom I received many great ideas. Dongfang Zhang was very generous with his time in helping me coat cathode samples on-demand. Thomas Tilp, Andrej Berg, and Matthias Schust were excellent mechan- ical engineers who embodied the skill and discipline of German engineering. Luis Zapata's conviction for precise mechanical design and insistence that I learn Solid- works has greatly informed the way I build my experiments. Nicholas Matlis asked very good questions and had a knack for reiterating concepts he learned. Others in- clude Giulio Rossi, Roland Mainz, Francois Lemery, Shaobo Fang, Michael Hemmer, Giovanni Cirmi, Oliver Muecke, Frederike Ahr, Hong Ye, Sergio Carbajo, and Chun Zhou. As if two exceptional research groups were not enough, I also had the fortune of working with the folks of the Nelson group at MIT during my latter 3 years at MIT from 2014-2017. I thank Ben Ofori-Okai and Prasahnt Sivarajah for eagerly including me in the echelon project and making our 3-person team communicative and efficient. That along with the fact that both of them were genuinely helpful people gave me a first experience of how effective team dynamics can work.