Strong Field Phenomena in Atoms and Molecules from near to midinfrared laser fields DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Yu Hang Lai, B.S. Graduate Program in Physics The Ohio State University 2018 Dissertation Committee: Dr. Louis F. DiMauro, Advisor Dr. Enam Chowdhury Dr. Ulrich Heinz Dr. Ezekiel Johnston-Halperin ⃝c Copyright by Yu Hang Lai 2018 Abstract paraStrong field atomic physics is the study of the interaction between an atom andan intense laser pulse such that the field strength is \not-so-small" compared to an atomic unit (50 V/A)˚ and so it could not be treated as just a perturbation to the atomic system. Depending upon the ionization potential of the target, typical laser intensity required to reach this regime ranging from ∼ 10 to 1000 TW/cm2. In the low frequency limit, the photoionization process can be interpreted as a tunneling process in which the atomic potential is \tilted" by the laser field allowing the electron to escape via quantum tunneling. The escaped electron wavepacket quivers in the strong laser field whose kinetic energy is characterized by the ponderomotive energy Up which is proportional to the laser intensity and the square of the wavelength. Electron recollision happens when the ionized electron is driven back towards its parent ion by the laser field and it leads to numerous intriguing phenomena such as high-order above-threshold ionization, non-sequential ionization and high-order harmonic generation. While most of the early experiments were performed in the near-infrared (NIR)(0.8 or 1 µm) wavelengths, an important advance over the last decade has been the emergence of intense mid-infrared (MIR)(∼ 2 − 4 µm) sources. The quadratic scaling of Up with wave- length benefits the study of recollision-driven phenomena and also enables the exploration of strong field interaction deep in the tunneling regime. In addition, the MIR regime isof particular interest for studying molecules due to the presence of vibrational resonances. In this dissertation, we explore several strong field phenomena in atoms and molecules with near and mid infrared fields, including: ii 1. A comprehensive experimental benchmarking of strong field atomic ionization the- ories. We performed a comparative study between experiment and theories of the total intensity-dependent ionization yield for different atom species at different laser wavelengths (0:4 − 4µm) at linear and circular polarizations in order to investigate the applicability of two commonly used strong field ionization theories, PPT and ADK. 2. Ionization and fragmentation of methane in vibrationally resonant MIR fields. We measured the mass spectra of fragments of methane irradiated by MIR fields which cover the resonant frequency range of the C-H bond stretching mode vibration (3:2 − 3:5µm). We observed significant enhancement in ionization and dissociation rate at resonant wavelengths compared with non-resonant wavelengths. 3. Electron recollision in tunnel ionization of C60 fullerenes in MIR fields. From the \soft" recollision for low energy electrons we found an unexpected suppression of the \low- energy structure which might be attributed to the induced dipole field; from the \hard" recollision for high energy electrons we demonstrated the applicability of the \laser-induced electron diffraction technique for imaging macromolecule and observed hint of laser induced deformation of the molecular structure. 4. Strong field double ionization with circularly polarized NIR fields. We searched for the existence of recollision effects in double ionization of magnesium, zinc and calcium at different wavelength in order to examine the validity of the classical interpretation forthe previously observed enhanced double ionization yield in magnesium irradiated by circularly polarized 0.8 µm fields. iii Dedicated to my parents iv Acknowledgments paraI have been very fortunate to have the opportunity to purse my PhD study at The Ohio State University. First I would like to express my gratitude to my scientific advisors, Prof. Louis DiMauro and Prof. Pierre Agostini, for offering me the opportunity to be part of their world-class research group. I am grateful that I could spend the last five and a half year working in their state-of-the-art laboratory. Thank Lou and Pierre for their instructions and guidance, I have learned so much from them. I would like to express my gratitude to my mentors and colleagues in the group. Many of the projects presented in this dissertation would not have been possible without the ideas and guidance by Dr. Cosmin Blaga and Dr. Junliang Xu. I have greatly benefited from working with Cosmin over the last few years. His experience and insight were critical in solving many problems we encountered. I have learned many experimental skills from working with him. In particular, I am glad to have the opportunity to learn how to build an optical parametric amplifier with him. Special thanks go to Junliang, who provided many theoretical ideas and calculations for many projects in the group. I gained a lot of theoretical knowledge from him. I am very grateful for his generous help and advices, and thank you for treating me as a brother. Many thanks go to Dr. Kaikai Zhang, who I worked with during my first two years in the laboratory. I have learned many essential experimental skills from him. Thank Dr. Hyunwook Park for his advice and encouragement. Thank Dr. Hui Xiong for his generous technical help during my first summer in the laboratory when I didn't even know how to operate a laser. I would like to acknowledge the students in the group who worked with me and con- v tributed to some research projects. Xiaowei Gong contributed to the methane experiments, pulse duration measurements and classical simulations for double ionization. Thanks a lot for your time and hard work. Kent Talbert worked with me on several ionization experi- ments over the last year. Thank you for helping me to perform the measurements efficiently. Also, I enjoyed discussions with my officemates, Tim Gorman and Greg Smith. I would like to acknowledge our collaborators who contributed to some of the results presented in this dissertation or other projects I have participated. Thank Prof. Mathias Kling's group for the collaborations on the C60 project, especially I want to thank Dr. Harald Fuest for visiting our laboratory to join our experimental runs and setting up the effusive oven. Thank Prof. Hirohiko Kono and his colleagues for providing important theoretical support. Thank Prof. Jens Biegert for giving me an opportunity to visit his and work in his laboratory in Barcelona. Special thanks go to Dr. Bruno Schmidt for sharing his phenomenal technique of mid- infrared pulse compression and providing helpful advice on building a dispersion-free SHG FROG. I enjoyed theoretical discussions with Dr. Elias Diesen and Prof. Jan-Michael Rost on the \low-energy structure, Dr. Fabrice Catoire on strong field ionization of60 C and Dr. Xu Wang on double ionization. I enjoyed collaborations and discussions with Dr. Enam Chowdhurys group on several projects which utilized our mid-infrared laser. Also, I want to express my gratitude to the teachers I met in high school and college who inspired me. Thank my high school physics teacher, Dr. Ada Lau, for her teaching and the extra time she spent on answering my physics questions after classes. Thank Prof. Pak Ming Hui, an excellent teacher I met in college. It was an invaluable experience to work with him on some theoretical problems. Thank Prof. Hin Wing Kui for letting me work in his laboratory during my final year in college. It was my first experience in experimental physics research. I really appreciate their advice and encouragement. Last but not least, I want to express my deepest gratitude to my dearest mother and father for their endless love and support over all these years. Thank you so much, I can't express how thankful and grateful I am. vi Vita Bachelor of Science, The Chinese University of Hong Kong . 2011 Graduate Associate, The Ohio State University . 2011 to present Publications H. Fuest, Y. H. Lai, C. I. Blaga, K. Suzuki, J. Xu, P. Rupp, H. Li, P. Wnuk, P. Agostini, K. Yamazaki, M. Kanno, H. Kono, M. F. Kling and L. F. DiMauro, “Diffractive imaging of C60 structural deformations induced by intense femtosecond mid-infrared laser fields” (submitted) Y. H. Lai, J. Xu, U. B. Szafruga, B. Talbert, X. Gong, K. Zhang, H. Fuest, M. F. Kling, C. I. Blaga, P. Agostini and L. F. DiMauro \Experimental investigation of strong-field- ionization theories for laser fields from visible to mid-infrared frequencies" Physical Review A 96, 063417 (2017) (Editors' Suggestion) Z. Wang, H. Park, Y. H. Lai, J. Xu, C. I. Blaga, F. Yang, P. Agostini and L. F. DiMauro \The roles of photo-carrier doping and driving wavelength in high harmonic generation from a semiconductor" Nature Communications 8, 1686 (2017) H. Park, A. Camper, K. R. P. Kafka, B. Ma, Y. H. Lai, C. I. Blaga, L. F. DiMauro and E. A. Chowdhury, \High-order harmonic generation in intense mid-IR fields by cascade 3-wave mixing in a fractal poled LiNbO3 photonic crystal" Optics Letters 42, 4020 (2017) A. Camper, H. Park, Y. H. Lai, H. Kagayama, S. Li, B. Talbert, C. I. Blaga, P. Agostini, T. Ruchon, and L. F. DiMauro \Tunable mid-infrared source of light carrying orbital angular momentum in the femtosecond regime" Optics Letters 42, 3769 (2017) D. R. Austin, K. R. P. Kafka, Y. H. Lai, Z. Wang, K. Zhang, C. I. Blaga, A. Yi, L. F. DiMauro, and E. A. Chowdhury \High spatial frequency laser induced periodic sur- face structure formation in germanium by mid-IR femtosecond pulses" Journal of Applied Physics 120, 143103 (2016) K.