Attosecond Light Pulses and Attosecond Electron Dynamics Probed using Angle-Resolved Photoelectron Spectroscopy Cong Chen B.S., Nanjing University, 2010 M.S., University of Colorado Boulder, 2013 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Physics 2017 This thesis entitled: Attosecond Light Pulses and Attosecond Electron Dynamics Probed using Angle-Resolved Photoelectron Spectroscopy written by Cong Chen has been approved for the Department of Physics Prof. Margaret M. Murnane Prof. Henry C. Kapteyn Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Chen, Cong (Ph.D., Physics) Attosecond Light Pulses and Attosecond Electron Dynamics Probed using Angle-Resolved Photoelectron Spectroscopy Thesis directed by Prof. Margaret M. Murnane Recent advances in the generation and control of attosecond light pulses have opened up new opportunities for the real-time observation of sub-femtosecond (1 fs = 10-15 s) electron dynamics in gases and solids. Combining attosecond light pulses with angle-resolved photoelectron spectroscopy (atto-ARPES) provides a powerful new technique to study the influence of material band structure on attosecond electron dynamics in materials. Electron dynamics that are only now accessible include the lifetime of far-above-bandgap excited electronic states, as well as fundamental electron interactions such as scattering and screening. In addition, the same atto-ARPES technique can also be used to measure the temporal structure of complex coherent light fields. In this thesis, I present four experiments utilizing atto-ARPES to provide new insights into the generation and characterization of attosecond light pulses, as well as the attosecond electron dynamics in transition metals. First, I describe a new method to extend attosecond metrology techniques to the reconstruction of circularly polarized attosecond light pulses for the first time. Second, I show that by driving high harmonics with a two-color linearly polarized laser field, quasi-isolated attosecond pulses are generated because the phase matching window is confined. Third, I present the first measurement of lifetimes for photoelectrons that are born into free-electron-like states compared with those that are excited into unoccupied excited states in the band structure of a material (Ni(111)). The finite excited-state lifetime causes a ≈200 as delay in the emission of photoelectrons. Finally, I describe direct time-domain studies of iv electron-electron interactions in transition metals with both simple and complex Fermi surfaces. In particular, I show the influence of electron-electron scattering and screening on the lifetime of photoelectrons. Dedication For my family Acknowledgements First of all, I would like to thank my advisors, Margaret and Henry for their mentoring, support and encouragement. They have provided me with incredible resources and opportunities in my Ph. D, as well as the connections with some of the best experts in attosecond and surface science community: Martin Aeschlimann (TU Kaiserslautern), Andreas Becker (JILA), Agnieszka Jaron-Becker (JILA), Mark Keller (NIST), Manos Mavrikakis (UW Madison) and Dan Dessau (CU Boulder). I gained lots of invaluable perspectives on science through the collaborations with them. I am also glad to work with the photoemission team in Kapteyn-Murnane group. Tory, thank you for teaching me so that I am able to go on and dive deeply into photoemission. Your optimism kept me motivational and patient during the difficult time of building a new setup. Piotr, thank you for providing so many ideas and new perspectives to the experiment. The discussions with you on physics upgrades my knowledge and help me grow professionally as a scientist. Zhensheng, thank you for working together with me during so many late nights in the lab. Those moments when we felt closer to the explanations to our data are truly unforgettable. Wenjin, thank you for the confidence in turning over the ongoing experiments to you. I am sure there will be more scientific findings to come. Many thanks to our collaborators in Germany, especially Sebastian Emmerich, Markus Rollinger, Stefan Mathias, Steffen Eich and Martin Piecuch from TU Kaiserslautern, and Moritz vii Ploetzing from Jülich. Thank you all for the knowledge and support I gained during my visit to Germany. I gratefully acknowledge the sample providers and theoretical support to our experiments. Many thanks to Mark Keller and David Miller who fabricated high-quality nickel samples for us and helped us understand the physics of graphene/nickel system and the magnetic properties of nickel. To Justin Shaw who provides us the permalloy sample. To Carlos for the simulations on high harmonic generation that constitutes tremendous theoretical support to our papers. Thanks to Guowen Peng and Tibor Szilvasi for the DFT calculations that help to unravel the magic of materials’ bandstructure. Thanks to the JILA machine shop their technical support, especially Hans Green, David Alchenberger, Blaine Horner, Kim Hagen, Todd Asnicar, and Kels Detra. To JILA computing group, especially J. R. Raith who made sure the computer well-behaved during the data-taking. To Ofer Kfir and Oren Cohen from Technion University for the excitement in circularly polarized harmonics. To Yue Cao for his instructions about ARPES theory and equipment. To Wei, for his advice on research and life and keeping me from getting lost in the difficult days. Many thanks to Dan Adams and his family for the wonderful Thanksgiving memories. To group members, past and present, especially Chengyuan Ding, Bosheng Zhang, Tingting Fan, Ming-Chang Chen, Ronny Knut, Chan La-o-vorakiat, Qing Li, Tenio Popmintchev, and Dimitar Popmintchev. viii Finally, and most of all, I thank my parents, my brother, my grandparents and the rest of my family for their great love and never-ending support. Contents Page Chapter 1 Introduction to Ultrafast ....................................................................................................... 1 1.1 Why Ultrafast ............................................................................................................... 1 1.2 Probing into the “Invisible” ......................................................................................... 4 1.3 Pump-Probe ................................................................................................................. 5 1.4 Ultrafast Lasers ............................................................................................................ 6 1.5 Femtosecond Solid-State Lasers .................................................................................. 7 1.6 Attosecond ................................................................................................................... 9 1.7 Beyond Attosecond ...................................................................................................... 10 1.8 Conclusion ................................................................................................................... 11 2 Attosecond Science and Thesis Overview ........................................................................... 12 2.1 Attosecond Dynamics Probed by Femtosecond Pulses ............................................... 14 2.1.1 High Harmonic Spectroscopy ........................................................................... 15 2.1.2 Electron Recollision-Based Probe .................................................................... 16 2.1.3 Attoclock ........................................................................................................... 17 x 2.2 How to Make Attosecond Pulses ................................................................................. 18 2.3 Attosecond Pump-Probe .............................................................................................. 22 2.4 Attosecond Pulse Characterization .............................................................................. 24 2.4.1 Ex-situ Characterization of Attosecond Pulses ................................................. 25 2.4.2 In-situ Characterization of Attosecond Pulses .................................................. 31 2.4.3 Applications of Sub-cycle IR Electric Field Characterization ........................... 32 2.5 Attosecond Time-Resolved Spectroscopy in Atoms ................................................... 33 2.5.1 Photoemission Time Delay in Atoms ............................................................... 34 2.6 Attosecond Time-Resolved Spectroscopy in Molecules ............................................. 36 2.6.1 Born-Oppenheimer and Single Electron Approximation ................................. 36 2.6.2 Photoemission Time Delay in Molecules ......................................................... 38 2.7 Attosecond Time-Resolved Spectroscopy in Solids .................................................... 39 2.7.1 Photoemission Time Delay in Solids ................................................................ 40 2.7.2 Photoemission Delay Reveals Intrinsic Material Properties ............................. 44 2.7.3 Attosecond Control of Charge Dynamics in Solids .......................................... 46 2.8 Organization of the Thesis ..........................................................................................