ABSTRACT Title of Document: A SCANNING SQUID MICROSCOPE FOR IMAGING HIGH-FREQUENCY MAGNETIC FIELDS Constantine P. Vlahacos, PhD, 2009 Directed By: Professor Frederick C. Wellstood Department of Physics This thesis examines the design and operation of a large-bandwidth scanning SQUID microscope for spatially imaging high frequency magnetic fields. Towards this end, I present results on a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 2 GHz and a sensitivity of about 52.4 nT per sample. By using a thin- film hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non- hysteretic SQUID and a flux-locked loop, the bandwidth limitation of existing scanning SQUID microscopes is overcome. The microscope allows for non-contact images of time-varying magnetic field to be taken of room-temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID to about 10 μm. The new readout scheme involves repeatedly pulsing the bias current to the dc SQUID while the voltage across the SQUID is monitored. Using a fixed pulse amplitude and applying a fixed dc magnetic flux allows the SQUID to measure the applied magnetic flux with a sampling time set by the pulse length of about 400 ps. To demonstrate the capabilities of the microscope, I imaged magnetic fields from 0 Hz (static fields) up to 4 GHz. Samples included a magnetic loop, microstrip transmission lines, and microstrip lines with a break in order to identify and isolate electrical opens in circuits. Finally, I discuss the operation and modeling of the SQUID and how to further increase the bandwidth of the microscope to allow bandwidth of upwards of 10 GHz. A SCANNING SQUID MICROSCOPE FOR IMAGING HIGH-FREQUENCY MAGNETIC FIELDS By Constantine P. Vlahacos Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park, in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2009 Advisory Committee: Professor Frederick C. Wellstood, Chair Professor Isaak D. Mayergoyz Professor Romel Gomez Professor Christopher Lobb Professor Bryan Eichhorn © Copyright by Constantine P. Vlahacos 2009 Είs μνήμην τοϋ πατρόs μου Παναγιώτης Βλαχάκος ii Acknowledgements It may not come as a surprise to anyone, but research is very much a social activity and as such your success depends on the people you interact with. It is strongly influenced by people who may or may not be part of your project or for that matter your lab. By asking a lot of questions and seeking advice I have often received constructive criticism, complaints, clever observations, or assistance which resulted in not only improving my research but more often than not I walked away having learned something new. This dissertation would not have been possible without the help and patience of many people. There are several people that I would like to acknowledge that have helped me professionally. At the University the person I would like to thank most is my advisor Professor Fred Wellstood whose guidance in everything that he has given me for the many years that I have been at the University. Under Fred’s guidance I have built or help to build several scanning probe microscopes, starting with the scanning microwave microscope and ending with the current scanning SQUID microscope that I address in this thesis. At the Laboratory for Physical Sciences I would like to thank my supervisor William Vanderlinde for all his help, unwavering support and patience. Also, I would like to thank Dr. Marc Manheimer, Dr. Kevin Osborn and Dr. Marshall Saylors all at LPS for their advice and generosity to loan me equipment for extended periods of time. I would like to thank my dissertation committee members, Professors Isaac Mayergoyz, Mel Gomez, Christopher Lobb, and Brian Eichhorn for taking time to read and comment on my thesis. I would like to thank Professor Isaac Mayergoyz for his iii guidance and support while I have been a graduate student in the Electrical Engineering department. I would like to thank the Center for Nanophysics and Advanced Materials which provided a generous and stimulating environment during my stay at the center. In particular I would like to thank Professor Richard Greene. I particularly learned a lot by giving talks and having to explain my work to others in his seminar. I would like to especially thank Professor Steven Anlage for introducing me to microwave microscopy and for his advice and discussions on superconductivity and microwave technology. I would like to thank Professor Christopher Lobb for guidance, support and friendship over the years that I have been at the University. I would like to thank colleagues and collaborators at Neocera, Inc., who greatly contributed to the 4 K SQUID microscope project. In particular, I thank Dr. Lee Knauss and Dr. Antonio Orozco and the people at Neocera for being very gracious hosts during my time on site working on the microscope. I would like to thank some of the people I worked closest with. John Matthews and Dr. Soun Pil Kwon both got me stated in working on this microscope and I deeply appreciate their help and advice. I especially thank John for all his help concerning Josephson device physics and writing wonderful software with Labview. Current students in the subbasement that I am indebted to and would like to thank are Benjamin Cooper, Tony Przybysz and Hyeokshin Kwon. I would like to thank Ben who is always helpful with advice, loaning equipment and an excellent sounding board for ideas, and Tony and Hyeokshin for bringing new ideas and energy into the lab. iv Other former students at the Center also helped with important advice and discussion. Dr. Andrew Berkley gave insightful observations and suggested changes in experimental design. Dr. Hanhee Paik offered friendship and useful advice from her experience working on the room temperature scanning SET microscope. I especially want to thank Dr. Sudeep Dutta whom I have known for many years for his always helpful advice on microwave microscopy, SQUIDs, quantum computing and excellent companionship in the lab. I thank Dr. Tauno Palomaki for his advice concerning timing measurements on Josephson junction devices. I also thank Drs. Rupert Lewis and Roberto Ramos who were always available with advice and especially to Rupert who gave me the microwave coaxial lines for my initial proof of concept experiment in a liquid helium dewar. There are several former students and colleagues I would like to acknowledge who assisted in training me and allowed use of their equipment. I would like to thank Dr. Randy Black whom I originally worked with in Fred’s lab. Randy helped on the early room temperature SQUID microscope and microwave microscope. His helpful advice on SQUIDs and all things mechanical is truly appreciated. I thank Dr. Ajay Amar who taught me the how to properly use a leak detector and for his advice and craftsmanship. I would like to thank Dr. Sojiphong Chatraphorn for his great friendship and conversation, and being a good lab partner in the same corner of the lab. I would like to thank Dr. Erin Fleet for his friendship and his advice concerning mechanical design. I have greatly benefited from their experience in scientific prototype design. I would like to thank Dr. Su-Young Lee for advice on building scanning SQUID microscopes. v I would like to thank the technical and support staff of the Center for Nanophysics and Advanced Materials. In particular, both Doug Benson and Brian Straugn helped me solve a wide array of technical problems such as the evaporation system, computer software and networking, machining parts, and other things I would throw at them. I would like to extend a very heartfelt thank you to both of them. I would like to thank Belta Pollard, Cleopatra White, and Grace Sewall for all the bureaucratic red tape and paperwork they have handled for me over the years so I could get my work done. I would like to thank the technical staff of the Physics Department. I would like to especially thank Jack Touart for his excellent help and advice in electronics and allowing me to use many of the Electronic Development Group’s equipment. I would like to thank Mr. Russell Wood, a former member of the technical staff of the Physics Department, for his excellent management of the student shop. He provided an excellent shop to allow me to efficiently construct the apparatus for my experiments. His advice on machining and designing scientific apparatus is truly appreciated. I would like to thank the support staff of the Physics Department. I thank Mr. Robert Dahms and Mr. Jesse Anderson for helping me order equipment and making purchases times on short notice. I thank Mr. Al Godinez for helping me with all my liquid helium orders and hunting me down when the Air Products truck would arrive with a delivery. Finally, I would like to thank my wife Sophia for her help, support and patience, as well as our daughters Lia and Irene who have brought much happiness to our lives. Finally, I would like to thank my mother for her support and confidence, and I dedicate this thesis to the loving memory of my father, Mr. Peter Vlahacos. vi Table of Contents Acknowledgements......................................................................................................iii List of Tables ................................................................................................................ x List of Figures.............................................................................................................