Ievgenii Borodianskyi Superradiant THz wave emission from arrays of Josephson junctions Superradiant THz wave emission from arrays of Josephson junctions arrays emission from wave THz Superradiant Ievgenii Borodianskyi Ievgenii Borodianskyi "Never memorize something that you can look up." - Albert Einstein ISBN 978-91-7911-178-6 Department of Physics Doctoral Thesis in Physics at Stockholm University, Sweden 2020 Superradiant THz wave emission from arrays of Josephson junctions Ievgenii Borodianskyi Academic dissertation for the Degree of Doctor of Philosophy in Physics at Stockholm University to be publicly defended on Wednesday 9 September 2020 at 13.00 in sal FR4, AlbaNova universitetscentrum, Roslagstullsbacken 21. Abstract High-power, continuous-wave, compact and tunable THz sources are needed for a large variety of applications. Development of power-efficient sources of electromagnetic radiation in the 0.1-10 THz range is a difficult technological problem, known as the “THz gap.” Josephson junctions allow creation of monochromatic THz sources with an inherently broad range of tunability. However, emission power from a single junction is too small. It can be amplified in a coherent superradiant manner by phase-locking of many junctions. In this case, the emission power should increase as a square of the number of phase-locked junctions.The aim of this thesis is to study a possibility of achieving coherent super- radiant emission with significant power and frequency tunability from Joseph-son junction arrays. Two types of devices are studied, based either on stacks (one-dimensional arrays) of intrinsic Josephson junctions naturally formed in single crystals of high-temperature cuprate superconductor Bi2Sr2CaCu2O8+x, or two-dimensional arrays of artificial low-temperature superconducting Nb/NbSi/Nb junctions. Micron-size junctions are fabricated using micro- and nanofabrication tools.The first chapter of this thesis describes the theory of Josephson junctions and how mutual coupling between Josephson junctions can lead to self-syn-chronization, facilitating the superradiant emission of electromagnetic radia-tion. The second chapter is focused on the technical aspects of this work, with detailed descriptions of sample fabrication and experimental techniques. The third chapter presents main results and discussion. It is demonstrated that de-vices based on high-Tc cuprates allow tunable emission in a very broad fre-quency range 1-11 THz. For low- Tc junction arrays synchronization of up to 9000 junctions is successfully achieved. It is argued that an unconventional traveling-waves mechanism facilitates the phase-locking of such huge arrays. The obtained results confirm a possibility of creation of high-power, continu-ous- wave, compact and tunable THz sources, based on arrays of Josephson junctions. Keywords: Josephson junction, Superconductor, ThZ emission, high-Tc. Stockholm 2020 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-181234 ISBN 978-91-7911-178-6 ISBN 978-91-7911-179-3 Department of Physics Stockholm University, 106 91 Stockholm SUPERRADIANT THZ WAVE EMISSION FROM ARRAYS OF JOSEPHSON JUNCTIONS Ievgenii Borodianskyi Superradiant THz wave emission from arrays of Josephson junctions Ievgenii Borodianskyi ©Ievgenii Borodianskyi, Stockholm University 2020 ISBN print 978-91-7911-178-6 ISBN PDF 978-91-7911-179-3 Printed in Sweden by Universitetsservice US-AB, Stockholm 2020 "If we knew what it was we were doing, it would not be called research, would it?” Albert Einstein Abstract High-power, continuous-wave, compact and tunable THz sources are needed for a large variety of applications. Development of power-efficient sources of electromagnetic radiation in the 0.1-10 THz range is a difficult technological problem, known as the “THz gap.” Josephson junctions allow creation of monochromatic THz sources with an inherently broad range of tunability. However, emission power from a single junction is too small. It can be amplified in a coherent superradiant manner by phase-locking of many junctions. In this case, the emission power should increase as a square of the number of phase-locked junctions. The aim of this thesis is to study a possibility of achieving coherent super- radiant emission with significant power and frequency tunability from Joseph- son junction arrays. Two types of devices are studied, based either on stacks (one-dimensional arrays) of intrinsic Josephson junctions naturally formed in single crystals of high-temperature cuprate superconductor Bi2Sr2CaCu2O8+x, or two-dimensional arrays of artificial low-temperature superconducting Nb/NbSi/Nb junctions. Micron-size junctions are fabricated using micro- and nanofabrication tools. The first chapter of this thesis describes the theory of Josephson junctions and how mutual coupling between Josephson junctions can lead to self-syn- chronization, facilitating the superradiant emission of electromagnetic radia- tion. The second chapter is focused on the technical aspects of this work, with detailed descriptions of sample fabrication and experimental techniques. The third chapter presents main results and discussion. It is demonstrated that de- vices based on high-Tc cuprates allow tunable emission in a very broad fre- quency range 1-11 THz. For low- Tc junction arrays synchronization of up to 9000 junctions is successfully achieved. It is argued that an unconventional traveling-waves mechanism facilitates the phase-locking of such huge arrays. The obtained results confirm a possibility of creation of high-power, continu- ous-wave, compact and tunable THz sources, based on arrays of Josephson junctions. i Sammanfattning Högeffekts, kontinuerliga våg, kompakta och inställbara THz-källor behövs för en mängd olika applikationer. Utveckling av energieffektiva källor för elektromagnetisk strålning i området 0,1-10 THz är ett svårt teknologiskt problem, känt som "THz-gap". Josephson-korsningar möjliggör skapandet av monokromatiska THz-källor med ett i sig brett spektrum av inställbarhet. Emissionskraften från en enda korsning är dock för liten. Det kan förstärkas på ett sammanhängande superradiant sätt genom faslåsning av många korsningar. I detta fall bör utsläppseffekten öka som en kvadrat av antalet faslåsta korsningar. Syftet med denna avhandling är att studera en möjlighet att uppnå koherent superradiantemission med betydande effekt och frekvensjusterbarhet från Josephson-korsningsgrupper. Två typer av anordningar studeras, baserade antingen på staplar (endimensionella matriser) av inneboende Josephson- korsningar som är naturligt bildade i enstaka kristaller av högtemperatursuprat-superledare Bi2Sr2CaCu2O8 + x, eller tvådimensionella matriser av konstgjorda låg temperatur superledande Nb / NbSi / Nb- korsningar. Korsningar i mikronstorlek tillverkas med hjälp av mikro- och nanofabriceringsverktyg. Det första kapitlet i denna avhandling beskriver teorin om Josephson- korsningar och hur ömsesidig koppling mellan Josephson-korsningar: er kan leda till självsynkronisering, vilket underlättar överstrålningsutsläpp av elektromagnetisk strålning. Det andra kapitlet är inriktat på de tekniska aspekterna av detta arbete, med detaljerade beskrivningar av provtillverkning och experimentella tekniker. Det tredje kapitlet presenterar huvudresultat och diskussion. Det demonstreras att enheter baserade på hög-Tc-koppar tillåter inställbar utsläpp i ett varierande brett frekvensområde 1-11 THz. För sammankopplingsmatriser med låg Tc uppnås synkronisering av upp till 9000 korsningar med framgång. Det hävdas att faslåsning av så mycket stora matriser underlättas av en okonventionell rörelsevågsmekanism. De uppnådda resultaten bekräftar möjligheten att skapa högeffekta, kontinuerliga våg, kompakta och inställbara THz-källor, baserade på matriser av Josephson- korsningar. ii List of appended papers This thesis is based on the following papers. I. Borodianskyi, E.A., Krasnov, V.M. “Josephson emission with fre- quency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa struc- tures.” Nat.Commun 8, 1742 (2017). Author’s contribution: I fabricated the sample, performed the measurements, contributed in data analysis and writhing of the manuscript. II. M. A. Galin, E. A. Borodianskyi, V. V. Kurin, I. A. Shereshevskiy, N. K. Vdovicheva, V. M. Krasnov, and A. M. Klushin “Synchroni- zation of Large Josephson-Junction Arrays by Traveling Electro- magnetic Waves” Phys. Rev. Applied 9, 054032 (2018) Author’s contribution: I have been taking active part in measurements and sample charac- terization and participated in writing the paper. III. A. A. Kalenyuk, A. Pagliero, E. A. Borodianskyi, S. Aswartham, S. Wurmehl, B. Büchner, D. A. Chareev, A. A. Kordyuk, and V. M. Krasnov “Unusual two-dimensional behavior of iron-based superconductors with low anisotropy” Phys. Rev. B 96, 134512 (2017) Author’s contribution: I helped with sample fabrication and low temperature electrical measurements. IV. A. A. Kalenyuk, A. Pagliero, E. A. Borodianskyi, A. A. Kordyuk, and V. M. Krasnov “Phase-Sensitive Evidence for the Sign- Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1-xNaxFe2As2 Josephson Junctions” Phys. Rev. Lett. 120, 067001 (2018) Author’s contribution: I helped with sample fabrication and low temperature electrical measurements V. M.A. Galin, F. Rudau. E.A. Borodianskyi, V.V. Kurin, D. Koelle, R. Kleiner, V.M. Krasnov, A.M. Klushin, “Direct visualization of iii phase-locking of large Josephson junction arrays by surface elec- tromagenetic waves”. To be pubished…ArXiv:2004.06623