Superconducting integrated THz receiver

The operation frequency of superconducting integrated THz receivers can be enhanced by replacing the commonly used elementary niobium with niobium nitride. This work presents the technology development of high-quality niobium nitride thin films and superconductor-insulator-superconductor multilayers along with the simulation and realization of high-frequency circuits for a superconducting integrated THz receiver using niobium nitride electrodes

New photonic architectures and devices for generation and detection of sub-THz and THz waves

The development of high-quality and reliable devices in the THz frequency region to fill the existing technological gap has become a major concern. This is chiefly motivated by the need of a widespread exploitation of the extensive variety of identified applications in this frequency region by a wide range of users, including the non-scientific community. The photonic approaches used for these purposes offer important and exclusive advantages over other existing alternatives, which have as a main representative the all-electronic technology, especially in terms of frequency range coverage, possibility of photonic distribution using optical fibers, weight and Electromagnetic Interference (EMI) immunity. Nevertheless, the optical techniques have traditionally provided with worse performance in terms of phase noise, tunability and dynamic range (in generation), and conversion ratio (in detection) when compared to state-of-theart all-electronic THz technology. The work accomplished in this thesis focuses on the design, development and validation of new photonic architectures and devices for both generation and detection of sub-THz and THz waves which overcome the drawbacks of optical techniques at this frequency region while maintaining all their advantages. In this thesis, several photonic sub-THz and THz generation systems have been developed using Difference Frequency Generation (DFG) architectures in which the DFG source is provided by an Optical Frequency Comb Generator (OFCG) and optical mode selection. Different devices and techniques are investigated for each part of the system before arriving to the final high performance synthesizer. Passively Mode-Locked Laser Diodes (PMMLDs) are firstly evaluated as integrated OFCG. An improved design of the OFCG is achieved with a scheme based on a Discrete Mode (DM) laser under Gain- Switching (GS) regime and optical span expansion by the use of a single Electro- Optical (EO) phase modulator. As optical mode selection, both high selective optical filtering and Optical Injection Locking (OIL) are used and evaluated. A commercial 50 GHz photodiode (PD) and an n-i-pn-i-p superlattice THz photomixer are employed as photodetector for Optical to THz conversion. The final reported system consists on an OFCG based on GS, OIL as mode selection strategy and an n-i-pn-i-p superlattice photomixer. This synthesizer offers a wide frequency range (60-140 GHz), readily scalable to a range between 10 GHz and values well above 1 THz. Quasi-continuous tunability is offered in the whole frequency range, with a frequency resolution of 0.1 Hz at 100 GHz that can be straightforwardly improved to 0.01 Hz at 100 GHz and 0.1 Hz at 1 THz. The measured FWHM at 120 GHz is <10 Hz, only limited by the measurement instrumentation. The system offers excellent frequency and power stability with frequency and power deviations over 1 hour of 5 Hz and 1.5 dB, respectively. These values are also limited by both the accuracy and uncertainty of the measurement setup. The performance achieved by this photonic sub-THz and THz synthesizer for most figures of merit matches or even surpasses those of commercial stateof- the-art all-electronic systems, and overcomes some of their characteristics in more than one million times when compared to commercial state-of-the-art photonic solutions. The detection part of this thesis explores the use of photonic architectures based on EO heterodyne receivers and the key devices that encompass these architectures: photonic Local Oscillators (LOs) and EO mixers. First results are developed at microwave frequencies (<15 GHz) using an Ultra-Nonlinear Semiconductor Amplifier (XN-SOA) as EO mixer and a GS based photonic LO. It is demonstrated how this LO device based on GS provides with a significant improvement in the performance of the overall EO receiver when compared to a traditional linearly modulated LO. Furthermore, this detection architecture is validated in an actual application (photonic imaging array), featuring scalability, flexibility and reasonable conversion ratios. After this, an EO heterodyne receiver is demonstrated up to frequencies of 110 GHz. The photonic LO employed is the abovementioned photonic sub- THz synthesizer developed in this thesis, while the EO mixer is an np-i-pn quasi ballistic THz detector. The first fabricated sample of this novel device is used, which is optimized for homodyne/heterodyne detection. The resulting sub-THz EO heterodyne receiver has conversion ratios around -75 dB. It works under zero-bias conditions, which together with the photonic distribution of the LO offers a high potential for remote detection of sub-THz and THz waves. In summary, new photonic architectures and devices are able to provide with state-of-the-art performance for generation of sub-THz and THz waves. In the case of EO heterodyne detection at sub-THz and THz frequency regions, photonic techniques are improving their performance and are closer to offer an alternative to all-electronic detectors. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------El desarrollo de dispositivos fiables y de alta calidad en el rango frecuencial de Terahercios (THz) con el fin de cubrir el actual vacío tecnológico se ha convertido en una importante inquietud científica. Esto está principalmente motivado por la necesidad de explotar el gran número de aplicaciones identificadas en esta región frecuencial por un gran número de usuarios, incluyendo a usuarios no científicos. El enfoque fotónico empleado para estos propósitos ofrece importantes y exclusivas ventajas sobre otras alternativas existentes, que tienen como principal representante a la tecnología electrónica, especialmente en términos de rango de frecuencia de funcionamiento, posibilidad de distribución fotónica con fibras ópticas, peso, e inmunidad electromagnética. No obstante, las técnicas fotónicas tradicionalmente han ofrecido peores prestaciones en términos de ruido de fase, sintonía y rango dinámico (en generación) y ratio de conversión (en detección) con respecto a la tecnología electrónica de THz en el estado del arte. El trabajo realizado en esta tesis se centra en el diseño, desarrollo y validación de nuevas arquitecturas y componentes fotónicos tanto para generación como detección de ondas de sub-THz y THz que permitan solucionar las desventajas de las técnicas ópticas manteniendo todas sus ventajas. En esta tesis, varios sistemas de generación de sub-THz y THz han sido desarrollados utilizando arquitecturas Difference Frequency Generation (DFG) en las que la fuente DFG es proveída por un Optical Frequency Comb Generator (OFCG) y selección de modos ópticos. Diferentes dispositivos y técnicas son investigados para cada parte del sistema hasta conseguir un sintetizador de altas prestaciones. Passively Mode-Locked Laser Diodes (PMMLDs) son inicialmente evaluados como OFCG integrados. Un diseño mejorado del OFCG es conseguido mediante el uso de un esquema basado en un láser Discrete Mode (DM) bajo régimen Gain Switching (GS) y expansión del ancho de banda óptico mediante el uso de un modulador de fase Electro-Óptico (EO). Como estrategia de selección de modos ópticos, tanto filtrado óptico altamente selectivo como Optical Injection Locking (OIL) son usados y evaluados. Un fotodiodo comercial de ancho de banda 50 GHz y un fotomezclador de THz de superred n-i-pn-i-p son empleados. El sistema de generación final que se presenta en esta tesis consiste en un OFCG basado en GS, OIL como técnica de selección de modos ópticos y un fotomezclador de THz de superred n-i-pn-i-p. Este sintetizador ofrece un rango de funcionamiento de 60 a 140 GHz, directamente escalable a un rango entre 10 GHz y valores más allá de un THz. Sintonía cuasi-continua es ofrecida en todo el rango de frecuencia de operación, con una resolución en frecuencia de 0.1 Hz a 100 GHz que puede ser directamente escalable a 0.01 Hz a 100 GHz y 0.1 Hz a 1 THz. El ancho de línea a 3-dB de la señal a 120 GHz es menor de 10 Hz, solo limitada por la instrumentación de medida. El sistema ofrece una excelente estabilidad en potencia y frecuencia, con desviaciones sobre una hora de operación de 1.5 dB y 5 Hz, respectivamente. Estos valores también están limitados por la precisión e incertidumbre de la instrumentación de medida. Las prestaciones conseguidas por este sintetizador fotónico de sub-THz y THz para la mayoría de figuras de mérito, igualan o superan aquellas de las mejores soluciones comerciales electrónicas en el estado del arte, y supera algunas de estas características en más de un millón de veces en el caso de soluciones fotónicas comerciales en el estado del arte. La parte de detección de esta tesis explora el uso de arquitecturas fotónicas basadas en receptores EO heterodinos y los componentes clave que forman estas arquitecturas: Oscilador Local (OL) fotónico y mezcladores EO. Los primeros resultados son desarrollados en el entorno de microondas (<15 GHz) usando un amplificador de semiconductor óptico ultra no lineal (XN-SOA) como mezclador EO y un OL fotónico basado en GS. Se demuestra como este OL basado en GS ofrece una mejora significativa de las prestaciones del receptor con respecto al uso de OL fotónicos tradicionales basados en modulación lineal. Además, esta arquitectura de detección es validada en una aplicación real (imaging array fotónico), ofreciendo escalabilidad, flexibilidad y ratios de conversión razonables. Tras esto, un receptor EO heterodino es demostrado hasta frecuencias de 110 GHz. El OL fotónico empleado es el sintetizador de altas prestaciones presentado en esta tesis, mientras que el mezclador EO es un nuevo detector de THz: el np-i-pn cuasi-balístico. La primera muestra fabricada de estos nuevos dispositivos, especialmente diseñados y optimizados para detección homodina y heterodina, es empleada. El receptor EO heterodino resultante ofrece ratios de conversión de -75 dB. Este dispositivo es capaz de trabajar sin alimentación, lo que unido a la distribución fotónica del OL, ofrece un gran potencial para detección remota de ondas de sub-THz y THz. En resumen, las nuevas arquitecturas y dispositivos fotónicos presentados en esta tesis son capaces de ofrecer prestaciones en el estado del arte para generación de ondas de sub-THz y THz. En el caso de detectores EO heterodinos en frecuencias de sub-THz y THz, las técnicas fotónicas están mejorando sus prestaciones significativamente y están cada vez más cerca de ofrecer una alternativa a detectores electrónicos en el estado del arte

Integrated photonics for millimetre wave transmitters and receivers

This PhD thesis entitled “Integrated photonics for millimetre wave transmitters and receivers” aimed at investigating the possibility of employing the uni-traveling carrier photodiode (UTC-PD) in millimetre wave (MMW) wireless receivers and, eventually, demonstrating a photonic integrated transceiver, by exploiting the concept of optically-pumped mixing (OPM). Previously, the UTC-PD has been successfully demonstrated as an OPM, by mixing an optically-generated local oscillator (LO) with a high frequency RF signal to generate a replica of the RF signal at a low intermediate frequency (IF), defined by the difference between the LO and the RF signal. This concept forms the foundation of this PhD thesis. The principal idea is to deploy the UTC-PD mixer in MMW wireless receivers to down-convert the high frequency data signal into a low frequency IF, where it can be easily processed and recovered. The main challenge to this approach is the low conversion efficiency of the UTC-PD mixer. For example, a conversion loss of 32 dB has been reported at 100 GHz. Also, the detection bandwidth in previous demonstrations was very narrow (around 100 Hz), which is too narrow to be useful in high-speed data communications. Consequently, a significant effort was made, in this thesis, to improve these parameters before the implementation in wireless receivers. The characterization and optimization works done in this thesis on the input parameters to the UTC-PD mixer have advanced the state of the art significantly. For example, conversion losses as low as 22 dB have been reported here. Also, the detection bandwidth has been increased to up to 10 GHz, allowing for multi-Gbps communication links. Based on these promising results, proof of concept wireless data transmission experiments were successfully conducted at different carrier frequencies (33 GHz, 35 GHz, and 60 GHz) using separate non-integrated UTC-PDs at the receiver with speeds of up to 5 Gbps. To the best of the author’s knowledge, this is the first demonstration of the UTC-PD at the receiver. Upon these successful demonstrations, further research was done on a photonic integrated circuit, which comprises UTC-PDs, lasers, optical amplifiers and modulators. The outcome of this research was the first demonstration of a photonic integrated transceiver. This transceiver is suitable for short distance communications and could find interesting applications in 5G and future networks, including: high definition (HD) video streaming, file transfer, and wireless backhaul

The Third International Symposium on Space Terahertz Technology: Symposium proceedings

Papers from the symposium are presented that are relevant to the generation, detection, and use of the terahertz spectral region for space astronomy and remote sensing of the Earth's upper atmosphere. The program included thirteen sessions covering a wide variety of topics including solid-state oscillators, power-combining techniques, mixers, harmonic multipliers, antennas and antenna arrays, submillimeter receivers, and measurement techniques

Development of terahertz vacuum electronics for array receivers

Heterodyne array receivers are employed in radio astronomy to reduce the observing time needed for mapping extended sources. One of the main factors limiting the amount of pixels in terahertz receivers is the difficulty of generating a sufficient amount of local oscillator power. Another challenge is efficient diplexing and coupling of local oscillator and signal power to the detectors. These problems are attacked in this dissertation by proposing the application of two vacuum electronic terahertz amplifier types for the amplification of the LO-signal and by introducing a new method for finding the defects in a quasioptical diplexer. A traveling wave tube (TWT) design based on a square helix slow wave structure (SWS) at 825 GHz is introduced. It exhibits a simulated small-signal gain of 18.3 dB and a 3-dB bandwidth of 69 GHz. In order to generate LO-power at even higher frequencies, the operation of an 850-GHz square helix TWT as a frequency doubler has been studied. A simulated conversion efficiency of 7% to 1700 GHz, comparable with the state-of-art solid-state doublers, has been achieved for an input power of 25 mW. The other amplifier type discussed in this work is a 1-THz cascade backward wave amplifier based on a double corrugated waveguide SWS. Specifically, three input/output coupler types between a rectangular waveguide and the SWS are presented. The structures have been realized with microfabrication, and the results of loss measurements at 1 THz will be shown. Diplexing of the LO- and signal beams is often performed with a Martin-Puplett interferometer. Misalignment and deformation of the quasioptical components causes the polarization state of the output signal to be incorrect, which leads to coupling losses. A ray-tracing program has been developed for studying the influence of such defects. The measurement results of the diplexer of a multi-pixel terahertz receiver operated at the APEX telescope have been analyzed with the program, and the results are presented. The program allows the quasioptical configuration of the diplexer to be corrected in order to obtain higher receiver sensitivity

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

Terahertz radiation from intrinsic Josephson junctions in Bi 2 Sr 2 Ca Cu 2 O 8+delta - dynamics, tunability, and applications

Terahertz (THz) radiation offers many new possibilities for applications, e. g., for spectroscopy, high-bandwidth data communication, medical diagnosis, and security screening. However, these potential applications are still largely unused because there are only a few devices available to generate these frequencies. Especially, from 0.3 to 2 THz there is a lack of coherent, compact, low-cost, tunable, and high-power emitters. Devices based on fast electronic circuits usually work at lower frequency and photonic systems at much higher frequencies. Unfortunately, there is no overlap of these technologies. To close this so-called THz gap one can use Josephson junctions which can convert an applied dc voltage into an ac current and, thus, are able to emit electromagnetic waves. According to the Josephson relation, an applied dc voltage of 1 mV translates into a frequency of 483.6 GHz. Especially, intrinsic Josephson junctions (IJJs) occurring in the high-temperature superconductor Bi 2 Sr 2 Ca Cu 2 O 8+delta (BSCCO) are attractive sources of radiation due to their large frequency range of, in principle, up to 10 THz, and the easy fabrication of hundreds of almost identical junctions stacked on top of each other. The challenging part is to achieve phase synchronization among the junctions to obtain high power emission that scales with the square of the number of junctions. Currently, IJJ stacks with emission powers up to tens of microwatts have been realized, and frequencies ranging from 0.2 to 2.4 THz can be generated. High-precision frequency measurements revealed linewidths down to some megahertz, which is already practical for applications. However, a device that combines all these features does not exist yet. Especially, at higher frequencies above 1 to 2 THz the emission power decreases strongly and the linewidth becomes broad. The group in Tübingen investigates THz generation from IJJ stacks in close collaboration with the groups of H. B. Wang (Research Institute of Superconductor Electronics, Nanjing University, China & National Institute for Materials Science, Tsukuba, Japan) and V. P. Koshelets (Kotel'nikov Institute of Radio Engineering and Electronics, Moscow, Russia). Within this collaboration, goals of this thesis were (1) to gain a deeper understanding of the mechanisms of THz generation from IJJ stacks and (2) to find ways how to tune and how to optimize the emission properties of the samples. As a third goal, if possible, some potential applications should be demonstrated. To understand the physical behavior in more detail and to study the influence of individual parameters on the system a three-dimensional numerical model for large stacks of IJJs was developed. Typically, such stacks with a large number of junctions strongly suffer from Joule heating, such that the temperature distribution becomes highly inhomogeneous and temperature dependencies of all involved physical quantities need to be considered. Based on combined heat diffusion equations and coupled sine-Gordon equations, covering both the thermal physics and the Josephson physics, numerical simulations were done allowing one to have a look into the dynamics of phase synchronization, hot spot formation, and the excitation of standing waves in the stack of junctions acting as a cavity for electromagnetic waves. The overall behavior of the system as well as effects of an external magnetic field were studied and compared to experimental data. In experiment, a simple array structure was investigated systematically to study the thermal and electric interaction of two nearby IJJ stacks. Moreover, ways to modify and tune the emission properties of BSCCO samples were studied. For instance, it was found that the emission power strongly depends on the position of the hot spot that develops at high-bias currents due to the strong self-heating of the samples. Also, a precise tuning of the emission power is possible by using a focused laser beam that locally heats the sample surface. Furthermore, charge carrier injection was used to change the doping level of the crystal affecting the emission properties. Since the long-term goal is to build a compact, tunable, and coherent device for a large field of applications at THz frequencies, some first, simple applications are presented which show that BSCCO stacks are suitable candidates as emitters. A compact THz emitter system was built, working at liquid nitrogen temperatures with a commercial 1.5 V battery making it cheap, portable, and easy to handle. Furthermore, spectroscopy experiments were done, showing that it is possible to detect gases like water vapor and ammonia.Terahertzstrahlung bietet viele neue Möglichkeiten für Anwendungen in Bereichen der Spektroskopie, breitbandiger Datenübertragung, medizinischer Diagnostik und bei Sicherheitskontrollen. Viele dieser Anwendungen werden jedoch nicht genutzt, da es nur wenige Bauelemente gibt, die in der Lage sind, Strahlung in diesem Frequenzbereich zu erzeugen. Insbesondere im Bereich zwischen 0,3 und 2 THz mangelt es an günstigen, kompakten, kohärenten und durchstimmbaren Strahlungsquellen mit hoher Intensität. Schnelle Schaltkreise in der Hochfrequenzelektronik arbeiten bei niedrigeren Frequenzen und photonische Systeme, wie beispielsweise Laser, bei höheren Frequenzen. Einen Überlapp der beiden Technologien gibt es jedoch nicht. Um diese sogenannte Terahertzlücke zu schließen, können z. B. Josephsonkontakte eingesetzt werden, die eine angelegte elektrische Gleichspannung in einen Wechselstrom umwandeln können und deshalb in der Lage sind elektromagnetische Strahlung zu emittieren. Bei einer Gleichspannung von 1 mV entsteht so gemäß der Josephsonrelation ein Wechselstrom mit der Frequenz von 483,6 GHz. Besonders intrinsische Josephsonkontakte (IJKs), die im Hochtemperatursupraleiter Bi 2 Sr 2 Ca Cu 2 O 8+delta (BSCCO) vorkommen, sind attraktive Strahlungsquellen aufgrund des hohen Frequenzbereichs bis zu 10 THz und der einfachen Herstellung von Stapel vieler hunderter, fast identischer Kontakte. Herausfordernd ist jedoch vor allem die Phasensynchronisation der oszillierenden Ströme in allen Kontakten, um eine hohe Ausgangsleistung zu erreichen, die mit dem Quadrat der Anzahl an Kontakten steigt. Momentan ist es möglich Stapel von IJKs mit einer Strahlungsleistung von einigen Zehn Mikrowatt herzustellen und Frequenzen von 0,2 bis 2,4 THz zu erzeugen. Präzise Frequenzmessungen ergaben Linienbreiten hinunter bis zu wenigen Megahertz, was für praktische Anwendungen bereits ausreichend ist. Ein Quelle, die all diese Eigenschaften vereint, existiert jedoch leider noch nicht. Insbesondere bei höheren Frequenzen über 1-2 THz bricht die Strahlungsleistung deutlich ein und die Linienbreite wächst. Die Untersuchung der Terahertzemission von IJK Stapeln in Tübingen findet in enger Kooperation mit den Arbeitsgruppen von H. B. Wang (Research Institute of Superconductor Electronics, Nanjing Universität, China & National Institute for Materials Science, Tsukuba, Japan) und V. P. Koshelets (Kotel'nikov Institute of Radio Engineering and Electronics, Moskau, Russland) statt. Im Rahmen dieser Kooperation waren die Ziele der vorliegenden Arbeit, (1) ein tieferes Verständnis für die physikalischen Vorgänge in Stapeln von IJKs zu gewinnen und (2) Wege zu finden, wie sich die Strahlungseigenschaften beeinflussen und optimieren lassen. Drittens sollte nach Möglichkeit untersucht werden, inwieweit bereits jetzt schon diese Art von Emitter in praktischen Anwendungen eingesetzt werden können. Um das physikalische Verständnis zu verbessern und um die Einflüsse verschiedener Parameter auf das System zu untersuchen, wurde ein dreidimensionales numerisches Modell zur Beschreibung großer Stapel von IJKs entwickelt. Da Joulesches Heizen bei Stapeln mit einer großen Zahl von Kontakten für gewöhnlich zu einem starken Selbstheizeffekt und zu einer extrem inhomogenen Temperaturverteilung führt, müssen bei der numerischen Beschreibung die Temperaturabhängigkeiten aller physikalischen Größen berücksichtigt werden. In Simulationen mit einem kombinierten System von Wärmeleitungsgleichungen zur Beschreibung der thermischen Physik und gekoppelten Sinus-Gordon-Gleichungen, welche die Josephsonphysik beinhalten, konnten Phasensynchronisation, die Entstehung von Hotspots und Stehwellen im Stapel, der als Kavität für elektromagnetische Wellen fungiert, beobachtet werden. Das generelle Verhalten des Systems und Effekte eines externen Magnetfelds wurden untersucht und mit Messdaten aus Experimenten verglichen. Auf der experimentellen Seite wurde die thermische und elektrische Wechselwirkung zweier nebeneinander stehender Stapel als einfache Arraystruktur systematisch untersucht. Weiterhin wurden Möglichkeiten gesucht, wie die Strahlungseigenschaften der Quellen manipuliert und optimiert werden können. Beispielsweise konnte gezeigt werden, dass die Strahlungsintensität stark von der Position des Hotspots abhängt, der bei höheren Strömen aufgrund des starken Selbstheizeffekts entsteht. Außerdem ist es möglich die Strahlungsleistung zu beeinflussen, indem ein Laser auf die Stapeloberfläche fokussiert wird und die Probe lokal heizt. Die Injektion von Ladungsträgern durch das Anlegen hoher Ströme führte zu einer Änderung der Dotierung des BSCCO-Kristalls und ebenfalls zu einer Veränderung der Strahlungseigenschaften. Da das eigentliche Ziel dieser Forschung in der praktischen Anwendung von Terahertzstrahlung liegt, werden im Rahmen dieser Arbeit auch einfache Möglichkeiten für Anwendungen präsentiert, die zeigen, dass BSCCO Stapel als Emitter geeignet sind. Es wurde ein kompaktes Emittersystem gebaut, welches zur Kühlung ausschließlich flüssigen Stickstoff benötigt und mit einer gewöhnlichen 1,5 V Batterie arbeitet, was es günstig, mobil und einfach zu verwenden macht. Ebenso konnte bei Spektroskopieexperimenten gezeigt werden, dass es möglich ist, Gase wie beispielsweise Wasserdampf und Ammoniak zu detektieren