Guided-Wave Superconducting Quantum Optoelectronic Devices

This thesis investigates a novel optoelectronic platform based on the integration of superconductive structures, such as thin films and micro-constrictions, with optical waveguides for ultra-fast and ultra-sensitive devices with applications including high-speed optical communications, quantum optical information processing, and terahertz (THz) devices and systems. The kinetic-inductive photoresponse of superconducting thin films will be studied as the basic optoelectronic process underlying the operation of these novel devices. Analytical formulation for the non-bolometric response is presented, and experimental photodetection in YBCO meander-line structures will be demonstrated. A set of superconducting coplanar waveguides (CPW) are designed and characterized, which support the operation of the devices at microwave frequencies. Microwave-photonic devices comprising a microwave transmission line and a light-sensitive element, such as a meander-line structure, are designed and measured for implementation of optically tunable microwave components. In order to support low-loss and low-dispersion propagation of millimeter-wave and THz signals in ultra-fast and wideband kinetic-inductive devices, surface-wave transmission lines are proposed, incorporating long-wavelength Surface Plasmon Polariton (SPP) modes in planar metal-dielectric waveguides. The theory of superconducting optical waveguides, including analytical formulation and numerical methods, is developed in detail. The implementation of superconducting optical waveguides is discussed thoroughly, employing conventional dielectric-waveguide techniques as well as optical SPP modes. Superconductive traveling-wave photodetectors (STWPDs) are introduced as a viable means for ultra-fast and ultra-sensitive photodetection and photomixing. A modified transmission line formalism is developed to model STWPDs, where light is guided through an optical waveguide and photodetection is distributed along a transmission line. As an appendix, a systematic approach is developed for the analysis of carrier transport through superconducting heterostructures and micro-constrictions within the Bogoliubov-de Gennes (BdG) framework. The method is applied to study the role of Andreev reflection and Josephson-like phenomena in the current-voltage characteristics of inhomogeneous superconducting structures. I-V characteristics are experimentally demonstrated in YBCO micro-constrictions with potential applications in millimeter-wave and THz devices

Electromagnetic Modelling of Superconducting Sensor Designs

The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy

Electron Beam Lithography patterning of superconducting and magnetic nanostructures for novel optical and spintronic devices

In this thesis novel, high-end superconducting and spintronic devices have been fabricated and characterized. In summary, the proposed work has been focused on the realization of nanowires, and more generally nanostructures, using the Electron Beam Lithography. Such a technology offers a powerful solution for nanofabrication able to conjugate spatial resolution, operation flexibility, and costs. Two main research fields has been explored: superconductive nanowires for advanced optical detection and nanostructures for magneto-resistance based devices

Investigating the feasibility of using nanobridge weak links as the active Josephson element in Rapid Single Flux quantum circuitry

Josephson junctions are used in present day voltage standards. To extend their use to AC voltage standards a high bandwidth, low-noise detector is required. A candidate component for this detector is a superconducting comparator based on Rapid Single Flux Quantum (RSFQ) circuits. The work presented here is a study to determine if nanobridge weak links can be used as the active Josephson element in these circuits. In order to achieve this an understanding of the nanobridge properties and in particular their critical currents is fundamental. We present simulations of a simple comparator using the circuit simulation software JSIM in order to study the effect of varying nanobridge parameters such as width, length, and loop area. These geometrical variables have an affect on the critical currents and loop inductances which subsequently effect device performance. Particular emphasis is given to investigation of how these parameters affect a key figure of merit, the grey zone width

Superconducting Multilayer Technology for Josephson Devices : Technology, Engineering, Physics, Applications

Within this book fabrication processes for high-quality Josephson junctions based on niobium and aluminum oxide as well as niobium nitride and aluminum nitride on various substrates are discussed. Techniques for achieving a planar chip topography and sub-µm lateral dimensions, aiding the realization of sophisticated Josephson devices such as SQUIDs, flux-flow oscillators and long Josephson junctions with artificial phase discontinuities, are presented in detail

Microwave and superconducting techniques for measurements on unconventional Josephson junctions

The frst part of the thesis describes the instrumentation, testing and analysis of a planar circuit designed for the measurement of the current-phase relationship of niobium-cobalt-insulator-niobium Josephson junctions. A detailed analysis method and fitting routine was developed but the results show that an irreducible mutual inductance places a limit on the accuracy of the chip for the intended measurement. The second part describes a study of the magnetic and microwave properties of a range of thin film niobium coplanar resonators with cobalt and normal metal layers. Magnetic measurements show a magnetic dead layer of 1.3 nm. The observed microwave losses are found to be two orders of magnitude higher than for high quality niobium films. Computer simulation shows that this is mainly due to conductive, rather than magnetic losses, and is in good agreement with the observed proportionality to cobalt thickness. Measurements of the temperature and magnetic field dependence of the losses and resonant frequency as a function of the cobalt thickness show no signs of the oscillatory thickness dependence reported in a number of other experiments. The temperature dependence of all films is found to be in good agreement with Mattis-Bardeen theory

A superconducting bandpass delta-sigma modulator for direct analog-to-digital conversion of microwave radio

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 291-305).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Direct analog-to-digital conversion of multi-GHz radio frequency (RF) signals is the ultimate goal in software radio receiver design but remains a daunting challenge for any technology. This thesis examines the potential of superconducting technology for realizing RF analog-to-digital converters (ADCs) with improved performance. A bandpass delta-sigma (AE) modulator is an attractive architecture for digitizing narrowband signals with high linearity and a large signal-to-noise ratio (SNR). The design of a superconducting bandpass AE modulator presented here exploits several advantages of superconducting electronics: the high quality factor of resonators, the high sampling rates of comparators realized with Josephson junctions, natural quantization of voltage pulses, and high circuit sensitivity. Demonstration of a superconducting circuit operating at clock rates in the tens of GHz is often hindered by the difficulty of high speed interfacing with room-temperature test equipment. In this work, a test chip with integrated acquisition memory is used to simplify high speed testing in a cryogenic environment. The small size (256 bits) of the on-chip memory severely limits the frequency resolution of spectra based on standard fast Fourier transforms. Higher resolution spectra are obtained by "segmented correlation", a new method for testing ADCs. Two different techniques have been found for clocking the superconducting modulator at frequencies in the tens of GHz. In the first approach, an optical clocking technique was developed, in which picosecond laser pulses are delivered via optical fiber to an on-chip metal-semiconductor-metal (MSM) photodiode, whose output current pulses trigger the Josephson circuitry. In the second approach, the superconducting modulator is clocked by an on-chip Josephson oscillator.(cont.) These testing methods have been applied in the successful demonstration of a super-conducting bandpass AE modulator fabricated in a niobium integrated circuit process with 1 kA/cm2 critical current density for the Josephson junctions. At a 42.6 GHz sampling rate, the center frequency of the experimental modulator is 2.23 GHz, the measured SNR is 49 dB over a 20.8 MHz bandwidth, and a full-scale (FS) input is -17.4 dBm. At a 40.2 GHz sampling rate, the measured in-band noise is -57 dBFS over a 19.6 MHz bandwidth.by John Francis Bulzacchelli.Ph.D