Search

This thesis describes research on Efficient Discrete Modelling of Axisymmetric Radiating Structures . Investigating the possibilities of surmounting the inherent limitation in the Cartesian rectangular Transmission Line Modelling (TLM) method due to staircase approximation by efficiently implementing the 3D cylindrical TLM mesh led to the development of a numerical model for simulating axisymmetric radiating structures such as cylindrical and conical monopole antennas. Following a brief introduction to the TLM method, potential applications of the method are presented. Cubic and cylindrical TLM models have been implemented in MATLAB and the code has been validated against microwave cavity benchmark problems. The results are compared to analytical results and the results obtained from the use of commercial cubic model (CST) in order to highlight the benefit of using a cylindrical model over its cubic counterpart. A cylindrical TLM mesh has not previously been used in the modelling of axisymmetric 3D radiating structures. In this thesis, it has been applied to the modelling of both cylindrical monopole and the conical monopole. The technique can also be applied to any radiating structure with axisymmetric cylindrical shape. The application of the method also led to the development of a novel conical antenna with periodic slot loading. Prototype antennas have been fabricated and measured to validate the simulated results for the antennas.

Electrically detected magnetic resonance (EDMR) is a sensitive spectroscopic technique, which can be used to readout few to single electron spins in semiconductor and carbon nanodevices for applications in solid state quantum information processing (QIP). Since only electrically active defects contribute to the EDMR signal, this technique can be used further to investigate defects and impurities in photovoltaic devices, in which they limit the sunlight-to-energy conversion efficiency significantly. Here, I employ X-band EDMR for semiconductor defect analysis and identify the most important recombination centres in Czochralski silicon with oxide precipitates, which can be intentionally grown to confine detrimental metallic impurities to inactive regions of the wafer in order to serve as a defect-free substrate for modern silicon photovoltaic devices. Those experiments show that oxide precipitation is accompanied by the formation of silicon dangling bonds. Furthermore, I describe a very promising route towards the fabrication and readout of few to single electron spins in carbon nanotube devices, which can be characterised structurally via transmission electron microscopy in order to relate their electrical and spin properties with their structure. Finally, I employ EDMR to read out electron spin states in donor-doped silicon field-effect transistors as a prerequisite for their application in QIP. I report on a novel cryogenic probe head for EDMR experiments in resonant microwave cavities operating at 0.35 T (9.7 GHz, X-band) and 3.34 T (94 GHz, W-band). This approach overcomes the inherent limitations of conventional X-band EDMR and permits the investigation of paramagnetic states with a higher spectroscopic resolution and signal intensity. Both advantages are demonstrated and discussed. I further report on a novel mechanism giving rise to the EDMR effect in donor-doped silicon field-effect transistors, which is capable of explaining why the EDMR signal intensities of the conduction electrons are enhanced by a factor of ∼100, while the donor resonance signals increase by a factor of ∼20 from X- to W-band only. The spin-relaxation and dephasing times are extracted from a series of pulsed-EDMR measurements and confirm this model. The author gratefully acknowledges funding from Trinity College Oxford, Department of Materials, EPSRC DTA, and Konrad-Adenauer-Stiftung e.V. (Begabtenförderung).

Levitating, controlling, and detecting the motion of mesoscopic objects is useful for inertial sensing and fundamental studies in quantum physics. We report the first experimental measurements of Meissner-effect levitation for a sequence of identical millimeter-scale neodymium magnets having varying strengths within a cm-scale superconducting aluminum coaxial quarter-wave stub cavity. We experimentally, theoretically, and analytically characterized Meissner-effect levitation within a 10 GHz superconducting aluminum coaxial quarter-wave stub cavity for a sequence of identically shaped millimeter-scale neodymium magnets having varying strengths (1.22-1.47 T). Magnet levitation within the cavity is accompanied by both gradual and abrupt shifts in the resonance frequency (with a height sensitivity as large as 400 MHz/mm) as well as changes in the total quality factor (8%-17%) as a function of temperature during the superconducting transition of the aluminum cavity. The experimental observation is confirmed with a cylindrical and spherical magnet. The controlled heating and cooling of the cavity show hysteresis in the frequency shift. Furthermore, the experimental observation has shown excellent agreement with finite element simulations, room temperature measurements, and a lumped element model.

Abstract: Information transfer between a microwave field and light, also known as microwave-to optical transduction, is a key area of research in quantum communication technology aimed at developing an interface between the superconducting processing devices operating at microwave frequencies and optical photons used for transferring quantum information over distance. Atomic vapors offer a good platform for such interfaces since they naturally support transitions at microwave and optical frequencies. In this work, we explored microwave-to-optical transduction in rubidium ensembles. We proposed a transduction method based on microwave-optical double resonance, where we can control the ensemble optical properties via non-linear interactions with a microwave magnetic field. With this method, we demonstrate the transduction of an audio signal from frequency-modulated and amplitude-modulated microwave fields to optical intensity modulation. We also present a protocol for engineering atomic spin polarization with microwave-assisted optical pumping and consider its applications for polarization-selective frequency conversion and microwave-controlled optical rotation.

The nature of dark matter is one of the most outstanding mysteries in science today. Possibilities for its composition range from black holes to heavy particles that can be detected by their collisions with atomic nuclei to light particles that act like a coherent field. Sensitive experiments have not yet been able to detect dark matter. A case can be made that the most promising dark matter candidate is the axion, which was invented to solve the strong charge-parity problem in the standard model of particle physics. If the axion exists, it could be produced in abundance and constitute a major component of dark matter.In the presence of a magnetic field, axions will be converted to an oscillating electromagnetic field with a frequency approximately corresponding to the axion mass. We can detect this electromagnetic field as a power excess spectrally coincident with the resonance of a microwave cavity. Each microwave cavity can provide good sensitivity in the search for axions over a limited frequency range, but we must consider new technologies to probe the full axion parameter space in a reasonable amount of time. Both as a testbed for innovative cavity and amplifier concepts and as a data pathfinder for higher frequencies, the Haloscope at Yale Sensitive to Axion Cold dark matter (HAYSTAC) was the first cavity experiment to use a dilution refrigerator and commission a squeezed-state receiver to circumvent the Standard Quantum Limit. The HAYSTAC Phase I and II cavity is a cylinder with an internal rod that rotates off-center and tunes the mode-of-interest resonance frequency between 3.4 and 5.8 GHz in a volume of 1.5 L. Recent theoretical work favors axions between 4 and 12 GHz. I examined various cavity designs to access higher frequencies in this range and determined that a seven-rod cavity design would be optimal with a volume of 1.7 L for a frequency range between approximately 5.5 and 7.5 GHz. I designed this seven-rod cavity, constructed it, and characterized it in detail for use on HAYSTAC. My cavity design allows access to this well-motivated axion mass range with high sensitivity by increasing accessible frequencies without sacrificing performance. This seven-rod concept can be extended to higher frequencies.

Abstract: This thesis describes the design, fabrication, and characterization of 3D superconducting microwave cavities for two applications. It first describes a cryogenically compatible microwave filter that is able to tune its resonant frequency by an unprecedented 5 GHz via deformation caused by a a helium pressure vessel. Then, it describes a novel microwave optomechanics device where such a cavity is coupled to an aluminized SiN membrane. It demonstrates that this device can be back-action cooled from the 450 mK base temperature of the cryostat down to 160 mK.