THz Mixing Using Y-Ba-Cu-O Josephson Junctions Fabricated With Focused Helium Ion Beam Irradiation
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THz Mixing Using Y-Ba-Cu-O Josephson Junctions Fabricated With Focused Helium Ion Beam Irradiation


This dissertation reports the results of the investigation of a novel Y-Ba-Cu-O helium ion damaged Josephson junction mixer. Mixers, also known as a heterodyne detectors, are crucial tools for the study of spectroscopy, due to their ability to achieve the needed high spectral resolution to resolve the fine features in spectral lines. Current state of the art heterodyne detectors rely on low transition temperature superconductors, which have demanding cryogenic requirements. The motivation behind developing a high transition temperature superconducting mixer is their ability to be implemented in space and balloon missions where relaxed cryocooling is important. Commercially grown Y-Ba-Cu-O thin films are acquired and utilized for the fabrication and testing of devices. I have developed a large scale fabrication process that allows high yield throughput of patterned chips on a 2-inch wafer. In this process several 5 x 5 mm devices are patterned with a planar spiral antenna, then etched to prepare for implementation of the junction at the center of the antenna. The junction is created by irradiating a focused helium ion beam on the Y-Ba-Cu-O film. These Helium Ion Josephson Junctions feature unique transport properties that are dependent upon the ion dose. For low doses of helium irradiation, the junctions have metallic barriers. For higher doses of irradiation, insulator barriers are created. The direct write process involved in the fabrication of the junction allows the junction size to be aggressively scaled down to produce high impedance devices. I have measured the DC electrical transport properties of several junctions with varying doses and widths to successfully demonstrate the ability to tune the impedance of the junction to the spiral planar antenna, which has proven difficult with other Josephson Junction mixers. The ability to match the junction impedance to the antenna improves the performance of the mixer and eliminates the need for complex circuitry that is currently used for impedance matching. The mixer performance of these devices were investigated by irradiating the junction with frequencies ranging from 90 GHz -- 2.5 THz. In addition a model was developed to fit the experimental measurements and further approximate the mixer performance from existing mixer theory. I demonstrated that an existing model for Josephson junctions can be adapted to fit the experimental data for the novel type helium ion Josephson Junctions. This model is also useful for other applications outside of mixers since it can be used to determine the fundamental parameters of a junction.

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