Dissertations, Theses, and Capstone Projects

Date of Degree

9-2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Timothy Benseman

Committee Members

Euclides Almeida

So Takei

Karl Sandeman

Ulrich Welp

Subject Categories

Condensed Matter Physics

Abstract

Devices microfabricated from the high-temperature superconducting cuprate Bi2Sr2CaCu2O8+𝛿 (Bi-2212) are a promising source of coherent terahertz radiation within the ’terahertz gap’ (0.3- 1.5 THz). A current-biased mesa patterned into the surface of a single crystal will produce electromagnetic radiation in the terahertz regime due to the naturally occurring Josephson junctions formed by the stacks of superconducting cuprate oxide planes in the material separated by insulating BiO and SrO barrier layers. Previous research has shown that by fabricating these mesas such that the geometry supports resonant cavity Fabry-Perot modes, the junctions within the mesa can phase- lock and coherently radiate in the THz regime, however the output power achieved by existing devices is so far still well below the threshold of power needed for applications like medical imaging and data transmission (≈ 1 milliWatt).

The purpose of this research is to improve upon and standardize the fabrication procedure of such devices. With the ultimate aim of producing at least 1 mW of coherent radiation in the frequency range of 0.5 - 1 THz at a targeted operating temperature of 77.4 K, a prototype was developed employing photolithographic techniques to tailor the sidewall angle, and ensure a thin base crystal to in order to minimize the self heating of the device. Lithographically patterned Bi-2212 antennae structures on top of the mesa structure were constructed in order to enhance the efficiency of the coupling of THz radiation to free space. In the process, successive annealing steps were introduced as a means of mitigating the effects of ion-milling on the surface of Bi-2212. A nondestructive method of crystal thickness was also developed exploiting the thickness dependence of Energy-dispersive X-ray spectroscopy (EDAX) and a calibration curve was developed.

In addition to microfabrication developments, this research also incorporated the characterization of Bi-2212 devices including measurements of the operational ranges and angle dependence of output power from mesa structured Bi-2212 THz emitters, an analytic determination of optimized mesa sidewall profile to minimize IJJ voltage spread, and finally experiments probing the magnetic properties of Bi-2212 to study Josephson plasma waves via transmission measurements through a Bi-2212 slab under small magnetic fields.

This dissertation consists of four chapters, beginning with an introduction of some of the relevant physics, followed by one chapter on engineering motivations for optimizing solid-state superconducting THz sources from Bi2Sr2CaCu2O8+𝛿, then a chapter on experimental progress on a prototype device of this type, and finally a brief overview on promising results in transmission through a Bi2Sr2CaCu2O8+𝛿 slab under small magnetic fields toward the experimental measurement of Josephson Plasma Waves.

Chapter 1: Introduction

Chapter 2: Engineering Motivations

Chapter 3: Lithography Process Development and Results

Chapter 4: Preliminary Josephson Plasma Wave Measurement

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