The exponential rise in global data traffic and the growing reliance on digital applications is pushing the bandwidth demands within data centers. The traditional hierarchical network architecture of data centers, primarily relying on electrical interconnects, faces scalability challenges including power dissipation, bandwidth limitations, and cooling requirements. Optical interconnects, using fibers and pluggable transceivers, emerge as a promising solution to these challenges, offering advantages such as electromagnetic interference resistance, high bandwidth, and efficient energy usage. This thesis explores the design, fabrication, and characterization of opto-electronic devices to be used as components for optical transceivers on a silicon photonics platform, which leverages the mature complementary metal-oxide semiconductor fabrication technology. Chapter 2 introduces the basics of waveguide theory alongside the principles behind defect-based avalanche photodiodes, phototransistors, and two-level system optical amplifiers. Chapter 3 details the design, simulation, and characterization of a high-responsivity silicon/germanium phototransistor, achieving over 1000 A/W in performance. Chapter 4 discusses the design and measurement of an all-silicon avalanche photodetector for near-infrared wavelengths mediated by deep-level defects. In Chapter 5, the focus shifts to enhancing the previously mentioned photodetector’s responsivity through slow light gain with subwavelength grating waveguide structures, with details on its design, simulation, fabrication, and characterization. Chapter 6 explores the development and analysis of an erbium-doped waveguide amplifier on a hybrid silicon nitride - tellurite platform, incorporating erbium ions via ion implantation. This thesis makes contributions toward realizing efficient silicon photonics-based data communication infrastructure, supporting the escalating demand for bandwidth while mitigating power consumption and improving system scalability. / Dissertation / Doctor of Philosophy (PhD)

Electronic microchips have now firmly plateaued in switching speed. A promising solution for increasing performance on unparallelizable tasks is to switch digital data purely in the optical domain on a photonic chip, as all-optical switching can reach up to terahertz speeds and beyond. Because ultrafast optical effects are weak phenomena, materials with extremely high nonlinear sensitivity must be developed, and very high optical intensities and coupling efficiencies are required to adequately switch data streams of light. In this thesis, unique all-optical platforms, waveguides, fiber-waveguide grating couplers, and an ultrafast optical switch were experimentally demonstrated as proofs-of-concept for the validity of densely integrated all-optical switches. Two horizontal slot waveguiding structures were designed and fabricated from scratch: a multiple horizontal slot waveguide with polycrystalline silicon sandwiching third-order nonlinear slots and a nonlinear cover-cladding with slot-like behavior over a thin crystalline silicon waveguide. Perfectly vertical grating couplers were then designed from a novel genetic algorithm, fabricated, and experimentally tested for both platforms with two promising nonlinear materials: silicon nanocrystals or a supra-molecular assembly, DDMEBT. Vertical grating couplers in the multiple horizontal slot waveguide achieved a theoretical coupling efficiency of 63% and an experimental coupling efficiency of 60%, which is the highest coupling efficiency into nonlinear slot waveguides to date. Vertical grating couplers for the cover-slot waveguide experimentally demonstrated a coupling efficiency of 38% and an extrapolated 1 dB bandwidth of 66 nm, the largest grating-coupled 1 dB bandwidth obtained for slot waveguides to date. A grating coupler was then designed to be included as one of two grating reflectors in a nonlinear resonator switch. Coupled mode theory and vectorial eigenmode propagation simulations were used to optimally design the grating coupler/resonator device, resulting in a record low footprint of 710 μm² per combined switch and fiber coupler device. The third-order nonlinear molecular material, DDMEBT, was, for the first time, successfully spun onto pre-patterned silicon-on-insulator chips with repeatable, defect-free results. Extremely sensitive experimental autocorrelation of the resonator's impulse response yielded output pulse durations as low as 600 femtoseconds. At high power and low pulse repetition rates, the switch's resonances redshifted by 4 nm with 4 dB of switching contrast, revealing an ultrafast Kerr effect which matched previous works. The resonator switch is therefore capable of modulating a single optical carrier frequency at 1 THz and switching an optical data stream at 500 GHz. These are the fastest switching speeds demonstrated by an integrated all-optical switch and validate the proof-of-concept needed for a future of densely integrated all-optical processing. / text

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An energy recovering linac (ERL) offers an attractive alternative for generating intense electron beams by approaching the operational efficiency of a storage ring while maintaining the superior beam quality typical of a linear accelerator. Two primary physics challenges exist in pushing the frontier of ERL performance. The first is energy recovering a high energy beam while demonstrating operational control of two coupled beams in a common transport channel. The second is controlling the high average current effects in ERLs, specifically a type of beam instability called multipass beam breakup (BBU). This work addresses each of these issues.;A successful 1 GeV energy recovery demonstration with a maximum-to-injection energy ratio of 51:1 was carried out on the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory in an effort to address issues related to beam quality preservation in a large scale system. With a 1.3 km recirculation length and containing 312 superconducting radio frequency (SRF) cavities, this experiment has demonstrated energy recovery on the largest scale, and through the largest SRF environment, to date.;The BBU instability imposes a potentially severe limitation to the average current that can be accelerated in an ERL. Simulation results for Jefferson Laboratory's 10 kW free electron laser (FEL) Upgrade Driver predict the occurrence of BBU below the nominal operating current. Measurements of the threshold current are described and shown to agree to within 10% of predictions from BBU simulation codes. This represents the first time the codes have been benchmarked with experimental data. With BBU limiting the beam current, several suppression schemes were developed. These include direct damping of the higher-order mode using two different cavity-based feedbacks and modifying the electron beam optics. Each method increased the threshold current for stability. Beam optical control methods proved to be so effective that they are routinely used in normal operation of the 10 kW FEL Upgrade.

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