Atmospheric turbulence in free space optical satellite downlinks negatively impacts link availability and bit error rate. These effects can be mitigated using a compensation system capable of measuring the incoming wavefront distortion and applying a suitable correction to the received signal. The traditional solution based on adaptive optics and the Shack-Hartmann wavefront sensor has limitations in bandwidth, system complexity, size, weight, and power consumption. Signal correction can also be accomplished using a novel single-chip silicon photonic solution. This work introduces a four-arm grating coupler structure acting as a wavefront sensing element that emulates the performance of the Shack-Hartmann wavefront sensor by giving local tip and tilt estimation. FDTD simulations and measurements have confirmed the presence of a monotonic relationship between incident angle, polarization, and coupler output which can be converted to phase estimation through a reconstruction algorithm. An array of four-arm couplers on a silicon photonic chip provides enough sampling to fully reconstruct the wavefront, providing benefits over traditional solutions such as higher bandwidth, reduced size and weight, and reduced cost. Scaling up the results of this work to a full device could provide a solution for free space optical satellite to ground links in remote and rural communities across Canada and around the world. / Thesis / Master of Applied Science (MASc)

Transparent ceramics are an important class of optical materials with applications in high-strength windows, radiation detectors and high-power lasers. Despite the many successful developments of the past decades, their challenging fabrication still needs to be perfected to achieve a better consistency in optical quality. In particular, ternary phase materials such as Yttrium Aluminum Garnet (YAG, Y3Al5O12), a long standing high-power laser host, require a precise control of stoichiometry, often beyond the precision of current analytical techniques, in order to reduce scattering losses and the presence of deleterious point defects. This work explores the potential of Laser-Induced Breakdown Spectroscopy (LIBS) for the quantitative analysis of ceramic compositions near stoichiometry. We have designed a compact and automated LIBS system to determine the plasma composition of sintered mixtures of Y2O3-Al2O3 near the garnet composition. The performance of our setup is evaluated and compared to conventional techniques. Optimized conditions for the acquisition of plasma emission spectra are discussed and the intensity ratios of Y+ and Al in the 300 to 400nm spectral range are analyzed using simple plasma models. The results show that, for the selected parameters of our experiments, the fluctuations in plasma temperature are minimal, and the stability of the plasma is improved. Current results show that ceramic compositions can be resolved within 1 at% in oxide and several suggestions are proposed to further increase the accuracy and precision of the method.

The recent and rapid growth of silicon photonics is driven by the ever-increasing demand for bandwidth inside and between data centres. Silicon photonics can offer an unparalleled performance in terms of scalability and power consumption with low-cost fabrication through the leveraging of CMOS fabrication techniques. This thesis describes research on the silicon micro-ring resonator modulator, a device which combines energy-efficiency with a compact footprint that is ideal for data centre applications. Both theoretical and experimental work is described in the context of improving the reachability, capacity and stability of the silicon micro-ring resonator modulator for inter/intra-data centre communication. Chapter 2 presents modeling work using MATLAB® that provides predictive results for both device-level and system-level performance. Chapter 3 studies the chirp characteristic of an over-coupled silicon micro-ring resonator modulator and its capability of generating a negative-chirp modulation. The resulting chirp-induced power penalty is measured to be as low as 2.5 dB after 100 km transmission. Chapter 4 focuses on the advanced modulation techniques that can be efficiently exploited for increasing the spectral efficiency in the typically band-limited system. A record single-polarization 104 Gb/s data rate per wavelength (direct-detect) was achieved by using digital signal processing to alleviate the modulation deficiencies that are specific to the silicon micro-ring resonator modulator. In Chapter 5, a generic resonance control method using intrinsic defect-mediated photocurrent is described and experimentally demonstrated to provide stability for the silicon micro-ring resonator modulator during high-speed operation. This control method can also lead to an “all-silicon” system without the need for power detection using germanium. / Thesis / Doctor of Philosophy (PhD)

The advent of two-dimensional (2D) materials has both transformed the fundamental understanding of processes and properties in condensed matter and accelerated the development of new technologies to tackle societal challenges related to sustainability and energy, information processing, and human health. However, many challenges linger before the unique promise of 2D materials can be fully exploited. This dissertation focuses on understanding the conditions of high-quality 2D material processing and the effect of defects on the properties of 2D materials. This work aims to adapt defect engineering processes that will enable a more precise control of the behavior of 2D materials for scalable implementation in devices. First, methods of isolating high-quality single-crystal monolayers of 2D materials with suitable dimensions for device applications are considered. More specifically, metal-assisted exfoliation is used to isolate large monolayers of molybdenum disulfide (MoS2). The frequency of monolayers obtained, their size and their quality, are assessed for different types of metal-assisted exfoliations. The role of strain and binding energy between each metal and MoS2 on the exfoliation results is presented. Next, the effects of defects on the properties of 2D materials are evaluated, with a focus on hexagonal boron nitride (h-BN) and MoS2. Non-deterministic defects obtained by heat treatment are considered for h-BN. The lattice distortions resulting from the presence of defects are evaluated. Electrical and optical properties changes obtained through defect creation are assessed. Some approaches for more deterministic defect placement and their study with nanoscale precision are presented. The results obtained using a focused electron beam and using nanoscale tip-matter interactions are described. A summary of the work with perspectives on the future developments in fundamental science and in device applications are provided to conclude the body of work.

Inspired by spatially varying refractive index profiles within the lens in human eyes which can mitigate chromatic aberration and allow high-resolution imaging, we aim to develop a scalable approach for manufacturing high-precision three-dimensional (3-D) gradient refractive index (GRIN) nanocomposites based on multi-component bulk Ge-As-Pb-Se (GAP-Se) glass-ceramics for their use in infrared imaging systems. This work extends our efforts towards optimizing processing protocols for nominally single-phase parent bulk glasses where we specifically attempt to address prior challenges associated with nano/micro-scale liquid-liquid phase separation of the as-formed bulk material. Key findings of this work illustrate new understanding of the process beyond prior efforts. Firstly, this process illustrates improvements to the glass' homogeneity over prior lab-scale melt-quenched melts due to the physical dispersion of Pb within the glass matrix. Secondly, the laser induced Pb-rich amorphous phase by a near-bandgap 2 μm laser, upon subsequent heat treatment undergoes crystallization resulting in the formation of high-index PbSe containing nanocrystals, that results in effective refractive index modification. Thirdly, while the desired high index PbSe containing phases resulted from the protocols developed, other crystal phases that reduce spatial resolution and induce unacceptable levels of scatter loss remain. Despite this drawback, losses in the glass as a function of base material depth were sufficiently low to allow creation of axial modification of the irradiated glass for the first time. While measurement of effective refractive index is not possible with our experimental instrumentation, the axial variation in crystal fractions with laser dose has been demonstrated, suggesting a corresponding axial gradation in the bulk material's refractive index. The degree of crystallinity in the glass-ceramic yielding effective refractive index changes has been shown to be modulated by the laser dose, providing a route towards spatially tailorable index change and modification to the composite's dispersion.

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The optical properties of a concentric nanometer-sized spherical shell comprised of an (active) 3-level gain medium core and a surrounding plasmonic metal shell are investigated. Current research in optical metamaterials has demonstrated that including lossless plasmonic materials to achieve a negative permittivity in a nano-sized coated spherical particle can lead to novel optical properties such as resonant scattering as well as transparency or invisibility. However, in practice, plasmonic materials have high losses at optical frequencies. It will be demonstrated that a properly designed passive optical spherical core impregnated with a gain medium and coated with a concentric spherical plasmonic nano-shell will have a "super resonant" (SR) lasing state. The operating characteristics of this coated nano-particle (CNP) laser have been obtained numerically for a variety of configurations and will be reported here. Once the optical properties of the isolated active CNP inclusion are established, several examples of optical metamaterials using them as inclusions will be presented and analyzed. In particular, the effective material properties of these optical MTMs will be explored using effective medium theories that are applicable to a variety of inclusion configurations. Two-dimensional (2D) mono-layers of these active CNPs, which form metafilms; three-dimensional (3D) periodic arrays of these active CNPs; and 3D random distributions of these active CNPs will be described. The effective permittivities and refractive indexes of these optical MTMs will be compared and contrasted to those of their active CNP inclusions. In addition to the active MTMs, some examples of nano-photonic applications enabled by the unique properties of these inclusions will also be presented. Specifically metamaterial pigments derived from exploiting the high absorption and low scattering properties of the passive CNP particle will be explored for possible use in color display technology as well as the use of the SR lasing state and localized plasmon resonance of the active CNP for nano-sensing applications.

Silicon photonics is the study and application of photonic systems, which use silicon as an optical medium. Data is transferred in the systems by optical rays. This technology is seen as the substitutions of electric computer chips in the future and the means to keep tack on the Moore’s law. Cavity optomechanics is a rising field of silicon photonics. It focuses on the interaction between light and mechanical objects. Although it is currently at its early stage of growth, this field has attracted rising attention. Here, we present highly sensitive optical detection of acceleration using an optomechanical accelerometer. The core part of this accelerometer is a slot-type photonic crystal cavity with strong optomechanical interactions. We first discuss theoretically the optomechanical coupling in the air-slot mode-gap photonic crystal cavity. The dispersive coupling gom is numerically calculated. Dynamical parametric oscillations for both cooling and amplification, in the resolved and unresolved sideband limit, are examined numerically, along with the displacement spectral density and cooling rates for the various operating parameters. Experimental results also demonstrated that the cavity has a large optomechanical coupling rate. The optically induced spring effect, damping and amplification of the mechanical modes are observed with measurements both in air and in vacuum. Then, we propose and demonstrate our optomechanical accelerometer. It can operate with a resolution of 730 ng/Hz¹/² (or equivalently 40.1 aN/Hz¹/²) and with a transduction bandwidth of ≈ 85 kHz. We also demonstrate an integrated photonics device, an on-chip spectroscopy, in the last part of this thesis. This new type of on-chip microspectrometer is based on the Vernier effect of two cascaded micro-ring cavities. It can measure optical spectrum with a bandwidth of 74nm and a resolution of 0.22 nm in a small footprint of 1.5 mm².

The extraordinary growth of parallelism in high-performance computing requires efficient data communication for scaling compute performance. High-performance computing systems have been using photonic links for communication of large bandwidth-distance product during the last decade. Photonic interconnection networks, however, should not be a wire-for-wire replacement based on conventional electrical counterparts. Features of photonics beyond high bandwidth, such as transparent bandwidth steering, can implement important functionalities needed by applications. In another aspect, application characteristics can be exploited to design better photonic interconnects. Therefore, this thesis explores codesign opportunities at the intersection between photonic interconnect architectures and high-performance computing applications. The key accomplishments of this thesis, ranging from system level to node level, are as follows. Chapter 2 presents a system-level architecture that leverages photonic switching to enable a reconfigurable interconnect. The architecture, called Flexfly, reconfigures the inter-group level of the widely-used Dragonfly topology using information about the application’s communication pattern. It can steal additional direct bandwidth for communication-intensive group pairs. Simulations with applications such as GTC, Nekbone and LULESH show up to 1.8x speedup over Dragonfly paired with UGAL routing, along with halved hop count and latency for cross-group messages. To demonstrate the effectiveness of our approach, we built a 32-node Flexfly prototype using a silicon photonic switch connecting four groups and demonstrated 820 ns interconnect reconfiguration time. This is the first demonstration of silicon photonic switching and bandwidth steering in a high-performance computing cluster. Chapter 3 extends photonic switching to the node level and presents a reconfigurable silicon photonic memory interconnect for many-core architectures. The interconnect targets at important memory access issues, such as network-on-chip hot-spots and non-uniform memory access. Integrated with the processor through 2.5D/3D stacking, a fast-tunable silicon photonic memory tunnel can transparently direct traffic from any off-chip memory to any on-chip interface – thus alleviating the hot-spot and non-uniform access effects. We demonstrated the operation of our proposed architecture using a tunable laser, a 4-port silicon photonic switch (four wavelength-routed memory channels) and a 4x4 mesh network-on-chip synthesized by FPGA. The emulated system achieves a 15-ns channel switching time. Simulations based on a 12-core 4-memory model show that for such switching speeds the interconnect system can realize a 2x speedup for the STREAM benchmark in the hot-spot scenario and a reduction of execution time for data-intensive applications such as 3D stencil and K-means clustering by 23% and 17%, respectively. Chapters 4 explores application-level characteristics that can be exploited to hide photonic path setup delays. In view of the frequent reuse of optical circuits by many applications, we proposed a circuit-cached scheme that amortizes the setup overhead by maximizing circuit reuses. In order to improve circuit “hit” rates, we developed a reuse-distance based replacement policy called “Farthest Next Use”. We further investigated the tradeoffs between the realized hit rate and energy consumption. Finally, we experimentally demonstrated the feasibility of the proposed concept using silicon photonic devices in an FPGA-controlled network testbed. Chapter 5 proceeds to develop an application-guided circuit-prefetch scheme. By learning temporal locality and communication patterns from upper-layer applications, the scheme not only caches a set of circuits for reuses, but also proactively prefetches circuits based on predictions. We applied this technique to communication patterns from a spectrum of science and engineering applications. The results show that setup delays via circuit misses are significantly reduced, showing how the proposed technique can improve circuit switching in photonic interconnects.

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