Current generation infrared (IR) photodetection requires a tradeoff between sensitivity and practicality. For applications requiring high sensitivity, the available options require varying levels of expensive and bulky cryocooling to reduce noise or to enable detection. Cheaper, more portable devices suffer from low quantum efficiencies or relatively slow recovery speeds. Computational and experimental studies have been performed to investigate the possibility of realizing a highly efficient, room temperature IR photodetector through plasmonic enhancement of a vanadium dioxide (VO2) bolometer. By incorporating metamaterial and plasmonic antenna geometries exhibiting near unity absorption and high field confinement, the photon flux requirement for a detection event can be significantly reduced. Several geometries were explored with promising theoretical performance, and improvements to these designs are suggested based on initial experimental results.

The favorable conductive properties of carbon nanotubes (CNTs) offer opportunities for constructing CNT-based nanocomposites with improved thermal conduction for a range of potential applications. Such lightweight composite materials are expected to have thermal properties that depend on their CNT volume fraction and operating temperature. The construction of CNT-based nanocomposites is challenged by the available processing methods for CNTs that are compatible with the construction of multi-laminated composite structures. The overall goal of this effort is to develop enhanced thermal properties in carbon nanotube-polymer composites that can replace traditional aerospace metallic materials to reduce the weight in space structures. The key innovation of this dissertation is in dispersing the carbon nanotubes onto a prepreg composite structure that sustains thermal storage and increase the thermal transport to support scientific instrumentation to more effectively radiate heat from a composite structure while increasing the thermal properties. The employed structures consisted of individual plies of IM7 prepreg composite with an embedded 8552 epoxy that were each coated with a CNT layer and then combined into the final composite structure using a vacuum-based hand layup technique for curing the 8552 epoxy. The composites were investigated by Raman spectroscopy, thermogravimetric analysis, thermal diffusivity, and differential scanning calorimetry. With varying the concentration of SWCNT up to 30 wt% to the IM7 prepreg composite, its heat capacity sustained over the tested temperature range and its through-thickness thermal diffusivity increased by 30% vs. the virgin composite material. By modeling, such additions of randomly oriented SWCNTs are suggested to increase the in-plane thermal conductivity by 120 to 150% over the temperature range of 120 to 470 K and by 30% in the through-thickness direction. A possible explanation of these improvements in the thermal conductivities are the reductions of the interfacial resistances between the SWCNTs, the 8552 epoxy, and the IM7 composite. The developed methods provide the opportunity for enhancing the thermal properties of a composite through the use of CNTs as additives. Such improvements would be particularly useful in aerospace applications for solar arrays, fairings, and thermal radiators.

This dissertation examines the fundamental physical properties and material processing methods required to design and fabricate the next generation of optical modulators based on the vanadium dioxide metal-insulator transition. All-optical devices capable of performing at GHz speeds, which are only limited by the laser pulse duration, were designed, fabricated and tested. Broad-band pump-probe experiments examined the femtosecond phase transition dynamics in vanadium dioxide when excited at a range of wavelengths from 400 nm to 1500 nm and indicate that THz modulation speeds could be achieved. In addition, fabrication methods and doping were explored as paths to tune the phase transition properties. The optical modulator design and material performance are discussed in the context of current state-of-the-art technology.

The integration of optical components with silicon complementary metalâoxideâsemiconductor (CMOS) technology may lead to the increase in information carrying capacity and reduction in power consumption necessary to continue the scaling the performance of microelectronic devices historically predicted by Mooreâs law. Silicon photonic structures that can guide light are well suited for such integration. However, the indirect band gap and relatively weak electro-optic responses of silicon provide challenges for chip-based lasing and modulation, two key functions necessary for an integrated photonic platform. For this reason, incorporation of materials possessing superior optical properties to silicon is actively being explored on silicon photonic platforms. The focus of this dissertation is to advance the scientific understanding and performance metrics of silicon-based optical modulators through hybridization with the actively tunable optical phase change material, vanadium dioxide (VO2). First, integration of VO2 onto a silicon ring resonator photonic platform and the subsequent electro-optic modulation of this hybrid structure are demonstrated. A tradeoff between extinction ratio and device response times is found when different VO2 patch lengths are utilized. Second, a platform in which VO2 is embedded within a silicon waveguide is realized. This embedded geometry increases interaction between the guided mode and VO2 in comparison to a geometry in which VO2 is placed on top of the silicon waveguide. Theoretical and experimental characterization through finite-difference time-domain analysis and temperature-dependent transmission measurements, respectively, demonstrates the tradeoff between extinction ratio and insertion loss as a function of VO2 patch length. Finally, the potential implementation of the hybrid silicon/VO2 embedded waveguide as an all-optical modulator with in-plane excitation is considered and its expected performance is compared to state-of-the-art all-optical modulators.

Zinc oxide is a wide band gap (3.37 eV) semiconductor with a high exciton binding energy (60 meV), of interest for optoelectronic applications due to both its ultraviolet (UV) and visible emissions. This dissertation shows four methods of tuning the UV band-edge emission of ZnO nanostructures. First, in a ZnO/MgO core-shell nanowire there exists a UV photoluminescence enhancement that varies with MgO thickness due to the formation of Fabry-Perot and whispering gallery optical cavity modes. Second, exciton-plasmon coupling in ZnO/MgO core-shell nanowires leads to enhanced emission of the UV luminescence. The combination of optical cavity formation and Ag plasmons leads to two enhancement mechanisms that depend on the MgO spacer thickness. Third, the structure of the MgO shell in the core-shell nanowires depends on the ZnO surface conditions. Changing the growth parameters of the ZnO nanowires tunes the ZnO m-plane surface conditions and thus the MgO shell structure. A smooth MgO shell supports enhanced UV photoluminescence through the formation of guided-wave optical modes, while a rough MgO shell supports neither the enhancement nor the guided modes. Fourth, temperature-dependent studies of exciton-phonon coupling elucidate that the vertically oriented nanowires, grown hydrothermally, have fewer defects and a sharper UV band-edge emission than the previously studied randomly oriented nanowires, further indicating that growth protocol has a critical influence on the UV optical properties of ZnO. Additionally, a novel structure, ZnO ânanopopcornâ showed strong exciton-phonon coupling, highlighting its high defect density. In summary, there is a range of mechanisms that can be used to enhance, modify, and control the UV band-edge emission in ZnO nanostructures, with vertically oriented ZnO nanowires having the most promise for enhanced UV emission optoelectronic and all-optical devices, from on-chip waveguides, lasers and LEDs, to scintillators and sensors.