Browsing by Subject "Photonics"

Now showing 1 - 5 of 5
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    Development and Characterization of Novel Garnet and Gold Thin Films for Photonic and Plasmonic Applications
    (2017-08) Dulal, Prabesh
    The massive amount of data that we produce and share today is the result of advancements made in the semiconductor and magnetic recording industries. As the number of transistors per unit area in integrated circuits continues to rise, power dissipation is reaching alarming levels. Photonics, which essentially is a marriage of semiconductor with laser technology has shown great promise in tackling the issue of power dissipation. The first part of this work focuses on optical isolators, which are essential to halt back-reflections that interfere with the laser source of the photonic systems. Novel terbium iron garnet thin-film optical isolators have been developed on semiconductor platforms and their magneto-optical properties are explored. Modesolver and finite-difference simulations are done to assess their device-feasibility and efficiency. Subsequently, a new photonic device has been developed using current semiconductor microelectronic fabrication techniques. Advancement in magnetic recording is equally vital to keep up with the demand for more data at faster speeds as the current perpendicular recording technique is fast-approaching its areal density limitations. Heat assisted magnetic recording (HAMR) is the next step in the evolution of hard drives. HAMR involves heating of magnetic media using plasmonic near field transducers (NFTs), which must be able to withstand elevated temperatures for extended times. The second part of this work presents a statistical crystallographic study of thermally induced deformation of Au NFTs. Subsequently, the most thermally stable crystallographic orientation for Au NFT has been determined that could lead to significant improvements in HAMR drive reliability.
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    Light Management in Chiral Optical Metamaterials and Photovoltaic Modules
    (2022-06) Cote, Bryan
    Engineered design of nanophotonic structures enables exemplary control over the generation and propagation of electromagnetic radiation. This thesis explores two promising applications of nanophotonic design, polarization control and photovoltaic light management. Materials with strong and selective interactions with circularly polarized light have wide ranging applications from document security to biological detection. We show that light emitting metasurfaces can be designed with tailored directionality and polarization state of photoluminescence outcoupling through the judicious placement of light emitters within the metasurface’s unit cell. Additionally, the effects of Mie resonances on the circular dichroism (CD) response of a chiral medium are studied. Large CD and dissymmetry factor enhancements are observed by designing the chiral medium to support spectrally overlapping electric and magnetic dipolar resonances. Lastly, the origin of the strong CD responses generated by chiral, single gyroid metamaterials is studied, a metamaterial design that can be fabricated through block copolymer self-assembly templating. The CD responses are found to be dominated by surface interactions, allowing for double gyroid metamaterials, which are achiral in the bulk, to support strong CD responses. The second half of the thesis examines methods to improve the light management in photovoltaic modules. We find that bifacial photovoltaic modules operate at high temperatures due to their increased absorption of rear-side incident light, decreasing their energy yield and operating lifetime. Spectrally selective photonic mirrors that simultaneously provide above-bandgap antireflection and sub-bandgap increased reflection are found to be a promising passive thermal management strategy for bifacial photovoltaics. A spectrally selective mirror is fabricated for outdoor field testing. Lastly, the optical losses in a four-terminal CdTe/Si tandem module are studied and mitigated though rational nanophotonic design.
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    Metal structures for photonics and plasmonics
    (2013-07) Park, Jong Hyuk
    The goal of this thesis is to investigate metal structures for photonics and plasmonics and to provide theoretical and experimental bases for their practical applications. Engineered micro- and nanostructures of a metal can efficiently manipulate surface plasmon polaritons (SPPs) - coupled photon-electron waves propagating along a metal-dielectric interface. Since SPPs are able to contain both characteristics of light and charge, exploiting SPPs can lead to novel optical behaviors, for example, concentration of light below the optical diffraction limit, generating large electric-field enhancements in confined regions. This unique characteristic of SPPs has opened up new opportunities for photonic and plasmonic applications such as surface-enhanced spectroscopy, subwavelength waveguides, optical antennas, solar cells, and thermophotovoltaics. However, while many fabrication techniques have been developed and utilized to prepare metal structures, some applications would still benefit from improved methods because SPPs are extremely sensitive to inhomogeneities on a metallic surface arising from roughness, impurities and even grain boundaries of a metal. To minimize the surface inhomogeneities of the metal structures and thus to exploit SPPs effectively, we introduced novel fabrication methods. First, the template-stripping method was employed to obtain high-quality silver films for SPPs in the visible wavelengths. The template-stripped films showed very smooth surfaces, leading to the improved dielectric function with high electrical conductivity and low optical loss. The dielectric function of the template-stripped films was compared with that of conventional films. As a result, the relation between the surface roughness and dielectric function of metal films could be derived. As another approach to reduce the inhomogeneities on a metal surface, we prepared single-crystalline silver films via epitaxial growth. Under controlled deposition conditions, single-crystalline silver films exhibited ultrasmooth surfaces with a root mean square roughness of 0.2 nm. Moreover, we observed that the absence of the grain boundaries can lead to an increase in SPP propagation length as well as precise patterning for metal structures. Beyond noble metals, we then introduced an effective route to obtain smooth patterned structures of refractory metals, semiconductors, and oxides via template stripping. The smooth structures of such materials can be favorable for many applications including thermal emitters, metamaterials, solar absorbers, and photovoltaics. We demonstrated that a variety of desired materials deposited on a thin noble metal layer can be peeled from silicon templates. After removing the noble metal layer, the revealed surfaces had very small roughness. This approach could easily reproduce structures via reuse of templates, leading to a low-cost and high-throughput process in micro- and nanofabrication. Finally, we showed that thermal excitation of SPPs in patterned metallic structures can provide tailored thermal emission. Typically, SPPs on metal structures are generated by using an optical source and then re-radiated as light, of which the emission angle and wavelength are determined by the geometry of the metal structures. However, since thermal energy can be another excitation source to create SPPs, heating of properly designed metal structures can result in tailored thermal emission. We experimentally demonstrated that at high temperatures, tungsten films with bull's-eye patterns exhibit tailored thermal emission with a unidirectional and monochromatic beam. In addition, since the thermal stability of the structures could be enhanced by coating with a protective oxide layer on the metal surface, the bull's-eye structures can be utilized as a novel radiation source. Overall, we pursued efficient engineering of SPPs in metal structures and development of improved fabrication methods for the metal structures. We believe that these results will promote the practical application of SPPs for electronic, photonic and plasmonic devices.
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    Modeling, Design, and Fabrication of Spectrally-Selective Mirrors for Photovoltaic Thermal Management
    (2020-07) Slauch, Ian
    A typical c-Si photovoltaic module will operate 20-30K above ambient temperature due to waste heat generated as it converts incident sunlight into electrical power. As temperature increases, the conversion efficiency drops by ~0.4%/K, reducing overall power output. Reducing the total amount of waste heat generated during operation would both lower the module operating temperature and improve its efficiency and energy yield. Waste heat is generated in the module in part due to parasitic absorption of sub-bandgap light that does not have enough energy to be useful for power conversion. Sub-bandgap reflection offers a method of preventing parasitic absorption, cooling the module, and increasing its efficiency. In this thesis, a time-independent matrix model is introduced to calculate module energy yield and waste heat generation through parasitic absorption, recombination, and electronic losses. The model considers the spectral and angular dependence of the optical properties of the module including modification by photonic structures, and is used to characterize and optimize the design of aperiodic photonic mirrors which selectively reflect sub-bandgap light from the module and enhance its energy yield. Importantly, these mirrors are designed considering weather and irradiance conditions typical for outdoor fixed-tilt module installations. As a result, it is shown that these mirrors are omnidirectional, achieving the required spectral selectivity regardless of the angle of incidence of sunlight or the geographic location of installation. Low-complexity mirror designs which are simple to fabricate offer the most potential for reducing the cost of energy. These designs are primarily anti-reflection coatings, but also avoid a rise in operating temperature while increasing energy output. Two simple designs are fabricated, integrated into modules, and tested outdoors. The fabricated mirrors have the desired spectral selectivity, and reduce module operating temperature by over 1K. Alternative strategies to reject sub-bandgap light, including reflection from the cell surface or cell rear contact, and backscattering from near the cell are also modeled and compared to result for reflection from the glass. Designing for the glass interface in particular allows maximization of the dual benefit, optical and thermal, of the mirrors.
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    Selective absorption of visible light by a plasmonic gold lattice
    (2021) Nixon, Matthew M
    Simulation, creation, and testing of metamaterial absorber utilizing plasmonic and metal-insulator-metal interactions to create selective absorption of light within the visible spectrum.