No description available.

Mid-infrared (mid-IR) detection and imaging over atmospheric transparent 3-5 μm and 8-12 μm bands are increasingly becoming important for various space, defense and civilian applications. Various kinds of microbolometers offer uncooled detection of IR radiation. However, broadband absorption of microbolometers makes them less sensitive to spectrally resolved detection of infrared radiation and the fabrication is also very tedious involving multiple complex lithography steps. In this study, we designed an optical nano-antenna array based detector with narrow frequency band of operation. The structure consists of a two-element antenna array comprised of a perforated metallic hole array coupled with an underneath disk array which trap incident radiation as dipole currents. The energy is dissipated as electron plasma loss on the hole-disk system inducing close to ~100% absorption of the incident radiation. This near perfect absorption originates from simultaneous zero crossing of real component of permittivity and permeability due to the geometrical arrangement of the two antenna elements which nullifies overall charge and current distributions, prohibiting existence of any propagating electromagnetic modes at resonance. Moreover, the continuous perforated film allows probing of the induced "micro-current" plasma loss on each nano hole-disk pair via a weak bias current. Such optical antenna design enables flexible scaling of detector response over the entire mid-infrared regime by change in the antenna dimensions. Furthermore, the development of simple nanoimprint lithography based large area optical antenna array fabrication technique facilitates formation of low cost frequency selective infrared detectors.

Plasmonics have attracted significant attention from the scientific community for photoelectrode technologies since they offer tunability, large light absorption cross-sections, transparency and potential scalability. On the downside, plasmonics exhibit large absorption cross sections in a narrow window compared to the total electromagnetic spectrum, leading to relatively small efficiencies. It has been already demonstrated, that coupling plasmonics with a Fabry Pérot nanocavity can significantly enhance the absorption of light in photoelectrode systems. Traditionally Fabry Pérot nanocavities are fabricated through clean room processes hindering their scalability. In this project, we report the successful enhancement of light absorption in a plasmonic photoelectrode system via a Fabry Pérot nanocavity that has been produced via solution methods. Furthermore, we report significant evidence of enhancement in the electrochemical properties of the system fabricated.

The area of nanophotonics has been the focus of researchers recently due to its high potential to overcome the limitations of scaling in electronic devices. One of the most popular devices in this field is the metasurface. A metasurface consists of a periodic or aperiodic array of spaced units called ’meta-atoms’, where the interaction between these neighboring elements provide unprecedented properties that cannot be obtained using a a regular array of antennas. By tuning the shape and structure of the meta-atoms, electromagnetic wave interaction with the metasurface can be manipulated to achieve a plethora of response characteristics. For active applications that require tunability of the response, a passive metasurface cannot be used to adapt to the varying operating conditions. Tunability of metasurfaces can then be achieved by using phase-changing materials. This type of materials can attain different optical properties by applying external stimulus such as heat, electric current, or laser pulses. The change in the optical properties would be beneficial for applications requiring reconfigurability or adaptation. In this thesis, I demonstrate the employment of volatile (Vanadium Dioxide) and non-volatile (Germanium Antimony Telluride) examples of phase-change materials to design reconfigurable metasurfaces operating at different bands in the infrared regime. I show metallic and dielectric-based structures that employ volatile and non-volatile phase-change materials, as well as apply physics such as plasmonics and bound states in the continuum to design and optimize metasurface structures for energy and biosensing applications. / Thesis / Doctor of Philosophy (PhD) / This thesis proposes methods to design and optimize reconfigurable and adaptive metasurfaces for energy harvesting, radiative cooling, and biosensing applications in the infrared range. The concept of phase-change metasurfaces is highlighted where a phase-change material (PCM) is employed to provide the tunable response. The properties of the PCM can be modified using several excitation methods such as thermal, electric, and laser excitation. The details of material selection, geometry configuration, as well as optimization procedures are demonstrated. Target applications for the study is harvesting from Earth’s ambient radiation around 10.6µm, adaptive cooling of spacecraft in the mid-infrared band 2.5 − 25µm, and trace biomarkers detection in the amide-I and amide-II bands (5.5−6.9µm). Full-wave numerical analysis was conducted using COMSOL Multiphysics software. Optimization was carried out using global optimization techniques implemented using Matlab and Python. The results show innovative designs for switchable absorbers, new approach for modeling of phase-change metasurfaces using deep learning, and employment of the physics of bound states in the continuum for the first time to implement a robust biosensing device. The results of this thesis would help advance the field of reconfigurable nanophotonics and related integrated applications.

Nanophotonics is the study and technological application of the interaction of electromagnetic waves (light) and matter at the nanometer scale. The field's extensive research focuses on generating, detecting, and controlling light using nanoscale features such as nanoparticles, waveguides, resonators, nanoantennas, and more. Exploration in the field is highly dependent on computational methods, which simulate how light will interact with matter in specific situations. However, as nanophotonics advances, so must the computational techniques. In this thesis, I present my work in various numerical studies in nanophotonics, sorted into three categories; plasmonics, inverse design, and deep learning. In plasmonics, I have developed methods for solving advanced material models (including nonlinearities) for small metallic and epsilon-near-zero features and validated them with other theoretical and experimental results. For inverse design, I introduce new methods for designing optical pulse shapes and metalenses for focusing high-harmonic generation. Finally, I used deep learning to model plasmonic colour generation from structured metal surfaces and to predict plasmonic nanoparticle multipolar responses.

This report presents evidence that in-situ reduction of metal ions bound to a cross-linked polymer surface does not always result in nanoparticle formation solely at the interface, as is commonly assumed, but also as much as 40 nm deep within the polymer matrix. Tetrachloroaurate ions were bound using a variety of multi-functional amines to cured films of SU-8 -- a cross-linkable epoxide frequently used for micro- and nanofabrication -- and then treated using one of several reducing agents. The resulting gold-nanoparticle decorated films were examined by X-ray photoelectron spectroscopy and by plan-view and cross-sectional transmission electron microscopy. Reduction using sodium borohydride or sodium citrate generates bands of interspersed particles as much as 40 nm deep within the polymer, suggesting both the Au(III) complex and the reducing agent are capable of penetrating the surface and affecting reduction and formation of nanoparticles within the polymer matrix. It is shown that nanoparticle formation can be confined nearer to the polymer interface by using hydroquinone, a sterically bulkier and less flexible reducing agent, or by reacting the surface in aqueous media with high molecular-weight multifunctional amines, that presumably confine Au(III) nearer to the true interface. These finding have important implications for technologies that apply surface bound nanoparticles, including electroless metallization, catalysis, nano-structure synthesis, and surface enhanced spectroscopy.

Bound electromagnetic surface waves can be excited by free-space waves on a corrugated conduction surface. These electromagnetic surface waves, called surface plasmon polaritons (SPPs), are coupled to a plasma of free charges, which travel together with the wave. We investigated the effect of separating metal corrugations from the smooth metal ground plane with a thin dielectric layer and show that SPPs can be excited via displacement currents. However, the SPP excitation resonances broaden and disappear as the dielectric thickness approaches 1% of the wavelength.

No description available.

Substrate-based nanostructures are of great importance due to their applications in microelectronic devices, chemical sensors, catalysis and photovoltaics. This dissertation describes a novel fabrication technique for the formation of periodic arrays of substrate-based nanoparticles. The prescribed route, referred to as dynamic templating, requires modest levels of instrumentation consisting of a sputter coater, micrometer-scale shadow masks and a tube furnace. The route has broad applicability, having already produced periodic arrays of gold, silver, copper, platinum, nickel, cobalt, germanium and Au-Ag alloys on substrates as diverse as silicon, sapphire, silicon-carbide, and glass. The newly devised method offers large-area, high-throughput capabilities for the fabrication of periodic arrays of sub-micrometer and nanometer-scale structures and overcomes a significant technological barrier to the widespread use of substrate-based templated assembly by eliminating the need for periodic templates having nanoscale features. Because this technique only requires modest levels of instrumentation, researchers are now able to fabricate periodic arrays of nanostructures that would otherwise require advanced fabrication facilities. / Mechanical Engineering

The ability to fabricate structures and engineer materials on the nanoscale leads to the development of new devices and the study of exciting phenomena. Nanostructures attached to the surface of a substrate, in a manner that renders them immobile, have numerous potential applications in a diverse number of areas. Substrate-supported nanostructures can be fabricated using numerous modalities; however the easiest and most inexpensive technique to create a large area of randomly distributed particles is by the technique of thermal dewetting. In this process a metastable thin film is deposited at room temperature and heated, causing the film to lower its surface energy by agglomerating into droplet-like nanostructures. The main drawbacks of nanostructure fabrication via this technique are the substantial size distributions realized and the lack of control over nanostructure placement. In this doctoral dissertation, a new pathway for imposing order onto the thermal dewetting process and for manipulating the size, placement, shape and composition of preformed templates is described. It sees the confinement of substrate-supported thin films or nanostructure templates by the free surface of a metal film or a second substrate surface. Confining the templates in this manner and heating them to elevated temperatures leads to changes in the characteristics of the nanostructures formed. Three different modalities are demonstrated which alters the preformed structures by: (i) subtracting atoms from the templates, (ii) adding atoms to the template or (iii) simultaneously adding and subtracting atoms. The ability to carry out such processes depends on the choice of the confining surface and the nanostructured templates used. A subtractive process occurs when an electroformed nickel mesh is placed in conformal contact with a continuous gold film while it dewets, resulting in the formation of a periodic array of gold microstructures on an oxide substrate surface. When heated the gold beneath the grid selectively attaches to it due to the surface energy gradient which drives gold from the low surface energy oxide surface to the higher surface energy nickel mesh. With this process being confined to areas adjacent to and in contact with the grid surface the film ruptures at well-defined locations to form isolated islands of gold and subsequently, a periodic array of microstructures. The process can be carried out on substrates of different crystallographic orientations leading to nanostructures which are formed epitaxially and have orientations based on underlying substrate orientations. The process can be extended by placing a metallic foil of Pt or Ni over preformed templates, in which case a reduction in the size of the initial structures is observed. Placing a foil on structures with random placement and a wide size distribution results, not only in a size reduction, but also a narrowed size distribution. Additive processes are carried out by using materials which possess high vapor pressures much below the sublimation temperature of the template materials. In this case a germanium substrate was used as a source of germanium adatoms while gold or silver nanostructures were used as heterogeneous nucleation sites. At elevated temperatures the adatoms collect in sufficient quantities to transform each site into a liquid alloy which, upon cooling, phase separates into elemental components sharing a common interface and, hence, resulting in the formation of heterodimers and hollowed metal nanocrescents upon etching away the Ge. A process which combined aspects of the additive and subtractive process was carried out by using a metallic foil with a high vapor pressure and higher surface energy than the substrate surface (in this case Pd foil). This process resulted in the initial preformed gold templates being annihilated and replaced by nanostructures of palladium, thereby altering their chemical composition. The assembly process relies on the concurrent sublimation of palladium and gold which results in the complete transfer of the templated gold from the substrate to the foil, but not before the templates act as heterogeneous nucleation sites for palladium adatoms arriving to the substrate surface. Thus, the process is not only subtractive, but also additive due to the addition of palladium and removal of gold. / Mechanical Engineering