Optical filters encompass a vast array of devices and structures for a wide variety of applications. Generally speaking, an optical filter is some structure that applies a designed amplitude and phase transform to an incident signal. Different classes of filters have vastly divergent characteristics, and one of the challenges in the optical design process is identifying the ideal filter for a given application and optimizing it to obtain a specific response. In particular, it is highly advantageous to obtain a filter that can be seamlessly integrated into an overall device package without requiring exotic fabrication steps, extremely sensitive alignments, or complicated conversions between optical and electrical signals. This dissertation explores three classes of nano-scale optical filters in an effort to obtain different types of dispersive response functions. First, dispersive waveguides are designed using a sub-wavelength periodic structure to transmit a single TE propagating mode with very high second order dispersion. Next, an innovative approach for decoupling waveguide trajectories from Bragg gratings is outlined and used to obtain a uniform second-order dispersion response while minimizing fabrication limitations. Finally, high Q-factor microcavities are coupled into axisymmetric pillar structures that offer extremely high group delay over very narrow transmission bandwidths. While these three novel filters are quite diverse in their operation and target applications, they offer extremely compact structures given the magnitude of the dispersion or group delay they introduce to an incident signal. They are also designed and structured as to be formed on an optical wafer scale using standard integrated circuit fabrication techniques. A number of frequency-domain numerical simulation methods are developed to fully characterize and model each of the different filters. The complete filter response, which includes the dispersion and delay characteristics and optical coupling, is used to evaluate each filter design concept. However, due to the complex nature of the structure geometries and electromagnetic interactions, an iterative optimization approach is required to improve the structure designs and obtain a suitable response. To this end, a Particle Swarm Optimization algorithm is developed and applied to the simulated filter responses to generate optimal filter designs.

The realm of the nano-world begins when things start getting smaller in size than one thousandth of the thickness of the human hair. Surface patterning at the nanoscale has started to find applications in information storage, self-cleaning of surfaces due to the "lotus effect", biocompatible materials based on surface roughness and many more. Several methods such as particle-beam writing, optical lithography, stamping and various kinds of self-assembly are widely used to serve the purpose of patterning smaller surface structures. However, globally much research is going into developing more efficient, reproducible and simple methods of patterning surfaces and in better controlling the order of these nanostructures. Researchers have always looked upon Nature to get inspiration and to mimic its model in engineering novel architectures. One of the methods used by this greatest artist (Nature) to make beautiful patterns around is through reaction diffusion based non-linear processes. Non-linear systems driven away from equilibrium sustain pattern only during the continuous dissipation of a regular flow of energy and are different from equilibrium processes that are converging towards a minimum in free energy (a. k. a. self-assembly). Dissipative pattern formation from micrometer to kilometers scale has been known but ordered patterns at nanoscale have never been achieved. In the process of thoroughly characterizing suitable substrates for nanoelectronics applications, we came across a remarkable process leading to the formation of highly ordered arrays of dimples on tantalum. The pattern formation happens in a narrow electrochemical windows which are functions of many parameters such as concentration, external applied voltage, temperature etc. After investigating the formation of dimples by performing spatio-temporal studies, we found that the underlying principles behind this unique way of engineering nano-structures have their roots in nonlinear interaction/reaction electro-hydrodynamics. We then have demonstrated the generality of this process by extending it to titanium, tungsten and zirconium surfaces. The pattern similar to Rayleigh-Bernard convection cells originates inside the electrochemical solution due to coupling among electrolyte ions during their migration across the electrochemical double layer (Helmholtz layer) and simultaneously imprints on the surface due to dissolution of metal oxide via etching. Based on these results we further postulate that, given appropriate electropolishing chemistry; these patterns can be formed on virtually any metal or semiconductor surface. The application of these nanostructures as nanobeakers for placing metal nanoparticles is also elucidated Highly porous materials such as mesoporous oxides are of technological interest for catalytic, sensing, optical and filtration applications: the mesoporous materials (with pores of size 2-50 nm) in the form of thin films can be used as membranes due large surface area. In the second part of this thesis, a new technique of making detachable ultrathin membranes of transition metal oxides is presented. The underlying concepts behind the detachment of membranes from the underlying substrate surface are discussed. The control on the size of the pores by modulating the voltage and concentration is also elucidated. The method is generalized by showing the similar detachment behavior on other metal oxide membranes.Thus, the results of this work introduces new techniques of engineering nanostructures on surfaces based on reaction-diffusion adaptive systems and contribute to the better understanding of electrochemical self-organization phenomena due to migration coupling induced electro-hydrodynamics. / Thesis / Doctor of Philosophy (PhD)

Platinum-Group Metal (PGM) nanostructures as advantageous alternatives to natural peroxidases have drawn great attention because of their superior catalytic activities, which can effectively enhance performance of enzyme-based in vitro diagnostics. The catalytic activity of metal nanoparticle peroxidase mimics can depend on their size, shape, elemental composition, and surface ligand of PGM nanostructures. Therefore, to develop optimal peroxidase mimics for a few bioanalytical and diagnostic applications, such as enzyme-linked immunosorbent assay (ELISA), it is important to investigate how structural aspects of PGM nanoparticles correlate with the ability of the nanoparticles to serve as functional mimics of protein peroxidase enzymes. In summary, this dissertation has studied: 1) iridium (Ir), platinum (Pt) and Ir/Pt bimetallic nanowire structures as peroxidase mimics, and the effect of different wires' length on their peroxidase-like activities and certain application of sandwich ELISA for the detection of carcinoembryonic antigen (CEA, a cancer biomarker); 2) ultra-small Ir nanoparticles, with an average size of 1.1 nm, supported by WO2.72 nanowire with high catalytic activity. Those Ir nanoparticles were applied to sandwich ELISA and competitive ELISA for sensitive detection of CEA and aflatoxin B1 (AFB1, a carcinogenic toxin), respectively; 3) the size effect of peroxidase mimics on their catalytic activities and performance in biosensing application, where Pd-Ir core-shell nanoparticles were used as a type of model peroxidase mimics. These studies may significantly stimulate further investigations of PGM nanostructures as peroxidase mimics and other potential applications in in vitro diagnostics.

Silicon has been the fundamental material for most semiconductor devices. As Si devices continue to scale down, there is a growing need to gain a better understanding of the characteristics of Si-based nanostructures and to develop novel fabrication methods for devices with extremely small dimensions. Ion beam implantation as a ubiquitous industrial method is a promising candidate for introducing dopants into semiconductor devices. Although the interactions between ion beams and Si nanostructures have been studied for several decades, many questions still remain unanswered, especially when the size of the target structure and the interaction volume of the incident ion beam have similar extents. Recent studies have demonstrated different potential use cases of ion beam interactions with Si nanostructures, such as Si nanocrystals (SiNCs). One of them is to use SiNCs embedded in a SiO2 layer as the Coulomb blockade for a single electron transistor (SET) device. In this work, we demonstrate the ion beam synthesis of SiNCs, as well as other ion beam interactions with Si-based nanostructures. To build the basic structure of a room-temperature SET, both conventional broad-beam implantation and a focused Ne+ beam from a helium ion microscope (HIM) were used for ion beam mixing. Subsequent annealing using rapid thermal processing (RTP) triggered phase separation and Ostwald ripening, where small nucleated Si clusters merge to form larger ones with the lowest surface free energy. Various ion implantation parameters were tested, along with different conditions during the RTP treatment. The SiNC structures were examined with energy-filtered transmission electron microscopy (EFTEM) to determine the optimum fabrication conditions in terms of ion beam fluence and thermal budget for the RTP treatment. Due to their small size and the resulting quantum confinement, SiNCs also exhibited optical activity, which was confirmed by photoluminescence spectroscopy on both broad-beam irradiated blank wafers and vertical hybrid nanopillar structures with embedded SiNCs. By scanning a laser probe over the sample and integrating the signal close to the emission peak, 1 μm-wide micropads with embedded SiNCs could be spatially resolved and imaged, demonstrating a new method of patterning and visualizing the SiNC emission pattern. To integrate SiNCs into vertical nanopillars for the fabrication of the SET, a fundamental study was conducted on the interaction between ions and vertical Si nanopillars. It was discovered that irradiating vertical Si nanopillars with ion fluence up to 2×1016 cm−2 immediately caused amorphization and plastic deformation due to the ion hammering effect and the viscous flow of Si during the irradiation. However, amorphization could be avoided by heating the substrate to above 350 °C, which promotes dynamic annealing. Several factors, including substrate temperature, ion flux, and nanostructure geometry, determine whether ion irradiation causes amorphization. Furthermore, at sufficiently high substrate temperatures, increasing ion fluence gradually reduced the diameter of the nanopillars due to forward sputtering from ions on the sidewalls. With a fluence up to 8×1016 cm−2 from broad-beam Si+, the diameter of Si nanopillars could be reduced by 50% to approximately 11 nm. Similar experiments were conducted on vertical nano-fin structures, which were thinned down to about 16 nm with Ne+ irradiation from the HIM. However, electrical measurements with scanning spreading resistance microscopy (SSRM) showed that the spreading resistance of the fins increased, even at a lower fluence of 2×1016 cm−2, which was too high for subsequent device integration. Nevertheless, these findings contributed to achieving the CMOS-compatible manufacturability of room-temperature SET devices and furthered our understanding of the fundamentals of ion interactions with Si nanostructures.

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