Publications

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A complementary metal oxide semiconductor (CMOS) compatible fabrication method for creating three-dimensional (3D) meta-films is presented. In contrast to metasurfaces, meta-films possess structural variation throughout the thickness of the film and can possess a sub-wavelength scale structure in all three dimensions. Here we use this approach to create 2D arrays of cubic silicon nitride unit cells with plasmonic inclusions of elliptical metallic disks in horizontal and vertical orientations with lateral array-dimensions on the order of millimeters. Fourier transform infrared (FTIR) spectroscopy is used to measure the infrared transmission of meta-films with either horizontally or vertically oriented ellipses with varying eccentricity. Shape effects due to the ellipse eccentricity, as well as localized surface plasmon resonance (LSPR) effects due to the effective plasmonic wavelength are observed in the scattering response. The structures were modeled using rigorous coupled wave analysis (RCWA), finite difference time domain (Lumerical), and frequency domain finite element (COMSOL). The silicon nitride support structure possesses a complex in-plane photonic crystal slab band structure due to the periodicity of the unit cells. We show that adjustments to the physical dimensions of the ellipses can be used to control the coupling to this band structure. The horizontally oriented ellipses show narrow, distinct plasmonic resonances while the vertically oriented ellipses possess broader resonances, with lower overall transmission amplitude for a given ellipse geometry. We attribute this difference in resonance behavior to retardation effects. The ability to couple photonic slab modes with plasmonic inclusions enables a richer space of optical functionality for design of metamaterial-inspired optical components.

State of the art grating fabrication currently limits the maximum source energy that can be used in lab based x-ray phase contrast imaging (XPCI) systems. In order to move to higher source energies, and image high density materials or image through encapsulating barriers, new grating fabrication methods are needed. In this work we have analyzed a new modality for grating fabrication that involves precision alignment of etched gratings on both sides of a substrate, effectively doubling the thickness of the grating. We have achieved a front-to-backside feature alignment accuracy of 0.5 µm demonstrating a methodology that can be applied to any grating fabrication approach extending the attainable aspect ratios allowing higher energy lab based XPCI systems.

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The manufacturing tolerances of a stencil-lithography variant, membrane projection lithography, were investigated. In the first part of this work, electron beam lithography was used to create stencils with a range of linewidths. These patterns were transferred into the stencil membrane and used to pattern metallic lines on vertical silicon faces. Only the largest lines, with a nominal width of 84 nm, were resolved, resulting in 45 ± 10 nm (average ± standard deviation) as deposited with 135-nm spacing. Although written in the e-beam write software file as 84-nm in width, the lines exhibited linewidth bias. This can largely be attributed to nonvertical sidewalls inherent to dry etching techniques that cause proportionally larger impact with decreasing feature size. The line edge roughness can be significantly attributed to the grain structure of the aluminum nitride stencil membrane. In the second part of this work, the spatial uniformity of optically defined (as opposed to e-beam written) metamaterial structures over large areas was assessed. A Fourier transform infrared spectrometer microscope was used to collect the reflection spectra of samples with optically defined vertical split ring from 25 spatially resolved 300 × 300 μm regions in a 1-cm2 area. The technique is shown to provide a qualitative measure of the uniformity of the inclusions.

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A unique, micro-scale architecture is proposed to create a novel hybrid concentrated photovoltaic system. Micro-scale (sub-millimeter wide), multi-junction cells are attached to a large-area silicon cell backplane (several inches wide) that can optimally collect both direct and diffuse light. By using multi- junction III-V cells, we can get the highest possible efficiency of the direct light input. In addition, by collecting the diffuse light in the large-area silicon cell, we can produce power on cloudy days when the concentrating cells would have minimal output. Through the use of micro-scale cells and lenses, the overall assembly will provide higher efficiency than conventional concentrators and flat plates, while keeping the form factor of a flat plate module. This report describes the hybrid concept, the design of a prototype, including the PV cells and optics, and the experimental results.

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A method for patterning on vertical silicon surfaces in high aspect ratio silicon topography is presented. A Faraday cage is used to direct energetic reactive ions obliquely through a patterned suspended membrane positioned over the topography. The technique is capable of forming high-fidelity pattern (100 nm) features, adding an additional fabrication capability to standard top-down fabrication approaches.

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Emerging nano-photonic and nano-opto-mechanical applications benefit from fabrication of complex three-dimensional structures. Creation of micrometer scale and sub-micrometer scale structures can be performed either additively, or subtractively. Additive techniques, where material is deposited, such as direct laser write, interferometric lithography, nano-origami and colloidal self-assembly have been used to create a wide array of complex sub-micrometer structures. Example of subtractive fabrication of three-dimensional structures, where material is removed, are less common.

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