Title: Grayscale-to-color: Single-step fabrication of bespoke multispectral filter arrays
Speaker: Calum Williams (Department of Engineering, University of Cambridge)
Time: 10:00 am – 11:00 am,
Date: Wednesday, February 20, 2019
Location: NASA LaRC, Bldg. 1200, Room 227 (Conference room)
Abstract: Multispectral filter arrays (MSFAs) are thin-film optical filter mosaics integrated atop image sensor pixels that provide spatially resolved color information. The Bayer filter—a mosaic composed of red, green and blue filters—is the most widely known MSFA, replicating color information that can be resolved by our eyes for imaging. MSFAs with a greater number of spectral bands are now found in applications as diverse as astronomy and biomedical imaging. MSFAs are typically fabricated using a dye/pigment-based approach or a multi-layer dielectric stack. Both approaches require: a variety of materials to achieve wavelength discrimination; relatively large thicknesses to achieve high efficiencies; and multiple lithographic steps (typically N-lithographic steps for N-spectral bands). Alternative nanophotonic approaches have been explored in recent years, however, broad spectral responses, overlapping multi-order resonances and structural features requiring ultrahigh resolution lithographic techniques have limited widespread application. Here, we experimentally demonstrate a novel approach for producing high efficiency (~80%), narrow linewidth (~50nm) MSFAs, operating across the UV—visible—near-infrared, using a single lithographic step. Grayscale lithography is utilized to generate customizable insulator thickness (≤200nm) profiles in metal-insulator-metal (MIM) geometries. MIM cavities exhibit strong Fabry-Perot resonances, whose spectral position is a function of insulator thickness, hence the spectral position of the transmission mode is dependent on lithographic exposure energy. To demonstrate the flexibility and commercial applicability of this approach, we fabricated MSFA designs with varying degrees of complexity and used them fixed atop commercial monochrome image sensors to successfully image a multispectral test scene.
Bio: Dr. Calum Williams obtained a BSc (Hons) in Physics from Cardiff University (2011), an MPhil (Dist.) in Micro & Nanotechnology at the University of Cambridge (2012), and in 2013 joined the Photonic Systems Development Centre for Doctoral Training at the University of Cambridge. He completed his PhD (Engineering) in 2017, with a thesis on ‘Plasmonic nanostructures for enhanced optical devices’. He is a now Postdoctoral Research Associate at the University of Cambridge and Junior Research Fellow at Wolfson College, Cambridge. This research is part of a multidisciplinary Cancer Research UK funded project. His research involves the application of concepts in nanophotonics to enhance biomedical optical imaging techniques to facilitate early stage cancer detection.
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