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Nanostructured Polarization Optics for Atmospheric Remote Sensing

The goal of this proposed project is to develop next-generation polarization imagers by incorporating customized pixel-by-pixel polarization and wavelength filter arrays implemented using optical nanostructures in silicon and related materials. Recent work has demonstrated that subwavelength-scale structuring of two optical materials (e.g. silicon and air) produces a composite material that has optical properties related to, but different from those of the constituent materials. By controlling the relative volume fraction and spatial configuration of these two (or more) materials, it is possible to engineer the effective optical properties of the nanostructure. Furthermore, these semiconductor fabrication processes allow the optical characteristics to be readily varied in relation to position on a wafer, enabling devices with multiple functionalities for wavefront engineering or focal plane arrays.

We intend to apply this multifunctional optical device design capability to a specific, high- impact atmospheric science goal of immediate NASA interest: simultaneously imaging the polarization state of light scattered by clouds and the phase of the water (ice or liquid) in the clouds. To reach this goal, we propose to build nanostructured optical polarization devices that will enable a new imaging system that can simultaneously determine cloud-phase while also measuring cloud polarization in the short-wave infrared (SWIR) spectral region of approximately 1.6–2.2 mm. This imager will use a pixelated focal-plane-array approach, in which adjacent pixels are overlaid with different filters that are each sensitive to a particular polarization state and wavelength. This is similar in principle to the Bayer filter that is widely used to provide red- green-blue (RGB) color imaging in commercial cameras, where a block of four pixels is overlaid with two green filters, one blue filter, and one red filter. Similar imagers are used in polarization imaging, using blocks of pixels each overlaid with different polarization filters [9]. We propose a unique hybrid of such an imager, in which each pixel is overlaid with a nanostructured optical element that transmits a desired polarization state at one of the SWIR wavelength bands used for cloud phase discrimination. This novel imaging system will revolutionize cloud polarization research by enabling simultaneous studies of cloud polarization and cloud phase in one instrument that can be more compact and light-weight than imaging systems that use bulk optical elements and often multiple detector arrays for the multiple polarization and spectral channels. Such a capability would enormously benefit studies of clouds with partially polarized, scattered sunlight. These studies, in turn, are crucial to NASA efforts to use space-borne polarization measurements to retrieve atmospheric aerosol and cloud properties, their associated radiative forcings, and their associated climate responses, which together constitute the largest uncertainties in climate science

The interdisciplinary nature of the proposed work brings together experts in microfabrication, nanophotonics, and optical remote sensing at two higher education institutions in Montana— Montana State University (MSU) and Flathead Valley Community College (FVCC). This integrative teaming will produce a new instrument development capability for imaging polarimeters using optimized multifunctional polarization and wavelength filters based on nanostructures. These multifunctional nanostructures will enable imaging systems with improved optical performance, fewer optical components, and lower overall weight and size, with important applications in NASA's mission portfolio, as well as technology transfer opportunities in Montana's emerging optics industry. Finally, this research effort will provide a number of valuable educational opportunities at the graduate, undergraduate and junior college levels, helping to address the workforce needs of both our nation's STEM priorities and the jurisdiction's growing technology economy.

Contact Info

Mail Wataru Nakagawa
Electrical and Computer Engineering
Montana State University
Bozeman, MT 59717
E-mail: Wataru Nakagawa
Phone: (406) 994-5956
Fax: (406) 994-5958
Website: Wataru Nakagawa
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