Pixelated Resonant Subwavelength Grating Filters for Greenhouse Gas Monitoring.

ABSTRACT We describe the design of pixelated filter arrays for hyperspectral monitoring of CO 2 and H 2 O absorption in the midwave infrared (centered at 4.25P m and 5.15 P m, respectively) using resonant subwavelength gratings (RSGs), also called guided-mode resonant filters (GMRFs). For each gas, a hyperspectral filter array of very narrowband filters is designed that spans the absorption band on a single substrate. A pixelated geometry allows for direct registration of filter pixels to focal plane array (FPA) sensor pixels and for non-scanning data collection. The design process for narrowband, low-sideband reflective and transmissive filte rs within fabrication limitations will be discussed. Keywords: grating, subwavelength, pixelate d, filter, sensor, hyperspectral 1. INTRODUCTION We design filters for an optical micro-sensor capable of qua ntifying the concentrations of greenhouse gasses in localized regions of the atmosphere. This sensor is a pixelated, hyperspectral arrayed optical component integrated with a focal plane array (FPA). The key optical component is a pixelated hyperspectral filter array. Filters with exquisite wavelength control are made possible by utilizing tailored resonant subwavelength gratings (RSGs) in the place of traditional multi-layer thin-film stacks. The differentiating technology is an inte grated RSG whose nanostructure can be geometrically scaled to access a wide center-wavelength range and maintain a very narrow waveband (0.1-10nm). The configuration of the RSG is an all-dielectric device that selectively reflects a wavelength of interest while transmitting the majority of the beam for further measurement. With an arrayed approach, highly-resolved data may be accumulated with a non-scanning, snapshot system. We present both reflecting and transmi ssive filter designs using the RSG structure. 1.1 Water and Carbon Dioxide Bands We currently have considerable absorption data from satellites in the longwave infrared (LWIR, 8-12P m). However, these absorption spectra are spatially low-resolution and integr ated through the entire atmosphere. With a local sensor, we can address effects associated with a population or region of a country and more fully understand changes of concentrations with altitude. This locally attributed information is necessary to accurately pinpoint climate changing cause and effect. Perhaps more importantly, the role of water vapor and clouds can be characterized with such data. Water vapor participation in the greenhouse effect is not well understood and may be the most significant in the process as it varies with temperature and altitude, and is the only gree nhouse gas that also exists in liquid and solid phases. Our proposed technology differs from satellite-based sensors th at integrate over the entire atmosphere. Local measurement, though difficult, allows a more accurate depiction of water and gas distributions across heights in the column of air extending from sea level to the upper atmosphere. The monitoring wavelength range that we utilize encompasses the midwave infrared (MWIR, 4-8P m) atmospheric waterbands and CO