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26 February 2020 Mid-wave infrared filtering in silicon subwavelength zero-contrast gratings
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Proceedings Volume 11290, High Contrast Metastructures IX; 112900K (2020)
Event: SPIE OPTO, 2020, San Francisco, California, United States
The advent of subwavelength dielectric gratings enables narrowband spectral filtering on a compact, low-loss, and readily fabricable platform. Subwavelength gratings realize narrow spectral features via coupling to laterally propagating leaky modes (guided mode resonance). Given their minimal number of layers and geometrically-tunable pass bands, these structures are particularly useful in infrared hyperspectral applications. We previously demonstrated long-wave IR (8 – 12 μm) infrared filters based on high-index contrast suspended silicon/air gratings. High-contrast gratings placed above a slab of the same index are zero-contrast gratings (ZCGs) and possess several advantages. In this study, we present mid-wave IR (MWIR, 3 – 6 μm) ZCG filters using air/Si/SiO2 gratings fabricated on commercial silicon-on-insulator wafers. Geometric parameters are optimized using a genetic algorithm. We demonstrate ZCG filters with quality factors (Q) as high as 175 at oblique incidence for a 4.4 μm wavelength, and with a background high-reflectivity window from about 4.0 to 5.5 μm. The filters are optimized for coupling to light polarized with the electric field perpendicular to the gratings (transverse magnetic, TM). We also demonstrate coupling to transverse electric (TE) modes under azimuthally oblique incidence. For the same mode order, TE modes are more weakly coupled than TM, and therefore enable narrower spectral linewidths. To obtain an experimental Q of 175, full conical mounting allows strong TM mode coupling for the background reflection, and weak TE mode coupling for a narrow transmission band. Experimental results closely agree with transmittance spectra calculated via rigorous coupled wave analysis. The ZCG approach also offers a means for the design and fabrication of 2D gratings that offer polarization independent operation. We present polarization-independent filter response on square and hexagonal lattice designs.
Conference Presentation
© (2020) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Michael Barrow and Jamie Phillips "Mid-wave infrared filtering in silicon subwavelength zero-contrast gratings", Proc. SPIE 11290, High Contrast Metastructures IX, 112900K (26 February 2020);

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