Far-infrared bands in plasmonic metal-insulator-metal absorbers optimized for long-wave infrared
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.53
Far-infrared bands in plasmonic metal-insulatormetal absorbers optimized for long-wave infrared Rachel N. Evans1, Seth R. Calhoun1, Jonathan R. Brescia1, Justin W. Cleary2, Evan M. Smith2,3, and Robert E. Peale1 1
Physics Department, University of Central Florida, Orlando, FL 32816, U.S.A.
2
Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH 45433, U.S.A.
3
KBRwyle, Beavercreek, OH 45440, U.S.A.
ABSTRACT
Metal–insulator–metal (MIM) resonant absorbers comprise a conducting ground plane, a dielectric of thickness t, and thin separated metal top-surface structures of dimension l. The fundamental resonance wavelength is predicted by an analytic standing-wave model based on t, l, and the dielectric refractive index spectrum. For the dielectrics SiO2, AlN, and TiO2, values for l of a few microns give fundamental resonances in the 8-12 Pm long-wave infrared (LWIR) wavelength region. Agreement with theory is better for t/l exceeding 0.1. Harmonics at shorter wavelengths were already known, but we show that there are additional resonances in the far-infrared 20 - 50 Pm wavelength range in MIM structures designed to have LWIR fundamental resonances. These new resonances are consistent with the model if far-IR dispersion features in the index spectrum are considered. LWIR fundamental absorptions are experimentally shown to be optimized for a ratio t/l of 0.1 to 0.3 for SiO2- and AlN-based MIM absorbers, respectively, with TiO2-based MIM optimized at an intermediate ratio.
INTRODUCTION Metal–insulator–metal (MIM) resonant infrared absorbers are thin film structures that can be fabricated by contact photolithography for night vision (8–12 μm wavelength long-wave infrared, LWIR) and hot-target tracking (3-5 μm wavelength midwave infrared, MWIR). They have been applied to IR bolometers [1] and to pyroelectric detectors [2] to improve responsivity. We reported [3] a study of SiO2-based MIM
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absorbers with fundamental resonances near 50 μm wavelength in the far-IR spectral region. The resulting resonant absorption spectrum comprised a simple fundamental resonance and harmonics, which accurately agreed a the simple analytic design formula based on a standing-wave model introduced in that paper. The “fundamental resonance” occurs at the longest wavelength of incident light that can excite a standing wave. If the index spectrum changes monotonically with wavelength, the fundamental resonance wavelength is single and unique. In contrast, for MIMs fabricated from dielectrics with strong LWIR dispersion and non-monotonically varying index spectra, multiple discrete incident IR wavelengths can excite the same standing wave along a given geometrical path [4], with a corresponding multiplici
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