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Resonant absorption in a Au rectangular grating. (a) Incident light is reflected, diffracted, or absorbed. The coordinate system, polarizations, and dimensions are defined. (b) Reflection, diffraction, and absorption efficiencies calculated for vertical incidence on a grating with , , and . The solid (dotted) lines are the results for the TM (TE) polarization. Diffraction is magnified and shifted upward for clarity. A sharp absorption peak at indicates a Wood’s anomaly. (c) Distribution of the component of the electric field (left) and its profile at (right), when the incident light is totally absorbed. A quarter standing wave is formed in the cavity. The incident electromagnetic fields are localized in the trenches and finally absorbed into the Au inner walls.
Fabrication of the infrared grating emitter. (a) Micromachined Si rectangular wall array is (b) coated with Au and then with Cr. (c) The flipped surface is glued onto a glass substrate with epoxy, (d) and finally the Si mold is dissolved by KOH at . (e) Scanning electron micrograph of a fabricated grating with , , and . One of the cavities is magnified in the inset. The smallest width fabricated is . The minimum thicknesses of the Au and Cr layers are 50 and , respectively. (f) The glass substrate of the emitting surface is glued onto a ceramic heater supported with two stems. The temperature of the grating is monitored with a thermocouple and stabilized within . The polarization direction of the radiation is indicated.
Polarized infrared emission from the fabricated grating emitters. (a) TM-polarized emission spectra from a grating with , , and measured through a wire-grid polarizer at various surface temperatures. Emission within about 5° from the normal direction was detected. The thermal emission has a single peak because higher-order resonances at shorter wavelengths are filtered out by the Planck distribution. A typical emission of the reference black body with an emittance of 0.94 is also shown, consistent with Planck’s theory. Sharp dips around 2350 and are due to the absorption by and in air, respectively. TE-polarized emission (not shown) was almost identical to a smooth Au surface. (b) Experimental (upper) and calculated (lower) emittances for representative gratings. The red and green lines represent the emission from the gratings shown in (a) and Fig. 2(e), respectively. The blue line has a grating with , , and . The solid (dotted) lines are for TM (TE) polarization. These emittances are determined from the spectra at , but those obtained at different temperatures are also similar; i.e., we found no effect of the temperature dependence on the dielectric constant of Au within the temperature range investigated. These emittances were also consistent with the measured reflectances.
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