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Bulk thermal emittance sources possess incoherent, isotropic, and broadband radiation spectra that vary from
material to material. However, these radiation spectra can be drastically altered by modifying the geometry of
the structures. In particular, several approaches have been proposed to achieve narrowband, highly directional
thermal emittance based on photonic crystals, gratings, textured metal surfaces, metamaterials, and shock waves
propagating through a crystal. Here we present optimized aperiodic structures for use as narrowband, highly
directional thermal infrared emitters for both TE and TM polarizations. One-dimensional layered structures
without texturing are preferable to more complex two- and three-dimensional structures because of the relative
ease and low cost of fabrication. These aperiodic multilayer structures designed with alternating layers of silicon
and silica on top of a semi-infinite tungsten substrate exhibit extremely high emittance peaked around the
wavelength at which the structures are optimized. Structures were designed by a genetic optimization algorithm
coupled to a transfer matrix code which computed thermal emittance. First, we investigate the properties of the
genetic-algorithm optimized aperiodic structures and compare them to a previously proposed resonant cavity
design. Second, we investigate a structure optimized to operate at the Wien wavelength corresponding to a
near-maximum operating temperature for the materials used in the aperiodic structure. Finally, we present a
structure that exhibits nearly monochromatic and highly directional emittance for both TE and TM polarizations
at the frequency of one of the molecular resonances of carbon monoxide (CO); hence, the design is suitable for
a detector of CO via absorption spectroscopy.
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