  Spectral imagers, the classic example being the color camera, are ubiquitous
in everyday life. However, most such imagers rely on filter arrays that absorb
light outside each spectral channel, yielding ~1/N efficiency for an N-channel
imager. This is especially undesirable in thermal infrared (IR) wavelengths,
where sensor detectivities are low, as well as in highly compact systems with
small entrance pupils. Diffractive optics or interferometers can enable
efficient spectral imagers, but such systems are too bulky for certain
applications. We propose an efficient and compact thermal infrared spectral
imager comprising a metasurface composed of sub-wavelength-spaced,
differently-tuned slot antennas coupled to photosensitive elements. Here, we
demonstrate this idea using graphene, which features a photoresponse up to
thermal IR wavelengths. The combined antenna resonances yield broadband
absorption in the graphene exceeding the 1/N efficiency limit. We establish a
circuit model for the antennas' optical properties and demonstrate consistency
with full-wave simulations. We also theoretically demonstrate broadband ~36%
free space-to-graphene coupling efficiency for a six-spectral-channel
metasurface. This research paves the way towards compact CMOS-integrable
thermal IR spectral imagers.
