Imaging Metasurfaces based on Graphene-Loaded Slot Antennas

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.