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. 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 ∼58% free space-to-graphene photodetector coupling efficiency, averaged over the 1050 cm ^-1 to 1700 cm ^-1 wavenumber range, for a four-spectral-channel gold metasurface with a 0.883 µm by 6.0 µm antenna pitch. This research paves the way towards compact CMOS-integrable thermal IR spectral imagers.