Conventional longwave-infrared (LWIR) sensors intrinsically have a fixed spectral band; adding wavelength tunability typically means the addition of optics such as filter wheels or other tunable band limiters. This type of optical arrangement is not only bulky and complex, but it can only tune the spectral band for all pixels in an IR focal-plane array (FPA) simultaneously. Now, researchers from Sandia National Laboratories and the Center for Integrated Technologies (both in Albuquerque, NM), the Naval Surface Warfare Center (Crane, IN), and the Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB (New Mexico) have integrated a graphene-enabled optical metasurface with a conventional IR sensor—an indium arsenide/indium arsenide antimonide (InAs/InAsSb) type-II superlattice (T2SL) detector—with the metasurface section covering the active pixel area. The transmittance of the metasurfaces can be spectrally tuned post-fabrication by applying a voltage to the graphene, changing the carrier density within the graphene and, as a result, the plasmonic response.
The single-pixel proof-of-concept device constructed by the researchers had an active area of about 500 × 500 μm in size, with a 50-nm-thick gold grating with a period of 1.2 μm on top of graphene, which is separated from the sensor by a 600-nm-thick layer of aluminum oxide (Al2O3). Tested over a spectral range of 850 to 1450 cm-1, or 11.6 to 6.9 μm, the experimental device showed a relative change in spectral response of more than 8%; simulations show greater changes for devices with better graphene quality. The end result could be LWIR hyperspectral cameras with pixels individually spectrally tunable via voltage changes. Reference: M. D. Goldflam et al., Appl. Phys. Lett. (2020); https://doi.org/10.1063/5.0007780.
