Metasurface-coupled 2D materials for uncooled LWIR imaging

Long wavelength infrared (LWIR) imaging systems, owing to their advantages in adverse environments, are crucial for target-acquisition tasks in DoD missions. Current detection and imaging technologies that cover the LWIR spectral region typically operate at cryogenic temperatures of 77 K or lower, which require expensive and bulky cooling systems that increase the overall size, weight, and power, and reduce the operational lifetime. Sivananthan Laboratories proposes to explore metasurface-coupled 2D materials for uncooled LWIR imaging. This approach combines the advantages of (a) large absorption in a 2D material, (b) enhanced optical coupling through a metasurface, (c) a readout integrated circuit (ROIC) integrable p-i-n junction device architecture, (d) cryogenic imaging performance but near room temperature operation with a high-quality and large-area crystalline material, and (e) layer stack transfer to a ROIC for less expensive integration. Our preliminary modeling results indicate that nearly complete absorption of 8-12 mm radiation can be achieved by a 600 nm-thick absorber that couples to a metasurface consisting of either space-filling pyramids on a square lattice or appropriately spaced cylinders on a triangular lattice. Our uncooled LWIR photodetector would contains only a 600 nm-thick absorbing 2D PdSe2 layer coupled to an appropriate non-absorbing (such as MoS2) metasurface. The salient features of 2D materials include 1) a much larger absorption coefficient, 2) an intrinsically low Auger generation/recombination rate because the approximately linear energy dispersion makes it impossible to satisfy both momentum and energy conservation in the Auger process, and (3) a scarcity of SRH centers because of the layered structures. Using a 2D material together with a metasurface would enable us to reduce the dark current by >600x relative to state-of-the-art HgCdTe FPAs, which would allow room temperature operation, but maintain the cryogenic-level performance of the state-of-the-art devices. The overall objective of this project in to identify a 2D material with high absorption in the LWIR region and to grow the material with a large area at a low cost, which would enable us to achieve the goals of the EPIC-LWIR solicitation. Specifically, Sivananthan Laboratories proposes to evaluate 2D materials—both experimentally and theoretically—, develop a COMSOL-based metasurface-coupled thin layer stack design for high and broadband absorption, and accurately predict the LWIR performance, such as dark current, noise equivalent differential temperature (NEDT) and detectivity (D*), identify the issues and develop mitigation plans for large area growth, 2D layer stack transfer to a ROIC of 20 mm-pitch, and integration into a 2048x2048 focal plane array (FPA), and recommend an approach for fabrication and demonstration in Phase II.