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III-V GaSb based strained layer superlattice (SLS) can provide broadband detection from 3 to 15 microns and beyond. However, the longer wavelength material has smaller external quantum efficiency (EQE) because of the smaller absorption coefficient and carrier diffusion length. The usual approach of using thicker material for higher EQE is not viable because of the limited carrier collection. In this work, we adopt a metasurface on top of a thin detector material to increase its absorption without increasing the thickness, thereby increasing EQE. The metasurface consists of an array of rings. These rings excite different optical responses in different wavelength regimes, thus yielding a broad detection spectrum. In the MWIR regime, they diffract light collectively, and in the LWIR and VLWIR regimes, they focus light as Fresnel lenses and excite localized surface plasmon polaritons (SPPs). Through these different optical interactions, the incident radiation is redirected and trapped in the detector volume. When the trapped light interferes constructively, the optical intensity builds up to a large value, leading to a large absorption in a thin material. A properly designed metastructure is thus able to achieve a large EQE in a broad spectrum. The thin absorbing layer also allows several of them stacking together to form a multi-color detector. Narrowband detection can be obtained in each color using circular posts instead of rings. By exploiting different optical field profiles under different optical responses, a voltage-switchable detection scheme can be implemented to detect four discrete wavelengths ranging from MWIR to VLWIR. The broad and narrow band designs thus show the flexibility of metastructures. To compare the present metastructure approach with the more conventional approaches using microlenses and Fresnel lenses that can also reduce dark current while maintaining a sizable QE, we optimize the respective structures and compare the merits under different approaches.
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