Flat transmissive lens designs for infrared sensor applications based on sub-wavelength gratings were analyzed using a finite difference time domain program (Lumerical). An initial design for broadband transmission with a design wavelength of 4μm exhibited a drop in the transmittance for large incidence angles that caused defocusing. This was investigated numerically and a phenomenological explanation is presented. The sub- wavelength structure was then optimized to work also for large incidence angles. The focal spot of the proposed lens was characterized using a finite element method package (Comsol).
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