Substrate-Independent Broad-Band Immersion Microlens Arrays with a High Coupling Efficiency for Infrared Focal Plane Arrays

The sensitivity of infrared (IR) focal plane arrays (FPAs) is often limited by a low pixel fill factor. Solid immersion microlens arrays address this problem by focusing the light reaching each pixel into the most sensitive part of that pixel. This strategy is used in CMOS image sensors but has not been industrially adopted for IR FPAs due to significant difficulties in integration with compound semiconductors. Here, we present an all-in-one solution for producing solid immersion microlens arrays compatible with various IR FPAs regardless of their substrate material. Our strategy is to use refractive lenses made of SiO2 and Si3N4 with very broad-band and efficient focusing abilities. Notably, our strategy works across a broad range of wavelengths with little performance degradation, meaning it is scalable to various applications. We implemented our method in short-wavelength IR FPAs and demonstrated 7.4 times improvement in quantum efficiency. This is the first demonstration of an immersion microlens array in a non-silicon infrared FPA.