We present the design and application of subwavelength diffractive lenses for integration with IR photodetectors. In addition to 100% fill factor, low f /numbers, and the ability for monolithic integration, subwavelength lenses offer high diffraction efficiencies yet require only single step fabrication. However, their design is made difficult due to the subwavelength nature, which precludes the use of scalar diffraction theory, and finite extent, which prevents the use of rigorous coupled wave techniques. To overcome these limitations we employ a design algorithm that uses the boundary element method as a vector model of diffraction. We apply our algorithm to the design of subwavelength lenses for integration with a microbolometer and a quantum well IR photodetector.