  Novel optical components based on metasurfaces (metalenses) offer a new
methodology for microlens arrays. In particular, metalens arrays have the
potential of being monolithically integrated with infrared focal plane arrays
(IR FPAs) to increase the operating temperature and sensitivity of the latter.
In this work, we demonstrate a new type of transmissive metalens that focuses
the incident light ({\lambda} = 3-5 {\mu}m) on the detector plane after
propagating through the substrate, i.e. solid-immersion type of focusing. The
metalens is fabricated by etching the backside of the detector substrate
material (GaSb here) making this approach compatible with the architecture of
back-illuminated FPAs. In addition, our designs work for all incident
polarizations. We fabricate a 10x10 metalens array that proves the scalability
of this approach for FPAs. In the future, these solid-immersion metalenses
arrays will be monolithically integrated with IR FPAs.
