  A limiting factor in the development of mid-infrared optics is the lack of
abundant materials that are transparent, low cost, lightweight, and easy to
machine. In this paper, we demonstrate a metasurface device that circumvents
these limitations. A flat lens based on antenna reflectarrays was designed to
achieve near diffraction-limited focusing with a high efficiency (experiment:
80%, simulation: 83%) at 45(o) incidence angle at {\lambda} = 4.6 {\mu}m. This
geometry considerably simplifies the experimental arrangement compared to the
common geometry of normal incidence which requires beam splitters. Simulations
show that the effect of comatic aberrations is small compared to parabolic
mirrors. The use of single-step photolithography allows large scale
fabrication.
