High efficiency near diffraction-limited mid-infrared flat lenses based   on metasurface reflectarrays
  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. 
Shuyan Zhang, Myoung-Hwan Kim, Francesco Aieta, Alan She, Tobias   Mansuripur, Ilan Gabay, Mohammadreza Khorasaninejad, Davi...
physics.optics