Metalenses at visible wavelengths: past, present, perspectives
  The so-called "flat optics" that shapes the phase of an incident wave in free space through subwavelength structures has a venerable history. In that domain, achieving high efficiency at large deflection angles requires a low absorption material, a suitable technological process, as well as suitably engineered nanostructures that makes them electromagnetically independent despite the small spacing required to spatially sample a rapidly varying phase. 
Philippe Lalanne, Pierre Chavel
physics.optics