RE:RE:RE:$92.10 Iron Ore this week !!!!!!The first time I knew about flat lens was last year in an article published in Science. The team that achieved the lens were scientists from Harvard from the team of Federico Capasso.
https://science.sciencemag.org/content/352/6290/1190
So I can tell you that this is not an easy problem.
I quote from here
https://www.seas.harvard.edu/capasso/2016/06/flat-lens-for-visible-wavelengths/
"Despite tremendous interest in planar lenses in the visible spectrum, there has been no solution that simultaneously satisfies the demands for high numerical aperture (NA) and efficiency, let alone for high end imaging applications. We introduce a breakthrough solution for this problem using titanium dioxide-based metasurfaces."
So perhaps "at the top of research" was not a good choice of words, but certainly, it's a problem worth a publication in Science. :)