  The numerical aperture (NA) of a lens determines its ability to focus light
and its resolving capability. Having a large NA is a very desirable quality for
applications requiring small light-matter interaction volumes or large angular
collections. Traditionally, a large NA lens based on light refraction requires
precision bulk optics that ends up being expensive and is thus also a specialty
item. In contrast, metasurfaces allow the lens designer to circumvent those
issues producing high NA lenses in an ultra-flat fashion. However, so far,
these have been limited to numerical apertures on the same order of traditional
optical components, with experimentally reported values of NA <0.9. Here we
demonstrate, both numerically and experimentally, a new approach that results
in a diffraction limited flat lens with a near-unity numerical aperture
(NA>0.99) and sub-wavelength thickness (~{\lambda}/3), operating with
unpolarized light at 715 nm. To demonstrate its imaging capability, the
designed lens is applied in a confocal configuration to map color centers in
sub-diffractive diamond nanocrystals. This work, based on diffractive elements
able to efficiently bend light at angles as large as 82{\deg}, represents a
step beyond traditional optical elements and existing flat optics,
circumventing the efficiency drop associated to the standard, phase mapping
approach.
