  Integration of metasurfaces with guided mode sources like waveguides have
opened new frontiers for on-chip optical integration. However, the
state-of-the-art in the field has targeted applications where long focal
distances over thousands of light wavelengths are needed. This regime where the
paraxial approximation holds enables inverse design of the metasurfaces with
weakly confining elements that are typically thicker than the wavelength in the
material. For short-focal length applications at distances less than
100{\lambda} where the paraxial approximation fails, and high numerical
apertures (NA) are necessary, a different approach is required. Here we
designed and experimentally demonstrated single-mode waveguide-integrated meta
beam shapers capable of redirecting the confined light into the free space and
focusing it at focal distances less than 100{\lambda} above the chip surface
into a tightly focused spot. Focal spot characteristics measured at 460nm
operating wavelength are approaching diffraction limited focusing across a
range of focal lengths, device footprints, and numerical apertures
demonstrating the robustness of our approach. Focal volumes smaller than
1{\mu}m^3 are demonstrated for a range of focal distances below 50{\mu}m. For a
device with some of the highest NA amongst integrated metasurfaces of 0.95 the
measured focal volume is as small as just 0.06{\mu}m^3 at a focal distance of
13{\mu}m. These on-chip integrated ultra-high NA meta beam shapers have the
potential to unlock new applications in quantum optical computing with trapped
ions, localized optogenetic neurostimulation, and high resolution in-situ
microscopy.
