  Sub-wavelength resolution imaging requires high numerical aperture (NA)
lenses, which are bulky and expensive. Metasurfaces allow the miniaturization
of conventional refractive optics into planar structures. We show that
high-aspect-ratio titanium dioxide metasurfaces can be fabricated and designed
as meta-lenses with NA = 0.8. Diffraction-limited focusing is demonstrated at
wavelengths of 405 nm, 532 nm, and 660 nm with corresponding efficiencies of
86%, 73%, and 66%. The meta-lenses can resolve nanoscale features separated by
sub-wavelength distances and provide magnification as high as 170x with image
qualities comparable to a state-of-the-art commercial objective. Our results
firmly establish that meta-lenses can have widespread applications in
laser-based microscopy, imaging, and spectroscopy.
