  In this letter, we experimentally report an achromatic metalens (AML)
operating over a continuous bandwidth in the visible. This is accomplished via
dispersion engineering of dielectric phase shifters: titanium dioxide
nanopillars tiled on a dielectric spacer layer above a metallic mirror. The AML
works in reflection mode with a focal length independent of wavelength from
wavelength of 490 nm to 550 nm. We also design a metalens with reverse
chromatic dispersion, where the focal length increases as the wavelength
increases, contrary to conventional diffractive lenses. The ability to engineer
the chromatic dispersion of metalenses at will enables a wide variety of
applications that were not previously possible. In particular, for the AML
design, we envision applications such as imaging under LED illumination,
fluorescence, and photoluminescence spectroscopy.
