  We demonstrate imaging over the visible band using a single planar
diffractive lens. This is enabled via multi-level diffractive optics that is
designed to focus over a broad wavelength range, which we refer to as an
achromatic diffractive lens (ADL). We designed, fabricated and characterized
two ADLs with numerical apertures of 0.05 and 0.18. Diffraction-limited
focusing is demonstrated for the NA=0.05 lens with measured focusing efficiency
of over 40% across the entire visible spectrum (450nm to 750nm). We
characterized the lenses with a monochromatic and a color CMOS sensor, and
demonstrated video imaging under natural sunlight and other broadband
illumination conditions. We use rigorous electromagnetic simulations to
emphasize that ADLs can achieve high NA (0.9) and large operating bandwidth
(300nm in the visible spectrum), a combination of metrics that have so far
eluded other flat-lens technologies such as metalenses. These planar
diffractive lenses can be cost-effectively manufactured over large areas and
thereby, can enable the wide adoption of flat, low-cost lenses for a variety of
imaging applications.
