  Optical metasurfaces are two-dimensional arrays of nano-scatterers that
modify optical wavefronts at subwavelength spatial resolution. They are poised
to revolutionize optics by enabling complex low-cost systems where multiple
metasurfaces are lithographically stacked and integrated with electronics. For
imaging applications, metasurface stacks can perform sophisticated image
corrections and can be directly integrated with image sensors. Here, we
demonstrate this concept with a miniature flat camera integrating a monolithic
metasurface lens doublet corrected for monochromatic aberrations, and an image
sensor. The doublet lens, which acts as a fisheye photographic objective, has a
small $f$-number of 0.9, an angle-of-view larger than
60$^\circ\times$60$^\circ$, and operates at 850 nm wavelength with 70% focusing
efficiency. The camera exhibits nearly diffraction-limited image quality, which
indicates the potential of this technology in the development of optical
systems for microscopy, photography, and computer vision.
