  Metasurfaces, composed of subwavelength scattering elements, have
demonstrated remarkable control over the transmitted amplitude, phase, and
polarization of light. However, manipulating the amplitude upon transmission
has required loss if a single metasurface is used. Here, we describe
high-efficiency independent manipulation of the amplitude and phase of a beam
using two lossless phase-only metasurfaces separated by a distance. With this
configuration, we experimentally demonstrate optical components such as
combined beam-forming and splitting devices, as well as those for forming
complex-valued, three-dimensional holograms. The compound meta-optic platform
provides a promising approach for achieving high performance optical
holographic displays and compact optical components, while exhibiting a high
overall efficiency.
