  Reflecting light to a pre-determined non-specular direction is an important
ability of metasurfaces, which is the basis for a wide range of applications
(e.g., beam steering/splitting and imaging). However, anomalous reflection with
100% efficiency has not been achieved at optical frequencies in conventional
metasurfaces, due to losses and/or insufficient nonlocal control of light
waves. Here, we propose a new type of all-dielectric quasi-three-dimensional
subwavelength structures, consisting of multilayer films and specifically
designed meta-gratings, to achieve perfect anomalous reflections at optical
frequencies. A complex multiple scattering process was stimulated by
effectively coupling different Bloch waves and propagating waves in the
proposed meta-system, thus offering the whole meta-system the desired nonlocal
control on light waves required to achieve perfect anomalous reflections. Two
perfect anomalous reflectors were designed to reflect normally incident 1550 nm
light to the 40{\deg} and 75{\deg} directions with absolute efficiencies higher
than 99%, and were subsequently fabricated and experimentally demonstrated to
exhibit efficiencies 98% and 88%, respectively. Our results pave the way
towards realizing optical meta-devices with desired high efficiencies in
realistic applications.
