  The angular response of thin diffractive optical elements is highly
correlated. For example, the angles of incidence and diffraction of a grating
are locked through the grating momentum determined by the grating period. Other
diffractive devices, including conventional metasurfaces, have a similar
angular behavior due to the fixed locations of the Fresnel zone boundaries and
the weak angular sensitivity of the meta-atoms. To alter this fundamental
property, we introduce angle-multiplexed metasurfaces, composed of reflective
high-contrast dielectric U-shaped meta-atoms, whose response under illumination
from different angles can be controlled independently. This enables flat
optical devices that impose different and independent optical transformations
when illuminated from different directions, a capability not previously
available in diffractive optics.
