  On the quest towards full control over wave propagation, the development of
compact devices that allow asymmetric response is a challenge. In this Letter,
we introduce a new paradigm for the engineering of asymmetry in planar
structures, revealing and exploiting unilateral excitation of evanescent waves.
We test the idea with the design and experimental characterization of a
metasurface for angular-asymmetric absorption. The results show that the
contrast ratio of absorption (the asymmetry level) can be arbitrarily
engineered from zero to infinity for waves coming from two oppositely tilted
angles. We demonstrate that the revealed asymmetry effects cannot be realized
using conventional diffraction gratings, reflectarrays, and phase-gradient
metasurfaces. This Letter opens up promising possibilities for wave
manipulation via evanescent waves engineering with applications in one-side
detection and sensing, angle-encoded steganography, flat nonlinear devices and
shaping the scattering patterns of various objects.
