  Inhomogeneous metasurfaces have shown possibilities for unprecedented control
of wave propagation and scattering. While it is conventional to shine a single
incident plane wave from one side of these metastructures, illuminating by
several waves simultaneously from both sides may enhance possibilities to
control scattered waves, which results in additional functionalities and novel
applications. Here, we unveil how using coherent plane-wave illumination of a
properly designed inhomogeneous metasurface sheet it is possible to realize
controllable retroreflection. We call these metasurfaces as "coherent
retroreflectors" and explain the method for realizing them both in theory and
practice. We show that coherent retroreflectors can be used for filtering
undesired modes and creation of field-localization regions in waveguides. The
latter application is in resemblance to bound states in the radiation
continuum.
