  The past decade has witnessed the development of a large variety of new flat
optics referred to as metasurfaces [1]. These metasurfaces are relying on
arrays of a large variety of phase shifting elements. This article aims at
presenting a novel type of flat optics able to perform wavefront shaping and
beam redirection with a high wavelength or angular selectivity, as well as an
intrinsic polarization selectivity. These new elements are not relying on
localized phase-shifting but on a distributed guided-mode, closely linked to
Resonant Waveguide Gratings (RWG). These new optical elements are referred to
as Meta Resonant Waveguide Gratings (MRWG). MRWGs have intrinsically rather low
aspect ratios and can be produced with existing high-throughput manufacturing
methods such as hot embossing and physical vapor deposition (PVD), making them
industrially attractive. Multiple design options linked to their many degrees
of freedom enable a wide range of optical properties, such as color-selective
free-space combiners and diffractive couplers configurations. The properties of
a simple MRWG are detailed, before unveiling a few variants to discuss the
influence of some key design parameters, and providing a general design method
for complex arrangements.
