  Conventional surface-relief gratings are inefficient at deflecting
normally-incident light by large angles. This constrains their use in many
applications and limits the overall efficiency of any optical instrument
integrating gratings. Here, we demonstrate a simple approach for the design of
diffraction gratings that can be highly efficient for large deflection angles,
while also offering additional functionality. The gratings are composed of a
unit cell comprising a vertically-oriented asymmetric slot-waveguide. The unit
cell shows oscillating unidirectional scattering behavior that can be precisely
tuned as a function of the waveguide length. This occurs due to interference
between multiple modes excited by the incident light. In contrast to
metasurface-based gratings with multiple resonant sub-elements, a periodic
arrangement of such non-resonant diffracting elements allows for broadband
operation and a strong tolerance for variations in angle of incidence.
Full-wave simulations show that our grating designs can exhibit diffraction
efficiencies ranging from 94% for a deflection angle of 47$^\circ$ to 80% for
deflection angle of 80$^\circ$. To demonstrate the multifunctionality of our
grating design technique, we have also proposed a flat polarization
beamsplitter, which allows for the separation of the two orthogonal
polarizations by 80$^\circ$, with an efficiency of 80%.
