In this Letter, we propose a metagrating consisting of simple rectangular bars for nearly unity anomalous diffraction with a large deflection angle. The analysis performed by the scattering-matrix method shows that such exceptional beam steering derives from the couplings of the two lowest propagation waveguide-array-modes and their constructive interferences. The tolerance of the incident angle for a high diffraction efficiency (e.g., >90%) is within a range of 33°. We also discuss that such an advantage still exists after considering a reasonable loss and dispersion. We envision that the proposed strategy may have wide use in the field of high-performance wavefront-shaping applications.
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