  Metagratings have been shown to form an agile and efficient platform for
extreme wavefront manipulation, going beyond the limitations of gradient
metasurfaces. Previous approaches for transmissive metagratings have resorted
on compound asymmetric inclusions to achieve single-channel near-perfect
diffraction. However, such complex inclusions are sensitive to geometric
parameters and lack the flexibility for arbitrary phase modulation, restricting
applications to beam deflection. Here, we show perfect unitary diffraction in
all-dielectric transmissive metagratings using rectangular inclusions by
tailoring their multipole interferences. Using this principle, we
experimentally demonstrate analog phase profile encoding of a hologram through
displacement modulation of CMOS-compatible silicon nitride nanobars,
manifesting broadband and wide-angle high diffraction efficiencies for both
polarizations and across the entire visible range. Featured with extreme
angle/wavelength/polarization tolerance and alleviated structural complexity
for both design and fabrication, our demonstration unlocks the full potential
of metagrating-based wavefront manipulation for a variety of practical
applications.
