  As a collective quasiparticle excitation of the magnetic order in magnetic
materials, spin wave, or magnon when quantized, can propagate in both
conducting and insulating materials. Like the manipulation of its optical
counterpart, the ability to manipulate spin wave polarization is not only
important but also fundamental for magnonics. With only one type of magnetic
lattice, ferromagnets can only accommodate the right-handed circularly
polarized spin wave modes, which leaves no freedom for polarization
manipulation. In contrast, antiferromagnets, with two opposite magnetic
sublattices, have both left and right circular polarizations, and all linear
and elliptical polarizations. Here we demonstrate theoretically and confirm by
micromagnetic simulations that, in the presence of Dzyaloshinskii-Moriya
interaction, an antiferromagnetic domain wall acts naturally as a spin wave
polarizer or a spin wave retarder (waveplate). Our findings provide extremely
simple yet flexible routes toward magnonic information processing by harnessing
the polarization degree of freedom of spin wave.
