If a system possesses a spin or pseudospin, which is locked to the linear momentum, spin-polarized states can exhibit backscattering-immune transport if the scatterer does not flip the spin. Good examples of such systems include electronic and photonic topological insulators. For electromagnetic waves, such pseudospin states can be achieved in metamaterials with very special artificial symmetries; however, these bulk photonic topological insulators are usually difficult to fabricate. Here we propose a paradigm in which the pseudospin is enforced simply by imposing special boundary conditions inside a channel. The symmetry-protected pseudospin states are guided in air and no bulk material is required. We also show that the special boundary conditions can be implemented simply using an array of metallic conductors, resulting in spin-filtered waveguide with a simple structure and a broad working bandwidth. We generate several conceptual designs, and symmetry-protected pseudospin transport in the microwave regime is experimentally indicated.
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