  Electromagnetic topological insulators have been explored extensively due to
the robust edge states they support. In this work, we propose a topological
electromagnetic system based on a line defect in topologically nontrivial
photonic crystals (PCs). With a finite-difference supercell approach, modal
analysis of the PCs structure is investigated in detail. The topological
line-defect states are pseudospin polarized and their energy flow directions
are determined by the corresponding pseudospin helicities. These states can be
excited by using two spatially-symmetric line-source arrays carrying orbital
angular momenta. The feature of the unidirectional propagation is demonstrated
and it is stable when disorders are introduced to the PCs structure.
