  A Cm planar photonic material (two-dimensional) including the mirror
reflection symmetry is explored where in the Dirac cones appeared at the
high-symmetry points of the Brillouin zone boundary. By implementing the
specific perturbation on the photonic crystal (PC), topological transition can
make a bridge between the trivial PC (ordinary insulator) and the topological
insulator including zero and non-zero spin Chern number (Cs), respectively. The
perturbation can be realized through the rotation of the Cm photonic crystal
around the nonzero angle to its initial position. Therefore, breaking the
mirror symmetry of the unit cell leads to mismatch of the symmetry planes of
the lattice. This modification results in the directional band gap & band
inversion which is the signature of the topological transitions. The mentioned
PC would be suitable for examination of the unidirectional transport of light
at the topological interfaces.
