When silicon atoms are arranged in a crystal, some electron waves get through the matrix and some get reflected. This interference creates a range of energies where electrons cannot exist. The introduction of impurities or defects allows one to control the flow of electrons, i.e., their conductivity. To recreate the same principle using light, the researchers built a checkerboard of perfect rows of tiny posts made of alternating materials. If you then insert a defect in the crystal, the photon can localize onto the defect.
Light waves can move along the defect.
If they move away from the defect, they are rebuffed by walls of perfect crystal. The researchers then built the defect so it makes a sharp turn. The light either turns or gets reflected back. With a slight adjustment in the placement of the posts, the researchers got the light to travel down the defect and turn without any loss in efficiency. All the light that was sent down the defect turned and continued on its way without being reflected.