Photonic crystals with topological defects
  We introduce topological defect to a square lattice of elliptical cylinders. Despite the broken translational symmetry, the long-range positional order of the cylinders leads to residual photonic bandgap in the density of optical states. However, the band-edge modes are strongly modified by the spatial variation of ellipse orientation. The $\Gamma-X$ band-edge mode splits into four regions of high intensity and the output flux becomes asymmetric due to the formation of crystalline domains with different orientation. The $\Gamma-M$ band-edge mode has the energy flux circulates around the topological defect center, creating an optical vortex. By removing the elliptical cylinders at the center, we create localized defect states which are dominated by either clockwise or counter-clockwise circulating waves. The flow direction can be switched by changing the ellipse orientation. The deterministic aperiodic variation of the unit cell orientation adds another dimension to the control of light in photonic crystals, enabling creation of diversified field pattern and energy flow landscape. 
Seng Fatt Liew, Sebastian Knitter, Wen Xiong, and Hui Cao
physics.optics