Influence of line defects on focusing in a two-dimensional photonic-crystal flat lens

We investigate in detail the influence of line defects on focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens. Through simulations, we find that a focusing can always be observed when a line defect in the lens is introduced along the light transmission direction and the width of the line defect is less than \lambda/2. However, there appear two focusings when the width of the line defect is more than \lambda/2. When the line defect is introduced along the direction perpendicular to the transmission, there is always one focusing.