Effect of rotational randomness on focusing in a two-dimensional photonic-crystal flat lens

Rotating the basic unit randomly is an important method for the transformation from periodical structures to disordered systems. In this work, we investigate in detail the influence of rotational randomness and surface modification on focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens by the finite-difference time-domain method and experimental measurements. We have found that focusing can appear within a large range of random rotation angles although the quality degrades with the increase of the rotation angle randomness. When the degree of randomness goes beyond a certain value (30 degrees), the focusing effect disappears. However, it is interesting that focusing can reappear with the modification of the surface of the random lens.