Unidirectional Invisibility in PT-Symmetric Cantor Photonic Crystals

In this paper, we investigate the nonreciprocity of reflection in parity-time-symmetric (PT-symmetric) Cantor photonic crystals (PCs). Two one-dimensional PCs abiding by the Cantor sequence are PT-symmetric about the center. The PT symmetry and defect cavities in Cantor PCs can induce optical fractal states which are transmission modes. Subsequently, the left and right reflectionless states are located on both sides of a transmission peak. The invisible effect depends on the incident direction and the invisible wavelength can be modulated by the gain-loss factor. This study has potential applications in tunable optical reflectors and invisible cloaks.