Type-II perfect absorption and amplification modes with controllable bandwidth in combined PT -symmetric and conventional Bragg-grating structures

We reveal previously unnoticed type-II perfect absorption and amplification modes of optical scattering systems. These modes, in contrast to their counterparts in recent works [S. Longhi, Phys. Rev. A 82, 031801 (2010); Y. D. Chong et al., Phys. Rev. Lett. 106, 093902 (2011)] which could be referred to as type-I modes, appear at a continuous region along the real-frequency axis with any frequency. The type-II modes can be demonstrated in the PT-symmetric and conventional Bragg-grating combined structures. A series of exotic unidirectional absorption and amplification behaviors are observed in the combined structures, making them have potential for use in versatile devices acting simultaneously as a perfect absorber and an amplifier. Based on the properties of type-II modes, we also propose structures with controllable perfect absorption and amplification bandwidths at any single or multiple wavelengths.