Unidirectional Bragg Gratings Using Parity-Time Symmetry Breaking in Plasmonic Systems

Abstract

Optical systems following concepts of parity-time (PT) symmetry have attracted significant attention because of their extraordinary behavior such as unidirectional reflectance or power oscillation. PT symmetric optical systems are realized by judiciously manipulating the complex refractive index to produce even- and odd-symmetric distributions for the real and imaginary indices, respectively. We propose two PT symmetric Bragg gratings based on step-in-width metal stripes and dielectric-loaded metal stripes operating with long-range surface plasmon polaritons. The gratings are designed to operate near 880 nm because optical gain can be conveniently provided by IR140-doped PMMA. Asymmetric reflectance is predicted in the proposed gratings based on modal and transfer matrix method computations. Moreover, we analyzed pulse reshaping and energy transport in generic gratings, and in the proposed plasmonic gratings, in terms of group and energy velocities. It is found that the group and energy velocities are dispersionless at the PT symmetry breaking point. Also, the group velocity dispersion can be inverted by changing the PT symmetric state from broken to unbroken or vice versa. Our designs are practical because a large left-right asymmetric reflectance contrast is produced for a wide range of physical dimensions. The proposed gratings are suitable as on-chip devices for optical processing providing new functionality such as switching and controlling the time delay of a data pulse without distortion.