Asymmetric optical amplifier based on parity-time symmetry
  We design and optimize a waveguide structure consisted of three segments of PT-symmetric coupler. It can amplify the difference mode with the attenuation of the sum mode, so performs an asymmetric optical amplifier. Also, the dependence of the amplification factor for the difference mode and the attenuation factor for the sum mode on the gain/loss is illustrated. 
Rujiang Li, Pengfei Li, and Lu Li
physics.optics