Giant amplification of modes in parity-time symmetric waveguides
  The combination of the interference with the amplification of modes in a waveguide with gain and losses can result in a giant amplification of the propagating beam, which propagates without distortion of its average amplitude. An increase of the gain-loss gradient by only a few times results in a magnification of the beam by a several orders of magnitude. 
Vladimir V. Konotop, Valery S. Shchesnovich, and Dmitry A. Zezyulin
physics.optics