Nonreciprocal wave propagation through open, discrete nonlinear   Schroedinger dimers
  We consider asymmetric (nonreciprocal) wave transmission through a layered nonlinear, non mirror-symmetric system described by the one-dimensional Discrete Nonlinear Schr\"odinger equation with spatially varying coefficients embedded in an otherwise linear lattice. Focusing on the simplest case of two nonlinear sites (the dimer), we compute exact scattering solutions such that waves with the same frequency and incident amplitude impinging from left and right directions have different transmission coefficients. The stability of some particular solutions is addressed. We show that oscillatory instability may lead to the formation of stable extended states coexisting with a nonlinear defect mode oscillating at a different frequency. Numerical simulations of wave packet scattering are presented. Gaussian wave packets with the same amplitude arriving from opposite directions on the dimer are indeed transmitted very differently. Moreover, asymmetric transmission is sensitively dependent on the input parameters, akin to the case of chaotic scattering. 
Stefano Lepri and Giulio Casati
nlin.PS