Dark solitons in dynamical lattices with the cubic-quintic nonlinearity

Abstract

Results of systematic studies of discrete dark solitons (DDSs) in the one-dimensional discrete nonlinear Schrödinger equation with the cubic-quintic on-site nonlinearity are reported. The model may be realized as an array of optical waveguides made of an appropriate non-Kerr material. First, regions free of the modulational instability are found for staggered and unstaggered cw states, which are then used as the background supporting DDS. Static solitons of both on-site and inter-site types are constructed. Eigenvalue spectra which determine the stability of DDSs against small perturbations are computed in a numerical form. For on-site solitons with the unstaggered background, the stability is also examined by dint of an analytical approximation, that represents the dark soliton by a single lattice site at which the field is different from cw states of two opposite signs that form the background of the DDS. Stability regions are identified for the DDSs of three types: unstaggered on-site, staggered on-site, and staggered inter-site; all unstaggered inter-site dark solitons are unstable. A remarkable feature of the model is coexistence of stable DDSs of the unstaggered and staggered types. The predicted stability is verified in direct simulations; it is found that unstable unstaggered DDSs decay, while unstable staggered ones tend to transform themselves into moving dark breathers. A possibility of setting DDS in motion is studied too. Analyzing the respective Peierls-Nabarro potential barrier, and using direct simulations, we infer that unstaggered DDSs cannot move, but their staggered counterparts can be readily set in motion.