Dark lattice solitons in one-dimensional waveguide arrays with defocusing saturable nonlinearity and alternating couplings

In the present work, we examine “binary” waveguide arrays, where the coupling between adjacent sites alternates between two distinct values C1 and C2 and a saturable nonlinearity is present on each site. Motivated by experimental investigations of this type of system in fabricated LiNbO3 arrays, we proceed to analyze the nonlinear wave excitations arising in the self-defocusing nonlinear regime, examining, in particular, dark solitons and bubbles. We find that such solutions may, in fact, possess a reasonably wide, experimentally relevant parametric interval of stability, while they may also feature both prototypical types of instabilities, namely exponential and oscillatory ones, for the same configuration. The dynamical manifestation of the instabilities is also examined through direct numerical simulations