Topological linear and nonlinear gap modes in defected dimer lattice with fourth-order diffraction

Abstract: In this study, we investigate the optical properties of linear and nonlinear topological in-phase, out-of-phase, and edge modes in a defected dimer lattice with fourth-order diffraction, encompassing their bifurcation characteristics, localized field distributions, and stability properties. Both focusing and defocusing nonlinearities are considered. Within the nontrivial regime, in-phase, out-of-phase, and edge modes can emerge within the gap. Numerical results reveal that three types of topological gap solitons bifurcate from their corresponding linear localized modes. The dimerization parameter can be tuned to drive the system from a trivial to a nontrivial configuration. We observe that as the strength of the fourth-order diffraction increases, the size of the spectral gap also enlarges, with this parameter effectively broadening the stability window for solitons. Our findings offer novel insights into the properties of both linear and nonlinear modes in topologically defected structures.