Stable discrete domain walls and quasi-rectangular solitons in quadratically nonlinear waveguide arrays

We predict the existence of optical domain walls and quasi-rectangular loca lized modes in waveguide arrays with quadratic nonlinearity. An analytical criterion for the stability of these localized modes is derived and confirm ed by direct numerical calculations. The instability gain near the respecti ve boundaries is calculated. The instability predicted is controllable by i nput power and therefore has a potential for use in new all-optical switchi ng schemes. Parameters of the waveguide arrays made from titanium-indiffuse d lithium niobate as well as the required power levels are estimated. (C) 1 999 Optical Society of America [S0740-3224(99)01410-1].