TWIN-HOLE DARK SOLITONS

It is generally believed that interaction between dark solitons is repulsive, and two solitons cannot form a bound state. Recently, it has been pointed out that parametric interactions in a diffractive quadratic nonlinear medium can support one-dimensional twin-hole spatial dark solitons [K. Hayata and M. Koshiba, Phys. Rev. A 50, 675 (1994)]. We demonstrate numerically and analytically that this twin-hole dark soliton is modulationally unstable. Instead, we show that, in the other physical context, wave mixing between the first and second harmonics due to cascaded second-order processes in a dispersive quadratic media can support stable twin- (or multi-) hole dark solitons. We explain the physical mechanism that allows these solitons to exist and analyze their stability numerically and analytically. © 1995 The American Physical Society.