Bifurcation of soliton families from linear modes in non-PT-symmetric complex potentials

Continuous families of solitons in generalized nonlinear Sch\"odinger equations with non-PT-symmetric complex potentials are studied analytically. Under a weak assumption, it is shown that stationary equations for solitons admit a constant of motion if and only if the complex potential is of a special form $g^2(x)+ig'(x)$, where $g(x)$ is an arbitrary real function. Using this constant of motion, the second-order complex soliton equation is reduced to a new second-order real equation for the amplitude of the soliton. From this real soliton equation, a novel perturbation technique is employed to show that continuous families of solitons always bifurcate out from linear discrete modes in these non-PT-symmetric complex potentials. All analytical results are corroborated by numerical examples.