Figure 7: External refraction of a unit-amplitude () spatial soliton at a nonlinear interface with when the incidence angle is (a) quasi-paraxial () and (b) nonparaxial () for . In (a), the trajectory of the beam in the second medium is essentially a straight line. In (b), the trajectory oscillates (“snakes”) around the straight-line path predicted by the analysis in Section 2. Calculations of the beam centre were obtained by fitting the numerical solution at each longitudinal position to a trial function of the form . Black dashed lines: best-fit trajectory.