  We have measured the photonic bandstructure of GaAs photonic-crystal
waveguides with high energy and momentum resolution using near-field scanning
optical microscopy. Intriguingly, we observe additional bands that are not
predicted by eigenmode solvers, as was recently demonstrated by Huisman et al.
[Phys. Rev. B 86, 155154 (2012)]. We study the presence of these additional
bands by performing measurements of these bands while varying the incident
light power, revealing a non-linear power dependence. Here, we demonstrate
experimentally and theoretically that the observed additional bands are caused
by a waveguide-specific near- field tip effect not previously reported, which
can significantly phase-modulate the detected field.
