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For scattering amplitudes in strong background fields, it is -- at least inprinciple -- possible to perturbatively expand the background to obtainhigher-point vacuum amplitudes. In the case of self-dual plane wave backgroundswe consider this expansion for two-point, one-loop amplitudes in pureYang-Mills, QED and QCD. This enables us to obtain multicollinear limits of1-loop vacuum amplitudes; the resulting helicity configurations aresurprisingly restricted, with only the all-plus helicity amplitude surviving.These results are shown to be consistent with well-known vacuum amplitudes. Wealso show that for both abelian and non-abelian supersymmetric gauge theories,there is no helicity flip (and hence no vacuum birefringence) on any plane wavebackground, generalising a result previously known in the Euler-Heisenberglimit of super-QED.

One-loop multicollinear limits from 2-point amplitudes on self-dual backgrounds