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Parametric scattering of microcavity polaritons into ghost branches

Polaritons of defined momentum and energy are excited resonantly on the lower polariton branch of a planar semiconductor microcavity in the strong coupling regime, and the spectrally and momentum resolved emission is analyzed. We observe ghost branches from scattering within the lower polariton branch, as well as from scattering to the middle polariton branch, showing the non-linear mixing between different branches. Extending the theoretical treatment of spontaneous parametric luminescence developed in Ciuti et al., Phys. Rev. B 63, 041303 (2001), the eigenmodes of the driven polariton system and its photoluminescence are modeled.