Angle-resolved measurements show unexpected emission resonances of the polaritons in a semiconductor microcavity. These resonances appear when more than one in-plane polariton mode is macroscopically occupied. The new resonances observed in angle-resolved luminescence do not lie on the expected polariton branches and possess different dispersion relations with negative effective mass. The experimental results can be well explained using an interacting polariton model that treats multiple scattering. The k dispersion of the luminescence resonances is reproduced using the Bogolubov approximation that deals with the macroscopic coherence of the signal, pump, and idler modes. This model also explains many puzzling features such as the stimulation of resonant Rayleigh scattering in the backward direction. In addition, the use of nonresonant control beams for coherent control of the polaritons is shown both experimentally and theoretically. The rich complexity of new phenomena in optically excited semiconductor microcavities can be attributed to the distinctive anti-Hermitian or anomalous coupling between polaritons
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