  A polariton Bardeen-Cooper-Schrieffer (BCS) state in a semiconductor
microcavity is an example of symmetry-broken states in open systems.
Fluctuations of the order parameter are an important tool to characterize such
a state. With the condensate formed by composite particles, the set of
zero-momentum fluctuations spans an infinite-dimensional electron-hole mode
subspace. We show that collective fluctuation modes with orbital angular
momentum different from that of the order parameter can be obtained with
terahertz radiation, and that a physical manifestation of such modes, which are
not Higgs modes, can be terahertz gain.
