  Semiconductor microcavities are widely used to study collective interactions
of cavity exciton-polaritons leading to their condensation phenomenon.
Exciton-light interaction is highly enhanced in such structures due to the
resonance enhancement of the electromagnetic field inside the high quality
cavity. Considerably lower interest is concerned to exciton states non-resonant
with the cavity mode for which the exciton-photon interaction is strongly
reduced due to the existence of the cavity. We point out here that these states
are responsible for non-classical light generation in semiconductor
microcavities.
