The nonlinear optical response of semiconductor microcavities in the nonper
tubative regime is studied in resonant single-beam-transmission and pump-pr
obe experiments. In both cases a pronounced third transmission peak lying s
pectrally between the two normal modes is observed. A fully quantized theor
y is essential for the agreement with the experimental observations, demons
trating that quantum fluctuations leading to intraband polarizations are re
sponsible for this effect.