As semiconductor fabrication technology developed, it became possible to fabricate high-Q semiconductor microcavities. Thus the study of semiconductor cavity QED started in the weak-coupling regime  and then entered the strong-coupling regime, where the exciton-photon coupling constant becomes larger than the exciton and cavity photon decay rates [50>]. In a high-Q semiconductor microcavity, the strong exciton-photon coupling leads to the formation of two new eigenstates of the exciton—photon coupled system, called “microcavity exciton polariton” states. The energy separation between the two polariton states increases as the exciton—photon coupling increases. This exciton—polariton normal-mode splitting is the solid-state analog of the vacuum Rabi splitting in the atom—cavity case. In the time domain, the strong exciton-photon coupling makes the spontaneous-emission process reversible; namely, the emission from the microcavity shows an oscillation, instead of the usual exponential decay. This is because photons emitted by excitons are reabsorbed and reemitted a number of times before exiting the cavity. Hence the excitation energy of the system is transferred back and forth between the QW exciton state and the cavity photon state, leading to a Rabi oscillation. This process can be modeled as a coupled pendulum system where the two pendulums correspond to the microcavity mode field at the frequency ωc and the exciton at the frequency ωex. The QW exciton state and the resonantcavity photon state form a simple system of two harmonic oscillators coupled through the light—matter interaction.