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Definition: oscillations of level populations (or quantum mechanical probability amplitudes) under the influence of an incident light field
If light interacts with a two-level system (e.g. an atom or ion with a ground state and an excited state), this can lead to a periodic exchange of energy between the light field and the two-level system. Such oscillations, which are called Rabi oscillations (with reference to the Nobel Prize winner Isidor Isaac Rabi), are associated with oscillations of the quantum mechanical expectation values of level populations and photon numbers. They can be interpreted as a periodic change between absorption and stimulated emission of photons, and can be modeled using the Bloch vector formalism. A competing process, which can prevent these oscillations, is spontaneous emission.
The angular frequency of the Rabi oscillations (2π times the number of Rabi cycles per second) is called the Rabi frequency. It is proportional to the amplitude (not the optical intensity) of the light field and to the dipole moment of the level transition.
When a solid-state gain medium is optically pumped, Rabi oscillations usually cannot be observed, since the upper and lower states actually consist of Stark level manifolds (containing multiple sub-levels) and coherence is quickly destroyed by phonons. Further, inhomogeneous broadening may occur, and the optical intensities are usually not spatially uniform.