Intensity dependent resonance light scattering

When low intensity monochromatic light is incident upon an atom in its ground state, the scattered light has the same frequency as the incident light, and is accordingly described by a δ-function spectrum. This result, which quantum mechanically is a simple consequence of energy conservation, may be understood from a classical perspective by imaging that the incident field drives a classical harmonic oscillator electric dipole. In the case of incident fields of great intensity, on the other hand, multiphoton effects come into play, producing a rather complicated spectrum with broadened and separated features, superimposed upon a δ-function part which remains, though with a magnitude which depends in a nonlinear way on the intensity of the incident field. The scattered field in this case is described by a correlation function which contains both a coherent and an incoherent part: = * + incoherent a relation which is a consequence of a similar relation obeyed by the current autocorrelation function for the driven atom