All spectroscopies are based on the polarization induced by light’s oscillating electric field. The electric field of an exciting beam induces a polarization in the illuminated material; in turn, the oscillating polarization will produce an output electric field (or, simply, emit light).
The illumination conditions affect the nature of the electric field-polarization interaction. When the excitation field is first turned on, the polarization is forced to follow the driving frequency of the electric field, so the light emitted by the polarization is of the same frequency as the excitation light. As light is continuously absorbed, the polarization and emitted light build up until they reach a steady-state. This steady-state process is called driven emissio