Stimulated Raman Scattering

This nonlinear process involves the interaction of a laser beam with the molecular vibrations in gases or liquids. In the presence of a strong external electric field, the charge distribution of a diatomic molecule is polarized. The molecule acquires a dipole that can interact with the driving field. The external field exerts a forceon the molecule proportional to E². This implies that there must be two input photons driving the interaction. Therefore two electric fields in (1) have the same frequency and represent the pump beam ωp which drives the interaction to provide amplification at ω_p − ω₀, where ω₀ is the frequency of the molecular vibration wave. In accordance with (1) three waves produce a fourth wave via the third-order polarization, and energy at the pump frequency can be transferred to the signal frequency ω_p − ω₀. In addition, nonlinear mixing processes create additional frequency components at ω_p−jω₀ and ω_p+jω₀ where j = 1, 2, 3, . . . .