Stimulated Raman Scattering
Raman scattering is a two-photon process . As illustrated in Fig. la, an incident photon is annihilated and a new photon of a different frequency is created. This implies that the scattering is inelastic, and the medium is left in a different energy state. When the scattered photon is lower in energy, or frequency, it is said to be Stokes shifted. The shift in frequency of the Stokes photon is related to a characteristic frequency of the medium. This is illustrated in Fig. 2a. Calling this characteristic material excitation frequency ω)v, the Stokes frequency is then ωs = ΩL — ωv, where ωL is the incident laser frequency. Thus, Raman (Stokes) scattering results in an excitation from the ground state of the medium to an excited state mediated by a two-photon inelastic scattering. The internal material excitation can involve the creation of an excited electronic state, an excited vibrational-rotational state, a lattice vibration, a spin flip in semiconductors, or electron waves in plasmas . The phenomenon occurs in gases, liquids, solids, and plasmas.