Fig.1. Principle of stimulated Raman scattering (SRS).
a, Energy diagram of SRS. If the difference frequency of pump and Stokes beams is resonant with a molecular vibration, SRS occurs and a molecule of the sample is excited from its ground state to the vibrational excited state. b, As a consequence a pump photon of the excitation field is annihilated (stimulated Raman loss = SRL) and a Stokes photon is created (stimulated Raman gain = SRG) as required from energy conservation.