Figure 2.
Energy level diagrams of different third order nonlinear induced polarizations. (a) When the energy difference between pump and probe beam is resonant with a vibrational transition of the molecule, a strong resonant CARS signal at the anti-Stokes frequency is emitted. (b) When the energy difference between pump and probe beam is not resonant with any vibrational transitions of the material, a weak but non-vanishing signal, known as the non-resonant background, is still generated at the anti-Stokes frequency. (c) Stimulated Raman loss (SRL) occurring at the pump field frequency has the opposite (180 degree lag) phase compared to the pump field. (d) Stimulated Raman gain (SRG) occurring at the probe field frequency has the same (zero degree lag) phase with that of the probe field.