Stimulated Raman Scattering

Stimulated Raman scattering takes place when an excess of Stokes photons that were previously generated by normal Raman scattering are present or are deliberately added to the excitation beam. The mode that is strongest in the regular Raman spectrum is then greatly amplified; all other weaker Raman-active modes are not present.

It was found that if a sample was irradiated with a very strong laser pulse, new non-linear phenomena could be observed in Raman effect. In comparison with continuous wave (CW) lasers with electric fields of about only 104 Vcm,-1 pulsed lasers with electric fields of about 109 Vcm-1 transformed much larger portions of incident light into useful Raman scattering and substantially improved signal-to-noise ratio.

Stimulated Raman scattering is an example of non-linear Raman spectroscopy. Very strong laser pulses with an electric field strength > 109 Vcm-1 transform up to 50% of all laser pulse energy into coherent beams at Stokes frequency u0 - um. Just for comparison, only 0.001% of laser light energy in spontaneous Raman scattering is transferred into Raman signal. Therefore, enhancement in a Raman signal of four-to-five orders of magnitude can be achieved in Stimulated Raman spectroscopy. The Stokes beam is unidirectional with the incident laser beam. Only the mode um (the strongest in the regular Raman spectrum) is greatly amplified. All other weaker Raman-active modes are not present. The Stokes frequency is so strong it acts as a secondary excitation source and generates the second Stokes line with frequency u0 - 2um. The second Stokes line generates a third one with a frequency of u0 - “ 3um etc.