Stimulated Raman in linear regime

[DrDu]

In the course of another thread I was lend to think about the Raman effect. I also read about the stimulated Raman effect and found that it is usually described as a third order nonlinear effect where a power of two of E is assumed to drive the nuclear vibration. I don't quite see why this is necessary. If I shine two lasers whose energy difference $\Delta \omega=\Omega$ equals that of the frequency of the vibrational mode, I would expect to get stimulated Raman scattering also from the ordinary Stokes transition from the vibrational ground state to the first excited state. I also don't see a fundamental threshold for this process, so if light is shone into a sufficiently large volume of Raman active material, I would expect spontaneous lasing. Do you see some flaw in these lines of though?

 

[BigJDubs]

I assume that the two-photon drive is necessary because it is usually mentioned in the literature, but I'm not sure why.The two-photon drive is necessary because it is much more efficient than a single-photon drive. In the presence of an electromagnetic field, stimulated Raman scattering is caused by the absorption of two photons of frequency Ω whose total energy is equal to the energy difference between two vibrational states. The two-photon drive allows the probability of such an event to be significantly increased, leading to a much higher efficiency of the process. In addition, if you were to use a single-photon drive for the stimulated Raman effect, then there would be a fundamental threshold for the intensity of the light required to cause the transition. This is because, in order to cause a transition, the single photon must have at least enough energy to cause the transition. With a two-photon drive, this threshold is not present, as the two photons need not each individually have the energy to cause the transition, as long as their combined energy is sufficient. Finally, while it is possible to get stimulated Raman scattering from the ordinary Stokes transition, this process will be much less efficient than with a two-photon drive, and so is not commonly used.