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?