# Stimulated Emission

1. Sep 12, 2009

### sokrates

I am trying to find an intuitive explanation for the stimulated emission phenomenon. I know the effect: a photon with the right frequency "interacts" with an excited electron to create a copy of itself( same phase, same amplitude, same state) putting the electron to a lower energy state...

But this doesn't fit as easily as the excitation of an unexcited electron...What I mean is, a photon with the right energy interacts with an electron and puts the electron into an excited state. This is much more intuitive and reasonable, at least for me.

I am trying to find a similar reasoning for the stimulated emission, is there one that you guys know of?

2. Sep 13, 2009

### conway

In many optical phenomena, the quantum mechanical picture is consistent with what we would expect from classical electromagnetic theory, with the atom considered as a harmonic oscillator. The atom is resonant with a particular frequency, but the relative phase of the atomic oscillator is almost completely random with respect to the incoming light. As the phase difference varies through 360 degrees, there is a point where the absorption from the incoming wave is a maximum. You can analyze this by taking the superposition of the incoming wave with the ordinary classical donut-shaped radiation pattern of an oscillating dipole. When this calculation is done, there is a shadow zone behind the oscillator where the incoming wave is diminished.

Let the phase shift another 180 degrees and the situation is reversed. The superposition of the two wave patterns now re-inforces the incoming wave. That's stimulated emission.

Both the absorption and emission processes described above depend on the strength of the incoming wave. For a given oscillator amplitude, the rate of absorption or emission is linear with the ambient field strength. There is also another kind of emission going on all the time which doesn't depend on the incident wave. It's just the same classical radiative donut pattern as calculated in all other directions (outside the above-mentioned "shadow zone") where there is effectively no significant interaction between the two wave patterns. That's spontaneous emission.