# Stimulated/Induced Emission

1. Feb 5, 2013

### moatasim23

How can an incident photon cause an atom to de excite??

2. Feb 5, 2013

### Jano L.

I do not know how photon can, but here is how EM wave can:

Imagine charged particle oscillating here and there according to the function

$$x(t) = x_0 \cos \omega t$$.

If the EM wave is such that the electric field oscillates in the same direction according to

$$E(t) = E_0 \cos (\omega t - \varphi),$$
I do not know how photon can, but here is how EM wave can:

Imagine charged particle of charge $q$ oscillating here and there according to the function

$$x(t) = x_0 \cos \omega t.$$

If the EM wave is such that the electric field oscillates in the same direction according to

$$E(t) = E_0 \cos (\omega t - \varphi),$$

i.e. there is some phase shift $\varphi$, the net power of the electric force is given by
$$P(t) = E(t) \dot x(t)$$

and can be positive or negative according to value of $\varphi$. If the power is negative, it means that the oscillating particles returns energy into the field and thus will decrease its oscillations - hence "stimulated emission".

i.e. there is some phase shift $\varphi$, the net power of the electric force is given by
$$P(t) = qE(t) \dot x(t)$$

and can be positive or negative according to value of $\varphi$. If the power is negative, it means that the oscillating particles returns energy into the field and thus will decrease its oscillations - hence "stimulated emission".

Of course atom is more complicated, but the idea is the same - whether there is absorption or release of EM energy depends on the mutual phase of the system and the EM wave.