Laser action and stimulated emission

[logearav]

Homework Statement

Revered Members,
An electron in ground state makes its way to an excited state upon absorption of photon of energy, equivalent to energy difference between ground state and excited state, and after some time, it decays by emitting the photon and returns to the ground state. This is called spontaneous decay.
For Laser action, population inversion and stimulated emission should occur.

Homework Equations

The Attempt at a Solution

Now let me explain the scenario
1) An atom(electron) in the energy level E1 absorbs a photon and goes to a state of higher energy say E3
2) While decaying from E3 to E1, it reaches a metastable state E2. Now, due to longevity of the stay in E2 than in E3, we can achieve population inversion in E2
My question is ,
1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission?
2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?

 

[Simon Bridge]

My question is ,
1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission?

Yes[/quote]2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?[/quote]
http://en.wikipedia.org/wiki/Stimulated_emission
E2-E1

The rest is just how you go about populating E2.

 

[logearav]

Thanks for the reply Simon. So I infer that the electron in the metastable state has already has an absorbed photon, and if we supply a photon of energy E2 - E1 externally, then this photon plus the absorbed photon are emitted and both are in same phase. Am I right?

 

[Redbelly98]

The electron in energy level E2 does not "have an absorbed photon"; it simply has energy. In fact it has an amount of energy E2-E1 more than it would have if it were in the ground state. When the electron makes a transition to level E1, that energy is converted to electromagnetic energy, i.e., the emitted photon.

Hope that helps.

 

[Simon Bridge]

+1 to Redbelly98:

It's not a useful picture to think of the electron as somehow holding-on-to a photon that gets released later.

The electron has absorbed a photon to get it to E3 already - that photon has been destroyed and it's energy converted electromagnetic potential energy for the electron.
When the electron decays to E2 it has to release some of that energy ... there are several ways it can do this but the most likely one by far is to release a single photon to carry off the energy difference.

 

[logearav]

Thank you Redbelly and Simon. So, when the electron releases photon when it comes to E2, we supply again a photon of energy E2 - E1 and the supplied photon acts as stimulating photon. Am I right?

 

[Redbelly98]

Yes.