Lasers & Photons: Stimulated Emission Explained

[Cheman]

When we have stimulated emisson as in a laser, a photon from a spontaneous emission makes an exited electron unstable which then falls back to its ground state releasing as 2nd photon. But why does the electron not just absorb the 2nds photon, move to an even higher energy level and then fall back to grounbd state releasing 1 very high energy photon?

Thanks in advance. :-)

 

[Cheman]

Can anybody answer this question? It just seems seems strange to me that when the photon arrives at the already exited electron it doesn't exite it any further - it just causes it to relax. Why?

 

[Dr Transport]

In most lasers, the pumping radiation has an energy which is less than the next higher metastable state, in a He Ne laser, you pump up to the Ne 3s state which lases down to either the 3p or the 2p state. In most gas alsers, the an electron is excited in a medium, in this case the He ions, then energy os transferred via collisions to the Ne ions because the He 2s and the Ne 3s are at nearly the same energy levels. Look in any Quantum Electronics book or Lasers text to see the energy diagrams.

 

[Cheman]

Sorry if I'm being dim, but how exactly does that answer the question? I was asking about why the photon doesn't exite the electron further instead of just making it fall and release another photon.

 

[Gonzolo]

My first thought is that there might not be an higher level that coincides with the absorbed energy. When an atom absorbs x eV (or Joules) of energy, the electron's new level must be x eV higher. If this level doesn't exist, the absorption can't take place.

Further corroboration is advised.

 

[Dr Transport]

That is correct...

 

[marlon]

When we have stimulated emisson as in a laser, a photon from a spontaneous emission makes an exited electron unstable which then falls back to its ground state releasing as 2nd photon. But why does the electron not just absorb the 2nds photon, move to an even higher energy level and then fall back to grounbd state releasing 1 very high energy photon?

Thanks in advance. :-)

First of all, the already present photon does not make an excited atom unstable...The fact that it is excited means that it is unstable because it will fall back to the ground-energy-level, yielding the emitted photon. Stimulated emission works because the photons present help to excite extra atoms. All these atoms then will fall back to their ground level and emit the photons. This is the lasing-mechanism.

Your question about releasing one very high energy-photon can be proved if you were to examin these processes quantummechanically. You will see that certain conservationlaws will need to be respected and these laws will make sure such a thing won't happen here.

Lasing can work in three or four stages depending on what atoms are involved. The four level mechanism is the most efficient because in the three level mechanism a basic condition for lasing is that at least half of the atoms in their ground state need to be excited...

regards
marlon

 

[Ring]

When we have stimulated emisson as in a laser, a photon from a spontaneous emission makes an exited electron unstable which then falls back to its ground state releasing as 2nd photon. But why does the electron not just absorb the 2nds photon, move to an even higher energy level and then fall back to grounbd state releasing 1 very high energy photon?

Thanks in advance. :-)

As you guessed both stimulated emission and stimulated absorption occur. But the ratio of the emission transition rate to absorption transition rate is proportional to the number of electrons in the excited state versus the number of electrons in the relaxed state.

This necessary population inversion is obtained by injecting energy into the system via optical pumping.

 

[Ring]

Oh and the reason the excited electron doesn't absorb the incident photon is because the incident photon's energy doesn't match the energy required to promote the electron to the next higher energy level.

 

[Cheman]

Thanks. But why is it that when the photon "hits" the already exited electron that not only is it not absorbed but it causes the electron to fall and emit a second photon which will of course be of the same frequency and in phase?

If you do not quite understand what I mean, take a look at the diagram from this link:
http://science.howstuffworks.com/laser6.htm

Thanks.

 

[Claude Bile]

Try looking at the problem from the wave point of view as well.

The photon causes the electron to oscillate at a frequency equal to the frequency of the photon. When the electron oscillates at this frequency, it radiates not only the incident photon, but the energy it had stored in the higher orbital as well in the form o a second photon. Because both photons originate from the same oscillator, they have the same frequency and phase.

Claude.

 

[Creator]

Thanks. But why is it that when the photon "hits" the already exited electron that not only is it not absorbed but it causes the electron to fall and emit a second photon which will of course be of the same frequency and in phase?

]

I understand your dilema, Cheman.

A complete understanding of stimulated emission must include quantum theory. The photon doesn't actually 'hit' the electron. In truth the oscillating field of the stimulating photon (wave) provides a 'perturbation' to the electron field which causes it to 'prematurely' drop to the lower (stable) energy level. It's purely a quantum effect (discovered by Albert in 1916).
What I mean by drop 'prematurely' is that it falls from the metastable state into the lower stable energy state before its normal average (spontaneous)decay time. The frequency of the perturbing photon must be a resonance frequency of the medium; that is, the frequency must be equal to the energy difference between the excited and ground state of the atom.

If the the resonant oscillating (photon) field perturbs the excited atom (before it has a chance to decay), the ensuing drop in energy level emits a photon of the same frequency, direction, phase and polarization as the stimulating photon.

Creator

--Save the whales; collect the whole set.--

 

[androz]

Hi all !

Ok, I'll try to give you some new informations (I don't have read all the replies so forgive me if I say something already said).

Well, it is possible that an electron in an excited state absorbs another (pump) photon to be in a much higher energy level. Such a mechanism is known as "upconversion" or "superfluorescence".

The principle is quite simple :
Suppose that in the energy diagram of the gain media (rare-earth ions for exemple), there are several levels that can absorb photons of the same energy (same wavelength). It is then possible, if the levels are imbricated, that more than one photon are absorbed by the same electron, by a two steps mechanism (and not a one step like two photons absorption that is another mechanism).
For exemple, you can take a thulium ion in ZBLAN glass (where phonon energy is less than in silica). It is possible with a pump laser of ~1150nm to excite the fundamental level to a higher level which rapidely relax to an intermediate level. From this level, the electron can radiatively relax to the ground by the emission of a photon (of less energy than the pump photon), or absorb another pump photon to get in a higher energy level. At such a pump wavelength, thulium can absorb up to three photons of the same pump. It is then possible to build a visible laser (emission at 485nm) by the use of a pump laser of higher wavelength !
In order an electron could absorb another photon, the cross section (probability of absorption) at the pump wavelength from an excited state must be high enough, and the phonon energy must be not too high to allow higher lifetimes of the excited states. That is why upconversion is difficult in silica, but much easier in ZBLAN glasses.

I hope you understood what I wrote, and that it gives you a beginning for complete answer...

 

[Cheman]

Thanks Creator. If you have any more information on this then I would happy for you to post it.