How does stimulated emission occur in lasers?

[wellorderingp]

I was reading about LASER production when I came by the concept of stimulated emission.

The book I was reading doesn't elaborate the topic much,so far I have understood what stimulated emission is but not how it happens.

When a photon strikes an excited atom the atom falls to it's ground state releasing another coherent photon.
To me, what should happen is that atom should gain energy and jump to a higher state(this is what normally happens).
Please elaborate this concept. Thank you.

 

[Simon Bridge]

The photon and the atom are involved in an electromagnetic interaction - a number of things can happen.
One of those things is that an atom may absorb the photon and end up in a more energetic state... or ionized.
It may be that nothing happens.

What actually happens is random, but the probability of each possible thing depends on the details.
i.e. to favor absorption, the incoming photon needs to have an energy corresponding to the next energy gap for the atom.
You'll no doubt be taught more about this when you get to the concept of "cross-sections".

 

[blue_leaf77]

In laser, one can identify two groups of photons: pump photons and emitted/laser photons. In normal condition most atoms lie in their ground state, in order to bring them to some excited state, they must be pumped up (i.e. excited) by illuminating them with pump light. Then, around the onset of lasing action, those excited atoms are going to decay owing to its finite lifetime and emit fluorescence light (spontaneous emission). These very first bunch of fluorescence photons trigger the other yet to decay atoms to undergo stimulated emission, releasing another coherent photon. These coherent photons are the emitted/laser photons.

 

[Andy Resnick]

Another aspect of laser operation is that the atoms need to have more than 2 states (ground, excited) available: a 2-state collection of atoms can never reach the lasing threshold (population inversion). Instead, an atom absorbs energy and goes to an upper excited state with a short half-life, then relaxes to a long-lived excited state- this allows sufficient population of excited atoms to accumulate.

 

[wellorderingp]

In laser, one can identify two groups of photons: pump photons and emitted/laser photons. In normal condition most atoms lie in their ground state, in order to bring them to some excited state, they must be pumped up (i.e. excited) by illuminating them with pump light. Then, around the onset of lasing action, those excited atoms are going to decay owing to its finite lifetime and emit fluorescence light (spontaneous emission). These very first bunch of fluorescence photons trigger the other yet to decay atoms to undergo stimulated emission, releasing another coherent photon. These coherent photons are the emitted/laser photons.

That explains "what" stimulated emission is. I am not able to understand "why" it happens that way.
As you stated ' the photons trigger the other yet to decay atoms to release coherent photons'.
The book I am reading gives a vague idea about what happens.
It states that-
In stimulated emission each incident photon encounters a previously excited atom and the optical field of the photon interacts with the electron. The result of interaction is a kind of resonance effect,which induces each atom to emit a second photon.
Please elaborate more on this. Thank you

 

[blue_leaf77]

Ah sorry, I thought what you are confused about was how can there be electrons in an excited state out of nowhere.
Then the answer to your question was explained by Simon Bridge. So electrons sitting in a certain level can undergo 3 distinct processes: spontaneous emission, stimulated emission, and absorption. Each of them has different chance to be happening, and this probability is related to cross sections, as Simon has said.

 

[wellorderingp]

Thank you very much.
But I also read that stimulated emission could be explained classically,using the dipoles and em theory.

 

[Simon Bridge]

But I also read that stimulated emission could be explained classically,using the dipoles and em theory.

Where did you read this?
Stimulated emission is properly understood in terms of quantum mechanics... there are always other ways to do the math though.

 

[Alastair McD]

When a photon strikes an excited atom the atom falls to it's ground state releasing another coherent photon. To me, what should happen is that atom should gain energy and jump to a higher state (this is what normally happens).

The energy of higher states are not exact multiples of the of the energy of the first excited state. Each step up is slightly less than the previous one. Therefore a second photon at the same frequency will not drive an excited molecule into a higher state. It produces a stimulated emission.