The Quantum Theory Behind Lasers and Masers

[scupydog]

Hi does anyone know where i can find an online version of the quantum mechanical theory behind lasers and masers. regards Dave.

 

[Naty1]

You can start at Wikipedia,
http://en.wikipedia.org/wiki/Laser

You'll find a lot of links in the various sections which may help...
There are lots of types...gas,solid state,chemical...this is a big field...

If that isn't enough, Google LASERS...

 

[scupydog]

Thx for that Naty but I've have done that, what i need is a quantum way in which lasers work and that i cannot seem to find.

ps Thx i found what i needed on wiki it was the free electron laser http://en.wikipedia.org/wiki/Free_electron_laser page that made it clear for thanks again.

 

[Cthugha]

Ehm, please note, that the FEL is a concept, which is a bit different from an usual laser.

For most common lasers, there is not so much qm behind it. Other strange concepts like VCSELs and polariton lasers need a lot more qm.

As a start, how familiar are you with spontaneous and stimulated emission and the concept of population inversion?

 

[scupydog]

Hi Cthugha, i understand spontaneous and stimulated emission as in, photon in photon out, energy level changes etc, but what i wanted to know is there a QM theory behind lasing ie for what reason does the incoming photon stimulate the emission of photons of the same wavelength etc and just wondered if anyone had put it online because i can't seem to find it which i thought was odd.

 

[Cthugha]

Well, in contrast to spontaneous emission, one can understand all stimulated emission effects using the classical or semiclassical description.

In case one indeed uses QM, you can treat stimulated emission using Fermi's golden rule. You might already have calculated some transition rates from the ground state to the excited state using this rule. Now the interesting thing is, that the transition rates in both directions are exactly the same. In most situations the transition rate for stimulated absorption equals the rate for stimulated emission. This is also why one needs population inversion and why there are no usual two level lasers: You would drive the emission and the absorption simultaneously, which would pretty much spoil the idea.

Unfortunately, I must admit, that I do not know a free online resource on the qm description of lasing processes, either.

 

[scupydog]

thx Cthugha i will have a look at Fermi's golden rule.

 

[Matterwave]

The quantum mechanics of it (a very basic and short version) is that when you have an atom in an excited state, and you shoot a photon at it, you can stimulate that atom to drop down to a lower state and release a photon. You shoot 1 photon, and you get 2 photons out. The mechanisms of this can be seen clearly in time-dependent perturbation theory where the photon is a "perturbation". If you can fill a tube of gas with a majority of excited atoms, then you can get a chain reaction with 1 photon producing 2 and 2 photons producing 4, etc, so that at the end you have a very intense beam.

 

[scupydog]

what i mean is... What is the mecanism for 1 photon in 2 photons out exactly matching the 1st photon...is this not a violation of energy conservation? thanks for your reply

 

[f95toli]

what i mean is... What is the mecanism for 1 photon in 2 photons out exactly matching the 1st photon...is this not a violation of energy conservation? thanks for your reply

No, because the INITIAL energy that puts one photon in an excited state (so that it can be emitted by "stimulation" from ANOTHER photon) comes from another source.
Sometimes this other source is a strong source of light (in many cases another laser) in which case this is known as "optical pumping".
In e.g. semiconductor lasers it is essentially the energy of the excited electrons that are converted to photons of the "right" frequency via stimulated emission, in gas lasers molecules in an excited state etc.
I.e. there is always a source of energy in the system, the whole point of the laser is that can "convert" this energy into coherent light.

It might actually be a good idea to read up on the theory of masers, they are -in my view- somewhat easier to understand than lasers (the principle is obviously the same, but the implementation is somewhat simpler in the case of a maser).