What makes a laser?

[xirrious]

This is not as newb of a question as the title would make it seem. The temporally and spatially coherent light we get from our lasers are a product of excited crystals (or gas, dye, etc) trying their damndest to stay in their happy ground state. We all know the basics of electromagnetism or the electroweak force as its known now (I assume, since this is the science forum).. And we know all elements have spectral signatures. This is how we know the composition of stars, planets, or any other celestial body. Photons carry immense information within them. All elements absorb and emit light characteristic to that element.

I am searching for a deeper answer for WHY we cannot use any element to produce laser light (even if its multiline). Whats the key im missing here? Theoretically any element should be able to lase, based on my understanding. This is obviously not true in practicality. Why? What characteristics do you need in an element for it to be a candidate for becoming a lasing medium?

I assume it has something to do with which valence shells are full, how many are full, which layer of each shell is occupied, etc, and perhaps also atomic weight, but I'm not a Ph.D.

If this is in fact a stupid question and I should know the answer already, I apologize in advance!:thanks:

 

[Cyparagon]

I am searching for a deeper answer for WHY we cannot use any element to produce laser light

You said it yourself:

All elements absorb and emit light characteristic to that element.

You need a population inversion to have lasing. Without knowing the specifics of excitation states, this simply means it is not feasible to excite the medium and keep it excited. The two biggest problems I see are either the efficiency is so abysmal that constructing the device is pointless, or that the energy requirement is so large, that things (like the lasing medium, or whatever contains the lasing medium), disintegrate.

A third issue you might not be aware of, is that even if you get something to lase, many lasing wavelengths are thus-far useless. Low power long wave IR for example.

 

[xirrious]

Your last point I'm much aware of, but you know better then I do surely that lasers are finding new applications in new fields daily.

Your first two answers were concise and exactly the type of answer I was looking for. You confirmed what I suspsected. These are the reasons I felt; without proof, made the most sense. Thanks Cyp! Rep when I can for an awesome helpful reply.

 

[xirrious]

To reiterate, THEORETICALLY, not including feasibility or efficiency, any element can be made to lase within its spectral signature, correct?

I FULLY understand the issues complicating this and why it may be practically impossible, this is a thought experiment of mine, nothing more.

Edit: Thought experiments involving physics is as close as I can get to an entheogenic experience possible, so that is my reason for this thread. Not to waste time with pointless questions.

 

[Sigurthr]

There's an old saying; "you can get anything to lase if you pump it hard enough". It's quite true.

 

[xirrious]

Thanks that's the bottom line I needed!

Obviously I never heard that old saying lol

 

[xirrious]

Okay so I did some more research to learn more about population inversion, more specifically though a more precise understanding of the difference between stimulated and spontaneous emission.. I've answered my own queston with the help of LPF. Thanks !

It's interesting to me the field associated with a passing by catalyst photon can influence an excited electron near by and cause it to emit a photon coherent with the catalytic photon, all without any absorption. See this is odd to me ass the electromagnetic field is created itself by photons, which are the boson counter part to the fermionic electron. Photons are allowed to split apart and recombinant due to quantum mechanics uncertainty principle , and THAT is the true reason lasers are possible. The catalyst photons cannot effect an electron without physical interaction, thus the uncertainty principle is the true culprit here.

Edit: to further extrapolate on this fact, the field itself associated with the catalytic photon is indeed only an apparition manifested by the uncertainty principle, the field itself is comprised of these virtual photons that occur wheb photons split. Virtual photons are simply photons existing on borrowed energy, and only become real in very specific situations. Such as an event horizon. Usually they exist for a unit or two of Planck time.. before recombining with their parent photon

 

[ultimatekaiser]

Yep. Congrats, you win a cookie

 

[xirrious]

What kind? I accept chocolate chip only.

 

[ultimatekaiser]

Sure. Have a go.

 

[xirrious]

Sensing thick sarcasm, but I'll enjoy my cookie none the less

 

[ultimatekaiser]

Haha. In all seriousness thanks for sharing

 

[xirrious]

Well, if someone didnt know, now they do..

 

[xirrious]

Added some more info to original conclusion post. I feel this info is good to know, thought admittedly hard to grasp at first unless you have a science background

 

[ultimatekaiser]

That's correct. Especially in the case of ruby. The material is self absorbing, unless the entire medium is saturated you don't get laser light for this reason. It just falls back to ground and is re-pumped by its own light..

 

[Roam]

I'm late to the party, but first of all inversion is not necessary for lasing. There is "LWI Effect" (lasing without inversion) which works by suppression of absorption, so you actually change the refractive index of a material, making something transparent which would otherwise be heavily absorbing. (Btw this was first predicted by Dr. Javan and demonstrated by Marlan Scully)

Secondly, yes you can get lasing in any atomic or molecular species. The issue is efficiency, for example if you have a 4 level system (HeNe, Nd yag, etc.) you only need dN = N2-N1 to start the laser. But for a 3 level system (e.g. ruby) you need more than 1/2 atoms to be in the excited state (because the lower energy level is just the ground). If N is the total number of atoms, then 2nd level population has to be N2=N/2 + dN/2! Much more that a 4-level laser! That's why they are often pulsed.

P.S. Sorry about the double posting.