I am aware of crystals being energised with internal photoelectric effect, and then, somehow they are provoked to release the energy at once? The last part is what i don’t understand. How can a photon force emission of other photons? Or am i getting this wrong entirely?
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The name of the thing you do not grasp is *stimulated emission*, the process by which a passerby photon grazes an electron and “coaxes” it to emit a second photon identical to the first. Despite all the great answers here so far, no one seems to have gone into detail for you on this specific process.
I only have a tenuous grasp of it myself, but what I can say is that it’s not that unlike how playing a specific tone on a musical instrument or speaker can cause the strings on nearby instruments to also vibrate at the same pitch.
Also, to understand this effect, you may have to partially unlearn what you think you know about electron orbital states. Judging from your post I can reasonably assume you understand that electrons are bound to exist only at specific orbital energies, and that they can hop between them, and will emit or absorb a photon when they do. This is a mostly correct picture. But if you took as an assumption that it means the interleaving space between orbits is fundamentally forbidden, that isn’t really the case.
With very few exceptions, an electron can exist basically wherever it damn well likes. The question is never, “Could it?”, but rather, “How unlikely is it?” Anywhere outside of the known orbitals is incredibly unlikely, granted. And if it does find itself in one of those rare places, external forces from the rest of the atom will make it quickly fall back in line. But it *can* go there. If only just for a moment.
I would draw an analogy to your home light switch. Flick it one way, flick it back. Note how it has that sudden “snap” to it. For all serves and purposes, the switch has only two states, on or off. But in the process of flicking it, it *does* pass through the intervening space, albeit very quickly. If you were to get very meticulous, you *could* pull the switch into the halfway position and hold it there. In the real world with friction, it could possibly even balance there without being held; though, if the switch were flawless and frictionless, it should always snap back to one of the two positions when you let go.
The electron is kind of the same way. It is possible for an outside force to sort of coax it into that pseudo-forbidden halfway point between a jump. And once the external force leaves, the electron has two options: snap back to where it came from, or complete the jump and snap to the new destination (and emit a photon in the process).
When the electron is in this transitional state, it oscillates with the same frequency as the photon it is about to emit. So, if a photon of that frequency grazes by, it can coax the electron to oscillate with it, at least briefly. Just like with sympathetic vibrations of musical instruments. This forced oscillation drags the electron into that pseudo-forbidden position, like playing with the light switch. And just as quickly as the photon came, it leaves. And the electron, dragged into that halfway state, can either snap back, or complete the jump and emit a photon just like the one that passed by. The latter case is stimulated emission.
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