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 key is population inversion, which means that there are more excited atoms than ground state atoms. Those are unstable and just looking for an excuse to emit light, and when they’re nudged by a photon, they’ll do so.
A ground state atom would absorb a photon instead, so there needs to be fewer of them around in order for light amplification to occur.
Imagine you got a bowl of marbles. You start to spin it to get them moving. They hit and collide, but after a time they start to move in same direction and pick up speed. Soon some of them start to have enough speed to jump out of the bowl.
And that is laser, basically. You excite a medium until it photons in it have enough energy to go past a barrier holding them back. They do this soon as they have enough energy. This can simply be done with two mirrors one of which is slightly transparent. You bounce light between these until it is powerful, constantly adding more, enough to pass through the transparent mirror. Adjusting the properties of the transparent mirror allows you to control properties if the laser.
A photon is energy. An electron can absorb that energy, and it moves further away from the nucleus. This is an interaction of the electromagnetic force. It’s like a bat. A bat has energy, and a ball can absorb that energy and move further away from the center of Earth. This is an interaction of the gravitational force. The only difference is that the bat not only has energy, it also has mass, so the mass is left behind after it loses its energy. A photon has no mass, so it is completely absorbed by the electron.
Now, imagine the electron falling back down towards the nucleus. This is more stable since the positive protons attract the negative electron. If further away is more energy, then closer is less energy. So the electron has to get rid of the energy it absorbed. The energy emitted takes the form of a photon.
This is not just lasers, this is how all light works. And not just visible light. Everything from cosmic rays to radio waves are photons with different amounts of energy. What’s important in a laser is that the source is made up of a very specific element with very specific energy gaps between possible states for the electron to be in. Picture a shelf. Before, when we hit the ball with the bat, it flew up to some arbitrary height, but with a shelf, there are only a handful of possible heights for the ball to be in. A laser works by making sure all the shelves came from the exact same manufacturer so all the heights are the same. That means only one type of photon is emitted.
There’s only one other thing I’d like to address in your question, that it happens “at once.” Which really isn’t the case. I can show you the math if you’re interested, but a typical 5 milliwatt, red laser pointer you’d use to play with your cat or present a PowerPoint emits about 17 quadrillion photons per second. So energy is flowing into the crystal at a constant rate and random electrons are absorbing it, and then randomly, they will release that energy as a photon, so it’s happening all over the crystal, all the time.
It’s a self-balancing equilibrium, if the electrons are a bit slow to release the energy, then there is more power incoming than outgoing. This means the number of excited electrons will increase. Since they all relax back to the ground state after a completely random amount of time, then more excited electrons means more decays per second. (Think of it like the odds of flipping a heads with one coin vs flipping 5 coins at once). So the power output will increase until it matches the power input. And a symmetric argument happens if the electrons are relaxing too quickly. They will output more power than is input so the number of excited electrons decreases until equilibrium is reestablished. At equilibrium, the number of photons that are excited.every second matches the number of electrons that decay, and this, photons emitted per second.
First, it doesn’t have to be a crystal. There are gas and liquid medium lasers. The important part is that the medium can have many excited electron states.
Think of the electrons like balls stacked in a pyramid. The ones on the bottom are most stable, they’re sitting on the ground (state). But in this example, no electrons can be put outside of the pyramid’s footprint. So you start stacking them. So you have a few less electrons above. And a few above that. It’s all still stable because there’s nowhere else for them to go. If they are given a bit of energy, they jump up a level, but then there IS space to fall down. So they do.
Lasers manipulate their medium in such a way, that the pyramid briefly (or continuously attemps to) inverts. Suddenly you have a lot of electrons on top that want to fall down.
Of course here the analogy breaks down a bit, because a physical pyramid of balls would immediately collapse. But electrons can have a delay. Still, the slightest nudge will bring it down. Interestingly, when a passing photon does so, the emitted photons aren’t random. They’re going to be closely matching the trigger photon. That’s why lasers emit a beam of (nearly) monochromatic light.
If an electron is in an excited state, it can drop down to a lower energy state and release a photon. This usually happens on its own (spontaneous emission), and is the principle behind fluorescent lamps, white Leds and so on.
However, it it can also be triggered by another photon, even if that photon is not absorbed. The photon is a disturbance in the electromagnetic field and it cam disturb the electrically charged electron enough to trigger a drop in energy level. This is called stimulated emission. Interestingly, in stimulated emission, the emitted photon gets synchronised with the incoming photon.
You don’t normally see much stimulated emission. That is because it is rare for electrons to be in an excited state. In most cases, if you do get a stimulated photon emitted, the chances are it will hit a ground state atom, and get absorbed again.
In a laser, the trick is to pump up the gain medium, so that the majority of electrons are excited, and it is rare to have electrons at ground state. This is called population inversion. In this context, the stimulated emission results in the amplification of light in the medium.
Pumping the laser medium was historically done with exceedingly bright light, using flash lamps similar to a professional photo flash. However, this is not the only way and it can be done with electrical current in semiconductor lasers or gas lasers.
Lasers are often not crystals. The atoms are energized *somehow* (exactly how varies).
They are *stimulated* to *emit* that energy as light. This is the “SE” part of a “LASER”. This step is key. When a photon passes through, it causes the atom to release its energy and create another photon. As the light bounces in the cavity it frees more and more photons all moving in the same direction, finally creating the beam.
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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