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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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.
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