# Stimulated emission in LASER

I have been reading about how lasers function : A photon is used for stimulated emission of electrons from the metastable state to a lower energy state. What I don't understand is : How can "giving" energy (in the form of photons) to electrons stimulate them to come to a lower energy state ? After stimulated emission , the old photon exists along with the new one and with the same energy as earlier, so how did it actually stimulate the electrons to fall ?

PS : I have just begun to learn phy, so pls ans as simply as possible :)

• I'm no laser physicist, but the key is having a population inverse in the gas. When you maintain more than half of the gas particles are in an electrically induced excited state, the few that do decay to lower states emit photons. Those photons then have a greater than 50% chance to hit an already excited particle and make it drop to the lower state, leaving you with 2 photons. If less than 50% of the particles are excited, any photons are more likely to hit non-excited particles and excite them, which means no net gain of photons.
– Jim
Jul 14, 2015 at 15:59
• So you have to use electricity to excite more than half the population of the gas and then you get stimulated emission when photons interact with already excited particles. But a proper answer needs more details than that. So I'll let the laser physics people take it from here
– Jim
Jul 14, 2015 at 16:01
• I think it comes from the fact that photon are Bosons and thus follow a statistical law that kinda help/force them to be in the same state (as opposit for the electron who cannot be in the same state at the same position). Sorry if it's kind of vague but I think that's the kind of answer you want. I'll let someone more expert answer in more detail Jul 14, 2015 at 16:46
• Stimulated emission can also be explained without the concept of photons, see Fain, B.; Milonni, P. W. (1987). "Classical stimulated emission". Journal of the Optical Society of America B 4: 78, dx.doi.org/10.1364%2FJOSAB.4.000078 Jul 14, 2015 at 19:31
• @jims bond thanks ! Well, the thing that's most confusing to me is how photons can make electrons fall by hitting them ? Shouldn't hitting them actually make them more energetic ? Jul 15, 2015 at 0:37