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 :)

  • $\begingroup$ 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. $\endgroup$
    – Jim
    Jul 14, 2015 at 15:59
  • $\begingroup$ 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 $\endgroup$
    – Jim
    Jul 14, 2015 at 16:01
  • $\begingroup$ 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 $\endgroup$
    – EigenDavid
    Jul 14, 2015 at 16:46
  • 1
    $\begingroup$ 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 $\endgroup$ Jul 14, 2015 at 19:31
  • $\begingroup$ @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 ? $\endgroup$
    – biogirl
    Jul 15, 2015 at 0:37

1 Answer 1


I guess your question is: how the electrons "know" they should jump down when the photons come in? Well, stimulated emission is a theoretical discovery by Einstein. The strict explanation requires quantum mechanism. A simple explanation is the electron at the upper level is too lazy to think about where should it jump. So it just follow the incoming photon, release the exact same energy and fall. If there is no incoming photon, it actually releases a random energy, which is called "spontaneous emission". If the electron at the lower level wants to jump back, it has to capture a photon, which is called "absorption". To amply the light, you will want to have more electrons at the upper level, which is called "population inversion".

Hope this helps.


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