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B Stimulated emission (LASER).

  1. Feb 19, 2017 #1
    The principle of operation of the laser is based on stimulated emission concept. In short, if a suitable energy photon hits the electron of an atom, in which the electron is in the excited state, there is a probability that the electron returns to the ground state. The photon emitted has the exact same phase of the photon that has stimulated.

    This appears to be an acquired principle, but I find no rigorous proof of why the two photons must be in phase with each other. A "heuristic" reasoning might be as follows. The electron should absorb the incident photon and jump to the higher level. Since there's a not higher level, you could say that the electron absorbs "virtually" the incident photon and then emit it instantly. At this point, just as instantly decay by emitting at the fundamental level coupled with the first the second photons. It is as if emits two photons simultaneously in which one is the exact copy of the other.

    Can this reasoning, although descriptive have a foundation.
    Last edited by a moderator: Feb 19, 2017
  2. jcsd
  3. Feb 19, 2017 #2


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    It is a result of the calculations. It can't get more rigorous than the mathematics.
  4. Feb 19, 2017 #3
    can you give me the track or as you set the calculation?
  5. Feb 19, 2017 #4


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    Textbooks should cover that, the original publications will certainly cover that as well. I don't have a link, but I don't think it is hard to find a derivation. It is certainly not highschool-level, however.
  6. Feb 19, 2017 #5
    I have several advanced texts of atomic physics and structure of matter, but the subject, specifically is not treated. Somewhere I read that one approach could be to the perturbation theory, but I find nothing exact.
    thank you anyway
  7. Feb 19, 2017 #6


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    See Introductory Quantum Physics , Knight and Gerry, Cambridge (2005), page 82. I don't know if it answers your question in full.
  8. Feb 20, 2017 #7
    Thank you very much
  9. Feb 21, 2017 #8
    In all the texts I have consulted, including the one recommended, I found nothing that would clarify explicitly because in the case of stimulated emission, the two photons are perfectly in phase with each other
    It seems an obvious principle, or assumption.
    I gave the following explanation.
    Suppose the case of spontaneous emission. The electron decays from the upper level to the lower level in an average time ## \tau ##. Its phase may be any. So in a material that has been excited and left to decay spontaneously, it produces incoherent light.
    Suppose now the case stimulated emission. Suppose also that the emitted photon has a phase difference with the incident photon. This phase difference ## \Delta \psi ## can assume any value from 0 to ## 2\pi ##. So we have in our process another degree of freedom that is completely arbitrary.
    Now how is it demonstrated the probability of electrostimulated decay does not depend on ## \Delta \psi ##. In other words you do not see why ## \Delta \psi ## should be a value or another, without a reason.
    then the simplest thing is that ## \Delta \psi ## = 0
    In another way, it can be said that the spontaneous decay is statistically random, while in the case of stimulated emission it is the effect of a cause.
  10. Feb 21, 2017 #9


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    That is a bit contrived, and the last sentence is incorrect, since stimulated emission is also a random process from a QM point of view.

    Without going into QED, the simple picture is that the atom acts like a small dipole being oscillated by the external field, and therefore will contribute to that field in phase with the stimulation.
  11. Feb 21, 2017 #10
    It is. In fact speak of causal event in MQ is not correct. for a moment I was deceived ... The stimulated emission also takes it with probability rate. However, or describes a theory with mathematical formulas and physical principles (symmetry, etc.) or is not satisfactory, or, perhaps this prof does not exist. perhaps in QED, there is the rigorous argumentation
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