While studying stimulated emissions, I was wondering how could a photon introduce directionality into another photon emitted by an excited electron just by mere stimulation.

How exactly does this 'stimulation' occur and how can this introduce directionality, because the stimulating photon and the emitted photon travel in the same direction.

[What I know is that if the excited electron(initially at rest) emits a photon, the photon can be emitted in any direction and the electron would recoil in the opposite direction to account for conservation of momentum in the system. But when stimulated emission happens why does the emitted photon travel in the same direction as the stimulating photon?]


1 Answer 1


The processes of stimulated absorption and emission of electromagnetic energy in quantized systems can be regarded as analogous to the stimulated absorption or emission of mechanical energy in classical resonating systems upon which a periodic mechanical force of the same frequency as the natural frequency of the system is impressed. In such a mechanical system, energy can be put in or taken out depending on the relative phases of motion of the system and the impressed force. The spontaneous emission process, however, is a strictly quantum effect. Quantum electrodynamics shows that there are fluctuations in the electromagnetic field. Because of the zero-point energy of the electromagnetic field, these fluctuations occur even when classically there is no field. It is these fluctuations that induce the so-called spontaneous emission of radiation from atoms in excited states.

In the ordinary atomic light sources there is a random relationship between the phases of the photons emitted by different atoms so that the resulting radiation is incoherent. The reason is that there is no correlation in the times that the atoms make their transitions. In laser light sources, on the other hand, atoms radiate in phase with the inducing radiation because their charge oscillations are in phase with that radiation. Since in a laser the inducing radiation is a coherent parallel beam formed by reflection between the ends of a resonant cell, the emitted photons are all in phase and act coherently.In practical devices the beam is unidirectional because of the coherence property which makes it possible to obtain essentially perfect collimation or focusing.

  • $\begingroup$ thanks! So is there a definitive equation describing the cause stimulations or rather that of the 'stimulation' effect'? Also by time reversal symmetry, if the stimulating photon and the emitted photon(let's assume they are in the same direction) are brought back to the point of collision with the parent electron(which emitted the photon due to stimulating effect), the electron has a choice and cannot distinguish between the two photons; hence, is it not that the principle of the stimulating effect does not time-reversally symmetric? $\endgroup$ Oct 19, 2016 at 18:56

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