How does stimulated emission work

A single photon interacting with an excited atom can therefore result in two photons being emitted. If the emitted photons are viewed as a wave, the stimulated emission will oscillate at the incoming light's frequency and be in phase coherent , resulting in amplification of the original light wave's intensity.

Figure 3 illustrates spontaneous a and stimulated b emission with the two coherent waves that result from the latter case. The primary problem in achieving stimulated laser emission is that, under normal conditions of thermodynamic equilibrium, the population, or number of atoms or molecules at each energy level, is not favorable to stimulated emission.

Because of the tendency of atoms and molecules to spontaneously drop to lower energy levels, the number at each energy level decreases as the energy increases. In other words, virtually all of the atoms or molecules are in the ground state for a visible-wavelength energy transition.

The reason that stimulated emission is difficult to achieve becomes apparent when considering the likely events surrounding the decay of an electron from an exited state with the subsequent and spontaneous emission of light. The emitted light could easily stimulate emission from another exited atom, but so few are available that the emission more likely will first encounter an atom in the ground state, and will be absorbed instead.

Because the number of atoms in an exited state is so miniscule in relation to the number in the ground state, the emitted photon has a much greater probability of being absorbed, rendering stimulated emission insignificant when compared to spontaneous emission at thermodynamic equilibrium. The mechanism by which stimulated emission can be made to dominate is to have more atoms in the excited state than in the lower energy state, so that emitted photons are more likely to stimulate emission than to be absorbed.

Because this condition is the inverse of the normal equilibrium situation, it is termed a population inversion. As long as there are more atoms in the upper energy level than in the lower, stimulated emission can dominate, and a cascade of photons results.

The first emitted photon will stimulate the emission of more photons, these subsequently stimulate the emission of still more, and so on. The resulting cascade of photons grows, resulting in the amplification of emitted light. If the population inversion terminates the ground state population becomes dominant , spontaneous emission will again become the favored process.

Matthew J. This process is called spontaneous emission. It is also possible that the emission is stimulated by incoming photons, which is called stimulated emission. The emission then goes into the same direction as the incoming photon.

In effect, the incoming radiation is amplified. This is the physical basis of light amplification in amplifiers and lasers. Of course, stimulated emission can only occur for incoming photons that have a photon energy close to the energy of the laser transition. See also our privacy declaration. If you wish to receive personal feedback or consultancy from the author, please contact him e.

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If you like this page, please share the link with your friends and colleagues, e. These sharing buttons are implemented in a privacy-friendly way! Sorry, we don't have an article for that keyword! An incoming photon stimulates an excited atom or ion to undergo a transition from the excited state to the ground state.

Questions and Comments from Users How can a photon sent to an excited atom cause emission of another photon rather than causing the electron to get even further excited to the next energy state? Answer from the author : If there is a suitable higher energy state, such excited state absorption can indeed happen. Answer from the author : The stimulation of the mission does not cost energy.

Answer from the author : Light interacts with atoms or ions, precisely speaking not only with single electrons within those. Answer from the author : The first question is somewhat philosophical. The reason for this is that an EM wave has an oscillating electric field and you have to include the electric field in the Hamiltonian.

Exactly the same argument applies. The probabilities of the various outcomes can be calculated using perturbation theory and specifically Fermi's golden rule.

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