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A Textbook of Engineering Physics by Avadhanulu, Kshirsagar says the following about Spontaneous Emission:

Photons of slightly different frequencies are generated. As a result, the light is not monochromatic.

It says the following about Stimulated Emission:

The spread of photon frequencies is relatively very narrow. As such the light is nearly monochromatic.

I do not understand why the frequency range of spontaneous emission will be wider and frequency range of stimulated emission will be narrow.

My friend told that the electrons from meta-stable states may also participate in Spontaneous Emission, thus making the light not monochromatic. But wouldn't that hold true in case of Stimulated Emission too?

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The rule of thumb is that with stimulated emission, an exact replica of the photon is created. Now this is not the exact truth, but is turns out to be a good approximation.

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field.

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    $\begingroup$ Ah, I understand how stimulated emission will produce photons of a very narrow frequency range. Is my understanding of how spontaneous emission has a wider frequency range because of the meta-stable states correct? $\endgroup$ – Siddharth Venu Oct 3 '18 at 18:58
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There is an inevitable Doppler broadening due to thermal motion in the gas; these even appears in solid state lasers.

As your friend stated, the population of excited states is never uniform; entropy is working against you.

However, it is always possible to make the lasing line a bit narrower, but this requires more purity in the medium, improved temperature control, and precision in the excitation.

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  • $\begingroup$ Is my understanding of how spontaneous emission has a wider frequency range because of the meta-stable states correct? $\endgroup$ – Siddharth Venu Oct 3 '18 at 19:00
  • $\begingroup$ You would need to study the line profiles to see if the broadening is due to a series of close lines, or if it is dominated by Doppler shifts. The statistics are quire clear, and you will study this in a quantum optics class. $\endgroup$ – Peter Diehr Oct 4 '18 at 19:11

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