Stimulated and spotaneous emission

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Main Question or Discussion Point

Why the light emitted in spontaneous emission is poly chromatic whereas the light in stimulated emissions is monochromanti?

If E1 and E2 be two energy level such that E2 >E1, in both emission the energy difference is fixed, so the frequency and so the wavelength thus the light emitted in both case should be monochromatic but why the light emitted in stimulidated emission is alone monochromatic ?
 

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Simon Bridge
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It's not. They may both be polychromatic if you want it to be.

A transition between a particular pair of energy states will always give the same wavelength ... but you don't normally get just one pair of energy states or just one photon.

You get a variety of colors when there are many different pairs of states involved producing lots of photons.

We usually build the devices relying on stimulated emission to be as monochromatic as possible.
With stimulated emission you can select which pair of states to use - the effect is particularly strong when your feed some of the stimulated photons back through the medium - as in a laser.

He-Ne gas (for instance) can be made to lase at a variety of different frequencies and steps have to be taken to suppress lasing at some of them.
 
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If E1 and E2 be two energy level such that E2 >E1, in both emission the energy difference is fixed, so the frequency and so the wavelength thus the light emitted in both case should be monochromatic
Yes, that is true. Assuming a jump between two specific energy levels, the frequency of the light be identical in both cases (monochromatic).

In spontanious emission the phase and the direction will be random. In simulated emission they are the same.

Maybe you can get various other frequencies from spontanious emission because other energy levels are involved. This does not happen with stimulated emission because the parameters are set (controlled) for only a single frequency.

[Edit] Didn't see Simon's post when I replied. What he said...
 

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