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I am currently reading Practical Flow Cytometry, fourth edition, by Howard M. Shapiro. The author says the following when discussing scattering:

Scattering, which explains both reflection and refraction, typically involves a brief interaction between a photon and an electron, in which the photon is annihilated, transferring its energy to the electron, which almost immediately releases all of the energy in the form of a new photon. Thus, light scattered by an object has the same (or almost the same) wavelength, or color, as the incident light.

This reminded me of stimulated emission. But I remembered that, in photonics (such as the case of solid state lasers), the wavelength of the emitted light is different from that of the incident light (for instance, when a laser diode of some wavelength is used to pump some gain medium, which then emits light of totally different wavelength). So I sought to do further research to clearly understand the difference between these two phenomena.

Wikipedia describes stimulated emission as follows:

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.

Surprising to me, this description seems to agree with that of scattering, in which the new, emitted light has the same wavelength (since wavelength is proportional to frequency) as the incident light. But this seems to contradict with what I know about how stimulated emission works in the context of photonics.

So what am I misunderstanding here? Doesn't stimulated emission emit light of different wavelengths, depending on the medium/material? What is the difference between scattering and stimulated emission? I would greatly appreciate it if people would please take the time to clarify this.

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In the case of scattering the incident photon isn't (usually) interacting with an excited electron, so we don't have the basic requirement for stimulated emission.

In the case of scattering the incident photon is absorbed, raising the electron's energy level (in an atom or molecule). The electron spontaneously drops down from this level to its original level (in the simplest case), emitting a single photon of the same frequency as the original photon. This emitted photon does not (except by chance) have the same phase, polarisation direction or propagation direction as the incident photon – very different from stimulated emission.

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Scattering of light is a general term for event or process where the incoming primary wave (in the simplest case, plane wave of single direction) interacts with material medium (in macroscopic theory) or with molecules/atom (microscopic theory), and this interaction leads to creation of secondary wave spreading from the locus of the interaction in directions different from that of the primary wave, with large part going in directions perpendicular to the primary wave propagation (transverse directions).

Scattering can be elastic (frequency of secondary radiation is the same as frequency of the primary wave) or inelastic (it is higher or lower than frequency of the primary wave).

An usual case of scattering is elastic scattering for frequencies far from resonance, so it is a process that does not require resonance (although it changes in case the primary wave is resonant with the medium). For example, blue light of the sky is result of mostly elastic Rayleigh-Mandelstam scattering of sunlight on the spatial inhomogeneities of index of refraction. If the medium is dense and homogeneous on the scale of the radiation wavelength, such as pure glass or pure water in case of visible light, scattering is severely suppressed, as there is lack of inhomogeneities and the induced secondary waves largely cancel in transverse directions.

Stimulated emission is a special kind of process where light of some frequency interacts with a molecule or medium that is in excited state with some energy and that molecule or medium radiates secondary waves of the same or very close frequency.

In general, when incoming primary wave interacts with such excited material medium/molecules, it can either make the system radiate 180 degrees off-phase secondary wave and then energy from the primary wave gets absorbed by the system (stimulated absorption), or launch an in-phase secondary wave that adds up constructively to the primary wave (stimulated emission).

Direction of the stimulated emission radiation is predominantly in the same direction that the primary wave is propagating but some radiation goes to other directions as well. Probability of this process is large only if the primary wave has similar frequency to the natural frequency at which the medium will radiate. It is a resonant process.

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    $\begingroup$ Spontaneous emission is a special kind of process... you mean stimulated emission? $\endgroup$ – Claudio Saspinski Jul 3 at 21:17
  • $\begingroup$ Oops, you're right, fixed it. $\endgroup$ – Ján Lalinský Jul 3 at 22:31
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    $\begingroup$ Thanks for the answer. Do you have any sources that substantiate your claim that large part of the scattered light goes in directions perpendicular/transverse to the primary wave? I was unable to find sources that state this. $\endgroup$ – The Pointer Jul 4 at 22:15
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    $\begingroup$ When I wrote that I remembered wrongly that scattering to the transverse directions is stronger than scattering into the direction of the primary wave. In fact, the opposite is the case: intensity in the primary direction is twice the intensity in perpendicular direction. See eng.uc.edu/~beaucag/Classes/Properties/ZimmIASLightScat.pdf Fig.7.5 $\endgroup$ – Ján Lalinský Jul 4 at 23:26
  • $\begingroup$ The photons from stimulated emission are identical in phase and direction to the stimulating photon. The reference you cite in the comment above does not mention stimulated emission at all. $\endgroup$ – Rob Jeffries Nov 1 at 22:03

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