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In both cases I see few low energetic levels, few upper energetic levels, some no-emission transitions between upper levels and main downwards transition called either fluorescence or laser.

Here is an example from article fluorescence:

enter image description here

And here is from YAG:Nd laser (four level scheme):

enter image description here

Are laser materials and fluorescent materials completely interchangeable or there are qualitative and/or quantitative differences between them?

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In laser materials there is actually fluorescence happening. Laser users call it spontaneous emission. The photon emitted by fluorescence has a random direction and his wavelength will be in the emission spectrum.

But the mechanism that is mainly occurring in laser is stimulated emission. To occur, stimulated emission need an excited medium and an incoming photon which will stimulate the emission and create an identical photon in terms of wavelength and direction. It is the explanation of the spatial and spectral property of laser light. Have a look at https://www.rp-photonics.com/stimulated_emission.html .

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  • $\begingroup$ Can't I take any fluorescenting material, then enlight it with appropriate wavelength and get stimulated emission? Since it fluoresces hence it has some excited electrons. Can't I stimulate them to de-excite? $\endgroup$ – Dims Dec 6 '19 at 16:40
  • $\begingroup$ No. You can make fluorescence with any laser material. But to make stimulation emission you need population inversion. See the DakkVader answer. $\endgroup$ – Aliceo Dec 6 '19 at 21:37
  • $\begingroup$ The system of terms is the same, look at the picture. I will pump material with a frequency, depicted with black arrow. Electrons will be excited, then go down by no-radiative transitions. Level S_1_0 will be overpopulated. Level S_0_2 will be underpopulated, because electrons will travel to S_0_0. I will be population inversion, won't it? Then I will enlight material with frequency, depicted with green arrow and it will be amplified. Where I was wrong? $\endgroup$ – Dims Dec 7 '19 at 7:12
  • $\begingroup$ In the fluorescence scheme you have a two energy level system (bold horizontal lines). You cannot make population inversion in a two level system. See the wikipedia article on population inversion: en.wikipedia.org/wiki/Population_inversion . In the YAG system there is 4 energy level. You can make population inversion and consecuently amplification. $\endgroup$ – Aliceo Dec 7 '19 at 9:53
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In a laser the process that is dominating is called stimulated emission as the name suggests, Light Amplification by Stimulated Emission of Radiation. This is possible when the medium is pumped enough to have more excited electrons in higher states than in the lower state. If $N_{lower} > N_{higher}$ we will have absorption instead. When $N_{lower} < N_{higher}$ we have what is referred to as population inversion. The threshold for lasing.

In a fluorescent material the process that dominates is spontaneous emission which doesn't require population inversion.

This fundamental difference is what sets them apart, other than that they are as you say quite similar.

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  • $\begingroup$ @PM2Ring Don't see how that answers his question but i edited and included this information as well. $\endgroup$ – DakkVader Nov 26 '19 at 9:13
  • $\begingroup$ Well, you aren't going to get much laser action if the medium isn't excited enough for a population inversion, are you? ;) OTOH, I guess you can locally excite a fluorescent material by passing a UV laser beam through it. $\endgroup$ – PM 2Ring Nov 26 '19 at 9:17
  • $\begingroup$ Yeah, that'd do the trick haha Thanks anyway for the input, no harm in clarifying things up. @PM2Ring $\endgroup$ – DakkVader Nov 26 '19 at 10:34
  • $\begingroup$ Are there any OTHER difference, except do we do pumping or not? Materials itself do know if they are fluorescenting or lasers? $\endgroup$ – Dims Dec 6 '19 at 16:42
  • $\begingroup$ @Dims It would depend to what degree you'd like to take the example. This is the most general and fundamental difference. For example a diode laser is closer to flourescence as the seed laser that produces the stimulating photons (the one usually incident on a pumped laser medium) is internally produced originally spontaneous emission. In the case of a diode laser the cavity decides which modes survive and thus it begins to lase. A flourescent material has no cavity typically fo r example and would not be called a laser. And has been said before, the dominating process is spontaneous emission. $\endgroup$ – DakkVader Dec 9 '19 at 7:01

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