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Let's assume that there is a cavity with a couple of mirrors and gain media between which possesses luminescence under some external excitation/pumping. Let the absolute quantum yield of the gain media be quite high - 50%, luminescence decay is approximately 100 µs and full width at half maximum is about 100 nm with maximum at 500 nm (something like blue-green light). All these parameters are quite real. So with such parameters what do we need to get lasing? And second question, what would the output spectrum look like?

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    $\begingroup$ For lasing you need a population inversion, which may not be achievable in all system. On the other hand, as a laser jock once told me, anything will lase if pumped hard enough (but that is because you will get non-linear effects, overcoming the issues causing the problem in the first sentence). $\endgroup$
    – Jon Custer
    Jan 15 '16 at 17:55
  • $\begingroup$ It's like with flying... you can make a barn door fly (I have seen that) or a lead cube (that I have not, but then, I have not been in the artillery), if you put a powerful enough engine on it. You don't need a cavity for a laser, either. The cavity is there to select a certain frequency, it's not essential for the amplification process. As for numerical predictions... that's not possible with your specifications. As Jon Custer said... one would need a level diagram of the system and a lot more information about the transitions. $\endgroup$
    – CuriousOne
    Jan 15 '16 at 18:18
  • $\begingroup$ In addition, in many cases luminescent emission goes down to the ground state, so absorption directly competes with stimulated emission. That is, the medium itself has high losses that must be overcome. $\endgroup$
    – garyp
    Jan 15 '16 at 18:22
  • $\begingroup$ @CuriousOne The cavity also provides feedback for oscillation. The oscillation function and the frequency-filtering function are, of course closely related, but you can have a filter without the sustained oscillation. $\endgroup$
    – garyp
    Jan 15 '16 at 18:25
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    $\begingroup$ garyp :curiousone is correct. A reasonably long lasing medium will generate a coherent output if well-pumped. It doesn't require a source oscillator as does a laser amp. $\endgroup$ Jan 15 '16 at 19:39
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Relevant link - not exact duplicate . It discusses the need for population inversion, and how to achieve it. A system with fewer than three (four if you want to limit power loss) energy levels is unlikely to be able to sustain population inversion, so while you could get "stimulated emission" it's unlikely that the output of such a medium would be coherent (there would be too many opportunities for spontaneous emission that destroy the coherence).

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  • $\begingroup$ Well, ok. That's fair. But if you noticed, I've pointed out that the luminecence decay time is 100 mks, which is quite long and means that we deal with phosphorescence and at least three lavel scheme. So it might be that I didn't emphasise that but my question is beyond the simple two level case. If your answer is still "the need for population inversion", that fine. But I wondered if there is something else $\endgroup$ Jan 15 '16 at 19:23
  • $\begingroup$ To be honest I skipped over that detail. I am not familiar with the abbreviation mks - milli kilo seconds sounds wrong. Can you please explain? Your title question was "every luminescent media", but you seem to be asking about a specific medium. $\endgroup$
    – Floris
    Jan 15 '16 at 19:26
  • $\begingroup$ yes, sorry. I mean microseconds $\endgroup$ Jan 15 '16 at 19:26
  • $\begingroup$ Yes, you are right. It's not exactly "every luminecent media", but in my experiance it's quite common parameters for liquids and solids that are able to glow, so that's why I discribed it like this $\endgroup$ Jan 15 '16 at 19:30
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    $\begingroup$ @CuriousOne: thanks, for your comment. It's very nice, but when you're talking about "specific mechanisms", that's exactly what I'm asking about. I'm just curious whether it is only a question of pumping power or not? For me it seems like one can increase the decay time from exited state and that'd be enough, but it still could be beaten by increasing the pumping power. In this way Jon Custer's friend is right. And of course we're not talking about any preactical laser, just the posibility to get lasing some lumiencent media. $\endgroup$ Jan 15 '16 at 20:04

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