You might have seen the absorption spectrum of ruby: it has distinct absorption peaks near 400nm and 550nm. Looking at its absorption graph, one could expect some fluorescence when excited from a ~450nm source - not as good as from a 400nm source, but absorption is still significant.

I've compared the fluorescence of a ruby rod when illuminated by 405 and 450-460nm LEDs. It appeared that fluorescence from a blue LED is extremely weak, many dozens of times weaker than from a 405nm LED. But from the absorption graph, the difference is not that great.

Does anyone have insight on why there is absorption, but almost no fluorescence of ruby when illuminated by a 450-460nm source? The same for a 532nm laser source (i.e. narrow band this time, but slightly higher energy than needed) - there is absorption, but almost no fluorescence.

Has someone seen fluorescence activation spectra in the literature? On the ruby state diagram I see that the main "up" transitions are activated by 420nm and 550nm photons, but it is not clear how wide the peaks are for crystalline (= almost perfect) synthetic ruby.

The goal is to find current / future semiconductor sources which would allow DPSS Ruby laser.

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    $\begingroup$ I don't understand your statement that there is no ruby fluorescence when excited by a 532 nm laser source. In static high-pressure physics we use 532 nm lasers all the time to excite the fluorescence of tiny ruby chips. There's lots of fluorescence with 532 nm excitation. Ruby fluorescence with 473 nm excitation is rather weak, but it gets much stronger at high pressures (i.e., pressures of 30 GPa and above). $\endgroup$
    – user93237
    Oct 21, 2017 at 23:19
  • $\begingroup$ @SamuelWeir There is fluorescence, but it is very weak when compared to 405nm excitation, many dozens of times weaker. It looks like it is some sort of non-radiative absorption. Are your ruby crystals somewhat pure? Does they fluoresce at normal pressure too? $\endgroup$ Oct 21, 2017 at 23:32
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    $\begingroup$ Yes, we use high quality synthetic ruby crystals. High quality alumina crystals measuring 20 microns in size or less with about 0.5% chromium as I recall. They glow brightly enough to be easily seen through a microscope with a red filter when hit by a focussed 5 mW laser at 532 nm. Have never tried 405 nm excitation. $\endgroup$
    – user93237
    Oct 22, 2017 at 2:11
  • $\begingroup$ @SamuelWeir I see your point then. They do glow red slightly and it is visible even without filter. $\endgroup$ Oct 22, 2017 at 13:38
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    $\begingroup$ The fluorescence from a 441 nm HeCd laser is strong enough to be useful, at a range of pressures under 500kbar, if that helps. There's easily visible fluorescence from 100um chips of ruby. $\endgroup$
    – Whit3rd
    Oct 24, 2017 at 7:59

2 Answers 2


So the fluorescence process is not a single step as are absorption or Emission with set energy levels. Fluorescence occurs in combination with radiation, energy loss in form of heat as well as phosphorescence sometimes. these all are depicted by a Jablonski Diagram whose picture is; https://prnt.sc/h8dnxy https://en.wikipedia.org/wiki/Jablonski_diagram

The internal conversion you see between absorption and fluorescence takes up energy and hence there is a lag of energy and fluorescence and absorption spectra don't match up. Similar the case for higher LED wavelengths. The issue with comparing fluorescence and absorption spectra is that they are two completely different processes with different mechanics. You cannot compare the two without taking in account the intricacies of the full process, have a look at the plots once again. You'll observe that even with excited states there is decline of energy. All this energy is also lost as heat or vibration which is not measured in a fluorescence spectra. Try comparing two fluorescence spectra that would give a more accurate picture.


From purely empirical plain sight observation, my synthetic laser ruby crystal glows bright red when excited with a 520nm green laser and also with a 450nm blue laser. The red emissions are quite obvious in broad daylight when using excitation laser power of 1 watt or greater.


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