Background / Experiment

I was surprised by this toot by @gigabecquerel, where the author shows red fluorescence in the thick bottom of a (drink) glass when exposed to a cheap consumer-grade green laser pointer laser. They also verified that it's not some kind of selective scattering or such of a minor red component in the original laser, by using a diffraction grating to definitely select a monochromatic beam.

I was able to reproduce these results (no filtering, though).

I was able to show this red fluorescence for

Thick-walled drinking glasses
Thin-walled -"-
Glass jugs
Glass bottles

I've not been able to reproduce with heat-resistant glassware (i.e., a glassware casserole dish), but that might be because of all walls being curved and it thus being hard to couple in a beam at the right angle without scattering too much green light, making the red undetectable.

I tried but could not reproduce with saturated sugar water (tested that for the refractive index being closer to that of glass, in hopes I could use its surface to more cleanly couple in beams). I didn't explicitly test water, but it would have struck me, if it flouresced in red, I'd think.

@gigabecquerel went on and showed stronger red fluorescence in organic material – olive oil. That threw all of my "dopants in partially cristalline lattice" theories out the window. That is, of course, assuming it's the same mechanism.

Question

What is the mechanism by which red light gets emitted by household glass when hit by a 532 nm laser?

Best Answer

The reddish colour produced by a green laser pointer passing through common glass objects is due to excitation of luminescence centres produced by the particular composition and matrix structure of the glass.

Fluorescence is when light excites a system that then almost immediately emits a lower energy photon. The system can be an atom, a molecule such a chlorphyll, or a luminescence centre in a crystal lattice or glass matrix. What typically happens is that a high energy photon excites the system which then quickly transitions non-radiatively (e.g. through vibrational transitions) to a lower energy state which, in turn, radiatively de-excites emitting a photon.

As mentioned by @Ed-V in the comments, the red fluorescence from olive oil is due to chlorophyl. Chlorophyll fluorescence was first reported by David Brewster in his 1833 article "On the Colours of Natural Bodies". Fresh extra virgin olive has a high chlorophyll content and fluoresces red (680 nm band) under a green laser, but poor quality or degraded olive oil will fluoresce yellow. See, for example: "Fun with fluorescence in olive oil", "Deeper Insight into Fluorescence—Excitation of Molecules by Light", or "Fluorescence Spectra Measurement of Olive Oil and Other Vegetable Oils".

Pure soda–lime–silica glasses are not fluorescent, but as also mentioned by @Ed-V, the reddish fluorescence in your glass is likely due to decolourizing agents acting as fluorescent "activators". Decolourizors are traditionally needed because iron (as Fe$_2$O$_3$) is an ubiquitous contaminant that gives a green tint to common glass, and for centuries manganese oxide has been added to glass to decolourize it. Modern glass may be decolourized by selenium or cerium, but I believe the essential physics and chemistry are all similar.

According to Kreidl, the fluorescence of manganese-doped glass is due to the transitions of Mn$^{2+}$ ions embedded in the glass matrix. Mn$^{2+}$ ions do not fluorescence when dissolved in water, but when dissolved in glass they can create luminescence centres that can fluorescence under ultraviolet in colours ranging from green to red depending the composition and structure of the surrounding glass matrix. Greenish fluorescence occurs when the Mn$^{2+}$ ions form part of the (mostly) SiO$_2 $ glass network, while red/orange fluorescence occurs when the Mn$^{2+}$ occupy more loosely bound interstitial locations. The exact colour emitted depends on the energy states of the luminescence centre formed by each Mn$^{2+}$ ion interacting with its specific neighbours.

For more detailed discussion of Mn$^{2+}$ in glass, e.g. which specific transitions may be involved, check out Section 1.4.2 of Pierre Mênassa's PhD thesis.

Since manganese is no longer common in modern glass, the actual cations producing the reddish fluorescence are likely not Mn$^{2+}$. Selenium doped glass fluoresces pink or orange, which I think is also consistent with what you observe (and with my own quick experiment with a green laser pointer and some glass tumblers). I can't find much information on cerium doped glass luminescence, but the peak of its fluorescence may be in the blue (see Marczuk or Herrmann et al).

It would be interesting to see what colour fluorescence is produced in your glass by UV radiation. (This might be hard to see, however, with typical consumer UV lamps.) Do the higher energy UV photons excite other colours? For example, a contribution from Mn$^{2+}$ (or other cation) green band might turn the combined fluorescence yellow.

Finally, I would not be surprised if your casserole dish does not visibly fluoresce. Such cookware may be made of borosilicate glass. I don't know the composition or quality of the glass in your dish, but I do know that high quality borosilicate glass is often used in fluorescent microscopy precisely because of its very low autofluorescence.