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This Thorlabs VRC4 card here uses a material that absorbs only NIR and emits in visible. The spec sheets don't list what chemical this is. Do you know materials that show similar emission properties even with weak laser excitation?

Karl
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user1155386
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    The pdf says ".. of a UV light beam and its focal ..", but the specs make it clear its actually IR. ;) One should say this must be an IR laser, and the visible light is just a faint glow. – Karl Aug 29 '19 at 06:29
  • Not possible except for non linear phenomena. See comment above. – Alchimista Aug 29 '19 at 08:55
  • @Karl Yep, they have a typo – user1155386 Aug 29 '19 at 19:13
  • @Alchimista Im not sure it must be an actual nonlinear effect. See my comment to the answer below. – Karl Aug 29 '19 at 19:37
  • @user1155386 Dont you want to ask about possible processes for this behaviour? – Karl Aug 29 '19 at 19:43
  • related: https://chemistry.stackexchange.com/questions/77410/is-it-possible-for-a-substance-to-absorb-a-longer-wavelength-of-em-wave-and-emit?rq=1 – Karl Aug 29 '19 at 19:53
  • @Karl TPA is a third order process. – Alchimista Aug 31 '19 at 08:32
  • @Alchimista TPA? – Karl Aug 31 '19 at 20:31
  • @Karl two photons absorption but I wanted to say second harmonic generation – Alchimista Sep 02 '19 at 08:52
  • @Alchimista Ja, but under a reasonably strong laser source, you might have a well populated $n=2$ state. From which you could easily get into a $n=3$ state, which decays to $n=1$ via visible fluoresence. You just need a substance where the lowest electronically excited state is within the IR energy range. Im a bit confused about the spectrum of this sensor sheet, which shows three IR absorption peaks. – Karl Sep 02 '19 at 20:59
  • @Karl The link doesn't work. But I see what you mean, it seems you have in mind something like azulene that in spite of being deeply blue and absorbing along the vis, has some transitions that from nir lead to vis emissions but again is a subtle mechanism based on states lifetime and I would say classifies as TPA. Also your mechanism should be non linear, at least if one does not start the analysis from the already excited state. But I think it is hardly what the op had in mind. In my opinion op could be satisfied with the answer by matt_black. And for details the link you have provided. – Alchimista Sep 03 '19 at 09:28
  • @Alchimista matt_black's answer does answer my question but partly. Green laser pointers employ strong IR sources that are then fed to SHG crystals. This card from Thorlabs seems to work even with weak to moderate laser excitation. So I was looking for chemicals that showed the same behavior but even with weak excitation. I edited the question to reflect this. – user1155386 Sep 03 '19 at 15:42
  • Check out https://www.rp-photonics.com/laser_viewing_cards.html, the IR light triggers phosphorescence of the card after it has been "charged" with visible light. – Paul Sep 03 '19 at 20:36
  • @Alchimista I updated the link, check under "graphs" there (VRC4 card). – Karl Sep 03 '19 at 22:01
  • @Paul Interesting link! Although the description specifically states: "As it is not necessary to charge the active region of the card before use, either CW or pulsed incident light will generate emission, even when the card is used in a darkened room." Maybe this does not employ some charging process. – user1155386 Sep 04 '19 at 05:43
  • It seems that the card either use a tpa mechanism or relies on the excitation of already excited states, as suggested by @Karl. I am not sure now if the latter classifies as non linear process, as Karl pointed out. First order parameters should be involved. However also a sequential abs mechanism implies that several photons are absorbed to leads to one photon emission. – Alchimista Sep 05 '19 at 09:42
  • Also posting to Physics SE might lead to an answer readily than here. – Alchimista Sep 05 '19 at 09:43

1 Answers1

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Such materials not only exist but are common

Most people will have seen an actual demonstration of materials that exploit the phenomenon of a substance that absorbs NIR light and emits visible light. Most Green laser-pointers are based on their use.

These usually rely on a semiconductor laser outputting near-infra-red (NIR) light at 1063nm wavelength which then passes through a crystal of KDP (mono potassium phosphate) or KTP (potassium titanyl phosphate) which emits green light at 532nm (which is the second harmonic of the original laser frequency). The mechanism involves non-linear optical processes (which usually require a fairly high intensity of light so won't happen as easily as phosphorescence, for example).

matt_black
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  • Like you said, the efficiency with SHG materials isn't that great, requiring high power lasers. I was surprised to see this card working with nominal power lasers as well, and I wasn't sure what kind of material this used. – user1155386 Aug 29 '19 at 19:12
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    Have you looked at the light curve they show? The card works at three different IR wavelenghts, but emits only at one VIS wavelenght. Also they explicitly say it is based on fluorencence. Could this be a double photon (or sequential) absortion, followed by vibrational relaxation and fluorencence? – Karl Aug 29 '19 at 19:35
  • @Karl Aren't double photon absorption cross sections quite small? Know of any material that has a large 2 photon absorption cross section? – user1155386 Aug 29 '19 at 21:47
  • @user1155386 Yes, they are,, I guess. Thats why I added the sequential absortion. The upper state of a first fluorescent transition is well populated under laser light, so you could easily get into a higher excited state. – Karl Aug 30 '19 at 06:44