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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?

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    $\begingroup$ 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. $\endgroup$ – Karl Aug 29 at 6:29
  • $\begingroup$ Not possible except for non linear phenomena. See comment above. $\endgroup$ – Alchimista Aug 29 at 8:55
  • $\begingroup$ @Karl Yep, they have a typo $\endgroup$ – user1155386 Aug 29 at 19:13
  • $\begingroup$ @Alchimista Im not sure it must be an actual nonlinear effect. See my comment to the answer below. $\endgroup$ – Karl Aug 29 at 19:37
  • $\begingroup$ @user1155386 Dont you want to ask about possible processes for this behaviour? $\endgroup$ – Karl Aug 29 at 19:43
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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).

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  • $\begingroup$ 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. $\endgroup$ – user1155386 Aug 29 at 19:12
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    $\begingroup$ 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? $\endgroup$ – Karl Aug 29 at 19:35
  • $\begingroup$ @Karl Aren't double photon absorption cross sections quite small? Know of any material that has a large 2 photon absorption cross section? $\endgroup$ – user1155386 Aug 29 at 21:47
  • $\begingroup$ @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. $\endgroup$ – Karl Aug 30 at 6:44

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