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It turns out that photomultiplier tubes can also see into the infrared, even when they're not supposed to. We built a PMT-based detector that was sealed up and

It turns out that photomultiplier tubes can also see into the infrared, even when they're not supposed to. We built a PMT-based detector that was sealed up and so we thought it'd be nice to have a camera on it. We went with an IR camera and IR illumination so we could have the camera on while the detector was running. We chose the wavelengths carefully so that the entire peak of the IR LED emitters would be in the "no response" zone of our PMTs. Turns out that, no, you could totally tell when those LEDs were on despite that being "impossible". We didn't do any research into this because it was completely feasible to just not run the camera and the detector at the same time, so we did that. (It also wasn't hard at all to choose an IR camera in the first place, so not a lot got wasted here.) PMTs are a little different than many other light detectors (this is how they get single-photon sensitivity!) but the hypothesis of photons doubling up seems completely reasonable. Perhaps we should have placed the IR emitter wavelength at at least twice the maximum sensitive wavelength of the PMTs? But then these detectors probably would have been sensitive enough to notice triple coincidences, so who knows....

exmadscientist 3y ago
The whole thing did in fact get written up here, though it doesn't talk much about the PMTs' IR sensitivity: https://arxiv.org/abs/1204.3500 https://arxiv.org/abs/1204.3500 The money plots for this discussion are Figure 7 ("this will totally work!") and Figure 9 ("this totally doesn't work!").
nonrandomstring 3y ago
Everyone can see those cheap security camera IR LEDs glow red? Right?
- tines 3y ago
  Yes, but that's more likely to be the LED emitting frequencies outside the range it's supposed to.
- NavinF 3y ago
  Yes and everyone can see 1310nm communication lasers (such as SFP modules) emitting red light when looking directly into one. It's because most IR lasers and LEDs are not really monochromatic. The visible light is very useful for debugging and there's no practical advantage in buying lasers or LEDs emit only one wavelength

exmadscientist 3y ago

The whole thing did in fact get written up here, though it doesn't talk much about the PMTs' IR sensitivity: https://arxiv.org/abs/1204.3500 https://arxiv.org/abs/1204.3500 The money plots for this discussion are Figure 7 ("this will totally work!") and Figure 9 ("this totally doesn't work!").

nonrandomstring 3y ago

Everyone can see those cheap security camera IR LEDs glow red? Right?

tines 3y ago
Yes, but that's more likely to be the LED emitting frequencies outside the range it's supposed to.
NavinF 3y ago
Yes and everyone can see 1310nm communication lasers (such as SFP modules) emitting red light when looking directly into one. It's because most IR lasers and LEDs are not really monochromatic. The visible light is very useful for debugging and there's no practical advantage in buying lasers or LEDs emit only one wavelength