2 ms·

Okay, so it's about two photons hitting the same molecule in quick succession. Why don't we see continuous infrared light, then? Surely light without pauses ha

Okay, so it's about two photons hitting the same molecule in quick succession. Why don't we see continuous infrared light, then? Surely light without pauses has way more ability to send two photons one after another than light with pauses.

4ad 3y ago
> Surely light without pauses has way more ability to send two photons one after another than light with pauses. No it's the opposite, SHG[1] requires huge spatial energy density which also means huge temporal density (power). A continuous wave of the required power would obliterate your retina (and your head). [1] https://en.wikipedia.org/wiki/Second-harmonic_generation https://en.wikipedia.org/wiki/Second-harmonic_generation
- z2h-a6n 3y ago
  It's not discussed in the linked article, but the paper it's based on argues against second harmonic generation and in favor of a two photon absorbtion process [1]. I'm not enough of an expert to comment on the rest of what you said, but I think I agree (based on skimming the paper) that the process still requires a high photon flux (i.e. high power desity). [1]: https://www.pnas.org/doi/full/10.1073/pnas.1410162111 https://www.pnas.org/doi/full/10.1073/pnas.1410162111 ; See second paragraph, and/or search the text for "SHG".
z2h-a6n 3y ago
4ad's comment has the right general idea. To explain in more detail: They compared pulsed light to continuous light with the same average power [1]. This means that the light intensity within the flux was much higher than the intensity of the continuous (since the pulses were short compared to the repetition rate). The proposed mechanism (and their result) depends on the square of the light intensity (because it requires simultaneous absorbtion of two photons), so the effect is scales quickly with increasing peak laser power, but (I'm guessing here) doesn't depend as strongly on average laser power. Additionally, I expect 4ad was right that sufficiently-intense continuous light would damage our eyes. My guess is basically that the timescales of two photon absorbtion and human perception are so different (i.e. two photon absorbtion is much faster), that the effects of the pulsed beam (which was repeating at 76 MHz in the experiment) can be treated basically continuous when considering human perception. [1]: https://www.pnas.org/doi/full/10.1073/pnas.1410162111 https://www.pnas.org/doi/full/10.1073/pnas.1410162111

4ad 3y ago

> Surely light without pauses has way more ability to send two photons one after another than light with pauses. No it's the opposite, SHG[1] requires huge spatial energy density which also means huge temporal density (power). A continuous wave of the required power would obliterate your retina (and your head). [1] https://en.wikipedia.org/wiki/Second-harmonic_generation https://en.wikipedia.org/wiki/Second-harmonic_generation

z2h-a6n 3y ago
It's not discussed in the linked article, but the paper it's based on argues against second harmonic generation and in favor of a two photon absorbtion process [1]. I'm not enough of an expert to comment on the rest of what you said, but I think I agree (based on skimming the paper) that the process still requires a high photon flux (i.e. high power desity). [1]: https://www.pnas.org/doi/full/10.1073/pnas.1410162111 https://www.pnas.org/doi/full/10.1073/pnas.1410162111 ; See second paragraph, and/or search the text for "SHG".

z2h-a6n 3y ago

4ad's comment has the right general idea. To explain in more detail: They compared pulsed light to continuous light with the same average power [1]. This means that the light intensity within the flux was much higher than the intensity of the continuous (since the pulses were short compared to the repetition rate). The proposed mechanism (and their result) depends on the square of the light intensity (because it requires simultaneous absorbtion of two photons), so the effect is scales quickly with increasing peak laser power, but (I'm guessing here) doesn't depend as strongly on average laser power. Additionally, I expect 4ad was right that sufficiently-intense continuous light would damage our eyes. My guess is basically that the timescales of two photon absorbtion and human perception are so different (i.e. two photon absorbtion is much faster), that the effects of the pulsed beam (which was repeating at 76 MHz in the experiment) can be treated basically continuous when considering human perception. [1]: https://www.pnas.org/doi/full/10.1073/pnas.1410162111 https://www.pnas.org/doi/full/10.1073/pnas.1410162111