Apparently the LED is on the order of 500 nm. Isn’t that essentially the same size as the actual wavelengths? (Just skimmed the article maybe this is discussed)
* Ok, actually pressed buttons that manipulated the electric field that was trapping the atom and watched the result on a display - lot of physics going on behind the scenes.
I would expect this to be somewhat of a problem with tiny LEDs. In an LED, you inject electrons and holes and you hope that a magical quantum process happens in which an electron and a hole meet, annihilate each other, and emit a photon. But this process is slow, and the electrons and the holes may wander around for a bit before combining. But in a very very small LED, smaller than the mean free path, I’d imagine you might have an issue where the electrons and holes frequently make it all the way across the device without recombining and manage to lose their energy as heat when they hit the opposite electrode. (I have not drawn the diagrams or checked the math here.)
(I took the relevant classes in grad school, but I’ve never done this sort of work academically or professionally, so no promises that I’m right.)
Not necessarily. Chiral gold nanocrystals can be as small as 10nm and still be excited by 808nm laser light causing two-photon absorption and emitting in the visible range.
However this reciprocity is frequently circumvented, because atoms and ions have a lot of energy levels. Instead of re-emitting the light, the atom may pass more quickly to another energy level, and from there it may emit light with a very different probability (and of a different frequency, i.e. this is fluorescence).
While in fluorescence light with a lower frequency is emitted, there is also the opposite case. In very intense light, e.g. from lasers, multi-photon absorption may happen. In that case there is also no reciprocity, because the atom has jumped an energy difference higher than that of the incoming photons. So it may re-emit light with a higher frequency.
With rubidium atoms, multi-photon absorption is very frequently used, for Doppler-effect-free spectroscopy (by absorbing photons that come from opposite directions, so that the effects of the movement of the atom will cancel). In comparison with other atoms, rubidium vapor cells are easy to procure, for spectroscopy experiments, or for use in frequency or wavelength standards, but they still are rather expensive, especially when enriched in only one of the two rubidium isotopes (e.g. if you want just rubidium 87, instead of natural rubidium, a Rb vapor cell may cost close to $1200).