[zh3]

The light is produced by electrons combining with holes, so the size of the material doesn't come into it (unlike an antenna). I've personally pushed* a single rubidium atom about in a quantum computer and watched it move by emitted light (rubidium atom: ~0.25nm, emitted light 420nm depending on excitation).

* 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.

[amluto]

Something akin to reciprocity still applies, no? You have a tiny rubidium atom, way too small to couple particularly strongly to the electromagnetic field at visible wavelengths. So it has a low cross-section for absorbing visible light. Won’t it necessarily radiate rather slowly as a result?

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.)

[lightedman]

"Something akin to reciprocity still applies, no?"

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.

[adrian_b]

There is some kind of reciprocity, e.g. when an atom absorbs light and it passes into a higher energy level, it will spontaneously emit light going back to the initial energy level, with about the same probability with which it has absorbed the light.

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).

[zh3]

Depends on the definition of slowly - thousands of photons per second upwards (and detectors are so sensitive these days that makes seeing easy). Lots of articles about on the subject, given the potential uses of this system.

https://www.cornishlabs.uk/tweezers

https://opg.optica.org/oe/fulltext.cfm?uri=oe-29-4-4858

[dragontamer]

It's probably more impressive that the LED was manufactured with light photons. I know it's "normal lithography" problems, but making a 500nm device out of 300nm or 400nm waves of light is downright impressive.

[jacquesm]

Have a look at what a high end CPU wafer mask looks like. It's nothing short of magic.