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This was an incredible read. I had no idea that light possessed momentum, especially angular momentum. > A potentially extraordinarily useful property of vorte

by pm90 6y ago

This was an incredible read. I had no idea that light possessed momentum, especially angular momentum. > A potentially extraordinarily useful property of vortex beams is that they do not interfere with each other if they all possess different twisting patterns. This means a theoretically infinite number of vortex beams can get overlaid on top of each other to carry an unlimited number of data streams at the same time. ... > The researchers began with a microring laser consisting of a ring of indium gallium arsenide phosphide only 7 microns in diameter in which light could flow in a loop, via a channel 650 nanometers wide. By varying the light pumped into this circle from microscopic arms on either side of this ring, the researchers could alter the orbital angular momentum of the beam emitted from the laser. Instead of emitting a single orbital angular momentum mode, they showed it could emit five distinct modes. > The scientists also developed a light detector based on tungsten ditelluride, which can act like a so-called Weyl semimetal, a material with properties lying between a conductive metal and a pure semiconductor. Their experiments found that different orbital angular momentum modes of light each generated unique patterns of electrical current within the photodetector, and they suggest this electronic method of detecting the orbital angular momentum of light could be scaled to work on microchips. Amazing stuff.

gibolt 6y ago
Absolutely agree! Wouldn't have dove into the article without your comment. Thanks! Does anyone know if there have been experiments confirming that light's angular momentum can be transferred like its linear momentum? Perhaps firing a rotating beam at a tiny sheet in space to induce spin and then reverse the light spin and confirm that the sheet eventually reverses. Since this technique could induce some extreme spin over long time/distance, I could imagine using it to pull crazy maneuvers in space. For example, aim a tiny slightly off balance disc at a distant planet. Once it started entering the planet's atmosphere, a correct entry angle and spin could take it from being head on to entering orbit.
- thatcherc 6y ago
  There are experiments like this! One is where circularly polarized light is used to spin up a small piece of glass to really high angular velocity (billions of rotations per second!) [1]. There are also atomic transition selection rules that depend photons transferring angular momentum into or out of an atom electronic orbitals. Essentially, when an electron jumps between the orbitals (s, p, d, etc from chemistry class), it can gain or lose angular momentum depending on its start and end state. But that momentum must be conserved, so in these cases a photon is emitted or absorbed to account for that angular momentum. The topic is kind of addressed here [2], but I'd have to dig up my old AMO notes to find a better reference. Lots of cool physics to dive into here if you're interested! [1] - https://phys.org/news/2018-07-world-fastest-rotation.html https://phys.org/news/2018-07-world-fastest-rotation.html [2] - https://en.wikipedia.org/wiki/Angular_momentum_coupling#Term_symbols https://en.wikipedia.org/wiki/Angular_momentum_coupling#Term...
- raziel2701 6y ago
  Yeah you can change the magnetization direction of materials by irradiating with circularly polarized light. Synchrotrons use circularly polarized x-rays to probe magnetic materials routinely. The underlying mechanism of these effects is simply the transfer of angular momentum.
dghughes 6y ago
I think this property of light is fairly new I recall reading about twisting light/torque light not that long ago. Searching brings up articles only a few years old but I think it was about ten yeas ago that I read about it.
- contravariant 6y ago
  Polarized light has been known for ages, I think this is some kind of coherent state that allows those bundles of photons to carry more angular momentum than normal. I think the classical solution also allows for these twisted EM waves, but I'm not 100% sure. Edit: To be precise I think it's something like group velocity but applied to angular momentum, but I'm not an expert.
- raziel2701 6y ago
  No this property of light has been known for ages, it's commonly known as circularly polarized light. There is a bit of fuckery here with trying to coin the term vortex laser to make it more marketable/sexier, but researchers have been playing with circularly polarized light for decades.
  - mturmon 6y ago
    I’m doubtful of this claim. I’m not an expert but I hear about vortex optical components at lunchtime conversation, and from what I understand these effects have only become exploited in the past 20 years or so. I'm open to correction or further context about this. See also the first paragraphs of https://en.wikipedia.org/wiki/Optical_vortex https://en.wikipedia.org/wiki/Optical_vortex, which distinguish between circular polarization from spin angular momentum, vs. orbital angular momentum which is modified by these vortex components.

by pm90 6y ago

gibolt 6y ago

Absolutely agree! Wouldn't have dove into the article without your comment. Thanks! Does anyone know if there have been experiments confirming that light's angular momentum can be transferred like its linear momentum? Perhaps firing a rotating beam at a tiny sheet in space to induce spin and then reverse the light spin and confirm that the sheet eventually reverses. Since this technique could induce some extreme spin over long time/distance, I could imagine using it to pull crazy maneuvers in space. For example, aim a tiny slightly off balance disc at a distant planet. Once it started entering the planet's atmosphere, a correct entry angle and spin could take it from being head on to entering orbit.

thatcherc 6y ago
There are experiments like this! One is where circularly polarized light is used to spin up a small piece of glass to really high angular velocity (billions of rotations per second!) [1]. There are also atomic transition selection rules that depend photons transferring angular momentum into or out of an atom electronic orbitals. Essentially, when an electron jumps between the orbitals (s, p, d, etc from chemistry class), it can gain or lose angular momentum depending on its start and end state. But that momentum must be conserved, so in these cases a photon is emitted or absorbed to account for that angular momentum. The topic is kind of addressed here [2], but I'd have to dig up my old AMO notes to find a better reference. Lots of cool physics to dive into here if you're interested! [1] - https://phys.org/news/2018-07-world-fastest-rotation.html https://phys.org/news/2018-07-world-fastest-rotation.html [2] - https://en.wikipedia.org/wiki/Angular_momentum_coupling#Term_symbols https://en.wikipedia.org/wiki/Angular_momentum_coupling#Term...
raziel2701 6y ago
Yeah you can change the magnetization direction of materials by irradiating with circularly polarized light. Synchrotrons use circularly polarized x-rays to probe magnetic materials routinely. The underlying mechanism of these effects is simply the transfer of angular momentum.

dghughes 6y ago

I think this property of light is fairly new I recall reading about twisting light/torque light not that long ago. Searching brings up articles only a few years old but I think it was about ten yeas ago that I read about it.

contravariant 6y ago
Polarized light has been known for ages, I think this is some kind of coherent state that allows those bundles of photons to carry more angular momentum than normal. I think the classical solution also allows for these twisted EM waves, but I'm not 100% sure. Edit: To be precise I think it's something like group velocity but applied to angular momentum, but I'm not an expert.
raziel2701 6y ago
No this property of light has been known for ages, it's commonly known as circularly polarized light. There is a bit of fuckery here with trying to coin the term vortex laser to make it more marketable/sexier, but researchers have been playing with circularly polarized light for decades.
- mturmon 6y ago
  I’m doubtful of this claim. I’m not an expert but I hear about vortex optical components at lunchtime conversation, and from what I understand these effects have only become exploited in the past 20 years or so. I'm open to correction or further context about this. See also the first paragraphs of https://en.wikipedia.org/wiki/Optical_vortex https://en.wikipedia.org/wiki/Optical_vortex, which distinguish between circular polarization from spin angular momentum, vs. orbital angular momentum which is modified by these vortex components.