There are two kinds of angular motion an object can have: about its own axis, and about an external axis. One example is the earth spinning on its axis, and the earth spinning about the sun. Due to its spin around its axis, we can say the earth has spin angular momentum, and due to its orbit around the sun, we can say that it has orbital angular momentum.
Photons, the particles of light, also have angular momentum. These tiny quantum wave-packets can spin about their axes and also orbit around their direction of propagation. Given the particle-wave nature of photons and other quantum objects, it’s difficult to envision, and also to talk about, just what it means for a photon to spin or orbit. Nonetheless, in observed cases of photons interacting with other particles, the conservation of angular momentum and other fundamental laws look like they’re obeyed if photons have spin and orbit angular momentum.
Spin angular momentum in photons is actually the reason light has polarization. The photons that make up a beam of light have one or both of two quantized spins, +1 or -1 (in units of hbar). If all the photons have one kind of spin, then the light is circularly polarized, while if all the photons have both kinds of spin (a quantum superposition of +1 and -1), then the light is linearly polarized. Wikipedia has a page on circular polarization with a helpful animation.
Photons can also have orbital angular momentum (OAM). If you think of a group of photons, you can imagine them spiralling around the centre of the beam, to create a vortex shape. However, you don’t need a group of photons: a single photon travelling in space can have orbital angular momentum, too. This angular momentum is also quantized: you can have OAM values of 0, +/-1, +/-2, … all the way to infinity. I again direct you to the Wikipedia page on OAM light for visualization purposes. While this was theoretically known for a long time, it was not until 1992 that scientists realized you could produce light with OAM in a lab. Since then, groups around the world have been making and studying light beams with OAM, and using them for a wide variety of applications.
OAM light, that is light having orbital angular momentum, can be created in a lab using special phase plates. This is a piece of glass with strategically placed thicker and thinner areas. Passing laser light with no OAM through this phase plate is enough to create light with OAM. Recently, researchers at SLAC (Stanford’s Accelerator Lab) have developed a new way of creating OAM light using their synchrotron. A synchrotron is an accelerator that speeds up electrons on a circular track, which creates electromagnetic radiation, just like moving electrons in an antenna creates radiation at radio frequencies. The synchrotron method of creating OAM is more complicated. However, it can create OAM light at many frequencies, all the way up to XRays. This broadens the range of applications for OAM light.
There already exist several applications for OAM light. One of them is to use the optical angular momentum degree of freedom to multiplex signals transmitted by light. In optical fibers or in air, you can transmit multiple messages at the same time by encoding each message into a different bands of wavelengths (exactly like different channels on a radio). You could also separate them into different polarizations, which allows you to send two times as many messages. If you also give each message a different OAM, then you can have many more times the number of messages going at the same time. One group in Italy showed that they could transmit 2.5 terabits of data over 1 meter in air using radio waves with OAM. Another group, with contributors from the US, Denmark and Isreal, showed that they could transmit 1.6 terabits of data over 1.1 km in an optical fiber.
Other applications range from the grand to the miniscule. Researchers in Scotland have proposed detecting the OAM of light reflected from distant astronomical objects to determine the speed with which they’re spinning, while a group in Australia has been using laser beams with OAM to trap and spin microscopic particles. It seems that ideas for how to make OAM light, and how to use, it are popping up everywhere simultaneously.
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