Objects that spin or spin are commonplace, from toy tops, spinners and figure skaters to water surrounding a drain, to tornadoes and hurricanes.

In physics, there are two types of rotational motion: spin and orbit. The movement of the Earth in our solar system illustrates this; Earth’s daily 360 degree rotation around its own axis is rotational rotation, while Earth’s annual travel around the sun is orbital rotation.

The quantity in physics defined to describe such a movement is the angular momentum (AM). MA is a conserved amount: given an initial amount of it, it can be broken down and redistributed among particles such as atoms and photons, but the total MA must remain the same. AM is also a vector: it is a quantity which has a direction, and this direction is perpendicular to the plane in which the rotational circulation occurs.

For the light particles in laser beams – photons – these two types of AD are present. Photons rotate, but do not rotate on their own axes; instead, the spin angular momentum (SAM) comes from the rotation of the photon’s electric field, and SAM can only point forward or backward relative to the direction of the beam.

Photons in laser beams can also have orbital angular momentum (OAM). The simplest laser beam in which photons have OAM is the ring beam: if you shine such a beam on the wall, it will look like a shiny donut or a ring with a dark center. The OAM vector also points forward or backward, and the OAM is the same for every photon in the beam.

In an article published in the journal *OPTICAL*, Professor Howard Milchberg of the University of Maryland and a research group demonstrate the surprising result that photons in a vacuum can have OAM vectors pointed laterally, 90 degrees to the direction of propagation – a literally orthogonal result. to the decades-long expectation that OAM vectors could only point forward or backward.

The research team, which in addition to Milchberg includes graduate student and lead author Scott Hancock and postdoctoral researcher Sina Zahedpour, did this by generating a donut pulse they call a “flying donut edge first.” (its more technical name is space-time vortex optics or STOV). Here, the ring hole is oriented laterally, and as the rotational circulation now occurs around the ring, the vector AM points at a right angle to the plane containing the ring. To prove that this laterally pointing OAM is associated with individual photons and not just the general shape of the flying ring, the team sent the pulse through a nonlinear crystal to undergo a process called second harmonic generation, where two red photons are converted to a single blue photon with a double frequency. This reduces the number of photons by a factor of 2, which means that each blue photon must have twice the laterally oriented OAM – which is exactly what the team’s measurements showed. The flying donut AM or STOV is the composite effect of a looping swarm of photons.

There are many potential applications of STOVs. For example, AM conservation embodied by jerky photons can make STOV beams resistant to disruption by atmospheric turbulence, with potential application to optical communications in free space. Additionally, since STOV photons must occur in pulses of light, these pulses could be used to dynamically excite a wide range of materials or to probe them in a way that exploits OAM and the donut hole.

“STOV pulses could play a big role in nonlinear optics,” says Milchberg, “where beams can control the material in which they propagate, allowing new applications in focusing, directing and switching beams” .

**Source of the story:**

Material provided by **University of Maryland**. *Note: Content can be changed for style and length.*

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