Science & Technology

Physicists show a swarm of photons somersaulting at a rock step

The STOV pulse (left) passing through the nonlinear crystal receives the second harmonic generation and generates a pulse to the right. Credits: Hancock, Zaheppour, and Milchberg / University of Maryland

From toy spinning tops and fidget spinners to figure skaters, rotating or rotating objects are common. And from the water that goes around the drains to the less-welcome tornadoes and hurricanes.

In physics, there are two types of rotational motion: spin rotation and orbital rotation. The movement of the Earth in our solar system illustrates these well. The daily 360 degree rotation of the Earth around its own axis is a “rotational” rotation, while the annual trip of the Earth around the Sun is a “orbital” rotation.

The amount of physics defined to describe such rotational motion is “angular momentum” (AM). The important thing about AM is that Conserved quantity.. Given the initial amount, it can be split and redistributed between particles (atoms, photons, pebbles, M & M, etc.), but the sum of AM must remain the same.Angular momentum vector.. This is a directional quantity, which is perpendicular to the plane in which the rotational circulation occurs.

These two types of AM exist in the light particles (photons) in the laser beam. Photons have spin, but it is unlikely that the photon is rotating around its own axis. Instead, spin angular momentum (SAM) results from the rotation of the photon’s electric field, which can only be directed forward or backward with respect to the beam direction. Laser beam photons can also have orbital angular momentum (OAM). The simplest laser beam with OAM photons is the “doughnut beam”. When you hit a wall with such a beam, it looks like a bright donut or a ring with a dark center. In this case, the OAM vector also points forward or backward. Thanks to quantum mechanics, the surprising fact is that OAM is the same for all photons in the beam.

In a treatise published on April 27, 2021 Optica, Professor Howard Milchberg’s group (IREAP / ECE / Physics) shows the surprising result that photons in vacuum can have a lateral orbital momentum vector at 90 degrees to the direction of propagation. OAM vector only Refers to the front or the back.

A research team that included graduate student and lead author Scott Hancock, postdoctoral fellow Sina Zahedpur (EE Ph.D. ’17), and Milchberg did this by producing donuts. pulse They dubb the “edge first flying donuts” shown in the figure (the more technical name is “spatiotemporal optical vortex” —STOV). Here, the angular momentum vector points at right angles to the plane containing the ring because the donut hole is sideways and rotational circulation occurs around the ring. To prove that this sideways OAM is associated with individual photons as well as the overall shape of the flying donut, the team sends pulses through a non-linear crystal (shown in the figure). , Performed a well-known process called “second harmonic generation”. , “Here, two red photons are converted into one blue photon with twice the frequency. This reduces the number of photons by half. That is, each blue photon should be twice as large as the landscape OAM. This is exactly what the measurement results show. As shown in the figure, the angular momentum of a flying donut (or STOV) (represented by a red and twice as long blue arrow) is a combined effect of a swarm of photons that somersault at the lock step.

STOV has many potential applications. For example, the conservation of angular momentum embodied by Somersalt photons makes the STOV beam resistant to decay due to atmospheric turbulence, and has potential applications in free-space optical communication. In addition, STOV photons need to be generated by pulses of light, so such pulses can be used to dynamically excite various materials or probe them using OAM and donut holes. I can do it. “STOV pulses can play a major role in nonlinear optics,” says Milchberg. “Being able to control the material that the beam propagates enables new applications in beam focusing, steering, and switching.”

See: “Second Harmonic Generation of Space-Time Optical Vortex and Conservation of Orbital Angle Momentum”, SW Hancock, S. Zahedpour, HM Milchberg, April 27, 2021, Optica.
DOI: 10.1364 / OPTICA.422743

Physicists show a swarm of photons somersaulting at a rock step Physicists show a swarm of photons somersaulting at a rock step

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