Researchers have uncovered that the spin of light can develop patterns like those of a spinning top: the spin vector moves along the surface of an inverted cone. Previously, this dynamic was thought to appear exclusively in particles with a magnetic momentum and not in photons. The discovery can potentially find applications in optical spintronics – a field in which information is transferred and processed using the spin of light instead of an electron as in traditional spintronics. The results of this RSF-supported study were published and featured as an Editors’ Suggestion in Physical Review B.

^{Credit: agsandrew / photogenica.ru}

All elementary particles, including electrons and photons (the quanta of light), possess energy and momentum, as well as are characterized by spin – an intrinsic angular momentum. These physical features play a crucial role in interactions between particles and with surrounding matter. For instance, a particle can change another body’s motion or cause it to spin by transferring momentum or angular momentum to it. For light, these properties can be described in two complementary ways: light as a stream of photons or an electromagnetic wave traveling through space.

Much like individual photons, electromagnetic fields carry energy, momentum, and angular momentum. In the meantime, the latter can be decomposed into orbital (spatial distribution) and spin contributions (polarization), which characterize light spin and can manifest themselves in various dynamic forms.

The spin angular momentum is a well-established quantity for particles with magnetic momentum such as electrons. This phenomenon occurs due to an electromagnetic field that causes spin to change its direction, making it move in a cone shape around the magnetic field. Similar patterns can be observed in a spinning top, the axis of which traces out a cone due to gravity. However, unlike electrons, photons do not possess an intrinsic magnetic moment; until recently, it remained unknown whether the spin angular momentum of an electromagnetic field can exhibit motion similar to that of electron spin precession.

Researchers from ITMO University, Harbin Engineering University (China), and Queen’s University (Canada) have shown that the spin of an electromagnetic field begins to rotate in a cone-shaped trajectory when the field is formed of two light waves of varied frequencies.

“In electrons, precession occurs when an electromagnetic field interacts with an external magnetic field. Since photons have no intrinsic magnetic moment, we assumed that a similar effect can be achieved by developing a special configuration of an electromagnetic field. For that, we settled on using two light waves with different polarizations and slightly different frequencies traveling in perpendicular directions. It turned out that the spin of the resulting field precesses akin that of an electron in a magnetic field. The role of an efficient magnetic field, in this case, is played by the configurations of light waves and precession dynamics are defined by the differences in their frequencies,”​ says Andrey Bogdanov, PhD in physics and mathematics, an associate professor, and a senior researcher at ITMO’s Faculty of Physics.

The calculations also showed that the behavior of spin rotation is proportional to the difference in frequencies of light waves used. If frequencies are similar, the spin rotates slowly and smoothly in a cone shape; otherwise, fast oscillations may appear alongside. This resembles a top – one that not just traces out a cone but also rocks back and forth during rotation.

“First and foremost, our study has a fundamental value – we were first to show that such unconventional motion of an electromagnetic field spin is possible. In the long term, this effect can be used to develop innovative data transmission methods. Today, scientists are actively working on spintronic devices, in which data is carried by the spin of elementary particles, most often electrons. We’re moving towards optical spintronics, which relies on the spins of electromagnetic fields (light) – a subject that our research team actively pursues,” concludes Abanoub Mikhail, a PhD student at ITMO’s Faculty of Physics.

Written by the press office of the Russian Science Foundation