Light consists of electromagnetic waves. Or, if put another way, the oscillations of electrical and magnetic fields, each with its own direction. This direction is determined by polarization – the physical quality that determines a light wave’s vector in space. If we were to compare light with waves of water, polarization would correspond to the direction of the flow.
There are two main “types” of polarization: linear and circular. In the case of linear polarization, the direction of the electric field changes along a line. This kind of polarization is widely used in products such as protective goggles for drivers or skiers. Circular polarization, on the other hand, is characterized by the rotation of the electric field along a spiral – either clockwise or anti-clockwise. It plays a major part in the interaction of light with substances – for instance, with molecules of DNA, sugar, or medical products such as chloramphenicol or ibuprofen. These, too, possess a spiral-like shape and thus interact well with circularly-polarized light. To detect such molecules with the help of light waves, scientists usually employ metasurfaces that replicate the “trajectory” of the rotation of light.
Although such structures have existed for a long time, their production remains a complicated technological task. Physicists from ITMO, however, have found a solution. They’ve developed a nanostructure that splits rightward- and leftward-oriented rotation of light with a near-100% accuracy. Another advantage of the technology is its ease of production.
At the core of the structure are ellipse-shaped silicon nanoparticles placed on a plate of glass. Each particle supports resonance, meaning it can hold light. The placement of the nanoparticles on the plate determines their mutual interactions. The particles also amplify one another’s resonance effect. In an ideal case, they produce an ideal resonator, which, in turn, creates so-called bound states in the continuum. In this special medium, particles can contain light energy within the structure for an infinite amount of time.
“Nanoparticles support two types of resonance: electrical and magnetic dipole resonances, which correspond to the oscillations of the electrical and magnetic fields of light. With the correct placement of the nanoparticles, we can ensure that these resonances correspond with the leftward and rightward circular polarization of light. But using just one type of resonance won’t allow for 100% accurate splitting of the polarizations. We’ve been able to combine the two resonance types with the help of reflectance from the glass that supports the nanoparticles. As a result, both resonances amplify one another and the structure becomes an ideal mirror for one type of circular polarization while allowing the other to pass through at all times. In other words, one stays on the metasurface and the other is reflected back,” says Mikhail Rybin, PhD, one of the project’s authors and a professor at ITMO’s Faculty of Physics.
The ability of the metasurface to retain light of a particular polarization makes it possible to accelerate its interactions with nearby spiral-like molecules. Additionally, changes in the intensity of reflected light within the nanostructure will indicate the presence of such particles in the examined substance. For these reasons, the developed structure can serve as a highly accurate detector of various substances – such as the aforementioned molecules of DNA, sugar, or chloramphenicol.
The invention may also be used to accelerate the completion of chemical and biological tests and to boost the quality of telecommunications signals. It also opens up new avenues for experiments concerning spun particles, such as electrons.
The study was conducted within the framework of Russian Science Foundation’s grant No. 24-72-10038.
