ITMO Physicists Use Magnetic Field to Teach Light to Ignore Defects

Light can scatter and be lost due to defects – random imperfections in the fabrication of a structure. This problem can be addressed from different angles. For instance, by using topological isolators. They don’t transmit light internally, but light can propagate along their boundary. Such structures reduce the impact of defects, but only at their boundary, where the wave travels along the surface and bypasses scratches without scattering backward. In their work, ITMO scientists Dmitry Vagin and Maxim Gorlach proposed a new solution to this problem. 

They developed a structure that not only minimizes the impact of defects but also focuses light in a special way. In a conventional lens, the path of light beams is reversible: the converging lens focuses light regardless of the direction it came from. In the new structure, this reversibility is distorted: it can act simultaneously as a converging or diverging lens depending on where the light is coming from. This effect is provided by a magnetic field and a special symmetry of the structure’s lattice.

The designed surface is intended for microwaves, as working with them is easier than with visible light. It all comes down to the wavelength: microwaves have wavelengths on the order of centimeters, which means the structural elements that control such waves can be relatively large – on the millimeter-scale. By contrast, visible light would require features hundreds of nanometers in size – roughly a thousand times thinner than a human hair – and fabricating such tiny features is much more difficult.

A metamaterial is an artificial structure made of a multitude of parts. Externally, the developed structure looks like an opaque dark plate; however, microwave radiation passes through it unimpeded. Its components, metal wires and cylinders, are made of a special material that responds to a magnetic field and controls the wave. In the model, the field is created by regular magnets located beside the surface. When subjected to a magnetic field, the material changes its properties, transforming the wave that passes through it. At the same time, the magnetic field is distributed throughout the structure in a way that it resembles a moving medium. That’s why light propagates unusually inside it.

“To control the wave, in our calculations we used yttrium garnet, a material responsive to the magnetic field. That’s what the cylinders are made of. When a wave enters them on one side, it passes the structure with a delay; when it enters on the other side, the delay is different. If we change the magnetic field by bringing the magnets closer or further away, the effect will be amplified or weakened correspondingly. This method can be used to increase or decrease the focal length of a non-reciprocal lens that focuses light only on one side and scatters it on the other,” explains Dmitry Vagin, the paper’s first author and a Master’s student at ITMO’s Faculty of Physics.

The researchers have already calculated how the material behaves when the wave enters from each side. Additionally, the calculations revealed that defects inside the structure have almost no effect on the wave’s passing: the signals that were randomly reflected from deformed and moved cylinders and wires cancelled each other out because of the magnetic field and the structure’s asymmetry.

Among the metamaterial’s practical applications is the production of photonic integrated circuits, a promising substitute for conventional microchips that have nearly reached their miniaturization capacity. Photonic chips use photons instead of electrons, which makes them faster and more energy-efficient. However, defects are too common in their production and this lowers the functionality of such devices. The new structure developed by ITMO physicists is a step towards a solution to this problem. Photonic integrated circuits are already actively used by some data centers: they process large amounts of data and the next-gen chips help transmit it faster.

In addition to protecting against defects, a lens based on the developed structure could also be useful for sensors: its outward-facing side defocuses incoming light, preventing the sensor from losing the image due to excessively bright light.

“At our frontier laboratory, we’ve been studying such non-reciprocal structures for the last five years. Last year, our team was the first to predict a new type of non-reciprocal electromagnetic response similar to the electrodynamics of axions – hypothetical particles that cause materials to interact differently with light. When subjected to radiation, a material’s surface produces charges that behave as single-pole magnets, even though usually magnets have two poles. Our new study continues this research. In recent years, a lot of scientific efforts were focused on decreasing the effect that defects have on optical properties of micro- and nanostructures. This study demonstrates a new approach to this problem. For now, we have implemented it in the microwave range, but potentially we can transfer the effect to photonic chips as well,” says Maksim Gorlach, the head of the frontier laboratory and a senior researcher at ITMO’s Faculty of Physics.

The study is supported by the program Priority 2030 and the Russian Science Foundation (grant No. 26-72-10054).