Physicists from ITMO and Harbin Engineering University have experimentally proven unique light scattering features in open optical systems with Mie resonators. Thanks to the discovered effects, it’s possible to control light absorption and gain coefficients, enabling high sensitivity of optical systems to external factors. These results pave the way to hypersensitive sensors and optical devices for information processing and transfer. The study was described in a paper published in Science Advances.

Credit: leungchopan / photogenica.ru

Optical structures are used in a great number of devices, from smartphones to lasers and sensors. The more advanced these devices become, the more compact, resilient, and efficient optical components they require. It is often the case that these technical requirements surpass the fundamental limitations of classical optics – and many of them can be overcome with the so-called non-Hermitian systems.

In classic, Hermitian, systems there are no energy losses or exchange with the environment, while non-Hermitian systems are open, they interact with their environments resulting in non-trivial optical effects. For instance, exceptional and diabolic points can appear in such systems. Exceptional points occur when several of the system’s resonances merge into a single point, demonstrating extreme sensitivity to external parameters. Diabolic points have to do with superscattering, a phenomenon where light scatters significantly more than in normal conditions. These effects make it possible to regulate the sensitivity of optical systems with high precision, controlling light absorption and gain coefficients and creating optical devices with target properties.

Until recently, the existence of exceptional and diabolic points in individual structures had only been predicted theoretically. Now, for the first time, a team from ITMO University and Harbin Engineering University has managed to experimentally demonstrate these effects in subwavelength photonic structures, Mie resonators. For this purpose, they used dielectric ring resonators, where they managed to “trap” light and then after precise calibration of resonator parameters create the conditions for exceptional and diabolic points. Near-field scanning demonstrated that inside the resonators, light isn’t just efficiently captured, but also significantly amplified.

“We observed the connection of several non-Hermitian singularities in light scattering, including the anapole state and superscattering. In the anapole state, electric and magnetic dipole moments interfere in such a way that the inner scattering is cancelled out and the resonator becomes optically “invisible.” In its turn, superscattering occurs when several resonances in the system overlay, multiplying light scattering. We have demonstrated that exceptional points always occur in the anapole state, while diabolic points occur in superscattering. Moreover, we have confirmed that exceptional points demonstrate extreme sensitivity: the smallest change in the resonator’s geometry, for instance, its height, leads to a great spectral shift in the resonance,” explains Andrey Bogdanov, the head of the research team and an associate professor at ITMO’s Faculty of Physics.

Additionally, the researchers learned to predict the exact parameters of the system in which exceptional and diabolic points occur. In the future, this will make it possible to design non-Hermitian photonic structures and use them to create more efficient and sensitive devices, such as hypersensitive sensors to detect disease biomarkers in the blood, monitor air or water contamination, or control material deformation. Thanks to the results of the study, it is now possible to develop new optical components for non-linear optics and topological photonics.

The project was authored by Nikolay Solodovchenko, a PhD student at ITMO, and Fan Zhang, a Master’s student at Harbin Engineering University (HEU). The study was part of ITMO’s 2030 Development Strategy and the roadmap of the joint ITMO-HEU campus and the International Center for Nanophotonics and Metamaterials in Qingdao.

Ksenya Desyatkova

Journalist

Catherine Zavodova

Translator