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Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

 Федеральный исследовательский центр «Красноярский научный центр Сибирского отделения Российской академии наук»

Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

Scientists have learned to lock electromagnetic waves in one-dimensional layered structures

17 June 2020 г.

Учёные научились запирать электромагнитные волны в одномерных слоистых структурах
Scientists at the Krasnoyarsk Science Center SB RAS and Siberian Federal University, together with colleagues from Taiwan, experimented for the first time in the world with the so-called bound states in a continuum in one-dimensional layered structures appearing due to the exact destructive interference of light with different polarizations. The study opens the way to the creation of high-quality controlled devices in photonics and spintronics. The results are published in the journal Nature Communications Physics.

A huge number of the observed physical phenomena is associated with waves: acoustic, electromagnetic, quantum-mechanical probability waves and others. Despite the fundamental differences, it is possible to find general patterns in the behavior of different waves. One of these intriguing phenomena, which is a bound state in the continuum, was predicted as early as in 1929 by physicists Neumann and Wigner for electron waves in quantum mechanics. This phenomenon is associated with the destructive interference or ability of transmitted and reflected waves to supress each other.

To better understand what the destructive interference is, one is to imagine two waves passing through each other on the water surface. If at the same point a crest of the first wave and a hollow of the second coincide, then the water surface will look undisturbed. In the case of quantum particles, the destructive interference of transmitted and reflected waves locks it at a point in space, although the particle’s energy is enough to leave the attraction zone.

Due to the complexity of the mathematical description, bound states in the continuum have long been regarded as “exotic ones”, attracting the attention of only theorists. In 1985, German theorists Friedrich and Wintgen proposed a simple model of an open quantum system which describes the locking of a quantum particle due to the destructive interference of two resonances. In 2008 this model formed the basis for the study of Krasnoyarsk physicists, Bulgakov and Sadreev, on the localization of light in a two-dimensional photonic crystal, which received experimental confirmation three years later. These studies gave rise to numerous articles on the bound state in the continuum for two-dimensional and three-dimensional photonic crystals. At the same time, such a phenomenon was believed to be in one-dimensional layered structures.

A team of scientists from Krasnoyarsk and Taiwan proved the opposite. Researchers at the Krasnoyarsk Science Center SB RAS and the Siberian Federal University showed the theoretical possibility of a bound state in the continuum in one-dimensional layered structures. For this purpose, physicists proposed a new model, which consists of only three layers. When an electron wave passes through the central layer, in which the magnetic field is rotated relative to the two extreme layers, it splits into two waves. When entering the third layer, these waves overlap and suppress each other, thus, the electron becomes locked and remains in a three-layer structure.

Experimental verification of theoretical constructions was not a simple matter. To work with electrons, it is necessary to create high-quality semiconductor structures, apply a magnetic field at very small scales and cool electrons at extremely low temperatures. Since the bound state in the continuum is a general wave phenomenon, scientists decided to create a similar system for light waves. In the framework of a joint international project, scientists from Taiwan made a three-layer photonic structure and made the necessary measurements based on the model and theoretical calculations of the Krasnoyarsk physicists.

One-dimensional photonic crystals are an optical analogue of the boundary domains with the same while a liquid crystal with a rotated optical axis is an analogue of the central layer with a rotated magnetic field. The liquid crystal is an anisotropic substance, i.e. it has different optical properties in different directions. As in the case of the electron problem, when the light from the photonic crystal is obliquely inclined onto the liquid crystal, the light wave splits into two, which destructively interfere upon entering the second photonic crystal. Thus, the light is locked in the defect layer, although its frequency is quite sufficient to leave the crystal.

“For the first time we have realized bound states in a continuum in one-dimensional layered media for optical waves. We have been able to experimentally show that it is possible to control the quality factor of such a system. The quality factor is a characteristic of an oscillatory system which shows how quickly the system loses its accumulated energy. Light energy cannot leave the bound state in the continuum; therefore, its quality factor is limited only by inherent losses in the materials themselves. By mechanically turning the optical axis of the liquid crystal, we increased or decreased the Q factor, approaching or moving away from the conditions for the realization of the bound state in the continuum. The liquid crystal is very sensitive to external effects, so further direction of the research is to demonstrate the control of the Q-factor using temperature or an external electric field. The models proposed in our study provide opportunities to create controlled devices in spintronics and photonics, ”says one of the authors of the research Pavel Pankin, Candidate of Physical and Mathematical Sciences, researcher at the L.V. Kirensky Institute of Physics, Krasnoyarsk Science Center SB RAS.

The study was supported by the Russian Foundation for Basic Research (grants No. 19-52-52006 and 19-02-00055).