ITMO Scientists' Invention Reduces the Size of Optical and Microwave Devices

Russian physicists from ITMO University, Saint Petersburg Electrotechnical University "LETI", and the Australian National University have developed a super resonator in the microwave range. This discovery will allow us to produce highly efficient compact elements for microwave technology and optical computers. The scientists' work is published in Advanced Materials.

Most modern household appliances work on the principle of controlling radio, acoustic and optical waves. With their help, we can heat objects, and record and transmit information. The core element of such systems is resonators, that is, devices that allow us to catch an incident wave and multiply its intensity. The properties of the resonators determine the quality of the technology designed to control light, sound or microwave oscillations.

A good resonator, capable of effectively capturing and retaining electromagnetic radiation, is usually large (compared to the wavelength of oscillations), but at the same time, it is required to be compact. Physicists from ITMO University, Saint Petersburg Electrotechnical University "LETI", and the Australian National University (under the supervision of Prof. Yuri Kivshar) found a solution to this problem: they decided to create a subwavelength resonator with sizes much smaller than a wavelength that is capable of concentrating electromagnetic energy as efficiently as possible.

"One of the main characteristics of a resonator is its Q-factor, in other words, the ability to accumulate incident electromagnetic energy," says Mikhail Odit, a staff member at ITMO's Faculty of Physics and Engineering and an associate professor at ETU "LETI", "but usually, the Q-factor decays rapidly when decreasing its size. Therefore, the creation of a compact and, at the same time, high-Q resonator becomes a great challenge. That's why we decided to use the so-called bound states in the continuum, known from quantum mechanics."

This phenomenon includes the interaction of radiation from two coupled resonances existing in one system, which can lead to either amplification or complete suppression of the resonator radiation.

"Our work shows that we can provide such a resonator geometry under which the emitted oscillations in the far field suppress each other," the researcher adds. "It happens when the resonator forms two types of oscillations that have a similar field shape and one frequency. If the oscillations are mutually subtracted, the resonator stops emitting energy, which, in fact, means a significant increase in its efficiency. In this case, the device remains compact."

To observe the described effect, it is necessary to choose the correct shape, size and material of the resonator. The researchers settled on a cylindrical device made of microwave ceramics with a high dielectric constant. But in order to find the size of the required cylinder with the required accuracy, dozens of resonators of different sizes would have to be manufactured. Therefore, it was proposed to make a small set of cylinders, the height of which would have a height twice higher than the previous one. Thus, the height of the smallest cylinder was only a quarter of a millimeter, and the largest was 15 mm. By combining a set of these resonators, it was possible to assemble a final sample of the desired height, while maintaining a height accuracy of only 1/4 mm.

"We needed a resonator with a high dielectric constant," says Sergey Gladishev, a Ph.D. student at ITMO University. "It is necessary for the best localization of electromagnetic energy in the resonator. This helped us see a clear resonance even in the presence of parasitic noise. Low loss microwave ceramics were our best choice. Its properties were retained even for cylinders with a thickness of less than a millimeter."

As a result, the scientists were able to define the optimal size of cylinders and experimentally observe super-resonant states. It was shown that already a five percent change in the resonator height led to a hundred times increase in its Q-factor. In fact, the researchers managed to obtain the highest Q-factor of the resonator for the given material, which could be further improved only by choosing an even better dielectric.

More information:
Mikhail Odit et al. Observation of Supercavity Modes in Subwavelength Dielectric Resonators, Advanced Materials (2020). DOI: 10.1002/adma.202003804

Provided by ITMO University