A study team from ITMO University and the Australian National University has discovered that various metasurfaces display the same behavior provided a symmetry breaking is added to their unit cells "meta-atoms". Asymmetry of meta-atoms brings about high-quality (high Q) resonances in the transmittance spectra of metasurfaces. These resonances are capable of numerous amplification of external signals. By exploiting the asymmetry, researchers were able to regulate the resonances and thereby an optical response, which is very desirable for practical applications. The research findings have been published in Physical Review Letters.
Quality factor (the so-called Q factor) is one of the most crucial features of a resonant system that defines the effectiveness of light-matter interaction and amplification of external signals. It demonstrates how well the structure can capture light. When the sample is decreased in size, particularly in thickness, its quality factor is also decreased considerably, making it inappropriate for practical applications.
In their latest research, a team of physicists from ITMO University and the Australian National University, led by Prof. Yuri Kivshar, have demonstrated a new physics of high-Q resonances. Researchers learned that the high-Q sharp resonances are mainly established by asymmetry of meta-atoms, and they virtually do not rely on the thickness of metasurfaces and the type of materials exhibiting a universal behavior for all types of such metasurfaces.
Thus, metasurfaces with broken symmetry can be employed to develop thin (less than the length of light) and extremely efficient lasers, sensors, and nonlinear radiation sources.
More significantly, the scientists established that high-Q resonances in asymmetric metasurfaces are ruled by bound states in the continuum. The latter are nonradiative states that appear when numerous resonances in a system interact in the regime of destructive interference subduing the radiative losses.
We have been studying bound states in the continuum for two years as part of a project supported by the Russian Science Foundation. At some point, we realized that the nature of high-Q resonances in metasurfaces is related to the physics of bound states of the continuum. It turns out that, by introducing an asymmetry, we can destroy bound states in the continuum and convert them to high-Q resonances. We analyzed a dozen asymmetric systems, found in various sources, in great detail and were able to show that the previously described effects were caused by bound states in the continuum.
Dr. Andrey Bogdanov, Research Fellow, International Research Center for Nanophotonics and Metamaterials, ITMO University.
The most important result of our work is that we were able to aggregate and summarize the results of a large number of works from various fields of photonics and radiophysics, all of which only have two things in common: the structure, which is an asymmetric metasurface, and the nature of observed phenomena, that of sharp and narrow resonances in spectral response. In older works, this was explained through the use of new terms. We, however, showed that all physical phenomena can be described via bound states in the continuum, a universal interferential phenomenon known to quantum physicists since the early 20th century.
Kirill Koshelev, Staff Member, International Research Center for Nanophotonics and Metamaterials, ITMO University.
According to the scientists, an in-depth understanding of optics of bound states in the continuum can help make simpler the process of developing materials with a definite optical response. Going forward, the researchers plan to use the results they attained to examine nonlinear optical effects in similar metasurfaces.