Optical resonators are fundamental in medicine, laser technology, astronomy, and many other modern-day fields. They are used to amplify light and generate beams in lasers and to detect gravitational waves in interferometers (e.g., LIGO); in medicine, resonators also serve as a basis for high-precision sensors that detect viruses and markers of various diseases.
Optical resonators are considered an excellent illustration of an open (non-Hermitian) system, which can exchange energy with the environment. As opposed to closed or self-contained systems, where energy is completely isolated in a certain volume, non-Hermitian ones interact with the outside world, which leads to surprising effects and behavior patterns.
One of such examples are exceptional points that occur when two resonances of the system merge, i.e. their frequencies and lifetimes coincide. This can be seen as the superposition of two shadows when one completely covers the other. Exceptional points are extremely sensitive to changes in external parameters, which is employed in optical and quantum sensors to detect the most minor disturbances in the surrounding space, as well as in quantum computing.
Exceptional points, nevertheless, have their own downsides, one of them being radiative losses. Such losses appear unavoidable as the system is open and some of its energy is constantly lost to the outside world in the form of radiation. This places substantial restrictions on their applications.
To address the losses issue and improve the use of exceptional points, an international team of scientists from Russia, China, Austria, and the UK suggested creating a single exceptional point out of two bound states in the continuum. These are unique states that do not emit energy, even inside an open system. The states are expected to serve as a “protective cocoon” shielding the point from radiative losses.
The team used numerical modeling to confirm their hypothesis and discovered a way to perform the procedure in dielectric metasurfaces, which are structured films made of a material with a high refractive index and used to localize light (wave concentration in a limited space of an inhomogeneous medium) and control its properties.
As a result, the researchers were able to obtain the new type of singularity – exceptional bound states in the continuum. In a non-Hermitian system, a singularity is a special state, some features of which diverge and thus result in aberrant behavior. Particularly, a singularity can emerge when two resonances combine into a single point, resulting in an expectational point.
Exceptional bound states in the continuum are a new type of states that combine two key phenomena that were previously thought to be incompatible. They are, on the one hand, highly sensitive to external perturbations (as exceptional points) but on the other – are protected against losses (as bound states in the continuum).
“Scientists used to believe that it was impossible to combine bound states in the continuum and create an exceptional point as it meant to ensure two contrary conditions: firstly that the system didn’t lose energy, and secondly, losses still remained. This caused a fundamental contradiction. We were able to overcome it by introducing an additional loss channel. This may seem trivial but it’s in fact a game changer for the problem. We took into account that energy can not only be emitted into outer space but also absorbed by the material itself. This assumption completely turns the situation upside down and allows us to combine two bound states in the continuum into one exceptional point. We can now say that these two striking phenomena of open-system (non-Hermitian) photonics, which have been actively studied for the last decade, have finally met,” said Andrey Bogdanov, an author of the paper and an associate professor at ITMO’s Faculty of Physics.
Andrey Bogdanov. Photo by Dmitry Grigoryev / ITMO.NEWS
As noted by the researchers, the predicted resonance states pave the way for high-precision optical sensors and can also be used in dielectric metasurfaces, including for detecting viruses or proteins. As the bound states evenly spread across the metastructure, they can efficiently interact with the thinnest layers of biological samples (about 100 nanometers), which allows scientists to measure the concentration of proteins or other markers using optical methods with higher accuracy, up to the detection of individual molecules.
The phenomenon will also be useful for creating energy-efficient nonlinear components for integrated photonics: optical transistors, modulators, and switches.
As of now, the team is making samples to observe exceptional bound states in the continuum. The experiment will be conducted with the Chinese researchers from Qingdao at ITMO’s International Research and Educational Center for Nanophotonics and Metamaterials. It will help verify the theoretical results and observe the merging of two bound states in the continuum into one exceptional point.
