A team of researchers from the University of Warsaw in Poland, the Institute Pascal CNRS in France, the Military Technical University in Poland and the British University of Southampton have shown that it is possible to master the so-called exceptional points. For the first time, physicists also observed the destruction of exceptional points from various degeneration points. You can read about the discovery that can contribute to the creation of modern optical devices in the latest nature communication.
The universe around us is made of elementary particles, most of which have their antiparticles. When a particle and an antiparticle i.e. matter and antimatter meet, annihilation occurs. Physicists have long been able to produce quasiparticles and quasiantip particles – elementary excitations: charge, vibrations, energy – trapped in matter, usually in crystals or liquids.
“The world of quasiparticles can be very complicated, although paradoxically the quasiparticles themselves help to simplify the description of quantum phenomena,” explains Jacek Szczytko of the University of Warsaw’s Faculty of Physics.
“Without quasiparticles, it would be difficult to understand the workings of transistors, light-emitting diodes, superconductors and some quantum computers. Even abstract mathematical concepts can become quasiparticles, as long as they can be implemented in physical systems. One such abstract concept is exceptional points.”
Theorists from the Institute Pascal CNRS in France, Guillaume Malpuech and Dmitry Solnyshkov explain.
“The so-called ‘exceptional points’ are specific system parameters that lead to the commonality of two different solutions that can only exist in lossy systems, that is, systems in which the oscillations slowly fade over time,” says Malpuech.
“They allow the creation of efficient sensors, single-mode lasers or unidirectional transport. Importantly, each exceptional point has a non-zero topological charge – a particular mathematical characteristic that describes the fundamental geometric properties and allows you to determine which exceptional point point will be the ‘antiparticle’ for another exceptional point,” adds Solnyshkov.
Scientists from the University of Warsaw and the Military Technical University in collaboration with researchers from the CNRS and the University of Southampton analyzed the optical resonator filled with liquid crystal. Liquid crystals are a special phase of matter in which certain directions are distinguished despite the liquid form.
For example, it can be probed by a light beam, which behaves differently depending on the direction of incidence relative to the optical axes of the liquid crystal. This function, combined with the easy tunability provided by an external electric field, is the basis for the operation of ordinary liquid crystal displays (LCD). Polarized light – that is, a specific direction of vibrations of the electric field of an electromagnetic wave – perfectly “feels” the direction of optical axes, and these are related to the direction of the elongated molecules of the liquid crystal.
“In the study performed, the liquid crystal layer was placed between two plane mirrors,” explains Wiktor Piecek of the Military Technical University in Warsaw. “The whole structure creates an optical cavity, through which only light of a specific wavelength can pass.”
This condition is met for the so-called cavity resonance modes, i.e. light of a certain color (energy), polarization and direction of propagation. This corresponds to a situation where a photon falling into the cavity can bounce several times between the two mirrors.
The presence of a liquid crystal, the orientation of which can be changed by applying a voltage, makes it possible to tune the energy of the cavity modes. Moreover, when the light is incident at an angle, the resonance state changes, which in particular can lead to different cavity modes crossing each other, ie having the same energy despite different polarization of the light.
For the specific orientation of the liquid crystal considered in the article, the two different cavity modes may only intersect for the four specific angles of incidence of light when an ideal structure without losses is considered. In fact, the light trapped in the cavity may escape through imperfect mirrors or be scattered.
Based on spectroscopic measurements, it can be determined how long the photon remains in the microcavity on average. In addition, due to the orientation of the liquid crystal layer, a difference was observed in the scattering of light polarized along and perpendicular to the axis of the liquid crystal. As a result, at the site of each degeneracy point for an idealized lossless cavity, a few so-called exceptional points were observed for which both the energy and the lifetime of the photon in the cavity are the same.
Mateusz Krol, the first author of the publication, describes the experiment: “In the tested system, it was observed that the position of exceptional points can be controlled by changing the voltage applied to the cavity. First of all, because the electrical bias is reduced, the exceptional points created on the basis of different degeneracy points come closer together, and for a low enough voltage they overlap each other, Since the approaching points have opposite topological charge, they destroy at the moment of the encounter, so they disappear, let no exceptional points.”
“This kind of topological singularity behavior, i.e. the destruction of exceptional points from different degeneracy points, has been observed for the first time. Previous work showed the destruction of exceptional points, but they appeared and disappeared at the very same degeneracy points,” adds. Ismael Septembre, a Ph.D. student at the CNRS.
Exceptional points have been intensively studied in many different fields of physics in recent years. “Our discovery will enable the creation of optical devices whose topological properties can be controlled by voltage,” concludes Barbara Pietka from the Faculty of Physics at the University of Warsaw.
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M. Król et al, Destruction of exceptional points from different Dirac valleys in a 2D photonic system, nature communication (2022). DOI: 10.1038/s41467-022-33001-9
Quote: Destruction of exceptional points of various degeneration points first observed (2022, October 14) retrieved on October 14, 2022 from https://phys.org/news/2022-10-annihilation-exceptional-degeneration.html
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