A team of researchers from the University of Warsaw in Poland, the CNRS Pascal Institute in France, the Military University of Technology in Poland and the University of Southampton in Britain have shown that it is possible to control so-called exceptional points. For the first time, physicists also observed the annihilation of exceptional points from different decay points. You can read about the discovery that may contribute to the creation of modern optical devices in the latest Nature Connections.
The universe around us is made of elementary particlesMost of them contain antiparticles. When a particle and antiparticle, i.e. matter and antimatter, meet each other, annihilation occurs. Physicists have long been able to produce quasiparticles and quasiparticles – elementary excitations: charge, vibration, energy – trapped in matter, often in crystals or liquids.
“The world of quasiparticles can be very complex, although paradoxically, quasiparticles themselves help simplify the description of quantum phenomena,” explains Jacek Szczytko of the University of Warsaw’s School of Physics.
“Without quasiparticles, it would be difficult to understand the workings of transistors, light emitting diodessuperconductors and some quantum computers. Even abstract mathematical concepts can become quasiparticles, as long as they can be implemented in them physical systems. Among these abstract concepts are exceptional points.”
Explains two theorists from the CNRS Pascal Institute in France, Guillaume Malbusch and Dmitriy Solnichkov.
“The so-called ‘exceptional points’ are specific system parameters that give rise to commonalities between two different solutions that can only exist in systems with losses, that is, those in which oscillations slowly fade over time,” Malbusch says.
“It allows the creation of efficient sensors, a single-mode laser, or unidirectional transmission. Importantly, each exceptional point has a non-zero topological charge – a certain mathematical feature that describes the basic geometric properties and allows you to determine which point will be the ‘antiparticle’ of another exceptional point” Solnichkov adds.
Scientists from the University of Warsaw and the Military University of Technology in collaboration with researchers from CNRS and the University of Southampton analyzed the liquid crystal-filled optical resonator. Liquid crystals are a special phase of matter in which certain directions are distinguished despite their liquid form.
The difference between the EP extermination considered earlier and this work. a Typical annihilation of the case of the airways where there is only one Dirac valley. EPs (EPs) are generated from DPs when the relative involuntary rate increases χ. On the contrary, they merge and form DP when the non-hierarchical relative declines. B The extermination of AP (EP) is described in this work, including the various valleys. 4 EPs are generated from 2 dp when the relative autonomic ratio increases. As it increases further, EPs meet and perish, leaving the system without any singularity. w Here is the file number. attributed to him: Nature Communications (2022). DOI: 10.1038 / s41467-022-33001-9
It can be checked, for example, by a ray of light, which behave differently depending on the direction of fall with respect to the optical axes of the liquid crystal. This feature, along with the ease of tuning by an external electric field, is the basis for the operation of common liquid crystal displays (LCDs). Polarized light – that is, a specific direction of the electric field vibrations of an electromagnetic wave – “sense” exactly the direction of the optical axes, and these are related to the direction of the particles elongated in the liquid crystal.
“In the research that was conducted, the liquid crystal layer was placed between two flat mirrors,” explains Wiktor Bekek of the Military University of Technology in Warsaw. “The whole structure creates Optic lumenthrough which only light of a specified wavelength can pass.
This condition is fulfilled for the so-called cavity resonance modes – that is, light with a certain color (energy), polarization and direction of propagation. This corresponds to a case in which a photon falling into the cavity can bounce several times between the two mirrors.
The presence of a liquid crystal, the direction of which can be changed by applying an electric voltage, allows the energy of the cavity modes to be adjusted. In addition, the state of the resonance changes when the light falls at an angle, which in particular can lead to the intersection of different cavity modes with each other, that is, to have the same energy despite the different polarization of the light.
For the specific orientation of the liquid crystal considered in the article, the two different cavity modes should only intersect for the specific four incidence angles of light when considering an ideal structure without any losses. Indeed, the light trapped in the cavity can escape through imperfect mirrors or be scattered.
The average time a photon stays within the microcavity can be determined on the basis of spectroscopic measurements. Moreover, due to the orientation of the liquid crystal layer, a difference in the scattering of polarized light along the . axis is observed liquid crystal. As a result, in the location of each decay point of a perfect lossless cavity, a pair of so-called exceptional points is observed in which the energy and lifetime of a photon in the cavity are similar.
Matthews Kroll, first author of the publication, describes the experiment: “In the tested system, it was observed that the position of the exceptional points could be controlled by varying the voltage applied to the cavity. First of all, as the electrical bias is reduced, the exceptional points that have been Created from different decay points from each other, and for a suitable low voltage, they overlap. Since convergent points have an opposite topological charge, they annihilate at the time of encounter, so they disappear, leaving no exceptional points.”
“This type of topological singularity behavior, i.e. the annihilation of exceptional points from different decay points, was first observed. Previous work showed the annihilation of exceptional points, but they appeared and disappeared at exactly the same decay points,” adds Ismail Seppi, Ph.D. Student at CNRS.
Exceptional points have been studied extensively in many different fields of physics in recent years. “Our discovery will allow the creation of optical devices whose topological properties can be controlled by voltage,” concludes Barbara Petka of the University of Warsaw’s School of Physics.
M. Kroll et al., Extermination of Exceptional Points of Various Dirac Valleys in a Two-Dimensional Photosystem, Nature Communications (2022). DOI: 10.1038 / s41467-022-33001-9
University of Warsaw
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