A synthetic dielectric surface has been developed by physicists from ITMO University where the propagation of electromagnetic waves is not hindered by current defects and can be regulated. It is the first of its kind and in the years ahead, such a structure will facilitate the development of more dependable optical devices and communication circuits. Details of their work can be found in Applied Physics Letters. The Russian Science Foundation supported the project.
The majority of materials can be grouped as either insulators or conductors: they can conduct electric current, or they can’t. However, there are a few that can conduct electricity but just at the thin level of the surface. These materials are called topological insulators. Their most uncommon property is that the current they convey is unaffected by all admixtures and likely defects of the material and “runs” without any losses. This exclusive property is known as a topologically protected edge state.
Topological insulators capable of conveying not only the electrical current but also “protected” electromagnetic waves will transform the development of optical technologies. In modern optical fiber and optical waveguides, the signal typically spreads in straight lines, scattering when encountering any defects or inflections. But if the waveguide is composed of a material that aids topologically protected edge states, the light could circumvent the hindrances and spread without any losses whatever the flaws it encounters on its way.
Previous research in this field revealed that under specific conditions, photonic crystals composed of ferrite cores can act as electromagnetic analogs of classical topological insulators (ferrites are magnetic materials derived from iron oxides and other metallic elements). But experimental set ups with ferrite rods turned out to be unwieldy, requiring several meters in length. Furthermore, they only permit creation of topologically protected edge states in the microwave frequency band. In the optical frequency band of visible and near-infrared light, magnetic materials demonstrate but a very feeble response.
A new solution for the development of topologically protected edge states in the optical band has been suggested by Alexander Khanikaev, a professor at ITMO University in St. Petersburg and the City University of New York, the US. The researcher was the first to propose that topological edge states can be formed within the structure of metamaterials. Synthetic compounds with exclusive electromagnetic properties, they can be used for reproducing the properties of electrons in solids, such as the spin (an intrinsic magnetic moment of an electron) and the synthetic fields that work upon them. Besides, it is possible to produce a metamaterial in which the distance between elements would be a lot smaller than the length of the wave of a band required. This was not possible with magnetic photonic crystals. Therefore, metamaterial-based devices can be not only practical but also quite compact.
The feasibility of Alexander Khanikaev’s concept was established by an international research team of physicists from Russia, Australia, and the US, who carried out several experiments at ITMO University. In the first set of experiments, topologically protected edge states were formed in a metal-based metamaterial. But the latter is incompatible to function at the optical band frequency because of their extensive light absorption. Hence, the following experiment, the researchers swapped to a dielectric metamaterial, or metasurface having one layer of synthetic structure.
The metasurface consisted of a foundation onto which the researchers attached meta-atoms composed of ceramic disks. These were arranged in a stringent order so that the distance between them would be lesser than a visible-light wavelength. Though the meta-atoms were created in a similar manner, they had a diverse orientation in the end structure: upward in one portion, downward in the other. It was at the domain wall, i.e. the border between these two portions, that topologically protected edge states were being formed.
Since the scientists only intended to verify the presence of topological edge states on dielectric metasurfaces, the experiments that they carried out were not in the optical frequency band, as it would have needed dealing with elements at the nanoscale. Instead, they chose a simpler method, examining the concept in the microwave frequency band. Eventually, their hypothesis proved correct: the excitation of an electromagnetic wave on the metasurface’s domain wall did make an edge state to happen. The physicists also showed that the edge state would happen even if the domain wall was not organized in a straight line but in a zigzag. Applied in the optical band, such a structure can offer a base for “protected” optical devices.
We managed to show that you could create a dielectric metasurface that would support topologically protected edge states. Drawing on these findings, we have already come up with a new experiment with meta-atoms of a simpler form that would be easier to reproduce at the nanoscale. This will give way to the development of next-gen optical devices supporting protected edge states in the optical frequency band.
Alexey Slobozhanyuk, Study Lead Researcher and Research Associate, Faculty of Physics and Engineering, ITMO University..
Topological photonic states pave the way to new possibilities for the management of electromagnetic waves.
The very opportunity of imitating the phenomena previously considered exclusive to electronic solid-state systems shows great potential for the development of the photonics of metamaterials with synthesized degrees of freedom. Our work experimentally shows that the pseudo-spin wave allows for manipulating the direction of the excitation and propagation of an edge state in metamaterials. In other words, it proves to be not only protected but also controllable. By upscaling our structure to make it work in the near infrared frequency band of optical telecommunications, topological states can become the ideal candidate for a variety of applications in integrated photonics.
Alexander Khanikaev, Project Head and Professor, ITMO University and City University of New York.
Also, researchers from the City University of New York and the University of Texas at Austin, the US, as well as from the Australian National University took part in the research project.