Metalattice with Strong Toroidal Dipole Response

Experimental metalattice-structure with a strong toroidal dipole response. (Source: ITMO)

Physicists have managed to create an experi­mental structure with a strong toroidal dipole response of the electro­magnetic field over a wide frequency range. This response is associated with a special confi­guration of electro­magnetic currents causing high concen­tration of the field. A special dielectric metal lattice was created to produce and measure the response. The results can be used to create non-scattering materials, as well as to control electro­magnetic fields.

It is not possible to create accurate sensors or data storage and processing devices without controlling electro­magnetic field properties such as energy concen­tration, direction of the oscil­lations, or polari­zation of waves. Regulating the inter­action of dipole responses associated with different current configurations in the field gives us the oppor­tunity to change electro­magnetic features of an object even to extent of making it invisible. This can be achieved by creating a structure combining two dipole types: conven­tional electric, and more complex toroidal dipole.

Until now, the toroidal dipoles experi­mentally observed by scientists were either very weak or existed only in an extremely narrow frequency range, which created complications for practical use. Moreover, the experi­mental structures were based on metals, which led to large energy losses. Scientists from ITMO Uni­versity along with their colleagues from Iran and Australia managed to overcome these diffi­culties. They were the first to develop a meta-lattice consisting of dielec­tric material with a toroidal dipole response domi­nating over a wide frequency range.

“We created a periodic structure which we then tested in a series of experiments to make sure that the toroidal dipole was strong enough. When studying the spectrum and distri­bution of the electro­magnetic field, we registered some features typical for toroidal dipole: the field was highly concen­trated and had a strong longi­tudinal component, which means that the direction of electro­magnetic field oscil­lations coincides with its propa­gation direction. This can be useful for creating molecular sensors or producing nonlinear effects in optics,” explains Andrey Sayansky, Ph.D. student at the Faculty of Physics and Tech­nology of ITMO University.

To create the meta-lattice, the scientists used indium and gallium phosphides. The refractive index of these dielec­tric materials is lower than the index of regular ones like germanium or gallium arsenide. However, the results showed that the more affor­dable medium dielectrics can also be used to avoid energy loss. Scientists hope that this will contribute to the more active research and practical appli­cation of such structures. Another major finding was that the meta-lattice toroidal response can be excited by any polari­zation wave. This will help expand the scope of application of meta-lattice-based materials and devices.

“We did not develop a non-radiating material, but we laid the foun­dation needed for its creation. Our findings are also suitable for a variety of other appli­cations. The principle of the toroidal dipole control demons­trated in our research can serve to create sensors, control light and transmit or store information,” says Andrey Miroshni­chenko, professor at the Univer­sity of New South Wales in Australia. (Source: ITMO)

Reference: A. Sayanskiy et al.: All‐Dielectric Metalattice with Enhanced Toroidal Dipole Response, Adv. Opt. Mat., online 13 July 2018; DOI: 10.1002/adom.201800302

Link: Dept. of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, Russia

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