PHOTOMETA

Better Materials with Meta Materials Metamaterials have novel and unique electromagnetic properties, not found in natural materials; yet some major challenges remain before they can be applied more widely. The PHOTOMETA project aims to promote the development of functional metamaterials, which could hold potential across a range of different applications, as Professor Costas Soukoulis explains The potential of metamaterials has generated a high level of interest in both the academic and commercial sectors, with researchers seeking to develop new materials with electromagnetic properties tailored for specific practical applications.

Unlike natural materials, metamaterials can offer a tailored electromagnetic response at almost any desired frequency, as Prof. Costas Soukoulis, coordinator of the PHOTOMETA project, explains. “The electromagnetic response achievable with metamaterials can be quite unusual and unconventional, for example leading to negative wave refraction or backwards propagation. Such a response is not possible with natural materials,” he says.

This leads to new paths for electromagnetic wave control, as all wave properties, such as frequency, polarization and direction of propagation, can be manipulated at high levels of precision. “This offers a means to more precisely control light, especially at frequencies where the response of existing natural materials does not offer many possibilities, for example THz frequencies,” explains Prof. Soukoulis.

The possibility to engineer the electromagnetic properties of metamaterials, combined with the fact that those properties are highly dependent on the design of metamaterial building blocks – with many design possibilities available – means metamaterials have great potential in a wide range of applications, including in sensing, monitoring and security and telecommunications. Yet there are some significant challenges to overcome before this wider potential can be realised, which forms a core part of the agenda of the EC-backed PHOTOMETA project. “The project’s overall aim is to demonstrate novel optical metamaterials and systems 16

with new functionalities, offering great advances in a variety of applications, ranging from quantum information processing to nano-lasers and wave sensors,” outlines Prof. Soukoulis. This work centers around four major points: “We aim to design and fabricate new optical metamaterials and metasurfaces for electromagnetic wave control. We also want to understand and reduce losses in optical metamaterials,” continues Prof. Soukoulis. “Thirdly, we aim to design dynamic metamaterials for tunable devices and THz generation, and also to manipulate and control optical forces using metamaterials.”

higher and higher rates; yet this is not always possible, which means that it is very difficult to achieve a strong resonant response beyond a certain frequency, resulting in saturation of the operational frequency,” outlines Prof.

Soukoulis. “The second major shortcoming of metallic optical metamaterials is their significant losses, due to the absorption of the metals, which increases with frequency.”

This factor alone may weaken the resonances and degrade the operation of metamaterials, representing a significant challenge in terms of the project’s wider goals. Prof. Soukoulis and his colleagues are investigating different approaches,

The electromagnetic response achievable with metamaterials can be quite unusual and unconventional, for example leading to negative wave refraction or backwards propagation.

Such a response is not possible with natural materials Functional metamaterials Researchers are taking quite a broad-based approach to this work, exploring novel designs and constituent materials for the development of different types of functional optical metamaterials. This research is built on a thorough understanding of how metamaterials behave and how their novel properties are realised. “Metamaterials behave as collections of oscillators, which need strong resonances in order to realise their exotic properties. Initially, this was demonstrated in the microwave region and subsequently there was a strong effort to go to optical frequencies,” explains Prof.

Soukoulis.

Controlling the properties of a metamaterial at higher frequencies is a significant technical challenge however. “In order to move up in frequency, electrons need to oscillate at

aiming to regain control of the properties of a metamaterial in the optical range. “To solve the frequency saturation problem, we have introduced the dark mode concept.

These non-radiating modes, if present in the metamaterial building blocks, can provide very strong resonances at very high frequencies, giving us the desired response. This concept can be supported in configurations with very few metallic parts, leading to reduced losses as well,” he says. An alternative approach to overcoming losses is to use the dark mode concept in an Electromagnetically Induced Transparency (EIT) scheme, while there are also other options. “We have incorporated gain in the metallic metamaterials, which can lead to loss compensation and restore their strong resonances. We are also exploring the potential of metamaterials made entirely by dielectrics,” continues Prof. Soukoulis.

EU Research

Figure Left: Like natural materials which acquire their properties from the properties of their individual atoms, metamaterials acquire their response from the response of their meta-atoms, like the split-ring resonators shown in the figure. A very important difference is that while atoms, and thus their properties, are given by nature the meta-atoms are man-made and they can be engineered at will, metamaterial response.

Figure Right: A metasurface composed of nanoscale metallic split ring resonators (SRRs) – white color in the background image -, if excited by a properly polarized wave with frequency equal to that of the SRR magnetic resonance, can give strong and broadband THz radiation emission, via optical rectification, which is induced by the non-linear response of the metallic SRRs. Work published in Nature Communications (Nature Communications 5, 3055 (2014); doi:10.1038/ncomms4055).

Figure Left: A novel and uncommon response that can be obtained with metamaterials is the so-called negative refraction, as shown in the figure, where a wave is negatively refracted at the two interfaces of a negative index metamaterial (NIM) slab placed in air. The NIM slab can be also impedance matched with air (i.e. if both permittivity and permeability are equal to -1), resulting to no-reflected waves at its interfaces, and thus total transmission.

PHOTOMETA Photonic Metamaterials: From Basic Research to Applications Project Objectives

“PHOTOMETA” project focuses on the theoretical study of novel artificial materials characterized here as metamaterials (MMs), [such as photonic crystals (PCs), negative index materials (NIMs), and plasmonics] which enable the realization of innovative electromagnetic (EM) properties unattainable in naturally existing materials.

Project Funding

MATERIALS, Meta Materials, Photonics, ERC-AG-PE3 - ERC Advanced Grant Condensed matter physics.

EU contribution: EUR 2 100 000.

Project Partners

• Professor Eleutherios Economou • Professor Maria Kafesaki • Dr George Kenanakis

Contact Details Figure Right: In our proposed novel dielectric laser design, the gain medium (green-color in top panel), which is bound by two silver stripes (grey-color), lases into the dark mode (blue and red peaks in the middle of the bottom panel) and the energy is therefore stored indefinitely between the two silver strips. When a scattering element is included (green strip in top-panel), the stored energy is able to radiate away in the form of a wave (blue and red crests). Here only one unit cell is shown, which is periodically repeated to form the actual radiating metasurface. Work published in Physical Review Letters (Phys.

Rev. Lett. 118, 073901 (2017); https://doi.org/10.1103/PhysRevLett.118.073901).

Wider potential The project’s work is largely exploratory in nature at this stage, with Prof. Soukoulis and his colleagues pursuing fundamental research into the properties of metamaterials.

However, Prof. Soukoulis is fully aware of their wider potential. “As we learn more about the properties of the metamaterials we study and we realize the possibilities offered by them, we become more and more excited with the subject. There are many open research directions where the PHOTOMETA research could be exploited. For example, quantum metamaterials, non-linear metamaterials and optical metasurfaces for full wave-front control,” he outlines. Metamaterials offer a unique platform for both basic and applied research, providing a sound platform for further investigation. “Although the research in this project is mainly exploratory, we always have specific applications in mind,” continues Prof. Soukoulis. “For example, sensing is one of the areas where metamaterials can offer revolutionary advancements, from microwaves, for the evaluation of dielectric materials for example, to the visible region.”

There are also many other potential applications of metamaterials, including shielding of high-frequency electronic devices, like cell phones, laptops, aircraft electronics and medical devices. The

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performance of these devices can be influenced by the presence of neighbouring electronic instruments, which Prof.

Soukoulis says can be a major problem. “This electromagnetic interference (EMI) can cause malfunction of sensitive devices, primarily medical equipment, and as a result, they can become unsafe, or even harmful,” he explains.

Researchers are looking at ways to shield these devices from this interference, and so enable them to continue functioning effectively.

“We have investigated experimentally the electromagnetic properties of graphenebased paint-like layers, large-scale polymeric composite films, 3D printed polymeric samples, and others, as possible candidates for EMI shielding,” says Prof. Soukoulis.

The project’s research has already opened up new paths of investigation, leading to a further extension of the initiative, so that Prof. Soukoulis and his colleagues can devote more time and energy to investigating this area. For his part, Prof. Soukoulis is convinced that metamaterials will be used widely in the future, in large part due to their unique properties, which encourages him to continue his research in this area in future. “We definitely plan to stay in this area, focusing mainly on basic research, but basic research that is associated with great potential in terms of applications,” he stresses.

Project Coordinator, Professor Costas Soukoulis FOUNDATION FOR RESEARCH AND TECHNOLOGY HELLAS, Greece N PLASTIRA STR 100 70013 HERAKLION Greece T: +30 2810 391380 E: soukoulis@ameslab.gov W: https://cordis.europa.eu/project/ rcn/107047_en.html

Professor Costas Soukoulis

Costas Soukoulis is a Senior Scientist in the Ames Laboratory and a Distinguished Professor of Physics at Iowa State University. He has been instrumental in creating the revolutionary fields of photonic crystals (PCs) and left-handed materials (LHMs), extending the realm of electromagnetism and opening exciting new applications.

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