Metamateriales plasmónicos

Metamaterials

The prefix meta-, from the Greek μετά, "beyond" is used to refer to a new type of device: metamerials. They may be defined as those materials or artificial devices, i.e. man-made, that have unusual electromagnetic properties not achievable by natural media and come from the designed structure and not its composition.

Metamaterials are of great importance in electromagnetism as they allow us to obtain materials with an adjustable refractive index, being able to obtain the most striking negative refractive indices, creating superlenses, which dramatically improve the quality of images, and securing means to obtain media with unusual or extreme constitutive parameters. Many applications can be achieved with metamaterials, such as modulators, filters, lenses, couplers and antennas.

We have been working in the design of devices with negative index. In this regard, several designs have been proposed and characterized thus obtaining a low loss negative index at optical frequencies. Another line of research focuses on the design of structures with artificial magnetic activity in the visible range.

Results

Magnetic Hot Spots in Closely Spaced Thick Gold Nanorings at NIR Wavelengths

Light–matter interaction at optical frequencies is mostly mediated by the electric component of the electromagnetic field, with the magnetic component usually being considered negligible. Recently, it has been shown that properly engineered metallic nanostructures can provide a magnetic response at optical frequencies originated from real or virtual flows of electric current in the structure. In this work [1], we demonstrate a magnetic plasmonic mode which emerges in closely spaced thick gold nanorings. The plasmonic resonance obtains a magnetic dipole character by sufficiently increasing the height of the nanorings. Numerical simulations show that a virtual current loop appears at resonance for sufficiently thick nanorings, resulting in a strong concentration of the magnetic field in the gap region (magnetic hot spot). We find that there is an optimum thickness that provides the maximum magnetic intensity enhancement (over 200-fold enhancement) and give an explanation of this observation. This strong magnetic resonance, observed both experimentally and theoretically, can be used to build new metamaterials and magnetic nanoantennas at optical frequencies.

References

[1] "Magnetic Hot Spots in Closely Spaced Thick Gold Nanorings", M. Lorente-Crespo, L. Wang, R. Ortuño, C. García-Meca, Y. Ekinci, and A. Martínez, Nano Letters 13 (6), 2654 - 2661 (2013).

NTC researchers demonstrate the first double-negative metamaterial in the visible spectrum

NTC researches from Universitat Politècnica de València have demonstrated experimentally a multilayer double-negative metamaterial working in the visible range [1]. In collaboration with researchers from the King's College London, our team has built a three-layer fishnet metamaterial that exploits a second-order magnetic resonance to reach the visible regime. The low-loss nature of the employed magnetic resonance, together with the effect of the interacting adjacent layers, results in a figure of merit as high as 3.34. A wide spectral range of negative index is achieved, covering the wavelength region between 620 and 806 nm with only two different designs. This result has been published in Physical Review Letters and and recognized as a milestone in optical metamaterials in Nature Photonics [2].

References

[1] "Low-Loss Multilayered Metamaterial Exhibiting a Negative Index of Refraction at Visible Wavelengths", C. García-Meca, J. Hurtado, J. Martí, A.Martínez, W. Dickson and A. V. Zayats, Physical Review Letters 106, 067402(1)-067402(2) (2011).

[2] "Past achievements and future challenges in 3d photonic metamaterials", C. M. Soukoulis, and M. Wegener, Nat. Photonics 5, 523 (2011).

Negative index metamaterial using coaxial plasmonic waveguides

We have developed a novel negative-index metamaterial [1] based on an array of plasmonic waveguides exhibiting backward modes. The metamaterial is unique in that it uses a fundamentally different approach to the usual resonant-based method of achieving negative index, by using the intrinsic backward properties of plasmonic waveguides which exist under certain conditions. The metamaterial operates throughout a big bandwidth within the visible spectrum, is polarization independent, and is subwavelength in all directions. An extension to 3D is also suggested. Recently, two papers have appeared in Optics Express [2] and Nature Materials [3] which confirm the results.

Figure

References

[1] "Coaxial plasmonic waveguide array as a negative-index metamaterial", F. J. Rodríguez-Fortuño, C. García-Meca, R. Ortuño, J. Martí, and A. Martínez, Opt. Lett. 34, 3325-3327 (2009).

[2] "Negative refractive index in coaxial plasmon waveguides", René de Waele, Stanley P. Burgos, Harry A. Atwater, and Albert Polman, Opt. Express 18, 12770-12778 (2010).

[3] "A single-layer wide-angle negative-index metamaterial at visible frequencies", Stanley P. Burgos, René de Waele, Albert Polman and Harry A. Atwater, Nature Materials 9, 407 - 412 (2010).

Double-negative polarization-independent fishnet metamaterial in the visible

We have designed a metamaterial that exhibits a truly negative refractive index in the visible spectrum for arbitrary light polarization [1]. This metamaterial is based on a high-order plasmonic resonance present in the so-called fishnet structure and overcomes the challenge of achieving a low-loss polarization-independent medium in the visible spectrum. The experimental result is the first demonstration of a multilayer metamaterial with simultaneously negative permittivity and permeability in the visible domain.

References

[1] "Double-negative polarization-independent fishnet metamaterial in the visible spectrum", C. García-Meca, R. Ortuño, F. J. Rodríguez-Fortuño, J. Martí, and A. Martínez, Optics Letters, Vol. 34, No. 10, pp. 1603-1605 (2009).

Metamaterials over flexible substrate

Researches at NTC have fabricated a metamaterial working at terahertz frequencies made over a flexible polypropylene substrate. The experimental measurements, in accordance with the numerical calculations, show the metamaterial reliance on the impinging electric field polarization. The structure’s symmetry yields purely electrical resonant responses eliminating bianisotropy effects. The widely used bendable polypropylene polymer may promote the insertion of metamaterial-based structures with special electromagnetic response in a number of objects of our daily lives such as textiles, automotive components, and polymer banknotes

References

[1] "Terahertz metamaterials on flexible polypropylene substrate", R. Ortuño, C. García-Meca, F.J. Rodríguez-Fortuño, J. Hurtado and A. Martínez, to be published.