Metamaterials: Nano antennas from        Asymmetric Split Ring Resonators (ASSRs)

Metamaterials can be described as materials (usually man-made) in which the geometrical arrangements and constituents are smaller than the wavelength of light that they interact with. These structural elements themselves are made of conventional materials, ultimately of normal atoms. Metamaterials represent the next level of structural organization of matter and can be designed to achieve advantageous and unusual electromagnetic properties.

They differ from photonic crystals in that photonic crystals are necessarily periodic and have structures with a repeat distance or lattice constant of the order of half the optical path length, while in metamaterials the  (nominal) lattice constant is less than half the optical path length. Also metamaterials need not be periodic.

At the optical scale, particularly in planar geometry, frequency selective surface or even nano antennas are valid descriptions. (The individual elements are size tuned for resonance at particular wavelengths). Also as we move through the frequency spectrum from RF through microwave, terahertz, infra red to visible, the ratio of  wavelength of light to structure size (nominal lattice constant) can reduce to around 5:1


Split Ring Resonators have been used in many areas of metamaterial research including the pioneering cloaking work in the microwave region. Here we use an extra split to give effectively two coupled resoantors. When exposed to IR light the induced electric field is concetrated at the ends of the arcs.

The asymmetric structures produce two resonances giving each a sharper peak (higher Q) than simple split rings. This is shown in the black trace above. (The spike around 4.3 microns is due to atmospheric CO2). The reflectance spectra for the above structures is coated with a test analyte, in this case the well known electron beam lithography resist, Poly(methyl methacrylate) (PMMA) is shown in red.  Two things to notice are, the large shift in wavelength caused by increasing the refractive index immediately above the rings, and secondly the extra feature at 5.8 micrometers. The latter is the enhanced signal from the molecular resonance in the PMMA ie the carbonyl resonance. This interaction is also known as a Fano resonance as it describes a fast Lorentz oscillator interacting with a slowly varying (in this case ASRR) oscillator. Both effects can be used for enhanced detection of organic materials.


People

  • Dr Nigel P Johnson
  • Dr Basudev Lahiri
  • Dr Ali Khokhar
  • Prof Richard De La Rue
  • Prof Scott McMeekin

Publications

  • 'Resonance hybridization in nanoantenna arrays based on asymmetric split-ring Resonators',         Basudev Lahiri, Scott G. McMeekin, Richard M. De La Rue, and Nigel P. Johnson , Applied Physics Letters 98, 153116 (2011) http://dx.doi.org/10.1063/1.3579537
  • 'Asymmetric split ring resonators for optical sensing of organic materials',                                           Basudev Lahiri, Ali Z. Khokhar, Richard M. De La Rue, Scott G. McMeekin and Nigel P. Johnson Optics Express. Vol. 17, Issue 2, pp. 1107-1115, (2009) http://dx.doi.org/10.1364/OE.17.001107
  • 'Magnetic response of split ring resonators (SRRs) at visible frequencies',
    Basudev Lahiri, Scott G. McMeekin, Ali Z. Khokhar, Richard M. De La Rue and
    Nigel P. Johnson, Optics Express Vol 18 No3 (02-01-2010)
    http://dx.doi.org/10.1364/OE.18.003210
  • 'Impact of titanium adhesion layers on the response of arrays of metallic split-ring
    resonators (SRRs)', Basudev Lahiri, Rafal Dylewicz, Richard M. De La Rue and
    Nigel P. Johnson, Optics Express, 18(11), pp. 11202-11208, 24th May (2010).                   
    http://dx.doi.org/10.1364/OE.18.011202
  • Metamaterials at Optical Frequencies: Fabrication and Measurements. Johnson NP, De La Rue RM, De La Rue SA  In: Applications of Metamaterials. Pub CRC Press, Ed Capolino F. Chapter 30:30.1-30.12 (2009) ISBN: 978-14200-5423-1