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Description
Kriti Agarwal1 Chao Liu1 Jeong-Hyun Cho1

1, University of Minnesota, Minneapolis, Minnesota, United States

Split-ring resonator (SRR) based metamaterials have been extensively studied due to their enhanced ability to confine light of wavelength several orders of magnitude larger than the dimension of the resonator. The strong confinement of incident light has been explored for the development of a diverse range of sensors including biological and chemical sensors that can assess the properties of the molecules in the vicinity of the split. The polarization dependent switching of resonant modes and resonant frequency of two-dimensional (2D) SRR structures presents a major hurdle in the widespread application of metamaterial sensors where the orientation of the SRR is hard to control. Furthermore, the 2D planar SRR structures do not undergo any resonance when the incident electric field is polarized perpendicular to the plane of the resonator. However, if the 2D metamaterials are self-assembled to form three-dimensional (3D) structures, novel optical properties can be achieved that cannot be realized in 2D planar SRRs. When six symmetric 2D X-shaped resonator segments fabricated on top of square surfaces are self-assembled, the resulting 3D cubic structure forms an 8-pointed star-shaped octagram SRR (OSRR). The octagram resonator consists of splits only at the corners of the cube, thus, achieving 3D SRR splits that are equally moderated by all three vectors (electric field, magnetic field, and wave) for every possible orientation of the OSRR. The 3D splits in OSRR achieve a perfectly isotropic (angle-invariant) transmission response. The strong OSRR coupling and angle-invariant amplitude offer a two-fold advantage i.e. a 25 times higher shift in resonant frequency (25 times higher sensitivity) than 2D SRRs and amplitude-based detection at low concentrations that cannot cause a measurable shift in resonant frequency. However, even with the enhanced sensitivity the constant need for replenishing antibodies used for binding the targeted molecules to the surface of the OSRR presents a major challenge and can also limit the detection capabilities for unknown target molecules. If the surfaces of the cube forming the OSRR are composed of porous nanomaterials (graphene oxide), the properties of the 3D metamaterials can be further tuned for desired sensory application. The strong affinity of GO functional groups towards all chemical and biological molecules and sieving properties of the porous GO layers provide an ideal surface for adhesion of targeted molecules for non-labeled sensing mechanisms. By varying the number of GO layers forming the 3D cubes, properties of the OSRR are tuned for control over sieving and molecular adsorption as well as extending the sensitivity of octagram sensors due to enhanced adhesion of targeted molecules.

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