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Hyperbolic metamaterials (HMM), a class of artificially engineered materials with a highly anisotropic permittivity response originating from opposite signs of the principal components of the electric tensor, have attracted significant interest in recent years due to their ability to manipulate the propagation light in exotic ways. Such materials enable distinctive optical phenomena such as negative refraction, super-resolution imaging, and enhanced spontaneous emission. Here we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM (composed of alternating, sputtered films of Ag and SiO2) to achieve high-sensitivity proximity detection of metallic and dielectric nanoparticles in a transmission dark-field configuration. The iso-frequency surface is unique in that propagation of light inside the HMM over the entire visible-range is allowed only for electromagnetic modes having tangential spatial frequencies kx exceeding the free-space wavevector k0 by over a factor of two (kx > 2k0). The nanoparticle detector consists of a 500-nm thick slab of HMM having an input side coated with ≈ 6-nm-thick Ag nano-islands (to couple light into the HMM) and an exit side consisting of a template-stripped ultra-smooth Ag surface. As a result of the optical bandgap of HMM, light illuminating the input surface at any angle (intensity I0) is effectively blocked from transmitting through the slab; only a vanishingly small evanescent-amount (intensity I1) leaks through, corresponding to an optical density OD = log(I1 / I0) ≈ 8 at λ0 = 633 nm. Bringing nanoparticles into deep-subwavelength proximity or contact with the pristine exit-surface of the detector opens up efficient transmission channels, corresponding to out-scattering of high-spatial frequencies into free space propagating modes (yielding an intensity for a given nanoparticle density, and an optical contrast ratio for detection defined as γ. FDTD simulations for an HMM structure having a perfectly flat exit-surface decorated with spherical gold particles of diameter 100 nm predict values of γ as high as ≈ 890. Two-dimensional finite-difference-time-domain (FDTD) simulations predict high-contrast detection of particles of diameter down to ≈ 10 nm whether composed of metals (Ag, Cr) or dielectric (SiO2).
We experimentally demonstrate that this HMM-based structure is capable of revealing in transmission spherical Au nanoparticles of diameter ≈ 40 nm deposited on its template-stripped surface. Incoherent light is used to illuminate the Ag nano-islands side of the detector, and a 100x objective lens (NA = 0.75) is used to collect the light exiting the template-stripped side of the device. The incoherent light is obtained by filtering a white-light LED source with a bandpass filter centered at λ0 = 633 nm (bandwidth = 92 nm). The use of broadband incoherent light, rather than a coherent laser source, minimizes the sizes of speckles created by the Ag nano-islands. Effective medium theory (EMT) predicts relatively constant type-II hyperbolic iso-frequency characteristic throughout this frequency band. The Au nanoparticles are randomly dispersed onto the surface, yielding an average inter-particle distance of ≈ 5 μm, as measured by scanning electron microscopy. The optical transmission images clearly indicate the presence of the Au nanoparticles on the detector surface, which appear as bright spots. The average contrast ratio γ for single Au nanoparticles is measured to be ≈ 22, with it being presently limited by residual roughness of the template-stripped Ag surface.
In conclusion, we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM to achieve optical proximity detection of nanoparticles, using a simple transmission scheme not requiring the use of dark-field optics. In this talk, we will further discuss recent results of fluorescence imaging performed under the same HMM platform, and efforts on expanding this work towards single-molecule imaging in a wide-field configuration. Due to its high sensitivity in deep-subwavelength proximity to a surface, this HMM-based device hints at promising applications in bio-chemical sensing, particle tracking and contamination analysis.
References:
[1] A. Poddubny, I. Iorsh, P. Belov, P. & Y. Kivshar, “Hyperbolic metamaterials,” Nat. Photon. 7, 958–967 (2013).
[2] H.N. Krishnamoorthy et al. “Topological transitions in metamaterials,” Science 336, 205–209 (2012).
[3] M.Y. Shalaginov et al. “Broadband enhancement of spontaneous emission from nitrogen-vacancy centers in nanodiamonds by hyperbolic metamaterials,” Appl. Phys. Lett. 102, 173114 (2013).
[4] T. Xu & H.J. Lezec, “Visible-frequency asymmetric transmission devices incorporating a hyperbolic metamaterial,” Nat. Comm. 5, 4141 (2014).
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