Silver nanowire hyperlenses
Final Report Abstract
Within the scope of the project the light propagation in silver nanowire metamaterials and their use for the creation of hyperlenses, which should allow the focusing of plane waves into subwavelength sized focal spots, was investigated using theoretical simulation techniques and experiments. Nanopores in aluminum oxide membranes were filled with silver using an electrochemical deposition process, which resulted in hexagonally, self-ordered nanowire arrays with a period of a=100nm representing hyperbolic metamaterials for visible and near IR light. To improve the homogeneity and long range periodicity of the nanowire array a lithographic prestructuring of the Al-substrates was performed using 3 beam interference lithography and resulting in perfect 2D hexagonal pore patterns with a long range periodicity of a= 300nm. To reduce the period towards the intended 170nm or smaller, anodization at reduced voltages was attempted. However, the larger diameter of the pores at the prestructured sites still hindered the effective spawning of additional pores in between. To determine the 2 principal values of the effective permittivity tensor of the nanowire array (uniaxial anisotropy) effective medium calculations, finite element simulations of the light propagation in the exact periodic wire array and angular resolved transmission experiments were performed. As expected the permittivity for fields parallel to the nanowires shows negative values and the permittivity for fields perpendicular to the wires positive values confirming the indefinite nature of the dielectric tensor, which is characteristic for a hyperbolic metamaterial. While for small metal fill factors on the order of 15% an overall good agreement of the permittivity values determined by the 3 different methods was observed, for larger fill factors the values derived by field plots from the finite element simulation showed a much larger discrepancy. This is attributed to the numerical errors/uncertainties, which amplified in the specific evaluation procedure. An alternative evaluation of the numerical data is therefore planned. Based on the permittivity values derived from EMT and experimental measurements two lens designs were simulated using finite elements software – an immersion lens, where a hyperbolically shaped trough is carved out of the metamaterial and a Fresnel zone plate lens, where a pattern of opaque stripes is placed at the top of a planar nanowire slab. Both designs represent cylindrical lenses where the focusing action only appears in one dimension. For both cases the formation of subwavelength focus spots is observed in the simulations for wavelengths in the visible and near IR. To access these spots the metamaterial has to be cut off at the focus depth leading to back reflections at the bottom interface and a slight widening of the focus spot. The use of a higher refractive index medium at the bottom to reduce impedance mismatch and back reflection might therefore be beneficial. Finally, the two different hyperlens designs were realized from the silver nanowire metamaterials. The immersion lends was carved out using ion beam milling and the Fresnel zone plate created by the deposition of an opaque Cr layer and subsequent local stripping by ion beam. A wedged bottom surface was created by ion beam milling as well to ensure the meeting of the correct focus depth at some point. While the optical characterization of the Fresnel lens is still to be performed, for the immersion lens a bright line was experimentally observed in transmission in a microscope, which might represent the focus. However, an enhanced area of transmission due to lowest absorption in this thinnest region of the immersion lens can not be completely excluded. Currently SNOM measurements are underway to investigate the focus position and width at the bottom of the hyperlenses when a plane wave is incident at the top.
Publications
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“Optical theorem and multipole scattering of light by arbitrarily shaped nanoparticles.”
A. Evlyukhin, T. Fischer, C. Reinhardt, B. Chichkov
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“The Interaction of Guest Molecules with Co-MOF-74: A Vis/NIR and Raman Approach.”
I. Strauss, A. Mundstock, D. Hinrichs, R. Himstedt, A. Knebel, C. Reinhardt, D. Dorfs, J. Caro
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“Experimental Demonstration of Surface Plasmon Polaritons Reflection and Transmission Effects.”
L. Zheng, U. Zywietz, A. Evlyukhin, B. Roth, L. Overmeyer, C. Reinhardt
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“Femtosecond time-resolved photoemission electron microscopy operated at sample illumination from the rear side.”
A. Klick, M. Großmann, M. Beewen, P. Bittorf, J. Fiutowski, T. Leißner, H.-G. Rubahn, C. Reinhardt, H.-J. Elmers, M. Bauer
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“Nanofabrication of High-Resolution Periodic Structures with a Gap Size Below 100 nm by Two-Photon Polymerization.”
L. Zheng, K. Kurselis, A. El-Tamer, U. Hinze, C. Reinhardt, L. Overmeyer, B. Chichkov
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“Nanoscale Broadband Deep-Ultraviolet Light Source from Plasmonic Nanoholes.”
L. Shi, J. R. C. Andrade, J. Yi, M. Marinskas, C. Reinhardt, E. Almeida, U. Morgner, M. Kovace
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“Omnidirectional Surface Plasmon Polaritons Concentration in 3D Metallic Structures.”
L. Zheng, A. Evlyukhin, L. Overmeyer, C. Reinhardt
