Optical Negative Refraction in Bulk Metamaterials of Nanowires
CALIFORNIA UNIV BERKELEY NANOSCALE SCIENCE AND ENGINEERING CENTER
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Metamaterials are artificially designed subwavelength composites possessing extraordinary optical properties that do not exist in nature. They can alter the propagation of electromagnetic waves, resulting in negative refraction 1, subwavelength imaging 2, and cloaking 3. First reported at microwave frequencies by using metamaterials made of an array of split ring resonators and metallic wires 4, negative refraction has been observed in two-dimensional 2D photonic crystals into the infrared IR region 5 8 and in surface plasmon waveguides at visible frequencies 9. In both cases, negative refraction is constrained in two dimensions and is limited to a narrow band of frequencies. An indirect observation of negative refraction in the mid-IR region was also reported in a semiconductor multilayer structure 10. Creating bulk metamaterials that exhibit negative refraction for visible light remains a major challenge because of substantial resonance losses and fabrication difficulties. Recent theoretical studies suggest that metamaterials consisting of metal wire arrays exhibit an optical response at frequencies far away from resonances 11, 12, in which electromagnetic EM waves propagating along the nanowires exhibit negative refraction at a broad frequency band for all angles 13. Moreover, the material loss is much lower than traditional metamaterials with similar functionality. We report observations of negative refraction in bulk metamaterials composed of silver nanowires with separation distance much smaller than the wavelength at optical frequencies Fig. 1A. A porous alumina template was prepared by electrochemical anodization 14, intowhich silver nanowires were electrochemically deposited. A 1-mm-wide slit, etched through a 250-nm thick silver film coated on the metamaterials, was illuminated by a collimated diode laser beam at different incident angles see left side of Fig. 1A.
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