BUTT ET AL . VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Plasmonic Band Gaps and Waveguide Effects in Carbon Nanotube Arrays Based Metamaterials Haider Butt, † Qing Dai, † Ranjith Rajesekharan, † Timothy D. Wilkinson, † and Gehan A. J. Amaratunga †,‡, * † Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom, and ‡ Sri Lanka Institute of Nanotechnology (SLINTEC), Lot 14, Zone A, EPZ, Biyagama, Sri Lanka A dvances in nanoscale fabrication al- low for the realization of artificial materials with properties that do not exist in nature, metamaterials. 1 They are composed of subwavelength magnetic structures, placed at close proximity to each other. 2 Due to mutual coupling between individual structures, they present proper- ties to incident electromagnetic radiation that are different from those associated with the material from which the structures are comprised of. We report the use of multi- walled carbon nanotubes (MWCNTs) as sub- wavelength structures to produce optical metamaterials that exhibit artificial dielec- tric properties and band gaps within the optical regime, paving the way toward in- teresting waveguide effects. Multiwalled carbon nanotubes first re- ported in ref 3 are very interesting materials and have been the focus of enormous re- search in the past decade. They are mostly metallic and are able to carry high current densities along their axis. 4 Apart from their myriad applications such as in field emission displays, 5 as electrodes in rectifiers, 6 solar cells, 7 and optical antenna arrays, 8 periodic arrays of vertically aligned carbon nano- tubes have also been used as photonic crystals. 9 Photonic crystals present periodi- city in their dielectric constant, which causes Bragg scattering of the incident electro- magnetic waves with comparable wave- lengths. This introduces band gaps, regions where no wave propagation is allowed. If these band gaps fall in the optical frequency range, they are called photonic band gaps. Therefore, for the carbon nanotube arrays to display photonic band gaps (in the op- tical regime), their lattice constants should be on the order of a few hundred nanome- ters. To the best of our knowledge, the carbon nanotubes based photonic crystals reported so far have lattice constants on the order of 1 μm, and they display photonic bands well outside of the optical regime in the terahertz range. Due to the technical difficulties in measuring the band gaps in these regimes, we see that so far only the optical diffraction and scattering measure- ments from periodic arrays of CNT have been reported. 9À11 We, however, report highly dense periodic arrays of multiwalled carbon nanotubes having a tube radius of 50 nm and lattice constants of 400 nm. When grown at such a close proximity, the metallic properties of the nanotube arrays can be used to achieve metamaterials display- ing an arti ficial negative dielectric constant toward incident light, causing reflection. We, for the first time, report the computational * Address correspondence to gaja1@cam.ac.uk. Received for review August 31, 2011 and accepted October 22, 2011. Published online 10.1021/nn203363x ABSTRACT Highly dense periodic arrays of multiwalled carbon nanotubes behave like low-density plasma of very heavy charged particles, acting as metamaterials. These arrays with nanoscale lattice constants can be designed to display extended plasmonic band gaps within the optical regime, encompassing the crucial optical windows (850 and 1550 nm) simultaneously. We demon- strate an interesting metamaterial waveguide effect displayed by these nanotube arrays containing line defects. The nanotube arrays with lattice constants of 400 nm and radius of 50 nm were studied. Reflection experiments conducted on the nanoscale structures were in agreement with numerical calculations. KEYWORDS: metamaterials . carbon nanotube arrays . plasmonic band gaps . optical waveguides ARTICLE