Cloaking dielectric spheres by a shell of metallic nanoparticles M. Farhat 1 , S. M ¨ uhlig 1 , C. Rockstuhl 1 , and F. Lederer 1 1 Institute of Condensed Matter Theory and Solid State Optics, Abbe Center of Photonics, Friedrich-Schiller-Universit¨ at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany Fax: +49(0)3641—9-47152; email: mohamed.farhat@uni-jena.de Abstract We describe a metamaterial made of randomly arranged silver nanospheres that is placed as a shell around a dielectric sphere with the purpose of cloaking it. The device is studied rigorously by full wave simulation and approximately by treating the shell of silver nanospheres within the Maxwell- Garnett theory. By both means it is concisely shown that such a metamaterial shell significantly reduces the visibility of the dielectric sphere. Advantages and disadvantages of such a cloak when compared to other implementations are disclosed. 1. Introduction In 2000 Sir John Pendry showed that a slab of negative refractive index could focus light in a special manner: it promises to overcome the well-known Abbe resolution criterion [1]. And since then, re- search on metamaterials gained momentum among the optics and acoustics communities; giving rise to many applications. The most intriguing outcome is possibly the invention of invisibility devices that hide objects from external observers by forcing light (or any other type of waves) to travel along curved trajectories [2]. Implementation of such cloaks came in sight by relying on metamaterials that provide extreme values of the permittivity and permeability. In the same vein, Al` u et al. proposed another tech- nique based on scattering cancelation which consists of coating the object to be cloaked by plasmonic layers to cancel the total scattered field [3]. In this contribution, we build upon this latter work, and fur- ther suggest to use a bottom-up metamaterial designing of such cloak instead of homogenous plasmonic layers. The advantages are a more efficient cloaking efficiency at higher frequencies, a possibly better tunability that avoids metallic shells with a minuscule thicknesses, and a back-up of a wide range of tech- nologies for its implementation based on colloidal nanochemistry. On top of reproducing the cloaking functionality using plasmonic shells, our work contributes to pave the way to consider metamaterials in the design process of functional devices. That is possible because the functionality of the cloak can be analyzed using full wave simulations, which take into account all the details of the individual metallic nanoparticles forming the shell, but also by considering them as an effective medium. It will be shown that in the plasmon resonance of the particles deviating results are encountered whereas at the operational frequency of the cloak, which is off-resonant, predictions by both methods perfectly agree. 2. Numerics To design a cloak on the base of the scattering cancellation technique the shell is required to have either a small positive (i.e. larger than zero but smaller than one) or a negative permittivity [3]. A single shell Metamaterials '2011: The Fifth International Congress on Advanced Electromagnetic Materials in Microwaves and Optics ISBN 978-952-67611-0-7 - 997 - © 2011 Metamorphose-VI