Low loss broadband polarization independent fishnet negative index metamaterial at 40 GHz Wissem Sfar Zaoui * , Ke Chen, Wolfgang Vogel, Manfred Berroth Universita ¨t-Stuttgart, Institute of Electrical and Optical Communications Engineering, Pfaffenwaldring 47, 70569 Stuttgart, Germany Received 25 December 2010; received in revised form 10 February 2011; accepted 18 February 2011 Available online 25 February 2011 Abstract We present a polarization independent fishnet negative index metamaterial at 40 GHz. The structure is investigated theoretically using finite element method simulations and experimentally by measuring the amplitude and phase of the S-parameters. The experimental setup for free space measurements of both transmission and reflection is hereby introduced. The internal properties are thereafter retrieved and show the double-negative behavior of the structure. This negative index metamaterial exhibits very high transmission (À0.13 dB), low reflection (À33.1 dB) and a high figure of merit (FOM = jRe(n)/Im(n)j = 42), where the real part of the refractive index is nearly À1 (Re(n) = À0.93) at 40 GHz. # 2011 Elsevier B.V. All rights reserved. Keywords: Metamaterial; Negative refractive index; Double negative; Fishnet; Free space measurement; Microwave 1. Introduction Metamaterials are artificially engineered materials, which exhibit properties that do not exist in nature. These composite structures give access to control the internal electromagnetic properties, i.e. the electric permittivity e and the magnetic permeability m, in such a manner that a variety of attractive phenomena like negative refraction and cloaking can take place. In recent years, negative index metamaterials (NIM), which have negative refractive index n rising from e < 0 and m < 0 simultaneously, have aroused interest of many researchers due to the promising planar perfect lens application that may overwhelm the diffraction limited conventional lens [1]. The basic idea to achieve negative m is to excite circular currents that generate a magnetic resonance, whereas negative e can be produced using continuous metallic wires for frequen- cies below the plasma frequency. This concept was first concretized by Smith et al. who utilized a combination of split ring resonators and metallic wires [2] and later by Dolling et al. using cut-wire pairs [3]. Another NIM structure called fishnet was reported in the same year by Zhang et al. and showed better performance due to the combined electromagnetic response of the wires and slabs [4]. However, the presented fishnet structures until now suffer from low FOM due to high losses and small negative index bandwidth due to the resonant behavior of the structure [5–9]. To enhance the efficiency of these NIMs, the well-known cross structure is modified, so that a better impedance matching to free space overlaps with the negative index region, giving rise to higher transmission and larger bandwidth. In this study, we present a polarization independent fishnet structure that is designed to have a refractive index n = À1 at 40 GHz www.elsevier.com/locate/photonics Available online at www.sciencedirect.com Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 245–250 * Corresponding author. E-mail address: wissem.sfarzaoui@int.uni-stuttgart.de (W. Sfar Zaoui). 1569-4410/$ – see front matter # 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2011.02.003