This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. INVITED PAPER Development of Bulk Optical Negative Index Fishnet Metamaterials: Achieving a Low Loss and Broadband Response Through Coupling By Jason Valentine , Shuang Zhang , Thomas Zentgraf, and Xiang Zhang ABSTRACT | In this paper, we discuss the development of a bulk negative refractive index metamaterial made of cascaded Bfishnet[ structures, with a negative index existing over a broad spectral range. We describe in detail the design of bulk metamaterials, their fabrication and characterization, as well as the mechanism of how coupling of the unit cells can reduce loss in the material through an optical transmission-line ap- proach. Due to the lowered loss, the metamaterial is able to achieve the highest figure of merit to date for an optical nega- tive index metamaterial (NIM) in the absence of gain media. The increased thickness of the metamaterial also allows a direct observation of negative refraction by illuminating a prism made of the material. Such an observation results in an unam- biguous demonstration of negative phase evolution of the wave propagating inside the metamaterial. Furthermore, the metamaterial can be readily accessed from free space, making it functional for optical devices. As such, bulk optical metama- terials should open up new prospects for studies of the unique optical effects associated with negative and zero index materials such as the superlens, reversed Doppler effect, back- ward Cerenkov radiation, optical tunneling devices, compact resonators, and highly directional sources. KEYWORDS | Metamaterials; nanoscale materials; negative index; optics I. INTRODUCTION Since the inception of negative index metamaterials (NIMs) in the microwave region [1], [2] there has been a constant drive to lower the operating wavelength into the optical regime [3]–[5]. A negative refractive index is not exhibited in naturally occurring materials due to lack of a high-frequency magnetic response. The ability to tune both the permeability and permittivity of a material allows scientists, for the first time, to fully explore the parameter space of electromagnetic properties. Achieving a material with a negative refractive index at optical frequencies is perhaps the most challenging implementation of the metamaterial concept as it requires engineering both Manuscript received July 21, 2010; revised September 23, 2010; accepted October 31, 2010. This work was supported by the U.S. Army Research Office (ARO) MURI program 50432-PH-MUR and in part by the National Science Foundation (NSF) Nano-Scale Science and Engineering Center CMMI-0751621. J. Valentine was with the National Science Foundation (NSF) Nano-Scale Science and Engineering Center (NSEC), University of California Berkeley, Berkeley, CA 94720 USA. He is now with the Mechanical Engineering Department, Vanderbilt University, Nashville, TN 37240 USA (e-mail: jason.g.valentine@vanderbilt.edu). S. Zhang was with the National Science Foundation (NSF) Nano-Scale Science and Engineering Center (NSEC), University of California Berkeley, Berkeley, CA 94720 USA. He is now with the School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. (e-mail: s.zhang@bham.ac.uk). T. Zentgraf is with the National Science Foundation (NSF) Nano-Scale Science and Engineering Center (NSEC), University of California Berkeley, Berkeley, CA 94720 USA (e-mail: zentgraf@me.berkeley.edu). X. Zhang is with the National Science Foundation (NSF) Nano-Scale Science and Engineering Center (NSEC), University of California Berkeley, Berkeley, CA 94720 USA and also with the Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: xzhang@me.berkeley.edu). Digital Object Identifier: 10.1109/JPROC.2010.2094593 | Proceedings of the IEEE 1 0018-9219/$26.00 Ó2011 IEEE