Development of Bulk Optical Negative Index Fishnet Metamaterials: Achieving a Low-Loss and Broadband Response Through Coupling

In this paper, we discuss the development of a bulk negative refractive index metamaterial made of cascaded “fishnet” 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 approach. Due to the lowered loss, the metamaterial is able to achieve the highest figure of merit to date for an optical negative 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 unambiguous 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 metamaterials 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, backward Cerenkov radiation, optical tunneling devices, compact resonators, and highly directional sources.