While the idea of a metamaterial-based invisibility cloak is fascinating, transforming the idea into reality is a daunting task. This is especially true at optical frequencies, where the required metamaterial structures have geometries with features sized at the nanometer scale. Making an invisibility cloak, or indeed many other devices made possible by metamaterials, requires the construction of a bulk (3-D) metamaterial, as opposed to a thin-film (2-D) version. Rather than trying to make a 3-D metamaterial using difficult and expensive nanolithographic techniques, researchers at Norfolk State University (Norfolk, VA) and Purdue University (West Lafayette, IN) have been working on a much simpler approach that now has allowed them to create an optical metamaterial 51 µm thick.
An anodic alumina membrane, 1 cm × 1 cm × 51 µm and naturally full of 35 nm holes, became the base material; silver was electrochemically plated in the holes, mostly filling them up and creating an irregular array of parallel nanowires. Angles of refraction in the material were studied at different wavelengths, revealing that the metamaterial has hyperbolic dispersion for wavelengths greater than 0.84 µm; in addition, the direction of refraction for 632.8 nm light was consistent with a refractive index of less than 1. Contact Mikhail Noginov at [email protected].