Optical cloaking achieved in visible spectrum

Compiled by Photonics Spectra staff

The Karlsruhe invisibility cloak has been refined so that it is now effective in the visible spectral range. “Seeing something invisible with your own eyes is an exciting experience,” said Joachim Fischer and Tolga Ergin, physicists and members of professor Martin Wegener’s team at Karlsruhe Institute of Technology’s Center for Functional Nanostructures (CFN).

In invisibility cloaks, light waves are guided by the material in such a way that they leave the invisibility cloak again as if they had never been in contact with the object being disguised; thus, the object appears invisible to the observer.

The invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. The device makes the curvature of a metal mirror appear flat, causing objects hidden underneath to become invisible. The metamaterial placed on top of this curvature looks like a stack of wood but consists of plastic and air. These “logs” have precisely defined thicknesses in the range of 100 nm. Light waves normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror. “If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum,” Fischer said.

An electron micrograph image of an invisibility cloak structure. The polymer-air metamaterial (“logs”) is colored blue; the gold-coated areas are in yellow. Courtesy of the Center for Functional Nanostructures.

Last year, Wegener’s team presented the first 3-D invisibility cloak and, up until that time, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When the structure was looked at from a third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could be produced for wavelengths from 1500 to 2600 nm.

This range is not visible to the human eye, but it plays an important role in telecommunications. The breakthrough was based on the use of the direct laser writing method developed by CFN. With the help of this method, it is possible to produce metamaterials – minute 3-D structures with optical properties that do not exist in nature.

Over the past year, the KIT scientists have continued to improve the already extremely fine direct laser writing method. They used methods that have significantly increased the resolution in microscopy. With this tool, they refined the metamaterial by a factor of two and produced the first 3-D invisibility cloak for nonpolarized visible light in the range of 700 nm. Findings appeared in the May 27, 2011, issue of Optics Letters (doi: 10.1364/OL.36.002059).

The group expects the work to influence dramatic improvements in transformation optics, metamaterials, lenses, solar cells, microscopes, objectives, chip production and data communication.