NSF Nanoscale Science and Engineering Center, UC Berkeley
Here, we experimentally demonstrate cloaking at optical frequencies. A dielectric carpet cloak is designed to hide object under a curved reflecting surface by mimicking the reflection from a flat surface, enabling broadband invisibility with low loss.
While the first experimental demonstration of an invisibility cloak was at microwave frequencies, scaling down conventional metamaterial building blocks to optical frequencies has remained a fundamental challenge and the cloak demonstrated only works at a single frequency. One of the ways to address these issues is the so-called carpet cloak. When the carpet cloak is placed on an object sitting on a flat reflecting surface, the object is compressed into the underlying surface. This results in the observer perceiving the whole system as the original reflecting surface, rendering the object invisible. Such a scheme is related to the topology of the singularity within the coordinate transformation. By crushing the object to a sheet rather than a point or a line, it avoids the usage of metamaterials with extreme indices. With further minimization on the anisotropy by utilizing a quasi-conformal map, the cloak can be readily made purely from isotropic dielectric materials with modest range of indices, working far away from material resonance. It leads in our cloak to low-loss operation in a wide wavelength range from 1400 nm to 1800 nm.