Cloaking at Optical Frequencies

2 The ability of rendering objects invisible using a cloak – not detectable by an external observer – for concealing objects has been a tantalizing goal 1-6. Here, we demonstrate a cloak operating in the optical regime. The cloak operates at a wide bandwidth and conceals a deformation on a flat reflecting surface, under which an object can be hidden. The device is composed of nanometer size silicon structures with spatially varying densities across the cloak. The density variation is defined using transformation optics to define the effective index distribution of the cloak. The prospect of optical cloaking has recently become a topic of considerable interest. Through the use of transformation optics 7-11 , in which a coordinate transformation is applied to Maxwell's equations, several designs for such a device were created 12-23. These designs are based on the idea of manipulating the structure of the cloaking medium so that the trajectory of light after interacting with the cloak is the same as that in an empty medium, without the cloak nor the object underneath. The external observer is therefore unaware of the presence of the cloak and the object. Such cloaks were recently experimentally demonstrated in the microwave regime using metamaterial structures with feature sizes in the millimeter to centimeter scale 24-25. Pushing this technology to the optical regime would greatly increase the potential application. This requires, however, nanometer control of the cloaking structure. Here we demonstrate a cloak in the optical domain using nanometer scale dielectric structures. We experimentally demonstrate an optical invisibility cloak that hides an object " under a carpet " with the help of a reflective surface. As shown in the sketch of Fig. 1, when an external observer looks at a deformed mirror, it detects the deformation in the reflected image (Figs. 1a, b). Following the theoretical work by Pendry and co-authors 20 and Leonhard and co-authors 11 , we designed and fabricated a cloaking device at optical frequencies capable of reshaping this reflected image and providing the observer with the illusion of looking at a plane mirror (Fig. 1c). One could then hide objects under those deformations without revealing their existence. 3 The cloaking device is composed of a spatially varying density of sub-wavelength 50 nm diameter silicon posts embedded in an SiO 2 medium, and the reflective surface consists of a distributed Bragg reflector (DBR). The distribution of posts induces a variation …