Background:

For years, cloaking devices with the ability to render objects invisible were the subject of science fiction novels while being unattainable in reality. Recently, the development of a design methodology deemed transformation optics has provided a tool by which space and light propagation can be transformed in unique ways. Utilizing transformation optics, the first experimental demonstration of cloaking was recently achieved at microwave frequencies using metamaterials possessing spatially varied magnetic resonances with extreme values for the permeability. However, scaling these metamaterials to optical frequencies and spatially structuring them into a cloak has been challenging. The use of resonant metamaterials with extreme values of permeability and permittivity also causes the underlying cloak to be narrow band and lossy. In our lab we have recently overcome these challenges in experimentally demonstrating the first optical cloak called a “carpet cloak.” This cloak is designed using a method called quasi-conformal mapping which eliminates the extreme material properties allowing the cloak to be both broad band and low loss.

Figure 1. (a) Schematic of carpet cloak with the cloak regions indicated. (b) SEM image of the actual carpet cloak. The width and depth of the cloaked bump are 3.8µm and 400nm, respectively.

Results:

The carpet cloak was fabricated in an SOI wafer where the Silicon layer serves as a waveguide in which the metamaterial cloak is made. The cloak consists of 110nm holes drilled through the Si layer with a specific spatial density that creates the proper optical transformation. The cloak is designed using a subset of transformation optics deemed quasi-conformal mapping which simplifies the required material properties. This simplification is what allows the cloak to be made from simple, spatially varying holes. Since the metamaterial is made from non-resonant Silicon, it is shown to operate over a large range of wavelengths. The carpet cloak is placed above a perturbation in a reflecting surface (curved surface in Fig. 1a,b). If light is incident on the curved surface alone (no cloak) it will perturbate the reflected beam, indicating the presence of the object. The carpet cloak’s function is to virtually compress the curved surface into a flat plane such that light incident on the cloak + curved surface will have the reflection of flat surface. In doing this, anything that is placed below the curved surface will appear as the flat plane surrounding it resulting in the object being hidden from view, or cloaked. 

Figure 2. (left panel)  SEM image of the carpet cloak indicating the architecture of the measured sample. (right panel) Experimental (left) and theoretical (right) performance of a (a) flat surface, (b) curved surface, (c) curved surface + cloak. It can be seen that the cloak has successfully transformed the curve surface back into a flat surface.

To measure the effectiveness of the cloak light was incident on a flat surface, a curved surface, and a cloak + curved surface (as seen in the right panel above: Fig. 2a, 2b, and 2c respectively).  The reflected beam profile was imaged and it was demonstrated that the cloaked curved surface is indeed transformed back to the original flat surface. Furthermore, the reflected beam profile was measured for both the curved surface alone and the curved surface + cloak over a range of wavelengths and was shown to be effective from 1400nm to 1800nm as seen below in figure 3a,b. By utilizing different technologies in fabricating the cloak, it is feasible to scale this design to the visible wavelengths in the future.

Figure 3. (a) Reflected beam profile of a curved surface + cloak over a range of wavelengths. (b) Reflection of a curved surface (no cloak) over the same range of wavelengths.

1. Jason Valentine, Jensen Li, Thomas Zentgraf, Guy Bartal and Xiang Zhang, “An Optical Cloak Made of Dielectrics”, Nature Materials 8, 568 (2009). view pdf