'Invisible objects' created sans metamaterial cloaking

A team of physicists has managed to make homogenous cylindrical objects completely invisible in the microwave range.

Contrary to the now prevailing notion of invisibility that relies on metamaterial coatings, the scientists from ITMO University, Ioffe Institute and Australian National University achieved the result using a homogenous object without any additional coating layers. The method is based on a new understanding of electromagnetic wave scattering.

The scientists studied light scattering from a glass cylinder filled with water. In essence, such an experiment represents a two-dimensional analog of a classical problem of scattering from a homogeneous sphere (Mie scattering), the solution to which is known for almost a century. However, this classical problem contains unusual physics that manifests itself when materials with high values of refractive index are involved. In the study, the scientists used ordinary water whose refractive index can be regulated by changing temperature.

As it turned out, high refractive index is associated with two scattering mechanisms: resonant scattering, which is related to the localization of light inside the cylinder, and non-resonant, which is characterized by smooth dependence on the wave frequency. The interaction between these mechanisms is referred to as Fano resonances.

The researchers discovered that at certain frequencies waves scattered via resonant and non-resonant mechanisms have opposite phases and are mutually destroyed, thus making the object invisible.

The developed technique made it possible to switch from visibility to invisibility regimes at the same frequency of 1.9 GHz by simply changing the temperature of the water in the cylinder from 90 degree C to 50 degree C.

First author Mikhail Rybin said that the theoretical calculations were successfully tested in microwave experiments. What matters is that the invisibility idea they implemented in their work can be applied to other electromagnetic wave ranges, including to the visible range. Materials with corresponding refractive index are either long known or can be developed at will.

The discovery of invisibility phenomenon in a homogenous object and not an object covered with additional coating layers is also important from the engineering point of view because it is much easier to produce a homogeneous cylinder, the discovery could prompt further development of nanoantennas, wherein invisible structural elements could help reduce disturbances.

The results of the study were published in Scientific Reports.