How to make objects invisible without metamaterial cloaking

April 14, 2015

The radio-frequency anechoic chamber used for the experiment (credit: ITMO University)

Physicists from ITMO University, Ioffe Institute and Australian National University have managed to make homogenous cylindrical objects completely invisible in the microwave range — without adding coating layers.

KurzweilAI has covered a wide variety of discoveries in metamaterals (“cloaking”); this method is based on a new understanding of electromagnetic wave scattering. The results of the open-access study were published in Scientific Reports.

The scientists studied light scattering from a glass cylinder filled with water, representing a two-dimensional analog of a classical problem of scattering from a homogeneous sphere (Mie scattering), the solution known for almost a century.*

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. It was possible to switch from visibility to invisibility regimes at 1.9 GHz by simply changing the temperature of the water in the cylinder from 90 °C to 50 °C.

“Our theoretical calculations were successfully tested in microwave experiments [but] can be applied to other electromagnetic wave ranges, including to the visible range,” said Mikhail Rybin, first author of the paper and senior researcher at the Metamaterials Laboratory in ITMO University.

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. For instance, invisible rods could be used as supports for a miniature antenna complex connecting two optical chips.

* 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. 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.

Abstract of Switching from Visibility to Invisibility via Fano Resonances: Theory and Experiment

Subwavelength structures demonstrate many unusual optical properties which can be employed for engineering of a new generation of functional metadevices, as well as controlled scattering of light and invisibility cloaking. Here we demonstrate that the suppression of light scattering for any direction of observation can be achieved for a uniform dielectric object with high refractive index, in a sharp contrast to the cloaking with multilayered plasmonic structures suggested previously. Our finding is based on the novel physics of cascades of Fano resonances observed in the Mie scattering from a homogeneous dielectric rod. We observe this effect experimentally at microwaves by employing high temperature-dependent dielectric permittivity of a glass cylinder with heated water. Our results open a new avenue in analyzing the optical response of high-index dielectric nanoparticles and the physics of cloaking.