Physicists make an object completely invisible in the microwave range

Physicists have 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
achieved the result using a homogenous object without any additional coating
layers. The method is based on a new understanding of electromagnetic wave
scattering. Physicists from ITMO University, Ioffe Institute and Australian
National University 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 achieved the
result using a homogenous object without any additional coating layers. The
method is based on a new understanding of electromagnetic wave scattering.

This is the radio-frequency anechoic chamber used for
the experiment.
Credit: ITMO University
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 work led to the first experimental observation of
an invisible homogeneous object by means of scattering cancellation.
Importantly, 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 °C to 50 °C. “Our
theoretical calculations were successfully tested in microwave experiments.
What matters is that the invisibility idea we implemented in our 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,” said Mikhail Rybin, first author of the paper and
senior researcher at the Metamaterials Laboratory in ITMO University.
The results of the study were
published in Scientific Reports. 
(Credit: Sciencedaily / ITMO University) 

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