Scientists make objects invisible in the microwave spectrum

Physicists have succeeded in making homogeneous cylindrical objects with a radius of 12mm and height of 42cm completely invisible in the microwave range. They managed this without using metamaterial coatings.

The complete research from ITMO University in Saint Petersburg is in Scientific Reports. The method used is based on a new understanding of electromagnetic wave scattering. To achieve the effect, the researchers studied the light scattered by a water-filled glass cylinder.

The university writes on its site that in this way a natural phenomenon is imitated, the so-called Miever scattering. For example, light scattering can be observed in the atmosphere if the diameter of particles in the atmosphere is comparable to the wavelength of the scattered light. Dust, pollen, smoke and microscopic water droplets are common causes of Mi Scattering. In the study, the researchers used water, whose refractive index can be changed by changing the temperature.

It turned out that a high refractive index can be associated with two scattering mechanisms, one related to the localization of light in the cylinder and a phenomenon where wavelength is important. The interaction between these two mechanisms is called Fanoresonance. The physicists found that the waves are scattered at certain frequencies through resonance and non-resonance mechanisms. Due to the opposite phases, the waves cancel each other out, making an object ‘invisible’.

This led to an experiment in which a homogeneous object was rendered invisible by destructive interference or ‘scattering quenching’. The technique made it possible to switch between visibility and invisibility on the same frequency of 1.9GHz by changing the water temperature from 90 to 50 degrees Celsius.

The big difference with many other studies into invisibility is that it does not require the use of a metamaterial. Materials with a similar refractive index already exist or can be developed.

The results could, for example, find application in the development of nanoantennas, where it may be practical to use ‘invisible’ structures as elements. Think of invisible rods to separate antenna chips where ordinary rods can cause interference by blocking the radio waves. The materials for this could be developed in such a way that they are invisible for a specific wavelength.

Credit: ITMO University