On the quest of electromagnetic invisibility

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When illuminated by and Electromagnetic Wave (EW), a metallic body will scatter the EW in
more or less every direction which is usually not desired. For example, backscattering (or
reflexion) can cause antenna blockage and forward scattering causes shadow zones. As the
incident field is null or very small in the shadow zones, targets found in these regions will not
be detected.

Reducing backscattering is a straight forward task by using radar absorbers
(https://www.privinvest.com/innovations/broadband-electromagnetic-radar-absorbers/?lang=fr)
However, if a metallic cylinder is considered, radar absorbers will increase the forward
scattering. The total scattering cross section of strongly scattering metallic objects can be
reduced by using various electromagnetic cloaks, which are complex structures with a
limited frequency bandwidth. Electromagnetic invisibility is possible when the total
scattering tends to zero.

In many applications, however, dramatic reduction of total scattering is not always required,
for instance, when backscattering reduction is the critical parameter. Moreover, the
direction of illumination is sometimes known and scattering reduction for other illumination
directions is not required. In these scenarios, it is possible to achieve the desired scattering
reduction using much simpler to manufacture devices.

Recently our work, on which we explore possibilities to reduce back-, forward and total
scattering from cylindrical bodies for plane-wave illumination from a certain direction has
been published in IOP Journal of Optics (https://iopscience.iop.org/article/10.1088/2040-8986/abaa62)

The proposed scattering-reduction device is a shell formed by several sectors of different uniform dielectric materials. We have compared the requirements on the material cover which are needed to reduce backscattering and forward scattering widths and outlined the corresponding design approaches. We have shown that all-dielectric and easily realizable structures can effectively reduce total scattering without using active materials or non-uniform anisotropic materials.

We have given two scenarios of coating as examples. In the first scenario, a 4-sectors coating has been presented as shown in Animation 1 (b). When illuminated by an EW, the bare metallic cylinder produces backscattering (Observation Point: B1) and forward scattering (Observation Point: F1). When coated with the 4-sectors coating backscattering is almost zero (Observation Point: B2) and the forward scattering is reduced by 56% and the incident plane wave starts to ‘form’ again as in can be seen near Observation Point: F2. In this case the total scattering reduction is of 23%.

In order to further reduce the forward scattering and thus reduce the shadow zone, a 16-sectors coating has been studied in the second scenario presented in Animation 1 (c). With the 16 sectors-coating, backscattering is almost zero (Observation Point: B3) as for the first scenario, forward scattering is dramatically reduced by 78% (Observation Point: F3) and total scattering is reduced by 65%.

It must be noted that, in this work we have considered a metallic cylinder having a diameter comparable to the wavelength which is mostly the case in naval applications. Also, we have put emphasis on producing realistic producible design of coatings. For our future and ongoing works in the field of Electromagnetic Compability, we are also constantly looking for industrial or academic partners.