On the quest of electromagnetic invisibility

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

Reducing backscattering is a straight forward task by using radar absorbers.

However, radar absorbers will increase the forward scattering if a metallic cylinder is considered. The total scattering cross section of strongly scattering metallic objects can be reduced by using various electromagnetic cloaks,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, achieving the desired scattering reduction is possible using 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 specific direction, has been published in the 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 uniform dielectric materials. We have compared the requirements on the material cover 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 reduce total scattering without using active or non-uniform anisotropic materials.

We have given two coating scenarios as examples. In the first scenario, a 4-sectors coating is presented 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), the forward scattering is reduced by 56%, and the incident plane wave starts to ‘form’ again, as can be seen near Observation Point: F2. In this case, the total scattering reduction is 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%.

In this work, we have considered a metallic cylinder having a diameter comparable to the wavelength, which is mostly the case in naval applications. We have also emphasized producing realistic, producible coating designs. For our future and ongoing work in the field of Electromagnetic compatibility, we are constantly looking for industrial or academic partners.