Metamaterial-based Cloaking – Is it Feasible to be Invisible?

Electromagnetic cloaking (‘invisibility technology’) is one among the most exciting concepts in metamaterials proposed in recent years [1,2]. It should make an object ‘invisible’ in radiofereqeuncy, microwave, or even optical part of electromagnetic spectrum. Obviously, there are many potential defense applications of this technology, spanning from aircraft radar invisibility to removal of unwanted blocking effects in crowded multi-antenna systems used on typical military ship platform, just to name a few [3]. Although the correctness of basic idea has already been proven both theoretically and experientially [1,2], practical engineering applications are still seriously limited. There are to two basic problems: pronounced losses and narrow operating bandwidth [4]. Actually, these problems are common for all applications based on passive metamaterials. They are caused by basic background physics rather than used technology (although it is usually believed so).

In this talk, two novel approaches (solely developed at University of Zagreb [4,5,6]), that attempt to solve these problems in cloaking defense applications, will be presented. The first approach is based on specific-goal-based optimization of design parameters of a passive cloak. It will be shown that it is possible to increase either the invisibility gain (up to a factor of 500) or the invisibility bandwidth (up to a factor of 2.5), but not both simultaneously. So, optimization significantly improves the cloaking performances, but, of course, it cannot beat out the background physics. Therefore, the cloaking bandwidth is still rather narrow (less than 1:1.01) as a direct consequence of basic dispersion-energy constraints. However, the use of novel active non-Foster metamaterials (metamaterials with embedded negative capacitors or/and negative inductors) elegantly goes around basic-energy dispersion constraints [5,6]. The non-Foster metamaterials offer extremely broad operation (more than 1:100 bandwidth).

In the final part of the talk, representative experimental results will be presented. Measured parameters of practical in-house-developed non-Foster metamaterials operating up to 1 GHz were used for determination of cloaking bandwidth. Obtained results revealed the cloaking bandwidth of up to 1:5, which fairly exceeds the performances of every passive cloak [4]. Thus, practical active invisibility cloaks may become available in coming years.

References:

[1] Pendry J., Schurig D., Smith D. ‘Controlling electromagnetic fields’. Science, (2006)  312:1780-1782. 10.1126/science.1125907
[2] Schurig D., Mock J., et.al, ‘Metamaterial electromagnetic cloak at microwave frequencies’, , Science, (2006), 314:977-980. 10.1126/science.1133628
[3] Kwon, D., Werner, D, ‘Transformation Electromagnetics: An Overview of Theory and Applications’, IEEE Antennas and Propagation Magazine, (2010), 52,1, 25-45
[4]     Hrabar, S., Sipus Z., Malcic I. ‘Broadening of Cloaking Bandwidth…’, Transformation Electromagnetics and Metamaterials, (ed. Kwon and Werner) Springer, (2014), 349-394
[5] Hrabar, S. Krois, I.,  Kiricenko, A., ‘Towards Active Dispersionless ENZ Metamaterial for Cloaking Applications’ , Metamaterials, (2010) 4, 2-3, 89-97.
[6] Hrabar, S. Krois, I., Kiricenko, A., ’ Negative capacitor paves the way to ultra-broadband metamaterials’, Applied Physics Letters. (2011), 99, 25, 25403-1-25403-4