On Broadband Active Cloaking

Although great progress has been made in the theory and realization of invisibility devices, the issue of narrow-bandwidth operation remains unsolved. In particular, physical bounds have been recently derived, based on Bode-Fano theory, that quantify the inherent difficulty in broadening the bandwidth of any linear, causal, and passive cloak. If we remove the passivity restriction, introducing active media into the system, it is then relevant to ask the following questions: 1. Can we overcome the Bode-Fano limit, at least theoretically, and to what extent? 2. Can we guarantee the unconditional stability of the active cloak? In this paper, we discuss some steps toward answering these general questions. We first offer an intuitive discussion of why a passive cloak cannot be broadband, while an active cloak might. Then, we use a 1-D example to show, for the first time, that active cloaks have indeed the potential to operate beyond the Bode-Fano bandwidth limit. Finally, we design a simple 3-D active cloak that exhibits a much broader bandwidth than any traditional passive plasmonic cloak. All our design examples of active cloaks are stable, and we discuss some useful conditions to ensure unconditional stability.