Active Scattering-Cancellation Cloaking: Broadband Invisibility and Stability Constraints

While 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 the Bode-Fano theory of broadband impedance matching, that quantify the inherent difficulty in broadening the bandwidth of any linear, causal, and passive cloak. If we relax the passivity restriction, introducing active media into the system, it is then relevant to ask the following questions: 1) Can the Bode-Fano limit on broadband invisibility be surpassed, at least theoretically, and to what extent? 2) Can we guarantee the stability of a broadband active cloak? In this article, we discuss some steps toward answering these general questions. We first offer an intuitive discussion of why a passive cloak cannot be arbitrarily broadband. 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 passive scattering-cancellation cloaks. All the designed active cloaks are stable, and we discuss some relevant conditions to ensure stability. Our results may pave the way for ultrabroadband invisibility devices.