Bounds on Herglotz functions and fundamental limits of broadband passive quasi-static cloaking

TL;DR

This paper derives new theoretical bounds on passive systems using sum rules, applying them to establish fundamental limitations on broadband passive cloaking in the quasi-static regime.

Contribution

It generalizes existing bounds on Herglotz functions and applies these to quantify the fundamental limits of broadband passive cloaking devices.

Findings

Established sharp bounds in transparency windows

Provided quantitative limitations on cloaking over finite frequency ranges

Results hold for objects of any shape

Abstract

Using a sum rule, we derive new bounds on Herglotz functions that generalize those given in (Gustafson and Sj\"oberg 2010) and (Bernland, Luger and Gustafson 2011). These bounds apply to a wide class of linear passive systems such as electromagnetic passive materials. Among these bounds, we describe the optimal ones and also discuss their meaning in various physical situations like in the case of a transparency window, where we exhibit sharp bounds. Then, we apply these bounds in the context of broadband passive cloaking in the quasi-static regime to negatively answer the following challenging question: is it possible to construct a passive cloaking device that cloaks an object over a whole frequency band? Our rigorous approach, although limited to quasi-statics, gives quantitative limitations on the cloaking effect over a finite frequency range by providing inequalities on the polarizabilty tensor associated with the cloaking device. We emphasize that our results hold for a cloak or object of any geometrical shape.