Active drains and causality

The concept of an active drain has been used recently to provide an optical element which manages to perfectly sink incoming electromagnetic radiation. Here I show that, without prior knowledge of the incoming signal, an element attempting to respond as an active localized drain cannot succeed.

Figure 1

The spectral “perfection” $|\epsilon E/D|$ of the active drain model as calculated from Eq. (6), for $T_4=1$, $T_2=2$, and three values of loss $\gamma$. A value over 100 implies signal cancellation to within 1%.