Effects of size and frequency dispersion in plasmonic cloaking

The plasmonic venue to realize invisibility and cloaking [A. Alù and N. Engheta, Phys. Rev. E 72, 016623 (2005)] is analyzed here in terms of its limitations and its frequency dispersion relative to the cloak size. Intrinsic limits due to causality and comparison with transformation-based cloaking techniques are discussed and analyzed. An interestingly simple low-dispersion cloak is also suggested for background materials with larger refractive index. These results may shed light on this scattering cancellation phenomenon, suggesting potential applications in scattering reduction and noninvasive probing.

Figure 1

(Color online) Scattering “gain” from a conducting particle with diameter $2a=1.5{\lambda }_0$ using two different cloaking designs. The scattering gain is obtained as the ratio of the total scattering cross section of the overall object with its cloak, divided by the total scattering cross section of the sphere alone.

Figure 2

(Color online) Scattering gain from a conducting particle, varying its diameter as indicated in the legend, for a cloak with permittivity given in Eq. (1) and (a) free-space permeability, (b) optimized positive permeability in order to cancel the residual scattering. For each curve the cloak outer radius has also been optimized for achieving minimum scattering at frequency $f_0$.

Figure 3

(Color online) Similar to Fig. 2, but for a background material with permittivity ${\epsilon }_b=10{\epsilon }_0$. In this case, the cloak permittivity is simply the same as free-space ${\epsilon }_0$ with no material dispersion. Note the large bandwidth of scattering reduction.