Electrically controllable soft optical cloak based on gold nanorod fluids with epsilon-near-zero characteristic

We propose an electrically controllable soft optical cloak based on a fluid system containing gold nanorods, which can be transformed from isotropic to anisotropic epsilon-near-zero (ENZ) state at a certain incident optical frequency due to the orientation of gold nanorods under an external electric field stimulus. Both effective medium theory and 3D finite element simulation demonstrate that, at the ENZ point, the scattering from arbitrary-shaped objects can be nearly perfect suppressed. The loss and aspect ratio of gold nanorods have an effect on the ENZ point and scattering suppression behavior. When different aspect ratio of gold nanorods is employed, the fluid has multi ENZ points and exhibits perfect suppression of scattering from objects at multiple incident optical frequencies. Because the orientation of gold nanorods depends on the strength of applied external electric field, the permittivity of fluid can be adjusted by external electric field and, as a result, the ENZ state and scattering suppression of objects can be controlled. The flexible, controllable, and multi-frequency responsive characteristics make the optical cloak possess potential use in soft smart metamaterial devices.