Limitation of the electromagnetic cloak with dispersive material

Based on causality, the limitation of the electromagnetic cloak with dispersive material is investigated in this letter. The results show that perfect invisibility cannot be achieved because of the dilemma that either the group velocity V-g diverges or a strong absorption is imposed on the cloaking material. It is an intrinsic conflict which originates from the demand of causality. However, the total cross section can really be reduced through the approach of coordinate transformation. A simulation of finite-difference time-domain method is performed to validate our analysis.