Revealing the truth about 'trapped rainbow' storage of light in metamaterials

It was claimed that the incident light can be gradually slowed down and finally trapped in a tapered metamaterial waveguide. Here we show that the energy incident from the input port of the tapered metamaterial waveguide will be totally reflected (instead of being trapped) due to the strong intermodal coupling between the forward and backward modes. The underlying physical mechanism for this strong intermodal-coupling is given. The occurrence of energy reflection is unambiguously proved using several independent methods, (1) the semi-analytical mode matching technique, (2) the numerical finite element simulation, (3) the requirement of energy and momentum conservation, and (4) an experimental verification at microwave frequency. The dream of 'trapped rainbow' for storage of light might still be possible if this intermodal coupling could be blocked, and our study provides a useful guidance for such an endeavor.