Optimizing low loss negative index metamaterial for visible spectrum using differential evolution: comment

In a recent paper, Zhao et al. [Opt. Express 19(12), 11605 (2011)], proposed the use of differential evolution technique to optimize figure of merit of a negative index metamaterial (NIM) for the visible spectrum. In this comment, we argue that certain ambiguities associated with the effective parameter retrieval should be also addressed in the paper for the accurate implementation of the technique for NIMs. Furthermore, the figure of merit reported in the paper is unrealistically large.