A negative refractive index occurs for one polarization if κ κ > ϵ ϵ r μ μ r ; in this case, ϵ ϵ r and/or μ μ r do not need to be negative. A negative refractive index due to chirality was predicted by Pendry and Tretyakov et al., and first observed simultaneously and independently by Plum et al. and Zhang et al. in 2009.