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In Fig. 4 we plot the frequency dependent conductivity obtained by solving the Maxwell equation numerically for ϵ = 0 , 0 . 1 , 0 . 5 and 0 . 99 with m $^{2}$L2 AdS = 0 , − 1 and − 2. We find a gap in the con- ductivity with the gap frequency ω$_{g}$ . For the same value of m $^{2}$L2 $_{AdS}$, the gap frequency ω$_{g}$ decreases with the increase of the constant ϵ . In each plot, the real part of the conductivity, Re[ σ ], approaches to a limit when the frequency grows. The limit for the case ϵ = 0 is one, but general it increases as ϵ increases. The imaginary part of conductivity Im[ σ ] becomes zero when ω → ∞ , but it goes to 

