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Normal-incidence transmission measurements are commonly used for determining the real part of the in-plane optical conductivities σ1 (ω) of graphene layers.We present an accurate expression for σ1 (ω) in a closed form for a multilayer graphene film supported on a finite-thickness transparent substrate.This form takes into account the coherent and incoherent multiple reflections of the system,whereas the traditional method assumes a semi-infinite substrate.The simulated results for graphene sheets with a layer number N ≤ 10 show that no matter what the transparent substrate is,the accuracy to which σ1 (ω) is determined by applying this expression is improved with no systematic error.Moreover,the layer number N can be exactly determined by simply dividing the σ1(ωp)value of N-layer graphene by the corresponding σ1 (ωP) of monolayer graphene,where ωp is the peak frequency of the ordinary dielectric functions imaginary part ε2 (ω) of graphene.