If you are using the Graphene for simulation in Comsol or Lumerical, you can consider a layer with thickness of the 1nm for one layer Graphene or multiple layers with the same thickness. If you consider more than one layer the method that people use is to multiply the surface conductivity to the number of layers. Even in Lumerical they consider a 2D material and they provided a option to scale the conductivity. But in my opinion for multiple layers you can consider the real thickness of Graphene, for instance if you have 5 layers then you will have 4*0.34nm thickness for Graphene with complex conductivity of 5 times greater than single Graphene sheet.

When doped. But not instead but as well. The frequency range is different.

You can go through this review article that talks about using graphene for SPR.

Due to its dynamic surface conductivity, it could be used as a plasmonic material. If you convert surface conductivity to the permittivity with specific thickness like 0.5nm or 1nm, you will get the negative real part of the permittivity function, which is as same as gold and other metals. but the frequency that the negative real part of permittivity starts (i.e. plasma frequency) varies for graphene with different doping levels. Also by changing the Fermi level of graphene via gating you can change the spectral point of the plasma frequency. The usual plasmonic frequency spectra for graphene is around THz band, which could be adjusted for desired application.

Thank you @Igor Goliney, Shawana Tabassum and @Sina Soleymani for answering.

However, I have another question. We know the general thickness of Graphene is 0.34nm*L, where L is the number of layer. Thus, when we will use Graphene as a plasmonic material should we add different layer of graphene to increase the thickness or we can just add a 30nm or 35nm layer of graphene thickness?

Thank You

If you are using the Graphene for simulation in Comsol or Lumerical, you can consider a layer with thickness of the 1nm for one layer Graphene or multiple layers with the same thickness. If you consider more than one layer the method that people use is to multiply the surface conductivity to the number of layers. Even in Lumerical they consider a 2D material and they provided a option to scale the conductivity. But in my opinion for multiple layers you can consider the real thickness of Graphene, for instance if you have 5 layers then you will have 4*0.34nm thickness for Graphene with complex conductivity of 5 times greater than single Graphene sheet.

@Sina Soleymani

For example, I want a graphene thickness of 10nm. For doing this, should I divide 10nm with multiple laayers or can I use just one layer of graphene with 10nm thickness?

Thank you

Personally I have never seen graphene with 10nm thickness in any paper, because it might have some properties like graphite too, and the on-plane permittivity might not be equal to the average of surrounding medium.

For instance if you want 10nm bulk graphene it means you have around 30 sheets of Graphene there. Separating them changes the situation and makes it difficult, because you will need very fine size of meshing there. If you want 30 layer 10nm and bulk is fine, but take care of out-of-plane permittivity function too. the following paper also used a multiple layers of graphene.

Article Active plasmonic switching at mid-infrared wavelengths with ...

As illustrated in a thesis published by A. VAkili graphene acts as a metal-dielectric interface in mid-infrared region (30 THz) when chemical potencial is 0.15.