In this example, we will consider a graphene metamaterial absorber where the absorption spectrum can be tuned by changing the chemical potential applied to the graphene. The structure of the device is similar to the plasmonic metamaterial absorber, apart from the the fact the top metal layer has been replaced with a 2D graphene sheet.
The absorption spectrum of the graphene absorber can be tailed by changing the key geometric parameters. Below are the two types of structures we will consider: uniform graphene sheet and graphene fishnet metamaterial.
We use a 2D graphene material model based on the surface conductivity of the graphene. For this example, a conductivity scaling of 2 is used for the graphene model to account for the two layers of graphene sheets used in . A 2D rectangle object is used to model the sheet, and there is no need to add a mesh override region over the graphene to resolve the thin layer. For the fishnet geometry, polygon objects with the refractive index of 1.53 (corresponding to the background index for dielectric) were used to pattern the 2d graphene sheet.
On the 'z min' boundary of the simulation region, a PEC (perfect electrical) boundary was used to mimic the perfect mirror. A background index of 1.53 was used.
The absorption spectra of the uniform and fishnet structures as a function of the chemical potential can be obtained by opening each of the simulation file (graphene_absorber_uniform.fsp and graphene_absorber_fishnet.fsp) and running the script file (graphene_absorber_ef_sweep.lsf). The key results from the simulation can be summarized as follows:
- The absorption spectrum of the graphene absorber is strongly dependent on the chemical potential
- A broader absorption spectrum can be obtained with the fishnet structure. Furthermore, this spectrum can be tuned by changing the geometry of the fishnet metamaterial.
Absorption spectra as a function of chemical potential (Fig. 5 and Fig. 6 from )
If you happen to know the chemical potential of the graphene, you can directly enter the value in the graphene material model. If not, you can use the charge transport solver to obtain the chemical potential as a function of applied voltage. Additional information on how to extract the chemical potential as a function of applied voltage can be found in the graphene electro-optical modulator example.
 A. Andryieuski and A. V. Lavrinenko, "Graphene metamaterials based tunable terahertz absorber: effective surface conductivity approach," Opt. Express 21, 9144 (2013).