In this example, we will calculate the absorption characteristics of a plasmonic metamaterial absorber based on . The absorption mechanism for these devices is mostly governed by the excitation of localized electromagnetic resonances, and is highly dependent on the geometry of the top metal layer and the thickness of the dielectric layer.
The plasmonic absorber is composed of a periodic array of subwavelength metal patches on top of a thin dielectric layer and a highly reflecting thick metal layer. For the optical simulation, we only need to simulate a single unit cell. The file plasmonic_absorber.fsp contains the structure shown in Fig. 1. The design parameters of the device can be defined in the 'Setup' tab of the 'model' object in the object tree.
Unit cell of MIM plasmonic metamaterial absorber
Both symmetric and anti-symmetric boundary conditions are used to reduce the simulation time. The two power monitors, 'R' and 'T', are used to calculate the transmission and reflection (and therefore, absorption) spectrum of the device. To be consistent with the simulations in reference 1, a Plasma (Drude) material called 'silver' was added to the material database and assigned to the top and bottom metal layers and a refractive index of 1.75 was used for the dielectric layer.
When the simulation plasmonic_absorber.fsp finishes, run plasmonic_absorber_RTA.lsf to create the images shown below.
R / T / A spectra (Fig. 2 from )
Magnetic field intensity at resonance
For these type of metamaterial absorbers, the spectral location of the resonance does not shift with the angle of incidence when the correct mode is excited. This is a highly desirable feature for infrared sensing applications as the detected light usually comes from many different angles. To verify that the desired absorption spectrum is robust for non-normal incident angles, one can take advantage of the broadband fixed angle source technique (BFAST) to simulate the absorption spectrum for a range of incident angles. The script plasmonic_absorber_angle_sweep.lsf uses BFAST to sweep the incidence angle for a broadband plane wave source. One can see from the figure below that the absorption spectrum has little dependence on the illumination angle, as expected.
Absorption as a function of wavelength and the angle of incidence for TE-polarization (Fig. 3 from )
 J. Hao et al., "Nearly total absorption of light and heat generation by plasmonic metamaterials," Phys. Rev. B 83, 165107 (2011).