# Graphene-Based Cylindrical Pillar Gratings for Polarization-Insensitive Optical Absorbers
## 3 Results
Figure 7 shows the absorption maps when the incidence angle is changed from 0\({}^{\circ}\) to 90\({}^{\circ}\) for both TE and TM polarizations. The maps show that the resonance at normal incidence splits into two arms for angular incidence. This behavior is typical of a grating structure that is derived by the absorption spectra. It is interesting to note that the absorption spectra do not split into two arms for certain resonant wavelengths and absorption is nearly independent of the source angle for specific angular ranges. Figure 5: Absorption spectra of the proposed structure under varying grating period for normally incident TE polarized plane wave. The waveguide Ta\({}_{2}\)O\({}_{5}\) thickness \(t_{\rm{Ta2O5}}\), and polymethyl-methacrylate (PMMA) pillars height \(t_{\rm{PMMA}}\) are fixed at 150 nm and 600 nm, respectively. Figure 6: (**a**) Absorption map with respect to the variation of Ta\({}_{2}\)O\({}_{5}\) layer thickness \(t_{\rm{Ta2O5}}\) under TE at normal illumination when the period, p, and PMMA pillar height, \(t_{\rm{PMMA}}\), are fixed at 600 nm and 600 nm, respectively. (**b**) Absorption map under TE normal illumination, when PMMA pillar thickness \(t_{\rm{PMMA}}\) is varied from 300 nm to 900 nm. The period, p, and waveguide, Ta\({}_{2}\)O\({}_{5}\), thickness \(t_{\rm{Ta2O5}}\) are fixed at 600 nm and 150 nm, respectively. To further elaborate these important results, the absorption spectra of the device for both TE and TM polarizations are shown in Figure 8 in an angular range of 0-20\({}^{\circ}\) for two different device periods. The absorption is limited within narrow bandwidths of \(\sim\)15 nm and \(\sim\)20 nm for the maps in (a,b) and (c,d) respectively over a considerable wide angular range of 20\({}^{\circ}\). The slope is about 0.5 nm/degree in the 0-10\({}^{\circ}\) range. Another important result is depicted in Figure 9 which shows absorption spectra in an angular range of 70-90\({}^{\circ}\). It is noted that the absorption is limited in narrow bandwidths of \(\sim\)25 nm and \(\sim\)10 nm for the angular ranges of 70-90\({}^{\circ}\) and 75-90\({}^{\circ}\), respectively in both (a) and (b) maps. Furthermore, the device behavior is similar under both TE and TM polarizations. Figure 8: Peak absorption wavelength when the incident angle is varied in range 0–20\({}^{\circ}\). (**a,b**) The device period p, the waveguide Ta\({}_{2}\)O\({}_{5}\) thickness t\({}_{\text{Ta}205}\), and PMMA pillars height t\({}_{\text{PMMA}}\) are fixed at 450 nm, 150 nm, and 600 nm, respectively. (**c,d**) The device period p, the waveguide Ta\({}_{2}\)O\({}_{5}\) thickness t\({}_{\text{Ta}205}\), and PMMA pillars height t\({}_{\text{PMMA}}\) are fixed at 600 nm, 150 nm, and 600 nm, respectively. Figure 7: Absorption maps of the device for varying incidence angle (0–90\({}^{\circ}\)) under both TE and TM polarizations. For (**a,b**) device period p, the waveguide Ta\({}_{2}\)O\({}_{5}\) thickness t\({}_{\text{Ta}205}\), and PMMA pillars height t\({}_{\text{PMMA}}\) are fixed at 450 nm, 150 nm, and 600 nm, respectively, whereas, for (**c,d**) the period p is changed to 600 nm. In short, the observations in Figure 8; Figure 9 prove the near insensitivity of the proposed device to the angle and polarization of the incident light in multiple angular ranges of around 20\({}^{\circ}\). Angular response of our device shows clear improvement as compared with results reported in [15] for a graphene-based absorber based on a one-dimensional grating where absorption spectra split into two arms from 0-20\({}^{\circ}\) under similar simulation conditions.