## 6 Conclusions  
This article proposed an ultrathin concentric hexagonal ring resonator (CHRR)-based metamaterial absorber for optical window applications. The overall size of the three-layered (Ni-AlN-Ni) MMA unit cell is 66 \(\times\) 60 \(\times\) 34 nm\({}^{3}\). The designed MMA exhibits strong absorptivity in the visible and infrared region with 96.3% average absorption from 380-2700 nm wavelength. The maximum absorption of 99.98% is attained at 572 nm, with above 90% absorption realized from 380-2700 nm. The proposed metamaterial absorber offers polarization-independent properties and stable oblique incidence angles for both TM and TE polarization. The analysis of metamaterial property, magnetic field, electric field, and surface current distribution is investigated to understand the absorption mechanism. Finally, a detailed comparison with the state of the art is performed in Table 1, which shows better acceptability of the proposed MMA over others in terms of unit cell size, absorption level, operational wavelength, etc. Therefore, the proposed MMA has great potential for applications of thermal emitters, infrared sensors, energy harvesters, etc. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference** & **Material** & \begin{tabular}{c} **Dimension** \\ **(nm\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} **Operation** \\ **Band (nm)** \\ \end{tabular} & \begin{tabular}{c} **Average** \\ **Absorption** \\ \end{tabular} & \begin{tabular}{c} **Absorption** \\ **Pick (nm)** \\ \end{tabular} & \
\begin{tabular}{c} ** MMA** \\ **Configuration** \\ \end{tabular} &
\begin{tabular}{c} **Polarization-** \\ **Insensitive** \\ \end{tabular} & **Year** \\ \hline
[28] & W, SiO\({}_{2}\) & 950 \(\times\) 950 \(\times\) 240 & 380–750 & 97.1\% & 99.99\% at & Square ring & Yes & 2022 \\
[36] & Ni, SiO\({}_{2}\) & 200 \(\times\) 200 \(\times\) 75 & 200–2000 & 80\% & 820 nm and & 7 hexagonal & \\  & & & & 2300 nm & nano-rings & N/A & 2021 \\
[37] & SiO\({}_{2}\), Au, Ag, Al and Ni & 800 \(\times\) 800 \(\times\) 130 & 400–700 & 92\% & 98\% at & Multilayer & \\  & & & & 470 nm & cylindrical & Yes & 2020 \\  & & & & & & shape & \\
[38] & rs43 SiO\({}_{2}\) and Al & 400 \(\times\) 400 \(\times\) 650 & 435–615 & - & 460 nm, & 4 square & No & 2020 \\  & & & & 520 nm and & resonators & & \\  & & & & & 570 nm & & \\
[39] & W, SiO\({}_{2}\) & 380 \(\times\) 380 \(\times\) 150 & 445–780 & 60\% & 99\% at & 7 round & N/A & 2019 \\  & & & & & 798 nm & nanodisks & \\
[40] &
\begin{tabular}{c} Tungsten \\ SiO\({}_{2}\) \\ \end{tabular} & 200 \(\times\) 200 \(\times\) 130 & 400–750 & 90\% & 465 nm and & ring & No & 2020 \\  & & & & 530 nm & resonator & & \\  & & & & 99\% at & Ring and 4 & \\
[41] & Cu- SiO\({}_{2}\)-Cu & 220 \(\times\) 220 \(\times\) 300 & 800–1500 & 95\% & 872.54 nm and & capacitor & \\  & & & & & 1008.69 nm & resonators & \\
[42] & Ti, Ge & 1400 \(\times\) 1400 \(\times\) 620 & 2000–14000 & 87\% & 99\% at & 4-layer single & Yes & 2020 \\  & & & 10490 nm & nanoring & & \\
[43] & Ti/Ge/ Si\({}_{3}\)N\({}_{4}\) & 4000 \(\times\) 4000 \(\times\) 2300 & 2000–14000 & 78\% & 98.6\% at & Periodic & \\  & & & 2300 & & 8530 nm & square & Yes & 2021 \\  & & & & & 99.5\% at & 6-layer & \\
[44] & SiO\({}_{2}\), Ti, MgF\({}_{2}\), Au, TiO\({}_{2}\) & 200 \(\times\) 190 \(\times\) 545 & 500–1700 & 82\% & 589 nm and & four-square & Yes & 2022 \\  & & & & 1097 nm & resonator & & \\  & Ni & & & & & 1097 nm & resonator & \\ Proposed &
\begin{tabular}{c} Ni \\ Aluminium \\ Nitride \\ \end{tabular} & \(66\times\) 66 \(\times\) 35 & 380–2500 & 96\% & 99.99\% at & Hexagonal & \\  & & & & 618 nm & ring & Yes & 2022 \\  & & & & & resonators & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison table. **Author Contributions:** Conceptualization, M.L.H., A.H. and T.A. ; formal analysis, M.L.H., A.H. and T.A. ; funding acquisition, M.T.I. ; investigation, M.L.H., A.H., T.A. and M.T.I. ; methodology, M.L.H., M.T.I. and T.A. ; visualization, M.L.H. ; writing--original draft, M.L.H.