# Graphene-Based Cylindrical Pillar Gratings for Polarization-Insensitive Optical Absorbers
## 1 Introduction  
Graphene has emerged as an attractive two-dimensional material due to its exceptional mechanical, electrical, and optical properties [1]. Monolayer graphene exhibits better absorbance than different dielectric and metals of the same thickness. A considerable amount of theoretical and experimental work has been published in recent years to explore the potential of graphene in optical and microwave absorbers in different spectral ranges of the electromagnetic spectrum for various applications such as biosensors, optical filters, modulators, and efficient photodetector [2; 3; 4].  
Monolayer graphene shows a constant absorption of about 2.3% over the visible and near-infrared wavelengths [5]. However, in the context of optical absorbers, this value is not enough which substantially limits its use in photonic devices. Therefore, enhancement of light absorption by monolayer graphene has drawn significant attention from researchers and several solutions have been proposed in the technical literature to achieve the perfect absorption. One- and two-dimensionaldielectric gratings exploit guided-mode resonance (GMR) to increase light-matter interaction. Light is effectively coupled to graphene if incorporated with such gratings [6]. By varying geometrical parameters of the grating structure, the resonant wavelength and the bandwidth of absorption are tuned. A number of numerical studies [7; 8; 9; 10] report perfect absorption of light in graphene incorporated dielectric gratings over diverse spectral ranges. Recently, near total absorption in monolayer graphene based on critical coupling was presented at a visible wavelength of 605 nm [11]. In some other recent studies, multilevel structures were investigated and enhanced graphene absorption was reported [12; 13; 14]. The main drawback of the multilevel structures is the complexity of the fabrication process since it requires the deposition of multiple material layers. Experimental and measured results for several fabricated graphene-based structures have also been presented [15; 16; 17; 18]. The maximum experimentally measured values of absorption reported are 35% at 0.73 \(\upmu\)m [15], 40% at 0.7 \(\upmu\)m [16], 45% at 10 \(\upmu\)m [17], and 99% at 1.48 \(\upmu\)m [18]. Apart from grating-based structures, perfect absorbers based on metamaterials have also been reported [19; 20; 21; 22].  
However, perfect, broadband, tunable and polarization-independent absorption of light in monolayer graphene for photonic applications is still a challenging problem and needs further research and exploration. In this framework, it is worth pointing out that the absorption spectra of resonant gratings are highly sensitive to the angle of incident light.  
In previous work, we demonstrated that graphene-based gratings are exploited to modulate the Fano-like signature of GMRs [23]. Moreover, two-dimensional arrays of rectangular gold nanopatches grown on monolayer graphene were used to experimentally demonstrate the polarization dependence of plasmonic gratings at normal incidence [24].  
In this study, we numerically investigate a two-dimensional (2D) dielectric grating through the rigorous coupled-wave analysis (RCWA) method. We demonstrate the effects of geometrical parameters of the 2D grating on its spectral characteristics of interest. We further study the response of the proposed device for the oblique incidence of a plane wave source. We show that it has a stable optical absorption of around 40% over a considerable wide angular range of 20\({}^{\circ}\). Electromagnetic field distributions at resonant frequencies are presented to explain the underlying physical phenomenon for optical absorption.