Review on Graphene-based Absorbers for Infrared to Ultraviolet Frequencies

The graphene-based absorbers are widely applicable and highly e cient. Graphene has very high electrochemical properties due to which tuning characteristics can be achieved with e cient and broadband absorption response. For this review paper, we have divided the graphenebased absorbers into three categories (Absorber sensors, Solar absorbers, and THz absorbers) based on their applications. We have presented a detailed discussion on various designs and their analysis in this paper. Absorber sensors are mainly applicable in biosensors for the detection of hemoglobin, urine biomolecules using the tuning properties of graphene, and are also applicable in medical, environmental, chemical, biological diagnostic applications. Solar absorbers are applicable in energy harvesting devices. Adding graphene layer in solar absorber design gives the highly e cient and broadband absorption response. THz absorbers are applicable in the THz applications in sensing and imaging devices. Some of the THz absorbers are improving the applications in the new eld of nanooptics with 2D material. Graphene and its excellent electrical and optical properties are applied in material designs which create new structures applicable in novel applications like sensing, imaging, solar energy harvesting, etc.

The graphene-based absorbers are widely applicable and highly ecient. Graphene has very high electrochemical properties due to which tuning characteristics can be achieved with ecient and broadband absorption response. For this review paper, we have divided the graphenebased absorbers into three categories (Absorber sensors, Solar absorbers, and THz absorbers) based on their applications. We have presented a detailed discussion on various designs and their analysis in this paper. Absorber sensors are mainly applicable in biosensors for the detection of hemoglobin, urine biomolecules using the tuning properties of graphene, and are also applicable in medical, environmental, chemical, biological diagnostic applications. Solar absorbers are applicable in energy harvesting devices. Adding graphene layer in solar absorber design gives the highly ecient and broadband absorption response. THz absorbers are applicable in the THz applications in sensing and imaging devices. Some of the THz absorbers are improving the applications in the new eld of nanooptics with 2D material. Graphene and its ex-cellent electrical and optical properties are applied in material designs which create new structures applicable in novel applications like sens-

Introduction
Day to day usage of electricity is increasing as the human mind is being more and more ad-  [5].
Metamaterial which is articially composed structures has gained remarkable attention due to their extraordinary physical characteristics such as symmetric transmission [6], perfect lens [7], negative refractive index [8], etc. The rst metamaterial-based absorber was designed by Landy et al. achieved a near-perfect absorption [9]. These absorbers were mainly fabricated using three layers consisting of a rst layer containing a patterned metallic design to achieve the impedance-matching and stop the reecting light.
To act as a resonance cavity and for spacing of electromagnetic waves to disperse the second layer was created of the dielectric layer which was then followed by the metallic plate layer to stop transmission [10]. The only drawback of these absorbers was that the absorption response was of narrowband [11] and a single-peak [12], dual-band [13], or multiband [14] with near-perfect (unity) [15] absorption response was achieved. But we need a highly ecient broadband solar absorber that can absorb the solar energy in most of the wavelength range including terahertz, ultraviolet, visible, infrared, microwave, etc. [16]- [19]. These solar absorbers are utilized in a wide range of applications such as solar energy harvesting, superlenses to optical structure designs [20]- [24].
We Recent studies also suggest that an absorber sensor can also detect SARS-CoV-2 [25]. Silica and its compounds are widely used for absorber sensor applications due to its remarkably high and solid uores-cence optical characteristics [26]. Gold is also widely used for designing absorber sensors due to its rich material properties i.e., biocompatibility, electro-optical properties, and simple manufacturing and alteration process [27]. Solar absorbers can be dened as absorbers with highly ecient absorption response over a broadband frequency range which should include the ultraviolet, visible, and infrared range and be utilized for the building block of the solar energy harvester. THz absorbers are mostly applicable for polarizers, THz-based photonics devices.
Graphene is a unique, only atom-thick, twodimensional (2D) element with remarkable electrical, crystal, optical characteristics that can be applied to wide use in a nowadays physics and other related elds [28]. Researchers have proved that a single layer of graphene can absorb 2.3% of white light with 0.1% of refractivity [29]. It is also proved that as the no. of the layer in graphene increases so does the absorption proving the linear relationship [30]. Graphene is also rigid and stable. Nearly several hundred nanometers graphene can also show ballistic transport [31]. Due to these advantages and tunability of graphene, graphene-metasurfacebased solar absorbers are in demand. Graphenebased absorber sensors are highly in demand due to their super sensitivity and high range of detection of dierent biomolecules. Graphene-based optical absorber sensor can detect the single-cell, cell-line, etc. [32]. Graphene gap layers provide increased tunability which improves the sensing in optical absorber sensors [33,34]. An optical ber covered by very thin sheet of graphene can be utilized as optical sensor for the detection of minor alteration in the refractive index of the aqueous analyte [35]. Body sweat can also be detected using a graphene-based absorber sensor and humidity sensor. Surface Plasmon resonance (SPR) absorber sensor can be utilized for DNA hybridization [36,37].

Design and analysis
The conductivity σ s of graphene is derived from where ε = permittivity; ε 0 = permittivity of vacuum; ω is the angular frequency; ∇ is the thickness of the single-layer sheet of graphene.
The conductivity is divided into two segments, intraband conductivity (σ intra ), and interband conductivity (σ inetr ). Here, k B is referred to as Boltzmann's constant, is the decreased plank's constant, T is the room temperature, µ c is referred to the chemical potential of graphene which is given as µ c = v F πCV DC /e, V DC is the gate voltage, V F is referred to as Fermi velocity, C stands for capacitance.
Here, k is the wave vector.

Absorber sensors
Absorber sensors are widely used to observe biomolecules which have wide applications in various elds such as medical, environmental, and industrial, etc. [40]- [42]. There are various kinds of absorber sensors are available i.e., optical [43], [44], electrochemical [45], physical [46]. As the optical absorber sensor is compact, low-cost, and due to the utilization of unique nanomaterials it is easy to fabricate. They can be classied into two classes including detection optical absorber sensors and labeled detection absorber sensors [47]. Direct type absorber sensors are cost-eective and SPR based detection is one of the important direct detection absorber sensors as it can be used to detect the biomolecules such as hemoglobin, urine, proteins, etc. [48]- [50]. Taya presented a slab waveguide using air and anisotropic material-based optical sensor for refractometry and sensing-based applications [51,52,53]. slab waveguide sensor for the detection of cancer cells based on anisotropic left-handed material [55]. Patel et al. reported a highly sensitive and tunable biosensor using phase change material for the detection of hemoglobin biomolecules that achieved the highest sensitivity of 1000 nm/RIU [56]. An environmental analysis can also be carried out using a graphene peptide-based absorber sensor [57]. We can also detect hemoglobin biomolecules with high sensitivity using graphene-based leaky-wave optical absorber sensor design [58]. Raman signals at various combination which is used for the biomedical diagnosis can also be detected using graphene gold nanoribbons substrate in an absorber sensor [59]. Sensitivity is calculated by measuring the dierence between the peak of two biomolecules and used as a performance measure for sensors. As the dierence between peak increases the quality of sensor is increased due to the improved sensitivity.
Graphene metasurface-based single spit ring resonator and double split-ring resonator SPR absorber sensor The graphene SPR sensor for the detection of hemoglobin biomolecules is presented in [44]. They have presented two kinds of split-ring resonators one with a single split ring and the other with the double split ring. The design was created using SiO2 substrate and gold split ring resonator with graphene spacer between substrate and resonator layer. The dimension of the design are as follows: L = 2.2 µm, substrate height H = 0.4 µm, graphene sheet thickness = 0.34 nm, resonator height g = 200 nm, radius of inner ring r2 = 0.4 µm, radius of outer ring r1 = 0.8 µm, width of split ring w = 0.2 µm. The structure proposed by them is presented in Fig.  1. The voltage (Vg) is given to graphene sheet to control graphene chemical potential (GCP). The graphene surface has also been appointed with the density Jx = Exσs, Jy = Eyσs. The concentration of hemoglobin and urine is varied in the range of 10 g/l to 40 g/l and 0-1.5 mg/dL to 10 mg/dL for various refractive index values, respectively. The design is investigated using COMSOL Multiphysics.
Absorption analysis has been carried out for the 1.65 µm to 2 µm spectral range as shown in Fig.  2(II). The sensitivity (S) analysis is also carried out given by equation 13 [60]:  concentrations of hemoglobin, urine, and air. The plot illustrates that the two biomolecules from urine and hemoglobin show maximum absorption in 1.5 µm to 2 µm for double split-ring resonator design and 1.65 µm to 2 µm for single split ring resonator design. The achieved highest sensitivities are 1972 nm/RIU for hemoglobin and 1604 nm/RIU for urine biomolecules, where RIU stands for Refractive Index Unit. The highest sensitivities achieved for SSRR structure and DSRR structures are 28785 nm/RIU and 26346 nm/RIU, respectively for hemoglobin-urine concentration. They have also shown the design to be wide-angle sensitive.
Graphene metasurface-based circular and split ring resonator absorber sensor Parmar et al. reported a graphene-based metasurface design for the detection of urine and hemoglobin biomolecules [61]. A tuning in absorption response is achieved by varying the size and shape of metasurface and graphene chemical potential (GCP). The detailed analysis via varying the physical parameters of design is carried out and results are presented in terms of absorption, electric eld, and sensitivity. The design is illustrated in Fig. 3(I) which is designed by taking a gold ground plane and then placing SiO2 substrate over it followed by a gold resonator separated by a single layer of the graphene sheet. The design parameters are as follows: ground plane thickness, Hg = 0.5 µm, sub- strate height Hs = 1.5 µm, graphene sheet height, Hg = 0.34 nm, resonator height, Hr = 0.5 µm.
The length of the structure, L is 7.5 µm. Res-onator design is varied for radius R = 2.6 µm, and R/2 = 1.3 µm and corresponding results are presented. The dierent views of structure including 3D, top, front views are illustrated in Fig. 3(I).
The proposed absorber sensor design is simulated using COMSOL Multiphysics with Finite Element Method (FEM). They have considered the periodic boundary conditions over the x and y-axis. The permittivity and permeability equations of metasurfaces are given in Eqs. (14)- (17). These parameters are obtained from the impedance and the refractive index as given in the Eqs. (14)-(17) [62].
where z is the impedance; S11 and S21 are reection and transmission coecients, respectively; n stands for refractive index; d is referred to unit element's maximum length; k0 is the wavenumber; m is the branch due to the periodic characteristic of sinusoidal function; µ is the permeability and ε is the permittivity. Figure 3(II) illustrates the absorption response for circular and split ring resonator of radius R for urine concentration ranging from 10 g/l to 60 g/l with the step of 10 g/l for various refractive indices. A large value of the refractive index is required for the maximum absorption. The absorption response is obtained for the wavelength range of 40.7 µm to 41.5 µm.
It is observed that the absorption response shifts to 41.25 µm from 41.15 µm, 41.1 µm from 41 µm for circular resonator and split ring resonator, respectively as seen in Fig. 3 (II) when we increase the refractive index and urine concentration. The absorption response is also investigated by halving the radius of the resonator and is presented in sensitivity of 40000 nm/RIU is obtained for circular metasurface with radius R/2.
Graphene-based C-shaped tungsten metasurface refractive index absorber sensor Patel and co-authors simulated a refractive index absorber sensor with high sensitivity using a Cshaped tungsten metasurface for the infrared region are illustrated in Fig. 4(I) [34]. Refractive index absorber sensors are applicable in medical applications [63,64]. The design is created by placing a ground plane of tungsten which is followed by a SiO2 substrate then a tungsten C-shaped resonator is separated from the substrate using a graphene layer sheet. The sensitivity of biosensors depends upon the shift in absorption peak as per the summation of biomolecules over the absorber sensor. The design parameters are as following: length of the structure, L = 2.2 µm, tungsten ground plane height, B = 0.2 µm, substrate layer thickness, S = 0.33 µm, the Cshaped tungsten resonator having length, L1 = 1.6 µm, width, W = 0.4 µm, and the height, C = 0.30 nm. The graphene spacer thickness, G, is 0.34 nm. To achieve high absorption, the substrate thickness is set higher than the tungsten ground layer thickness. This shape also helps to trap the waves in the graphene and substrate layer and graphene's high conductivity also helps to achieve high absorption. The detailed analysis is carried out by varying the various parameters and exploring the concerned absorption plot.
The absorption response of the design for various compositions ranging from 10 g/l to 40 g/l with the step of 10 g/l with corresponding refractive index is shown in Fig. 4(II) for the wavelength range of 1.7 µm to 1.95 µm. The optimized design is also determined by varying various parameters such as substrate height, resonator height, ground layer thickness, etc. from the absorption and reection plots. This study concludes that the increase in resonator thickness reduces the absorption rate. The same phenomena are observed while increasing the width of the C-shaped resonator. But when we increase the substrate thickness the absorption rate also increases. same. The absorption plot showcases the peak at 1800 nm and 900 nm for hemoglobin and air design, respectively stating the shift of 900 nm with the high sensitivity of 2571 nm/RIU.
Graphene-based C-I shaped array-based refractive index absorber sensor Patel et al. presented a highly sensitive graphenebased refractive index absorber sensor using arrays of C and I-shaped gold metasurface [65]. The metasurface design is presented in Fig. 4(III). The structure is designed by placing gold C-I shaped arrays over a single layer sheet of graphene which is placed over a substrate of silicon dioxide which is followed by a ground plane of gold material. The design parameters are mentioned here: the length of the structure, L = 2 µm, silicon dioxide layer height, TS = 0.6 µm, the height of C-I shaped resonator, TG = 0.1 µm, height of biomolecule layer, TB = 1.4 µm, and the width of C-I shaped resonator, C = 0.55 µm. This design is simulated for a dierent composition of hemoglobin biomolecules ranging in between 10 g/l to 40 g/l with the step of 10 g/l. The analysis is carried out in the wavelength range of 0.2 µm to 1.2 µm and the absorption peak is achieved at 0.73 µm and 0.8 µm indicating the shift of 70 nm for the refractive indices of 1.34 and 1.36 [66] giving the sensitivity of 3500 nm/RIU. The design is also analyzed for variation in physical parameters which indicates the increase in the substrate height increases absorption rate, while the high absorption rate is noted for the low thickness of the ground layer. Fig. 4(IV) illustrate the electric eld and magnetic eld responsible for the design with a various wavelength which indicates the absorption is low for 0.59 µm and high absorption is achieved for the rest of the wavelength indicated by red color.
Graphene-based refractive index absorber sensor with various combinations of the resonator including circular, square and plus shape arrays Patel et al. reported a comparative sensitivity analysis of refractive index absorber sensor for various patterns of circle, square, and plus shape resonator as illustrated in Fig. 5 [67]. The metasurface is designed by rst placing a substrate of silicon dioxide, followed by an array of gold resonators separated from the substrate by a thin layer of graphene spacer. The hemoglobin biomolecules layer is placed over a gold array resonator. The variation of a different combination of circle, square, and plus-shaped resonator is demonstrated in Fig. 5(II). The design parameters are as follows: length of the structure, L = 2.5 µm, silicon dioxide layer height, TS = 0.5 µm, graphene single-layer thickness, g = 0.34 nm, gold array resonator height, TG = 0.3 µm, hemoglobin biomolecule layer thickness, TB = 0.6 µm, and the diameter of circular disks array resonator, GD = 500 nm. The design is simulated in COMSOL Multiphysics software.
The design is analyzed for various combinations of circular, square, and plus-shaped resonator and concerned absorption responses are presented in Figs. 6(a-e) for the wavelength range of 0.4 µm to 0.55 µm. The absorption responses are achieved for four dierent compositions of hemoglobin biomolecules ranging between 10 g/l to 40 g/l with the step of 10 g/l and the corresponding refractive indices are also veried experimentally. The 10 nm shift is observed for the hemoglobin molecules for refractive indices of 1.34 RIU and 1.36 RIU giving the sensitivity of 500 nm/RIU. The highest sensitivity is achieved for a 3×3 circular disk array resonator design. While the 3×3 square array, circular-square array, square circular array, 2×1×2 square array, 3×3 plus-shape array, and plus and square shape array achieved the highest sensitivity of 300 nm/RIU, 400 nm/RIU, 300 nm/RIU, 450 nm/RIU, 266 nm/RIU, 400 nm/RIU, respectively. So, the optimized resonator combination of 3×3 circular disk array is achieved and then various physical parameters are varied to observe their eect on absorption response. The comparison is also carried out for air and hemoglobin biomolecules and the 120 nm shift is achieved with the sensitivity of 353 nm/RIU is determined.

Solar absorber
The fabrication process of metamaterial design is complex due to its three-dimensional designs and losses. Solar absorbers are in demand due to their frequency-dependent resonating response in the ultraviolet, visible, and infrared regions. Metasurface absorbers have remarkable utilization in designing broadband absorbers. We can also add the layer of graphene to achieve broadband absorption. A highly ecient broadband solar absorber can be designed using a monolayer graphene sheet created by a carbon atom positioned in a honeycomb lattice structure [68]. The term broadband absorption emerged due to the coupling of the layers, the broad bandwidth of absorption increases as the no. of layer increases [69,70]. A specic absorption range can also be attained using the Q-factor during the designing of absorbers and resonators [71]. An absorber must be optically thick but physically thin so that it can achieve a high absorption response [72]. Patel et al. presented a broadband absorber for the visible range using phase change material and the average absorption of 92.86% was achieved for the frequency range of 500 THz to 740 THz [73].
Graphene-based broadband solar absorber based on C-shape rectangular sawtooth metasurface Patel et al. presented a wideband graphene-based solar absorber for the near-infrared range placing a rectangular sawtooth at the outer surface of the Cshape resonator [74]. The design is created by rst placing tungsten ground layer which is then followed by a silicon dioxide substrate them tungsten made C-shape resonator and the rectangular sawtoothshaped resonator is placed at the outer surface of C-shape. The resonator and substrate layer are separated by a monolayer sheet of graphene. The threedimensional view of the structure is presented in Fig.  7(a). The design parameters are: the length of the structure is 2 µm, the tungsten-made ground layer height is 200 nm, the silicon dioxide-based substrate is 330 nm height, the graphene sheet is of 0.34 nm thickness. The C-shape resonator width, length, and height are 400 nm, 1600 nm, 146 nm, respectively. The length and width of the rectangular sawtooth are set to 0.1 µm, 0.2 µm, respectively. The distance between two consecutive rectangular sawtooth is kept at 0.2 µm. Data related to tungsten is determined by the Drude model [75].
The results in terms of absorption, fermi energy plots, electric and magnetic elds are achieved in the wavelength range of 0.71 µm to 1.1 µm presenting the near-infrared (NIR) region. The detailed analysis by varying the position of the rectangular sawtooth is also carried out and related plots are presented in Fig. 7(b). The results indicate that for only C-shaped resonator the average absorption of 66.7%, resonator with rectangular sawtooth positioned at the inner surface of C-shape achieves the average absorption of 86.6%, and the resonator with rectangular sawtooth positioned at the outer surface of C-shape achieves the highest average absorption of 91.8% in the NIR region. The comparative analysis of two designs one with graphene layer and one without graphene layer clearly indicates that the graphene improves the absorption response by a very high margin. This solar absorber is also angle-insensitive and also polarization-insensitive. Analysis of the design is also carried out by varying the design parameter and optimal design is determined. Graphene-based broadband solar absorber using uniformly placed gold resonator array Patel et al. simulated a broadband graphene metasurface-based solar absorber for the NIR regime using a uniformly placed gold resonator array [76]. This paper investigated the eect of the multilayer structure and the pattern of gold resonators on absorption response. This resonator layer is placed over a silicon dioxide substrate layer and these two layers are separated by a single layer of a graphene sheet and below the substrate, there is a ground layer of gold material. They have presented ve vari-ous designs by varying the thin square gold resonator pattern and adding these to multilayer substrate of which three variations are illustrated in Figs. 8(a-c). The various geometrical parameters of the designs with the variation for comparative analysis are the height of the gold ground layer, hg is varied between 80 nm to 120 nm, the substrate height, hs is varied in the range of 80 nm to 120 nm, too. While the thin square gold resonator thickness, h l is varied from 30 nm to 70 nm. The reectance can be given as described in Eq. (18): (Reprinted with the permission from [74], copyright Elsevier).
When Z(ω) and Z0(ω) are of same values, the reectance will be zero, and this is also possible when µ(ω) and ε(ω) are same. We obtain metamaterials results in the two most important forms of reection and absorption. The absorption can also be explored using Eqs. (20)-(25) [77] ∆ × E = iωµ0 H (20) Q tot abs = A(ω)F (ω)dω (25) where Q abs is the power absorbed by the component; Qinc is the power reaching the solar panel from the sun; Q tot abs is total power. A design is also explored using the R-L-C circuit as shown in Fig. 8(d) [78]. Metal induces the resistance R, inductance L, while the capacitance C is caused due to the gap between the metal layer. Eqs. (26)-(28) derives the normalized impedance: The design is simulated using the COMSOL Multiphysics software using the FEM computation method. The results are investigated in the form of absorption, reectance, and transmittance for the frequency range of 155 THz to 425 THz. From the absorption result, it is clear that the metasurface with two thin square alternate layer checkers array-based design and metasurface with two thin square identical layer checkers array-based design achieved the highest average absorption of 85% in the NIR regime. While metasurface with thin square two identical layers array-based design, metasurface with thin square single layer checkers array-based design, and metasurface with thin square single layer array-based design achieved average absorption of 82%, 72%, and 71%, respectively. The design presented in Fig. 8(c)  parameters to analyze their eect on absorption response as this particular design achieves high average absorption. The optimized parameters for the designs are then determined, where square gold resonator height is 70 nm, substrate height of 120 nm, and gold ground layer thickness is 120 nm. Fig.  8(e) presents the refractive index analysis of the design for 155 THz to 425 THz frequency range and the refractive index is computed by substituting the reection and transmission coecients in Eqs. (26-(28) and Eq. (16). Fig. 8(e) clearly shows the negative refractive index proving the metamaterial behavior in the whole NIR region. Due to the broadband response of the design, this can be implemented as a unit block for energy harvesting devices.
Graphene-based plus and square shape resonator-based broadband and highly ecient solar absorber Patel et al. presented a graphene-based plus and square shape resonator-based broadband and highly ecient solar absorber [79]. The design is created by placing silicon dioxide over a ground plane of the gold layer then a gold resonator of square and silver resonator of plus shape which is separated from a substrate using a thin layer of the graphene sheet. The design's three-dimensional view is presented in Fig. 9(a). The design parameters are: ground plane thickness, G_H = 150 nm, substrate thickness, S_H = 150 nm, graphene sheet height is of 0.34 nm, length and width of the structure is 800 nm, width of gold and silver material resonator, S_W = G_W = 150 nm, and the resonator thickness, H = 150 nm. The proposed structure is simulated using COMSOL Multiphysics software. They have investigated the two variations of the design one where they have added the thin graphene layer and one where it is not added.
The results are presented in form of absorption and reectance for 200 THz to 1600 THz as illustrated in Figs. 9(b-c). A detailed analysis by varying various physical parameters is also carried out and the related results are illustrated in Figs. 9(d-e). The average absorption of 92.72% and 89.9% is achieved for the design where graphene is present and where it is not present, respectively. The highest average absorption of 97.51% is reported with enhancement in absorption due to the graphene layer positioned above a substrate keeping all the waves inside the substrate layer. The only exception is the ultraviolet region where the average absorption is high for solar absorbers without graphene. The absorption response of these two variations is illustrated in Figs. 9(b-c). The absorption response with respect to variation in gold resonator width and silver resonator width in the range of 150 nm to 250 nm and 100 nm to 200 nm, respectively as illustrated in Figs. 9(d-e). The plot clearly indicates the reduction in absorption response as the width of both the gold and silver resonator increases as it covers more space over the substrate layer increasing the reectance. The design with graphene sheet achieves the average absorption of 85.48% in infrared, 97.51% in visible, and 89.57% in ultraviolet regime in the frequency range of 200 THz to 1600 THz. An average absorption of 92.72% is achieved for the whole solar spectrum and due to the broadband and high eciency, it can be used as a unit block for energy harvesting instruments.
Graphene-based L-shape metasurface based wideband solar absorber Charola et al. reported a tri-layer L-shape metasurface-based wideband solar absorber for the visible and ultraviolet region [80]. The proposed de-sign is created by metal-dielectric-metal formation, where the L-shape resonator and ground layer are of tungsten material and the substrate is of silicon dioxide as shown in the three-dimensional and front view of the structure illustrated in Fig. 10(a-b). The design is created by placing an L-shaped resonator placed uniformly over a substrate. This design with various top metallic structures is fabricated by scrutinizing the dip-coating techniques reported in [81] and electron beam lithography reported in [82,83]. The design parameters are: length of the design, L = 133.33 nm, ground layer thickness, B1 = 29 nm, substrate height, B2 = 50 nm, resonator height, B3 = 20 nm, length of L-shape, S = 75 nm, width of L-shape, W = 25 nm, and the distance between consecutive L-shape is set to 58.33 nm.
The design is simulated in COMSOL Multiphysics 5.4 and the results in terms of absorption, the electric eld intensity is presented for the frequency range of 340 THz to 1150 THz as illustrated in Fig.  10(c-e). The design achieves an average absorption of 92.2% in visible and 74.1% in the ultraviolet region. The L-shape metallic structure has remarkable capabilities of absorbing, concentrating, and indicating the incoming EM waves in the proper direction for both grazing and oblique incidence. The design is further explored for the eect of variation in physical parameters on absorption response. The eect of change in angle of incidence on absorption response indicates that the design achieves more than 80% average absorption for incidence angle between 0 o and 40 o as presented in Fig. 10(d). The electric eld intensity of the proposed structure at 611 THz is presented in Fig. 10(e).

THz absorber
THz absorbers are tunable absorbers that are designed in the THz far-infrared frequency range. Furthermore, the terahertz spectrum is full of various applications including wireless communication, security purposes, and imaging applications which all need an ecient absorber that can perfectly absorb the energy from waves [84]- [86]. Tao et al. presented the rst-ever THz absorber based on metamaterial which achieved 70% absorption response at 1.3 THz [87].
Graphene gold grating-based THz absorber Guo et al. reported a hybrid graphene gold grating-based design to obtain improved nonlinear eects in the THz frequency range [88]. The design is presented in Fig. 11(a)  in the x -axis direction and with the periodicity of p which is assumed to be expanded to innity in the y-axis direction. The signicant nonlinear response is mostly owing to the localization and intensication of the electric eld at the postulated structure's absorption resonance, as well as graphene's large nonlinear conductivity at low THz frequencies. The corresponding absorption results are presented in Fig. 11(b). This design can supposedly be utilized for various applications based on the new nonlinear optics eld of 2D material. This can be applied in THz frequency generators, signal processors, and wave mixers. The suggested hybrid grating's strong eld connement inside nanoscale trenches and along graphene can be exploited to accelerate dipole prohibited transitions on the atomic scale, too [89,90].
Graphene-based circular array-based absorber Dave et al. presented a graphene-based circulararray based terahertz absorber for far-infrared (FIR) frequencies [91]. Design is made by a silicon layer and a single layer sheet of graphene and the circular array is positioned as shown in Figs. 12(a-b). A length is set at 7500 nm and height of the design is 1500 nm, the radius of the graphene sheet is 3500 nm, and the gap between the two adjutant circles is 2000 nm. The presented structure is simulated using FEM in COMSOL Multiphysics for the frequency range of 1 THz to 7 THz and the results in form of absorption, reectance, transmittance, and phase variation. The transmittance coecient (Tij), reectance coecient (Rij), and phase (φij) can be dened using Eqs. (29)-(31) [92]   Results illustrated in Fig. 12(c) indicated the reduction in absorption as the value of graphene chemical potential increases. The plots indicate the change in resonance frequency values as the radius of the smaller circle varies. The phase variation also changes as the GCP and radius of the small circle vary as illustrated in Fig. 12(d). The maximum phase variation of about 85 o is observed for the GCP of 0.9 eV. To conclude maximum absorption of 85% is achieved for a smaller circle radius of 250 µm and 450 µm. The tunable ability of this structure can be utilized to design THz sensor, modulator, polarizer, or absorbers.
Tunable multi-stacked-based THz absorber based on graphene-grating Parmar et al. reported a tunable multi-stacked THz absorber based on graphene grating for the FIR frequency range [93]. The design of the implemented structure is presented in Fig. 13(a) and we can observe the grating structure formed from gold wires and the stacked graphene layer. In the free space around the gold wire and between two graphene layers silica material is placed. The eective height of the design H = 1000 µm, grating structure width W = 300 µm, the gap between two graphene layers is 50 µm. In the center of the design a gold circle of radius r = 50 µm, is positioned. The absorption, reectance, and transmittance analysis of the structure is carried out for the frequency range 1 THz to 3 THz. From port 1 the grating structure is excited with transverse magnetic mode. The variation in absorption with respect to incidence angle variation is illustrated in Fig. 13(b). This design can be the unit element of many THz-based applications in sensing and imaging elds. 238 "This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited (CC BY 4.0)."