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Abstract
Nanostructured-based broadband perfect absorbers are of great interest in a wide range of applications, including spectroscopy, energy harvesting, and thermal photonics, etc. Here, a planar square meta-ring of nickel (Ni) is presented, which has the potential to absorb a large operational wavelength starting from 400 nm to 3000 nm. The proposed device model comprises a configuration of three layers with a top and bottom metal of Ni and a middle dielectric layer of aluminium nitride (AlN). The designed square ring metamaterial absorber (SRMMA) can attain an average absorption value of more than 90% over a large waveband from 400-3000 nm. Moreover, it can also maintain its absorption value up to 70% for oblique incident angles for both the wave polarization cases (TE and TM). Furthermore, it also exhibits the polarization-insensitive absorption response under the influence of different polarization angles, which is attributed to the four-fold symmetry in the geometry of the square meta-ring. Furthermore, our SRMMA also reduces fabrication complexity and cost due to its simplified design architecture. These promising features of the proposed SRMMA may have widespread applications in solar energy harvesting, thermal emission, and other optoelectronic devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metamaterials are artificially engineered materials designed to interact with electromagnetic (EM) waves and to use them in implementing a plethora of microwave and optical devices [1–8]. Metamaterials with their subwavelength meta-atoms or unit cells can manipulate the EM waves and perform several interesting functions such as chiral imaging [9], optical reflectors [10,11], nano-antennas [12], flat lenses [13,14], optical vortex beam [15], plasmon-induced transparency [16], Bessel beam generation [17,18], Cherenkov radiation [19] and perfect absorption [20–22]. These intriguing and exotic attributes of metamaterials have made them very popular among the research community [23]. In this proceeding, we will focus on the perfect absorption aspect of the metamaterials, which has indispensable applications in the field of microwave, communication, and nanophotonics [24–29].
After the first-ever demonstration of metamaterial absorbers by Landy et al. [30], researchers proposed various kinds of absorbers ranging from microwave and optical regimes [31–35]. Broadly, metamaterials can be grouped into two streams: 1) narrowband absorbers and 2) wideband absorbers. The narrowband absorber can be used in sensing [36] and filtering [37], whereas, wideband absorbers are immensely employed in applications like radar cross-section reduction (RCSR) [38], stealth technology [39], photovoltaics, and thermal emitters [26,40], etc. One standard configuration of a metamaterial absorber is the sandwiched dielectric between two metal layers. Mostly, the top and bottom surfaces consist of plasmonic metals like gold (Au), silver (Ag), aluminum (Al), etc., and the middle layer is usually a lossy dielectric. Nowadays, plenty of other metals, namely chromium (Cr), tungsten (W), bismuth (Bi), iron (Fe), and titanium (Ti), are also employed to construct plasmonic nanostructured absorbers [41–43]. In addition, the various metal nitrides such as titanium nitride (TiN) [44], zirconium nitride (ZrN) [45], vanadium nitride (VN) [46], and hafnium nitride (HfN) [47] are also used for designing perfect absorbers for solar thermo-photovoltaics applications.
Designing broadband, miniaturized, and highly efficient metamaterials is a daunting challenge. In literature, researchers have proposed various methods to achieve higher bandwidths, such as multi-layered structures [44,48], composite or 2D materials like graphene [49] and black phosphorus [50], and multiple resonators [12,51] in a supercell. Although these methods produce wideband absorption but from a practical point of view, these multi-layer or multi-resonance structures are ineffective and have a lot of drawbacks such as high cost, complex structures, difficult fabrication process, and bulky size. These mentioned challenges encourage and motivate the research community to explore metamaterial-based wide bandwidth absorbers. In [52], Luo et al. proposed a wideband metamaterial absorber in the visible range. The design is based on cylindrical arrays of the dielectric sandwiched with Ni metal. Its operating range is 400-700 nm, with an absorption rate of above 90%. Another cylindrical hole approach is presented in [53] by Qi et al. to design broadband absorbers ranging from UV to near-infrared wavelengths with an absorptivity of over 90%. A conical frustum-like multi-layer metamaterial absorber is reported in [54], achieving 480 nm to 1480 nm bandwidth. In [55], Zhang et al. proposed a metamaterial structure with a cascade of three metals. It achieves a 90% absorption in the 490-825 nm range. In addition, fractals, self-similar repeated geometric structures, have also been employed to overcome the low bandwidth issues. The self-similar features allow these structures to achieve multiple resonances and increase the bandwidth [56,57]. In 2021 [40], an ultra-wideband nanostructured Ni-based plasmonic absorber is also reported in which fractal-based hexagonal rings are employed to enhance the absorption window. This fractal-based absorber attained more than 80% absorption value in large optical wavelengths from 200 nm to 4000 nm. Besides, the elliptical-rings-based fractal design is also investigated in 2020 [58], which showed above 90% absorption value and covered the entire visible wavelength spectrum from 400-750 nm. Several other structures like cylindrical arrays, hole-arrays or multi-layer architectures, etc. [55,59,60] have been reported. However, these approaches are either too complicated to fabricate or have lower absorption bandwidth. A more practicable approach is to design the subwavelength unit cell size and geometry in such a way that it can interact with the incident waves over a larger operating range of wavelengths which can produce a large absorption window as compared to the previously discussed metamaterial absorbers. Attaining ultrawideband absorption features from a single-layer device architecture is challenging and difficult to achieve due to the inherent bandwidth limitation.
In this communication, the authors propose a simple and highly efficient ultrawideband metamaterial absorber-based simple and cost-effective design architecture, in which a plasmonic metal Ni is employed as the top and bottom surfaces. The high-indexed dielectric material, AlN is inserted between Ni's top and bottom surfaces. The basic purpose for selecting Ni as a constitutive plasmonic metal is its high tolerance at incalescent temperatures while supporting a large absorption bandwidth owing to its high imaginary refractive index component. Furthermore, it has high corrosion resistance and low cost as compared to the available noble metals, namely Au, Ag, and Al. The designed SRMMA exhibits a phenomenal broadband absorption response spanning from the visible to short-infrared spectral wavelengths (400 nm – 3000 nm). Furthermore, this high bandwidth is achieved without compromising any of the essential attributes, such as polarization insensitivity, design simplicity, and wide-angle stability up to ${60^\circ }$ under TE and TM excitation. Compared to the existing broadband metamaterial absorbers in this spectrum, our SRMMA has promising features such as simple design specifications, polarization insensitivity, high bandwidth, and angular stability. The proposed SRMMA may have some exciting applications in thermal emission, solar photovoltaics, and spectroscopy.
2. Design setup and theory of the SRMMA
Figure 1 depicts the schematic of the proposed Ni-based SRMMA. The meta-unit cell comprises a metal/dielectric/metal-based device structure. The top of the unit cell is a Ni square ring, which is the main building block of the proposed SRMMA. The length of the square ring is optimized as 50 nm with a thickness of 25 nm. The Ni metal is deposited over the dielectric layer having a thickness of 15 nm. The sandwich dielectric layer consists of ALN, which has a high relative permittivity of 8.6, which is taken from the built-in material library of the CST-studio [61]. The loss tangent of ALN is 0.0003, and hs denote the height of the dielectric spacer. Using a high permittivity dielectric increased the capacitance and allowed us to reduce the size of the meta-unit cell. Since the resonance frequency is $1/2\pi \sqrt {LC} $, with an increase in capacitance, one can achieve the same resonance with a smaller meta-molecule. The bottom layer of the unit cell is also made up of Ni metal with a thickness of 10 nm, preventing the EM wave's transmission through the bottom layer. The overall periodicity P of the meta unit-cell is 75 × 75 nm2. The proposed meta-molecule is simple and symmetrical in design, reducing the device's cost and fabrication complexity.
Fig. 1. Pictorial illustration of the proposed SRMMA, (a) schematic of the proposed unit cell of square-ring of the Ni, (b) side-view of the proposed unit cell of square-ring of Ni, which shows the configuration of all the three-layers, and (c) three dimensional (3D) view of the periodic arrays of the square-ring of Ni in which incident and reflected optical light are shown, while θ is the incident-angle of the input optical light and φ indicates the rotational angle (polarization angle) of the optical light.
The EM response and field distribution are simulated utilizing the 3D frequency-domain EM simulation method of CST studio. We employed the unit cell boundary conditions in the x-y direction and used an open-add space in the wave propagation direction, i.e., the z-axis.
The absorption of the EM waves can be understood using the transmission line theory. The SRMMA is designed such that the impedance of the metamaterial matches with the impedance of the incident EM waves. This condition mitigates the reflection of the EM waves. Further, the high-indexed dielectric of AlN is used to trap the EM waves inside the absorber. The following Eq. (1) describes the absorption phenomena in the SRMMA [40,58]:
In the above Equation, $A(\omega )$ is the absorption, $R(\omega )$ represents the reflection, and $T(\omega )$ is the transmission coefficient. The reflection and transmission coefficient can be expressed in terms of the scattering parameters as shown in the following Eq. (2):
The relationship between the scattering parameters and the impedance of the SRMMA can be explored using the following Eqs. (4) and (5) [62]:
In the above Eqs., n represents the effective refractive index, k represents the wave-vector, and d is the height of the SRMMA. As discussed earlier, ${S_{21}}$ is close to zero. From Eq. (5), it is evident that by controlling the impedance of the SRMMA we can control the absorption of the EM waves.
3. Results and discussion
In this section, we explore the spectral absorption features of the discussed SRMMA by indenting plane waves with a wavelength in the range of 400 nm to 3000 nm. The discussed metamaterial is firstly studied under the normal excitation of transverse electric (TE) and transverse magnetic (TM) waves. Figure 2(a) illustrates the simulated absorption of SRMMA. The SRMMA shows above 90% absorption levels in the 400 nm to 3000 nm wavelength span, which covers the entire visible and short-infrared region. The plot also shows the reflected and transmitted power, which is close to zero in the band of interest, and affirm the efficient performance of the designed SRMMA. Furthermore, the absorption response for both TE and TM polarized waves is identical, which owes to the four-fold symmetry of the design.
Fig. 2. Optical characteristics of the proposed SRMMA, (a) scattering parameters of the proposed SRMMA, green solid-line demonstrates the absorption curve while blue- and red solid-line represent the reflection and transmission curve respectively, (b) effective parameters of the proposed SRMMA, red solid-line denotes the absolute magnitude of the permittivity and blue solid-line corresponds to the absolute magnitude of the permeability, and (c) the absolute magnitude of the normalized effective impedance of the proposed SRMMA
The almost perfect wideband absorption of the proposed SRMMA results from the nearly perfect impedance matching and zero transmission due to the Ni layer at the bottom with a thickness greater than the skin depth of incident waves. A perfect matching results in a zero reflection coefficient [40]:
In above Eq. (6), ${Z_M}$ is the impedance of the SRMMA and ${Z_o}$ is the impedance of the free space. As can be observed that when ${Z_M}$ and ${Z_o}$ are close to each other, then reflection reaches to its minimum value, and we know that $A(\omega )= 1 - \Gamma (\omega )$, absorption will be maximum. These impedances (${Z_M}\; \& \; {Z_o}$) can also be represented in terms of effective permittivity permeability and s-parameters, as shown below. As ${\mu _M}$ = ${\varepsilon _M}$, then ${Z_M} = 1,$ and from Eq. (9), S11 is zero, and absorption reaches to 100%.
In the above Eqs. (11) and (12), is k the wavenumber and calculated as $k = \omega /c$ whereas d is the substrate height. It is clear from Eqs. (11) and (12) that if ${S_{11}}$ and ${S_{21}}$ go to zero, then ${\varepsilon _M}$ and $\textrm{}{\mu _M}$ becomes equal. As a result, ${Z_M}$ and ${Z_o}$ becomes equal, reducing the reflection coefficient to a null. The Fig. 2(b) shows the magnitude of ${\varepsilon _M}$ and $\textrm{}{\mu _M}$ which is close to each other in the desired band (400-3000 nm). The normalized impedance is also plotted in Fig. 2(c) and it is close to unity, demonstrating the perfect matching with the free space.
Figure 3 shows the electric field distribution over the surface and the current distribution at the top and bottom surfaces of the SRMMA for two different wavelengths, i.e., 624 nm and 1917nm. It can be observed that the maximum of the electric field is confined at the edges of the square meta-ring, thereby inducing quadruple-like plasmonic resonances, as shown in Fig. 3(a). Considering λ = 1917nm, the E-field is maximally distributed at the top and bottom portion of the square meta-ring, which produces dipole-like resonances. Overall, these two resonances excite the localized surface plasmons (LSPs), which enhance absorption. Furthermore, the current distribution analysis is also studied at the top and bottom surfaces of the proposed SRMMA, as depicted in Fig. 4. Figure 4(a) represents the current distribution at the lower wavelength, λ = 624 nm, it is noticed in this Fig. 4(a), the direction of currents are parallel to each other, and these parallel arrangement of the currents produce electric resonances, which cause absorption inside the proposed SRMMA [63].
Fig. 3. Physical demonstration of the electric field distribution at the surface of the proposed SRMMA, (a) λ = 624 nm and (b) λ = 1917nm.
Fig. 4. Physical demonstration of the current distribution at the top and bottom surfaces of the proposed SRMMA, (a) current distribution at the top surface for λ = 624 nm, (b) current distribution at the bottom ground-surface for λ = 624 nm, (c) current distribution at the top surface for λ = 1917nm, and (d) current distribution at the bottom ground-surface for λ = 1917nm.
Besides, Fig. 4(b) highlights the surface current distribution at the higher wavelength, λ = 1917nm. Similarly, the top and bottom also provide the parallel arrangements of the currents, which induces an electric resonance at higher operating wavelength points. Overall, these electric resonances induce dipole-like phenomena, which helps to achieve the wide absorption response of the proposed SRMMA.
The absorptivity calculated so far is under the normal incidence of the EM wave. However, in real scenarios, an EM wave can also excite the metasurface at oblique incidence. Similarly, the polarization of the EM waves is also not fixed, and it can be TE or TM polarization. Therefore, the reflection coefficient can be evaluated for TE and TM modes using the following Eqs. [40,58]:
In the above Eqs. (13) and (14), ${\theta _i}$ is the incident angle and ${\theta _t}$ is the transmitted angle. When we have normal incident EM waves, the reflection coefficient merely depends on the impedance (Zm) of the metamaterial, as free-space impedance (Z0) is a constant value of 377 Ω. However, in the case of an oblique incident case, it also depends on the incident and transmitted angles of EM waves. When the incident angle changes, the reflection coefficient changes and absorption ultimately varies. As we keep increasing the obliquity, the anisotropy of the structure increase, which changes the impedance of the meta-atom, and the matching condition breaks. Consequently, as the mismatch increases, the absorption decreases. One way to tackle this problem and make the proposed SRMMA more practical for deploying in the real environment is through the design's simplicity. The proposed SRMMA demonstrates wide-angle stability due to the steady rise of the anisotropy with θi. As shown in Fig. 5(b), the discussed SRMMA exhibits an above 80% absorptivity for an incidence angle of 50° for TE polarization mode and above 70% for θ = 60° excitation angle. For TM mode, the stability is even preferable with above 85% absorptivity with an incidence angle of 60° as shown in Fig. 5(b).
Fig. 5. Optical absorption characteristics of the proposed SRMMA under different operating conditions, (a) absorption under the influence of different oblique incident angles for TE-polarized optical light, (b) absorption under the influence of different oblique incident angles for TM-polarized optical light, and (c) absorption under the influence of different polarization angles of the input optical light.
Polarization insensitivity is one of the foremost attributes for the practical application of the proposed SRMMA. The SRMMA is immune to polarization angles due to the four-fold symmetry in the design. The polarization insensitivity is analyzed by varying the polarization angle of a normal incident wave from 0° to 90° in regular steps of 30°. Figure 5(c) displays the polarization insensitivity of the proposed SRMMA. As the polarization angle changes, the plasmonic modes orientation also changes, but due to the horizontal and vertical symmetry of the design, the absorption remains unaffected.
The effect of different parameter variations on absorption is also included in our analysis. The impact of major structural parameters is captured in Fig. 6. In Fig. 6(a), we have swept the length of the square ring from 30 nm to 70 nm in the wavelength range 400 nm to 3000 nm. At lower lengths from 30 nm to 40 nm, the average absorption is almost around unity but then decreases at higher wavelengths starting from 1500 nm. At the more considerable length of 70 nm, the absorption decreases rapidly and goes below 90% in the band of interest. From 40 nm to 60 nm, the absorption is stable and mostly over 90%, with 50 nm being the closest to the optimal length. Thereafter, in Fig. 6(b), the width of the proposed SRMMA is varied from 10 nm to 30 nm, and its effect on the absorption response is explored. The average absorption remains over 90% for the desired optical window throughout the 10 nm to 30 nm widths range. However, it can be observed that at lower width values, the absorption is better at higher wavelengths.
Fig. 6. Optical absorption of the proposed SRMMA under the influence of various design parameters, (a) absorption characteristics under the variation of length of the square meta-ring (L), (b) absorption characteristics under the variation of the width of the square meta-ring (W), (c) absorption characteristics under the variation of thickness of the square meta-ring (tm), (d) absorption characteristics under the variation of thickness of the dielectric substrate (hs), and (e) absorption characteristics under the variation of periodicity of the unit cell (P).
Furthermore, the influence of the thickness of the top Ni meta-ring on the absorption is also observed by varying the thickness from 10 nm to 35 nm. From Fig. 6(c), we can notice that the thickness has no significant effect on the absorption, which is, on average over 90%. After that, we analyzed the effect of the height of the substrate over the absorption. The height of the substrate is an important parameter, and it affects the bandwidth as well as the absorption response of the SRMMA. By increasing the thickness of the substrate, the resonance frequency decreases and the fringing effect increases, which observed in Fig. 6(d). The substrate height varies from 5 nm to 30 nm in Fig. 6(d) over the required wavelength range. As we increase the height, the absorption at lower wavelengths starts to decrease and even goes below 80% for 20 nm to 30 nm substrate height, whereas at higher wavelengths, the response is almost similar. Next, the variation in the periodicity of the unit cell is observed in the absorption. The periodicity varies from 65 nm to 90 nm in Fig. 6(e). The overall absorption is over 90% for all the mentioned values; however, it slightly decreases to below 90% for P = 65 nm around (1000 nm-1200 nm). The geometric parameters of the proposed metamaterial-based structure determine its impedance. In our proposed SRMMA, all the contributing design parameters, including L, W, tm, hs, and P have been optimized to obtain the desired matching conditions over a large optical wavelength (400-3000 nm). The variation in these design parameters greatly influences the absorption characteristics of the proposed SRMMA. Therefore, when we vary the values of these parameters, the impedance of the proposed SRMMA changes and absorption varies. Finally, the design parameters, especially L and hs, significantly alter the absorption characteristics of the proposed SRMMA. The length (L) of the square meta-ring determines the overall effective size of the proposed SRMMA, and the dielectric spacer helps in trapping the optical light inside it, therefore, the thickness (hs) has a significant impact on the absorption features. So by proper optimization and carefully selecting these parameters, the absorption window can be adjusted.
In Table 1, the performance attributes of the different plasmonic absorbers are compared with the present SRMMA. In addition, Table 1 highlights the design topology, absorption bandwidth, layers pattern, and angular stability analysis of all the previously published work. Compared with the SRMMA, most of the discussed absorbers are composed of multiple-stacked layers arrangement or cover only a limited optical wavelengths. In contrast, our absorber holds a single-layer device configuration with a simple and planar square meta-ring, which is easily fabricable. Furthermore, it exhibits an ultra-broadband absorption bandwidth from 400 to 3000 nm, which is difficult to achieve with the simple design specification. So, in light of all the mentioned performance parameters, our SRMMA fulfills the demand of an ideal absorber and would be a valuable addition in the stream of nanoscale absorbers.
Table 1. Comparison study of some of the previously reported nanostructured absorbers with the present study
4. Conclusion
In summary, ultra-broadband absorption characteristics were realized with the simple and planar design of a square-shaped meta-ring of Ni. The proposed SRMMA achieved more than 90% absorption value, covering the long spectral wavelength spanning 400 to 3000 nm. The proposed absorbing metamaterial was composed of an ultrathin single-layer device configuration, an upper square-shaped meta-ring of Ni, which helped to interact with the incident optical light, thereby causing broadband absorption. The lower ground metallic film of Ni played a key role in suppressing the transmission of optical light. The middle layer of AlN assisted in trapping the light inside the dielectric spacer. The influence of oblique incident and polarization angles was also explored to observe the absorption trend of the proposed SRMMA. Furthermore, the effect of design parameters of square meta-ring as well as dielectric spacer layer were also investigated and studied. The physical absorption phenomena was also explained in the light of the surface electric field and current distribution by considering two plasmonic resonances generated in the proposed SRMMA. Finally, the promising characteristics of the planar nanostructured absorber make it an exciting candidate for various applications, including thermal emission, solar harvesting, and other optoelectronic devices.
Funding
King Abdullah University of Science and Technology (Innovative Technologies Laboratories).
Acknowledgement
The authors would like to acknowledge research funding to the Innovative Technologies Laboratories from King Abdullah University of Science and Technology (KAUST).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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