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Abstract
Metamaterial-based nano-scale absorbers have been becoming very popular in the modern era due to efficiently absorbing solar radiation to revamp the performance characteristics of thermal emitters and solar thermophotovoltaics (STPV) systems. Here, we explore and implement an ultra-broadband nanostructured metamaterial absorber (NMMA), which comprises a stack of alternating nano-squares of TiN and TiO2 mounted over the dielectric substrate backed by a metallic sheet. The numerical simulations and electromagnetic (EM) characteristics of the proposed NMMA have been investigated by employing the finite difference time domain (FDTD) EM tool. The numerical results indicate that the average absorption of the NMMA reaches 96% in the wavelength range from 200-3000 nm (from ultraviolet to mid-infrared), and the minimal absorption is also above 90% in a continuous large operating spectrum ranging from 200-2800 nm. Surprisingly, the absorption features of the designed nano-absorber remain stable under the influence of oblique incident-angles for both the polarization states (TE & TM). Furthermore, the proposed nano-absorber manifests polarization-insensitive behavior due to the presence of four-fold symmetry of the proposed structure. Large operational bandwidth, miniaturized structure, and the use of thermally stable refractory metal TiN make this NMMA an appealing candidate for the applications of thermal emission, solar thermophotovoltaics, and other opto-electronic devices. In addition, the design of this absorber is also scalable to other operating spectrums through carefully selecting the materials and optimizing the geometry of the proposed structure.
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1. Introduction
Metamaterials are artificially manufactured synthetic structures, which have been attracting remarkable attention due to their unprecedented light manipulating characteristics. Due to their exotic and unusual features, they have the capability to manipulate, control, and alter the properties of incident electromagnetic waves [1,2]. These artificial materials are comprised of metallic and/or dielectric sub-wavelength meta-atoms/meta-molecules with specific arrangement and configuration, providing significant physical properties such as negative refractive index, super lensing, EM cloaking, and inverse Doppler Effect, which cannot be realized with the existing natural materials [3–6]. These particular characteristics of these artificial materials pave the way for research community to use them in enormous applications, including meta-holograms [7], plasmonic sensors [8], optical filters [9], photo-detectors [10], and perfect absorbers [11,12], etc. Generally, EM metamaterial absorbers are composed of a three-layer device configuration with upper and lower sheets constructed with metals, and the middle-layer comprises a dielectric material. Metamaterial absorbers attained huge attention of the world community after Landy's seminal work on the perfect narrow-band absorber, which was presented in 2008 [13]. Thereafter, a remarkable interest has been aroused towards this exciting field, and researchers started to explore the metamaterial-based absorbers for various applications in the microwave, terahertz (THz), visible, and infrared (IR) spectrum [14–17]. Initially, people tried to investigate single, dual, and multi-band perfect absorbers for different operating frequencies [13,18,19]. Subsequently, the era of wideband metamaterial absorbers began, and scientists tried to conceptualize the different techniques to attain a large operational bandwidth [20,21] and the most frequent methods to obtain a high absorption bandwidth from metamaterial-based structures are multiple stacked layers and multi-resonance [20,21].
For the last couple of years, EM absorbers have been widely employed in the ultraviolet (UV), visible, and infrared (IR) band owing to the remarkable interest in their implementation of photovoltaics, thermal emitters, and energy harvesting [11,12,22]. Subsequently, Wang et al., in 2013, numerically designed metamaterial nanostructure absorber and it showed limited operating wavelength because it faced destructive interference between the transmitted and reflected EM wave components, forcing the light to be confined in an absorbing thin-film [23]. For instance, consider a multi-band metamaterial absorber reported by Xie et al. in 2018, the absorber could produce four narrow-band absorption peaks by employing a perforated rectangular resonator in the unit-cell [24]. Moreover, some investigations require at least multiple resonators to cause various absorption peaks [25]. For example, Khan et al. proposed a cross-shaped metasurface absorber, in which the absorption resonances and the corresponding spectral bandwidth could be remarkably enhanced by adding 4 extra nano-rods at the different locations of crossed-shaped structures [26]. There have been structures based on the ten's pairs of metallic-dielectric layers with a wide variety of geometry and size, leading to the concept of ultra-broadband absorber due to the combination of several resonators that overlap each other in the broad wavelength range [27,28]. Likewise, Hoa et al. reported a nano-absorber in which a top meta-structure of 10 metal/dielectric layers are stacked and form a frustum. This device shows greater than 90% absorption characteristics starting from 480-1480 nm [29]. Also, in 2017, an absorber designed by Zhu et al. consisted of 44 pairs of Au/Si dual hexagons surrounded by 32 pairs of Au/Si rings, topping the device thickness of 100 nm and 200 nm for Au and Si, respectively. This structure could absorb light from the wavelength span of 800 nm to 2000 nm with an efficiency of more than 80% [30]. Liu et al. presented an absorber by utilizing 9 pairs of rectangular-shaped Fe/Si with the thickness of 155 nm and 10 nm, respectively, placed on a metallic dielectric film, which depicts an average absorption of 96% in the wide spectral range of 300-3000 nm [31]. However, the main drawback of such absorbers is using tens of paired metallic-insulator layers, leading to difficulty in fabrication process and restrict their implementation in practical applications due to higher cost.
Over time, different materials have also been investigated to attain a broadband spectrum of the nano-structure metamaterial absorbers. Au and Ag are the noble metals that were successfully utilized in the unitary absorbers [32,33]. However, these metals are not well-suited under high-temperature operating environments and lack their feasibility in integrating silicon fabrication processes due to low melting points [34]. On the contrary, the refractory materials such as TiN with extraordinary performance in an extensive wavelength spectrum has provided to induce EM resonances and overcome the obstacles owing to its striking features such as melting point as large as 2930°C, resulting in thermal and chemical stability, and also it is CMOS compatible, making it a desirable material for various applications [22,34]. Dang et al. presented an absorber that comprises Ti truncated-pyramid resonators, designed on SiO2 and TiN film, exhibited a high absorption rate of 95% from 390 to 1171 nm [35]. Regarding the thermal stability of refractory metals, it is notable to mention that a broadband absorber experimentally demonstrated by Chirumamilla et al. is composed of TiN nano-pillars. The discussed absorber shows an average absorptivity of 94% in a broad spectral wavelength; more importantly, its thermal efficacy was inspected by exposing it with high temperatures environments for 24 hours. Finally, it retained its structural geometry under thermal treatment with the temperature of 1473 k, paving the way for justifying the thermal stability of this material [36]. Another interesting point about plasmonic TiN is its ohmic junction with TiO2, which helps to enlarge the spectral absorption [37,38]. However, although there were various kinds of reports from different material and geometry to achieve broadband absorption, it is still a challenge to design a nano-structure to meet the requirements of an ideal absorber, such as trapping the light in the large operating wavelength spectrum with high efficiency, thermal stability, last but not least having simple structure to lessen the cost of fabrication process.
In this work, we devise and exhibit a novel and new design of metamaterial-based nano-structure, which absorbs the large operating wavelength ranging from UV to IR spectrum. The following absorber is based on the nano-squares grating of 5 layers of TiN/TiO2 arrays mounted on TiO2/TiN films. This absorber could offer an average absorption rate of 96% in a continuous wavelength span from 200 nm to 2800 nm, covering UV, visible, and MIR EM spectrum with the total thickness of 380 nm, and this nano-scale multi-layer metamaterial absorber is a new addition in the stream of solar metamaterial absorbers.
2. Design methodology and analytical treatment
The pictorial diagram of the meta-unit cell of the proposed NMMA is demonstrated in Fig. 1. It is composed of metallic/dielectric nano-square pairs grown over a dielectric substrate TiO2 covered by a bottom ground-sheet of TiN. The relative permittivity of TiN is taken from Palik [39] and the complex refractive index data of TiO2 is extracted from Ref. [40]. The nanostructured absorber is comprised of two pairs of TiN/TiO2 layers and a TiN metallic-layer at the top of the stacked structure. The designed NMMA resembles with the hyperbolic metamaterial (HMM), as HMM also composed of multiple stacked layers to attain the desired characteristics [41]. The height and width of the first pair (from bottom to top) of metal and dielectric are 50 nm and 240 nm, respectively. Also, the corresponding heights for the second pair are 50 nm and 170 nm, respectively; the fifth top layer is a TiN nano-square resonator with width = 170 nm and height = 30 nm. The dielectric film thickness TiO2 layer is 50 nm, and the opaque ground layer is TiN with a thickness of 100 nm, canceling the light transmission. Interestingly, the nanostructured absorber is based on a 380 nm thick structure, which utilized only three layers of TiN nano-square resonators (purple color) and two layers of TiO2 nano-square patches (yellow color) in constructing the top metasurface. Moreover, the periodicity of the meta-unit cell is 300 nm. The overall unit cell thickness and size lie in the subwavelength scale rule of the metamaterial. These characteristics validate the implementation of an ideal nanostructured absorber with high absorption efficiency and a favorable thin-film structural size to reduce the fabrication process.
Fig. 1. Schematic illustration of the proposed NMMA along with its physical geometric dimensions, P = 300 nm, W1 = 240 nm, W2 = 170 nm, h1 = 100 nm, h2 = 50 nm, h3 = h4 = h5 = h6 = 50 nm, and h7 = 30 nm, respectively (a) 3D periodic arrays of the proposed NMMA, (b) single unit-cell depiction of the proposed NMMA.
We have utilized the 3D time-domain finite-difference (FDTD) method in the simulation to calculate the electromagnetic response and electromagnetic field distribution [35]. Periodic-boundary conditions are employed in the x-y direction, and a perfect-matching layer (PML) is used in wave propagation (z-axis) direction. In this proposed model, the stacked layers of TiN/TiO2 produce different plasmonic resonances, and middle dielectric lossy material helps to trap these resonances, and the TiN metallic sheet beneath the TiO2 functions as a perfect mirror for reducing the transmitted waves. In metamaterial-based structures, absorption performance A (ω) is defined as Eq. (1) [11,22].
where R(ω) and T(ω) are the frequency-dependent reflected and transmitted light of the under normal-illumination, $R(\omega )= {|{{S_{11}}} |^2}$ and $T(\omega )= {|{{S_{21}}} |^2}.$ Due to employing TiN ground layer, the light failed to transmit, so T(ω) 0, so the absorption of the metamaterial can only be controlled with the reflection component stated as Now R(ω) is the only parameter on which we should focus to increase the absorption rate of the absorber. Impedance matching theory helps to minimize this reflection parameter. So that a metasurface behaves as an absorbing structure if its impedance is exactly synchronized with the impedance of the free-space. To clarify this, we analyze the relationship between the structural characteristics and impedance and employ impedance matching conditions as Eqs. (3) and (4) [22].3. Results and discussion
This section studies the spectral absorption features of the designed NMMA. Therefore, we can employ suitable material and geometry to achieve the meta-impedance close to 1; in that way, the metamaterial could reach the maximum absorption value. The simulated absorption of the proposed NMMA is presented in Fig. 2(a), which exhibits an absorption value of larger than 90% in the wavelength span of 200–2800 nm along with the transmitted and reflected power.
Fig. 2. (a) Absorption, reflection, and transmission spectra of the proposed NMMA under normal incident light. (b) Complex impedance characteristics of the proposed NMMA.
Using the inversion of the S-parameters, the impedance values are computed and visualized to understand the concept of perfect absorption. Figure 2(b) clearly shows that, for the large wavelength spectrum including ultraviolet, visible, and infrared (200-3000 nm), the effective impedance is approaching 1, and the imaginary part is near 0; thus, the reflection is minimum. Because of the good impedance matching phenomena, the total absorbance is greater than 90% in the above-mentioned region [11].
The efficiency of the proposed NMMA is simulated under various structures to better understand its physical mechanism. From Fig. 3(a), it is clear that the maximum peak is at 693 nm (visible region) when no metasurface layer is deposited on a thin TiO2 spacer and an opaque TiN film. Subsequently, by adding the first pair with height and width of 50 nm and 240 nm, respectively, on the TiO2 dielectric film, the structure has a wave peak at 557 nm, and more importantly, there is a remarkable change in the absorption efficiency for longer wavelengths, especially for those in the range of more than 1500 nm. Achieving a better absorption performance, we need to add another pair on the first pair so that all the operating wavelengths can reach to maximum efficiency; it is also clear that the absorber meets the requirements of a good absorber for the operating wavelength of 200 to 2800 nm. However, it is of paramount importance to adjust the absorber to have a continuous flat wavelength band in the whole operating spectrum. This need made us add another metallic TiN resonator as the last layer on top of the two previous pairs; and it is obvious that TiN square can deliver the large absorption performance at the shorter and longer wavelength region and makes the operating waveband flat and continuous with the high absorption of more than 90% in a long-wavelength region of ultraviolet, visible, near and mid-infrared (i.e., from 200 nm to 2800 nm). We have combined these nano-square resonators and gradually changed the resonators’ size to design a broadband nanostructured absorber. The absorption behavior, which induces due to the resonance wavelengths, is highly dependent on the geometric sizes of the metallic resonators.
Fig. 3. (a) Absorption features of the proposed NMMA with different combinations of stacked-layers of nano-squares, (b) Absorption features of the proposed NMMA under different combinations of the widths of the stacked-layers of nano-squares, (c) Absorption features of the proposed NMMA with different heights of stacked-layers of nano-squares, and (d) Absorption features of the proposed NMMA under different heights of the dielectric substrate.
Next, the effect of structural parameters, including the width of the TiN/TiO2 nano-square patches, on the absorption performance of the proposed NMMA has been studied. In this process, the geometric parameters are selected as, h1 = 100 nm, h2 = 50 nm, h3,4,5,6 = 50 nm, h7 = 30 nm, and P = 300 nm. Only the widths (W1 & W2) change while the other parameters are kept fixed for realizing the influence of the size of the patches. Figure 3(b) demonstrates the absorption performance for the proposed NMMA under an equally step (Δw = 30 nm for both the W1 and W2) of increasing the widths of square patches. As can be seen, the ultra-broadband absorption retains in a large operating band with high efficiency, suggesting that the more the widths of the square patches, the greater absorption efficiency in the longer wavelength can be achieved. This phenomenon is because the larger resonators have higher coupling effectiveness at longer wavelengths because of their larger dimensions. Also, the refractory metal employed in this structure has a similar plasmonic response to that of the noble metal nanoparticles [42]. Furthermore, the effect of thicknesses of metallic/dielectric pairs and dielectric thickness (TiO2) are investigated. Here, the height of first and second pair (h3, 4, 5, 6) are changed, and the other parameters are kept constant as follows: W1 = 240 nm, W2 = 170 nm, P = 300 nm, and h7 = 30 nm. Figure 3(c) demonstrates the absorption spectra for different values of (h3, 4, 5, 6) in the wavelength range of 200 to 3000 nm and the thickness varies from 10 to 50 nm. The more the thickness of the square resonators, the flatter the absorption curve and wider the absorption bandwidth. When the thickness of the each square resonator is selected as 50 nm, the absorption bandwidth expands from 200 to 2800 nm with high efficiency (i.e., absorption more than 90%). By increasing the thickness of the resonators, electromagnetic waves with longer wavelengths are also trapped; as we can see the significant changes occur at longer wavelength. Moreover, Fig. 3(d) shows the absorption spectra of the proposed NMMA under an equally increasing of the thickness for the TiO2 film layer, while other parameters are remained unchanged. It is observed that the proposed NMMA retains it large operating bandwidth while the absorption peaks are shifting at the longer wavelengths.
In addition, the impacts of oblique incident-angles are also explored on the absorption efficiency of the proposed NMMA, as illustrated in Fig. 4. Therefore, the incident and the refracted waves’ angles significantly correlate with the reflection coefficient as given in Eqs. (6) and (7) [16].
Fig. 4. Absorption features of the proposed NMMA with oblique incident angles, (a) TE mode, and (b) TM mode.
Polarization insensitivity is the most desirable feature of the metamaterial absorber for various practical applications. As a result of its four-fold symmetrical geometry, the NMMA shows the same absorption for all the polarization states of impinging light, as shown in Fig. 5. In addition, the NMMA also provides good and stable absorption for both polarizations (TE & TM excitations). It is observed that the proposed NMMA retains its absorption characteristics while changing the incident angle of both the wave-polarization. Figures 4(a) and (b) demonstrate an oblique irradiation feature for TE and TM polarizations while incidence angles were varied from θ = 10° to θ = 90° (with Δθ =10 ° step size). As can be seen, the proposed NMMA can maintain a strong absorption for an incident angle up to 60°. The extended absorption spectra are mainly obtained from combined conditions of irradiation oblique electromagnetic wave excitation and the nano-square resonators, resulting in efficient coupling [38]. It is notable to mention that, with increasing the incident angle, the absorption spectra exhibit a gradual decline in the absorption bandwidth due to the presence of a high anisotropic effect at larger angles and scattering of waves [43]. Regarding TE and TM polarizations, the absorber maintains and confine incident light efficiently even at higher angles, so that the absorber traps light with an absorption rate of more than 70% up to θ = 60°for TE mode and over 75% for TM mode in a continuous waveband from 200 nm to 3000 nm. The proposed metamaterial structure is far better in absorbing TM polarization than TE polarization since the nature of resonance is largely dependent on the polarization states and angle of the irradiation [43].
To have a better insight into the optical resonance of the structure, the resonant electric field (|E|) and magnetic field (|H|) distributions in the x-z plane under TM polarized incident light (owing to the symmetry design of the structure, it's the same with TE polarization) are simulated, and the results are demonstrated for the wavelengths such as λ1 = 350 nm, λ2 = 1600 nm, and λ3 = 2800 nm in Figs. 6(a)–6(f), as the proposed NMMA has a continuous bandwidth with high efficiency, the mentioned wavelength are responsible for showing how the resonators trap the light at the various operating wavelength of the device. For example, at a shorter wavelength range (λ1 = 350 nm), the energy of the electric field is mainly confined at the upper side of the structure (Fig. 6(a)), meaning that the resonant absorption produced at the edge of the smallest thickness TiN layer (top one), and the rest of electric field is accumulated at the edge of the third patch of TiN on the surface. At the same time, most of the magnetic field (|H|) is located at TiO2 layers, sandwiched between TiN resonators, as shown in Fig. 6(b). As can be seen from Figs. 6(c, d), by increasing the wavelength to λ2 = 1600, the maximum proportion of the electric field concentrates at the edge of the first pair of TiN/TiO2 from the bottom; the also magnetic field is produced at the interface of last TiN/TiO2 layer and third square-patch on TiN/TiO2 film. To analyze the behavior of the structure in longer wavelengths, λ3 = 2800 nm is chosen. Now, the lowest patch on the surface is more responsible for trapping light, so it localizes the electric charges gathered around the first pair of the TiN/TiO2 from the bottom, as demonstrated in Fig. 6(e), also the magnetic field shifts into TiO2 film substrate (Fig. 6(f)). These characteristics verify that the structural size in the proposed solar absorber creates the multiple resonant modes, leading to the formation of continuous and flat broadband absorption with high efficiency between λ = 200 nm and λ = 3000 nm.
Fig. 6. The distribution of the electric (|E|) and magnetic (|H|) fields in the x-z plane at the operating wavelength of (a, b) 350 nm, (c, d) 1600 nm, and (e, f) 2500 nm.
Figure 7(a) depicts the emissivity of the proposed nanostructure for the operating wavelength > 3000 nm. The grating layers are also encapsulated by TiO2 layer which is at the top of the proposed structure. When the thickness of encapsulation layer are varied from t = 240-300 nm with a step size of 20 nm then the absorption efficiency is remain between 74% to 76% respectively. From the Fig. 7(b), the operating wavelength (700-1200 nm), which has higher absorption efficiency at t = 280 nm, it can be employed to use as a solar energy harvester. Furthermore, a comparative analysis between the recently reported broadband metamaterial absorbers in the literature and the present study has been carried out, as shown in Table 1. The analysis shows that the proposed absorber shows the promising results and better in performance than the others reported in literature.
Fig. 7. (a) Emissivity of the proposed nanostructured an (b) Absorption performance of the proposed NMMA under the different thickness of encapsulation film of TiO2.
Table 1. Comparative analysis of reported broadband metamaterial absorbers with the present study.
Nanostructured absorbers composed of other refractory metals, such as Cr, Ti, V, and W, could still provide exceptionally high absorption over an extended wavelength range, as illustrated in Fig. 8(a). However, when ordinary noble metals such as Au and Ag are utilized to substitute TiN in the absorber, the absorption performance suffers significantly. Figure 8(b) shows that these plasmonic metal-based absorbers exhibit different absorption effects; shorter wavelengths exhibit several sharp and narrow-band absorption peaks, whereas longer wavelengths have the lowest and flat absorption curve. Therefore, a system based on refractory metals may achieve substantially more absorption than a system based on common noble metals- their different plasmonic resonant characteristics are largely responsible for this reason. Noble metals can also support significant surface plasmon resonances in the visible and near-infrared ranges; however, refractory metals may also exhibit strong plasmon resonances due to their high imaginary parts in the large wavelength range only with broadband modes [44]. Figure 8(c) displays the average absorption of absorbers comprised of metals such as Ag, Au, V, W, Ti, Cr, and TiN. The average absorption of refractory metals is greater than that of noble metals for this proposed nano-structure with the same geometrical features. Approximately 88%, 94%, 89%, 93%, and 96% of electromagnetic energy can be absorbed by metamaterial absorbers formed of V, Cr, W, Ti, and TiN respectively, while Ag and Au trap 43% and 51% of the incident light, respectively.
Fig. 8. (a) Absorption spectrum for different refractory metals including Tungsten (W), Chromium (Cr), Vanadium (V), Titanium (Ti), and Titanium nitride (TiN), (b) Absorption spectrum for noble metals such as Gold (Au), and Silver (Ag). (c) Comparative analysis of average absorption of the proposed NMMA for different metals.
4. Conclusion
In summary, the polarization-insensitive and ultra-wideband NMMA, which possessed a metal/dielectric nano-squares stack, was devised and investigated. Multiple plasmonic resonances generated by each contributing layer in the nano-squares stack were combined to achieve the wideband absorption characteristics for a large operating wavelength. At normal incidence, the proposed NMMA expressed an average absorption value of 96% from a wavelength span ranging 200 nm to 3000 nm, and it also showed continuous waveband absorption features between 200 and 2800 nm with efficiency greater than 90%. Furthermore, the absorption spectra remain outstanding under the effect of a wide incident angle range, signifying absorption efficiency of higher than 70% in the whole mentioned spectrum from 200 nm to 3000 nm for both the linearly polarization states, which leads to better performance in harsh environment. In addition, other refractory metals, namely tungsten, chromium, vanadium, and titanium, had also been examined to implement the metamaterial absorber, and the average absorption is still over 86%. By and large, this proposed nanostructured-based architecture can offer the platform to implement perfect metamaterial absorbers for potential applications in STPV, light trapping, and imaging.
Disclosures
The authors declare no conflicts of interest.
Data availability
The associated data may be obtained from the authors upon reasonable request.
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