High Absorption Broadband Ultra-Long Infrared Absorption Device Based on Nanoring–Nanowire Metasurface Structure

Abstract

Long-wave infrared (LWIR) broadband absorption is of great significance in science and technology. The electromagnetic field energy is absorbed by the metamaterials material, leading to the enhanced light absorption, from which the Metal–Dielectric–Metal (MDM) structure is designed. FDTD simulation calculation indicate that the bandwidth within which the absorber absorption ratio greater than 90% is 11.04 μm, and the average absorption rate (9.10~20.14 μm) is 93.6%, which can be accounted for by the impedance matching theory. Upon the matching of the impedance of the metamaterial absorber with the impedance of the incident light, the light reflection is reduced to a minimum, and increase the absorption ratio. Meanwhile, the good incidence angle unsensitivity due to the metasurface structural symmetry and the characteristics of the electromagnetic field distribution at different incidence angles. Due to the form regularity of the nanoring–nanowire metasurface structure, the light acts similar in different polarization directions, and the surface plasmon resonance plays a key role. Using FDTD electromagnetic field analysis to visualize the electric field and magnetic field strength distribution within the absorber, the electromagnetic field at the interface in the nanoring–nanowire metasurface structure, promote the surface plasmon resonance and interaction with damaged materials, and improve the light absorption efficiency. Moreover, the different microstructures and the electrical and optical properties of different top materials affect the light absorption. Meanwhile, adjusting the absorption layer thickness and periodic geometry parameters will also change the absorption spectrum. The absorber has high practical value in thermal electronic devices, infrared imaging, and thermal detection.

1. Introduction

Metamaterials (MMs) are composite materials that rely on artificial structures to achieve abnormal electromagnetic responses, possessing many peculiar properties that cannot be obtained from naturally occurring materials [1,2]. Electromagnetic metamaterials are composite materials with artificially designed structures that exhibit extraordinary physical properties not found in natural materials [3,4]. The external electromagnetic characteristics of metamaterials are related not only to the material properties, but also to the size and geometric shape of the artificial structures, making metamaterial design highly flexible. Electromagnetic metamaterials contain specific microstructural units, whose sizes are typically smaller than or close to the wavelength of electromagnetic waves [5,6]. Through precise design and regulation of parameters such as the shape, size, and arrangement of these microstructures, unique responses and manipulations of electromagnetic waves can be achieved [7].

The electromagnetic properties of metamaterials endow them with great potential in various applications, including light manipulation [8], imaging [9], sensing [10,11], superlenses [12], and other processes [13]. Among these, metamaterial absorbers (MMAs) have been a significant research topic in recent years, as they can confine and completely absorb incident electromagnetic waves at the subwavelength scale, showing promising applications in detection [14], thermal emitters [15], energy harvesting [16], cooling [17], and other fields [18]. Many metamaterial absorbers operate based on the principle of surface plasmon resonance (SPR), which is highly frequency-selective and can only excite effective SPR within a very narrow frequency range [19,20,21]. To adapt to a broader application scope, broadband absorption in the long-wave infrared region can be achieved in MMAs through strategies such as designing multi-layer structures [22], adjusting structural parameters [23], and selecting/modulating material properties [24]. Due to the resonant characteristics of MMAs, their absorption frequencies are closely related to their structures, and most structures exhibit narrow-band absorption. Therefore, broadband absorption has attracted significant attention.

Currently, numerous studies focus on achieving broadband absorption from the visible to infrared bands, commonly using strategies that integrate multiple coplanar resonators to generate multiple localized surface plasmon resonances (LSPRs) and expand the absorption bandwidth [25,26]. By optimizing the size and shape of the resonators, different resonators can resonate in different frequency ranges, thereby increasing the overall absorption bandwidth. Additionally, the performance of absorbers is limited by the number of resonators and the coupling effects between adjacent resonators. Reasonably adjusting the spacing between resonators and adopting periodic or aperiodic arrangements can enhance broadband absorption. Another broadband absorption strategy involves tapered alternating metal–dielectric multi-layer structures [27], where increasing the number of stacked layers can couple the electromagnetic responses of different tapered structures, expanding the overall absorption bandwidth. However, increasing the number of layers not only increases the heat capacity (a larger heat capacity leads to a higher noise-equivalent temperature difference) but also raises the requirements for manufacturing processes.

To achieve simple-structured broadband absorption, dielectric materials with high intrinsic absorption, such as silicon nitride and silica, are used in MMA design [28]. Furthermore, exciting multiple resonance modes through appropriate structural parameters can achieve multi-peak or broadband absorption [29,30]. For example, Fei Ding et al. utilized titanium disks to simultaneously excite localized and propagating surface plasmon resonances, achieving broadband absorption in the near-infrared band [31]. Under electromagnetic wave irradiation, the free electrons on the surface of the titanium disks undergo collective oscillations, forming surface plasmon waves. The interaction between these surface plasmon waves and incident electromagnetic waves localizes electromagnetic field energy near the titanium disk surface, enhancing the absorption of electromagnetic waves and realizing broadband absorption in the near-infrared region.

Researchers have gradually expanded the study of broadband absorption in metamaterials from the visible range to the infrared band [32,33]. However, there are few reports on absorbers operating in the longer-to-ultra-long wavelength range (8–22 μm). Studies in the long-wave infrared (LWIR, 8–14 μm) band are related to infrared imaging and detection, while absorption characteristics in the very-long-wave infrared (VLWIR, 14–30 μm) band are associated with interstellar target detection in space and tracking of missiles, satellites, and other aircraft [34,35,36]. In this work, we propose a new design and structure for achieving broadband absorption in the LWIR region. The Ti-Si-Si₃N₄-SiO₂-Ti absorber, featuring composite dielectric layers, realizes perfect absorption performance through coupled surface plasmon resonance and intrinsic absorption, achieving an average absorptivity of nearly 93.6% and a bandwidth of up to 11.04 μm where the average absorptivity exceeds 90% in the target LWIR band. By rationally manipulating the geometric parameters and material types of the metamaterial absorber, we achieved broadband, high-absorption, polarization-insensitive, and angle-insensitive absorption.

The absorber in this study exhibits significant absorption capacity in the target LWIR range, making it suitable for applications such as thermal emitters, infrared imaging, and photodetectors. Additionally, due to its insensitivity to polarization and incident angles—almost all incident waves are absorbed by the resonators—the designed absorber can be used in infrared imaging, thermal detection, and thermoelectronic harvesting.

2. Models and Methods

In the process of simulating and calculating the structural performance and electromagnetic field distribution of the absorber, numerical calculations were performed using the three-dimensional Finite-Difference Time-Domain (FDTD) method [30,37,38]. A planar light source in the 8–22 μm range was selected as the incident light for the structure, Initially, it is set that the incident light is perpendicularly incident along the z-axis direction. Subsequently, the wide-angle response analysis is carried out by adjusting the included angle between the incident light and the z-axis. Under the array mode, periodic boundary conditions were applied in the x and y directions of the absorber, while perfectly matched layer (PML) boundary conditions were used in the z direction. The refractive index parameters of Ti, SiO₂, and Si in the LWIR band were obtained from Pailk, and the parameters for the lossy dielectric layer silicon nitride were derived from Junaid [39].

As shown in Figure 1a, the designed absorber is a multilayer composite micro–nano structure. From Figure 1b, the top layer consists of periodically arranged unit cells, each containing a nanoring and four surrounding nanowires. Key dimensional parameters are labeled, including the inner and outer radii r₁, r₂ of the nanoring, the distance d from the nanowire to the center, the width w and length b of the nanowire, and the unit period P. Figure 1c illustrates the side-view layer structure: from top to bottom, the layers are a patterned functional layer (Ti), composite dielectric layers (Si, Si₃N₄, SiO₂), and a metal reflective layer (Ti). The thicknesses of each layer, h₁, h₂, h₃, h₄ and t are labeled. Through multiple parameter optimization simulations, the structural parameters listed in

Table 1. Optimized structural parameters (μm).

P	d	b	w	r1	r2	h1	h2	h3	h4	t
2.9	0.9	1	0.2	0.4	0.7	0.02	0.3	0.3	0.5	0.2

were obtained.

The thickness of the metal reflective layer (Ti) is greater than the skin depth, so the transmittance is zero.

$$\mathsf{\delta }=\sqrt{{\displaystyle \frac{2}{\mathsf{\omega }{\mathsf{\mu }}_{\mathrm{Ti}}{\mathsf{\sigma }}_{\mathrm{Ti}}}}}$$

(1)

From Equation (1), for the bottom Ti material, by referring to relevant materials, we obtained the values of μ and σ of the Ti material [40]. From this, we can conclude that within the wavelength range of 8–22 μm, the thickness t of the Ti material of the absorber is much greater than the skin depth of the Ti material. Due to the limitation of the skin depth of the titanium material, when the propagation depth of the electromagnetic wave exceeds this value, its amplitude will decay exponentially to a negligible level, resulting in the T(ω) approaching 0.

However, in the x, y, and z directions, the mesh step length is set to 0.02 μm. The transmission monitor is placed 1 μm below the model, and the reflection monitor is placed 2 μm above the model.

3. Results and Discussion

Figure 2a shows the absorption, reflection, and transmission curves of the absorber. The absorber exhibits characteristic absorption peaks at wavelengths of λ₁ = 9.28 μm, λ₂ = 10.63 μm, λ₃ = 14.84 μm, and λ₄ = 18.38 μm, with an average absorptivity of 93.6% and a bandwidth of 11.04 μm. Figure 2b presents its relative impedance characteristics. When the input impedance of the absorber is close to the free-space impedance—i.e., impedance matching—electromagnetic waves can enter the absorber more smoothly, reducing reflection loss.

$$\stackrel{\mathrm{–}}{\mathrm{z}}={\displaystyle \frac{{\mathrm{z}}_{\mathrm{eff}}}{{\mathrm{z}}_0}}$$

(2)

$$\mathrm{R}\left(\mathsf{\omega }\right)={\left|{\displaystyle \frac{\stackrel{\mathrm{–}}{\mathrm{z}}-1}{\stackrel{\mathrm{–}}{\mathrm{z}}+1}}\right|}^2$$

(3)

According to Equations (2) and (3) [41,42], when the impedance of the metamaterial absorber Zeff matches the impedance of free space Z₀, the R(ω) of the absorber is minimized. This means that when the Re is 1 and the Im is 0, it has the best absorption performance. As in Figure 3c, the Re fluctuates around 1, and the Im fluctuates around 0 In the wavelength range of 8–22 μm. This indicates the reason why the absorber proposed in this paper expresses perfect absorption effect [43].

Under the wavelengths corresponding to the absorption peaks in the first half of Figure 2a, the absorber exhibits good impedance matching, which enables the electromagnetic waves to be efficiently absorbed by the absorber. Under the wavelengths corresponding to the absorption peaks in the second half of Figure 2a, the absorber suppresses reflection and transmission through comprehensive electromagnetic mechanisms such as mode coupling and multiple scattering. Even if the impedance characteristics change, high absorption can still be achieved, allowing electromagnetic waves to efficiently penetrate into its interior [44,45]. The internal structure—including the dielectric materials of the Si-Si₃N₄-SiO₂ layers and the top Ti layer’s patterned design—dissipates electromagnetic wave energy through mechanisms such as dielectric loss and magnetic loss. In summary, the absorber achieves high absorptivity in specific bands by optimizing its effective impedance characteristics, minimizing reflection and transmission, and demonstrating excellent absorption performance.

Figure 3a–d show the electric field distribution in the X-Y plane at different resonant wavelengths, while Figure 3e–h depict the electric field distribution in the X-Z plane. The figures reveal that the electric field is strong in the nanoring and nanowire structures, whereas the magnetic field is weak, indicating that the resonance mode in these structural parts of the absorber is dominated by surface plasmon resonance (SPR) [46,47]. When the frequency of the incident electromagnetic wave matches the oscillation frequency of electrons on the material surface, the SPR effect is excited, significantly enhancing the electric field on and near the surfaces of these structures, localizing energy and forming a strong electric field distribution [48,49]. Figure 3i–l shows the magnetic field distribution in the X-Z plane. The strong magnetic field distribution in the composite dielectric layers indicates that the absorption mode in the absorber’s dielectric layers is dominated by the Fabry–Perot (F-P) cavity effect [50]. The multi-layer structure (Si-Si₃N₄-SiO₂) of the absorber forms a structure similar to an F-P cavity. Electromagnetic waves undergo multiple reflections and interference between layers; when resonance conditions are met, energy accumulates within the cavity, and electromagnetic waves of specific wavelengths form standing waves or enhanced electromagnetic field distributions in the region of the nanoring and nanowires, further increasing the field strength in this area. In summary, the SPR effect localizes and enhances the electromagnetic field on the surfaces of the nanoring and nanowires, while the F-P cavity effect accumulates energy in the absorption dielectric layers at specific wavelengths through interference [33]. These two effects work together to endow the absorber with superior absorption characteristics in the 8–22 μm range.

Figure 4a shows different top structures and their corresponding absorption curves. Case 1 represents the proposed top structure of the absorber in this paper, consisting of a nanoring and four surrounding nanowires. Its absorption curve exhibits a significantly wider wavelength range with absorptivity greater than 90% compared to other cases. Case 2 features a square ring and four surrounding nanowires, with absorption peaks and sustained intervals inferior to those of Case 1. Case 3 contains only a nanoring, showing a high-absorption bandwidth notably narrower than Case 1. Case 4 has only four nanowires, with an absorption curve similar to Case 3 and a high-absorption bandwidth also smaller than Case 1. Through overall comparison, Case 1 demonstrates the best performance in the bandwidth range where absorptivity exceeds 90%, maintaining efficient absorption across a broad wavelength range, thus justifying its selection. Figure 4b presents the absorption curves for different top patterned functional layer materials. The materials provided—titanium (Ti), chromium (Cr), tungsten (W), and germanium (Ge)—all interact with electromagnetic waves via the surface plasmon resonance (SPR) effect at specific wavelengths [51]. The absorption curves reveal that the intensity and bandwidth of absorption peaks vary with different materials: some exhibit sharp peaks, indicating high absorption efficiency and narrow bandwidth at specific wavelengths, while others show relatively gentle peaks with broader bandwidths [52]. This relates to the materials’ loss mechanisms, such as Ohmic loss in metals and electron-hole recombination loss in Ge, both of which affect the shape and bandwidth of absorption peaks [53]. By comparing the absorption curves of different materials, Ti was ultimately selected as the top material for the proposed absorber in this study.

As shown in Figure 5, the influence of structural parameters of the top patterned functional layer on the absorption performance of the absorber is presented. In Figure 5a, when the period P increases, the characteristic size of the overall electromagnetic response of the structure increases, leading to a decrease in resonance frequency and a red-shift in the absorption peak [54]. An appropriate P promotes the superposition of multi-mode electromagnetic resonances, broadening the absorption bandwidth. P affects the impedance matching between the structure and free space: good matching increases the average absorptivity, while poor matching reduces it due to increased reflection. In Figure 5b, as the distance d from the inner side of the nanowire to the structural center increases, the nanowire shifts outward relative to the center, altering the electromagnetic distribution and equivalent resonance parameters of the structure. This results in a decrease in resonance frequency and a red-shift in the absorption peak [55]. Adjusting d appropriately can optimize the multi-mode coupling effect and broaden the absorption bandwidth. Figure 5c,d show that increasing the length b and width w of the nanowire expands the area of electromagnetic response, lowering the resonance frequency and causing the absorption peak to red-shift. When b and w are optimal, the coupling between nanowires and electromagnetic waves is enhanced, improving energy loss efficiency (such as Ohmic loss) and impedance matching, thereby increasing the average absorptivity [56,57]. Figure 5e,f indicate that increasing the inner and outer radii r₁ and r₂ of the nanoring enlarges the overall size of the nanoring, decreasing the resonance frequency and red-shifting the absorption peak. The difference between r₁ and r₂ affects the diversity of SPR modes within the ring; a suitable radius difference increases mode superposition and broadens the bandwidth.

As shown in Figure 6, the influence of the thickness of different layers in the absorber on its absorption performance (absorption peak position, absorption bandwidth, and average absorptivity) is presented. In Figure 6a, when h₁ increases, the electromagnetic response characteristics of the top structure change, leading to a red-shift in the absorption peak. This is because h₁ affects the coupling strength between the top patterned functional layer and electromagnetic waves, as well as the resonance mode; changes in thickness alter the local electromagnetic field distribution, thereby adjusting the resonance frequency. h₁ also influences the energy loss mechanism (such as the damping effect of surface plasmon resonance) [58]. Appropriately increasing h₁ may broaden the absorption bandwidth, but if the thickness deviates from the optimal value, the average absorptivity may decrease due to poor impedance matching or suppressed resonance modes. In Figure 6b–d, Si-Si₃N₄-SiO₂ serves as the composite dielectric layer, and changes in thickness affect interlayer interference and the Fabry–Perot (F-P) cavity effect [59,60]. If h₂, h₃, and h₄ increase, the optical thickness of the F-P cavity increases, causing the absorption peak to tend to red-shift; conversely, it may blue-shift. Additionally, the thickness of the dielectric layers influences multiple reflections and energy accumulation of electromagnetic waves within the layers. Suitable thicknesses can enhance constructive interference, broaden the absorption bandwidth, and increase the average absorptivity; inappropriate thicknesses will disrupt the interference conditions, leading to a narrower absorption bandwidth and reduced average absorptivity [61,62]. In summary, each structural parameter influences the red-shift or blue-shift in absorption peaks, the width of the absorption bandwidth, and the level of average absorptivity by regulating electromagnetic resonance modes, SPR effects, and impedance matching [63]. In this study, FDTD simulation calculations were used to optimize these parameters to achieve ideal absorption performance.

As shown in Figure 7a, the absorption spectrum exhibits high absorptivity over a wide wavelength range, and the overall change in the absorption spectrum is small as the incident angle varies. This indicates that the absorber maintains high absorption performance at different incident angles. As shown in Figure 7b, the absorption spectrum of the absorber in the TE mode is similar to that in the TM mode, maintaining high absorptivity over a wide wavelength range, and the change in the incident angle has no significant impact on the absorption spectrum. By comparing Figure 7a,b, it can be seen that the absorber has polarization insensitivity. As shown in Figure 7c, when the incident light changes from the TM mode to the TE mode, as the polarization angle varies, the absorption spectrum still maintains a high absorptivity and does not show a significant decline due to the change in the polarization state [64,65]. This indicates that the absorption performance is stable under different polarization states, that is, the absorber has polarization insensitivity. The polarization insensitivity and polarization independence of the absorber are of great significance in practical applications [66,67,68].

As can be observed from

Table 2. Compare the band range with absorption over 90% for the different structures and the mean absorption in this wavelength range.

References	Band Range with Absorption over 90%	Mean Absorption in the Band Range	Structural Layers
[69]	5 μm (8–13 μm)	96.7%	Ti-SiO2-Ti
[70]	7.23 μm (8.98–16.21 μm)	94.1%	Cr-Ge-Si3N4-Ti
[71]	0.71 μm (354–1066 nm)	97%	Ti-SiO2-Al
[72]	6 μm (8–14 μm)	94.50%	Ti-Ge-Ti
Proposed absorber	11.04 μm (9.10–20.14 μm)	93.6%	Ti-Si-Si3N4-SiO2-Ti

[69,70,71,72], the proposed absorber features a wider wavelength span and a higher absorption rate as opposed to those suggested in other publications. As a result, it has outstanding application situations in the a of thermal transmitters, infrared imaging, and photodetector devices.

4. Conclusions

In general, we simulated and calculated the performance parameters of an absorber with a nanowire–nanoring metasurface structure composed of Ti-Si-Si₃N₄-SiO₂-Ti. The results show that this absorber can achieve broadband absorption of over 90% in the wavelength range from 9.10 μm to 20.14 μm, with an average absorption efficiency of 93.6%. Then, we verified the insensitivity of the absorber to the incident angle and its polarization independence. Additionally, we simulated the electromagnetic field distribution at four resonant wavelengths to explore the physical mechanism of broadband absorption. Finally, we took the structural parameters as variables and simulated the changes in absorptivity to optimize the parameters. We also compared and analyzed the influence of the top-layer metal microstructure and material type on the absorption performance.