Investigation of an Ultra-Wideband Optical Absorber with the Bandwidth from Ultraviolet C to Middle Infrared

Abstract

In the development of optical absorption technology, achieving ultra-wideband high absorption structures that span from the 200 nm ultraviolet C region to the 5800 nm mid-infrared range has been a significant challenge in materials science. Previous studies have shown that few optical absorbers can simultaneously achieve an absorption rate above 0.900 and cover such a vast spectral range. This study presents an innovative seven-layer composite structure that successfully addresses this long-standing technical issue. Through a carefully designed layered architecture, the researchers employed COMSOL Multiphysics (version 6.0) for detailed numerical simulations to verify the optical performance of the structure. The structural design features two key innovations. In the layered composition, the bottom (h1), h3, and h5 layers are made of metallic Fe, while the layers above them (h2, h4, and h6) use SiO₂. The top layer is composed of a discontinuous cylinder Ti matrix. The first innovation involves the use of an inwardly recessed square design on the metallic Fe planes of the h4 and h6 layers, achieving high absorption across the 600–5800 nm range. The second innovation involves the use of the discontinuous cylinder Ti matrix for the top layer, which successfully enhances absorption performance in the 200–600 nm wavelength range. This structure not only employs relatively low-cost metals and oxide materials but also demonstrates significant optical absorption potential. Through numerical simulations and precise structural design, this study provides new ideas and technological pathways for the development of ultra-wideband optical absorbers.

1. Introduction

Metamaterials are ingeniously designed materials defined by their distinctive periodic structures, where unit cells are smaller than the wavelengths of electromagnetic waves they interact with [1,2]. Through sophisticated structural engineering, these materials enable unprecedented control over electromagnetic properties. By carefully manipulating their geometry, unit cell design, and spatial arrangement, engineers can precisely engineer metamaterials to exhibit unique responses to electromagnetic fields that transcend the capabilities of conventional materials. This exploration now covers an ultra-broadband range with high absorptivity across various spectral regions for different applications [3,4,5]. Radio frequency and microwave absorbers are sophisticated engineered structures designed to strategically attenuate and dissipate incident electromagnetic waves across a wide and complex spectrum, ranging from low-frequency radio waves to high-frequency terahertz radiation [6,7]. These microwave absorbers are precisely constructed to convert electromagnetic energy into heat through various complex mechanisms, effectively preventing unwanted electromagnetic reflections, scattering, and interference in critical applications. Terahertz (THz) metamaterial absorbers represent a cutting-edge class of engineered electromagnetic structures that operate within the critically important terahertz frequency spectrum, bridging the gap between infrared and microwave radiation [8,9]. The unique structural design of THz metamaterial absorbers enables them to exhibit remarkable performance in advanced sensing, imaging, and spectroscopic applications, particularly in the challenging terahertz frequency range.

Metamaterial absorbers operating in the mid-infrared spectral region offer unique opportunities for critical applications in diverse fields, including molecular sensing and spectroscopy, thermal imaging and infrared sensing, energy harvesting and thermophotovoltaics, and optical communications and data transmission [10,11]. The majority of solar energy is concentrated in the near-infrared [12,13] and visible spectra [14,15]. As a result, there is a strong preference for using broadband metamaterial perfect absorbers to optimize the efficiency of solar energy technologies. These absorbers are designed to capture a wide range of wavelengths, maximizing energy absorption across the spectrum. This approach not only improves the overall performance of solar cells and other solar energy devices but also contributes to the advancement of more sustainable and efficient energy solutions. By effectively utilizing the energy within these critical ranges, broadband metamaterials play a key role in enhancing the future of solar power generation. Solar cells, especially organic and perovskite solar cells, can degrade over time when exposed to ultraviolet (UV) radiation. UV absorbers can help mitigate this degradation, improving the stability and lifespan of solar panels. UV absorbers can be designed to capture energy across a broader range of the UV spectrum, which can be particularly beneficial for increasing the overall energy conversion efficiency of solar cells.

UV-absorbing materials, commonly used in UV sensors to detect UV radiation levels, are essential in environmental monitoring (e.g., UV index sensors), industrial applications (e.g., quality control), and security. Also, some UV absorbers can change their absorption properties in response to the presence of specific chemicals or biological agents. This makes them valuable for creating sensors that can detect hazardous substances or pathogens. Therefore, ultraviolet absorbers have strong potential in solar photovoltaics, thermal emission, and sensor applications [16,17,18]. Designing an optical absorber capable of absorbing light from 200 nm ultraviolet to 5800 nm mid-infrared wavelengths can significantly enhance the light absorption efficiency of photovoltaic materials, thereby improving the energy conversion efficiency of solar cells. Furthermore, in thermal imaging or infrared detectors, such an absorber can selectively capture energy from ultraviolet to infrared light and convert it into heat, thereby improving the system’s precision and sensitivity. This makes it highly applicable in fields such as environmental monitoring, military, and medical imaging. Such an absorber is critical for sensors, as it can capture a wide range of wavelengths from 200 nm to 5800 nm, greatly benefiting applications like environmental monitoring, remote sensing, and multispectral imaging. It improves sensor sensitivity and accuracy, aiding in the detection of various substances or the identification of specific wavelength features. However, to date, there are few optical absorbers that can span such a wide range of 200 nm ultraviolet to 5800 nm mid-infrared wavelengths and provide multifunctional capabilities in a single material.

In this study, we investigated an absorber capable of covering a broad wavelength range from 200 to 5800 nm. This multi-layer metamaterial absorber features three key innovations that contribute to its enhanced performance. First, the materials used in its design are cost-effective and commercially available: SiO₂ (an oxide) and metals such as Fe and Ti, instead of noble metals like Ag and Au. The SiO₂ layer can be used to modify the light propagation path, helping optimize the distribution of light within the absorber and enhancing the absorption efficiency. When incorporated into a multilayer structure, it can alter light reflection and refraction, thereby improving absorption. Additionally, as a low-reflection layer, SiO₂ helps minimize light loss, directing more light into the absorber. Fe is a material with high electrical conductivity and strong light absorption characteristics. Its high conductivity allows it to efficiently absorb light and convert it into heat, particularly in the mid-infrared and visible light ranges. This contributes to an increased absorption rate in optical absorbers. At certain wavelengths, metallic materials exhibit surface plasmon resonance effects, which significantly enhance absorption, especially in the mid-infrared spectrum. Ti can effectively absorb light in the visible and ultraviolet ranges, especially when used as a thin film. This enhances the overall absorption rate of the optical absorber. When combined with other materials, such as SiO₂, Ti can create specific interface effects that further boost absorption at particular wavelengths. Therefore, we used SiO₂, Fe, and Ti to design our investigated optical absorbers.

This choice significantly reduces the material cost, making the absorber more economically viable for practical commercial applications. Second, the design incorporates two middle Fe layers in a square structure, as opposed to a continuous plane. This structural modification enhances the absorber’s efficiency by improving its performance in absorptivity across the extended wavelength range of 2000 to 5800 nm, achieving high absorptivity due to the unique interaction of the two-interlayer square Fe structure with incident light. The distinct geometry of the Fe layers supports efficient trapping, contributing to a broad absorption spectrum. Third, the top layer features a discontinuous cylindrical Ti matrix, a design choice that plays a crucial role in enhancing absorptivity at shorter wavelengths [19]. The discontinuous cylinders help scatter and trap light more effectively, achieving an absorptivity greater than 0.900 for wavelengths below 600 nm. This high level of absorption in the ultraviolet to visible range demonstrates the effectiveness of the top layer’s structure in improving the overall performance of the absorber. This study presents a highly efficient and cost-effective solution for broadband light absorption across a wide spectral range. The use of inexpensive materials and innovative structural designs offers promising potential for practical applications in fields such as solar energy harvesting and infrared sensing.

2. Structure of the Investigated Ultra-Wideband Optical Absorber with the Bandwidth from Ultraviolet C to Middle Wavelength Infrared

The intricate geometric architecture of the unit cell (with the w1 = 300 nm) of the investigated multi-layered metamaterial absorber is meticulously detailed in Figure 1, revealing a sophisticated seven-layer configuration designed for optimal electromagnetic wave absorption. For our optical absorber design, we utilized the material properties of Ti, SiO₂, and Fe directly from COMSOL’s built-in material database. This includes their complex refractive indices, wavelength-dependent optical dispersion curves (n(λ) and k(λ)), dielectric constants, and absorption coefficients. The structure exhibited precisely engineered layer thicknesses: a foundational Fe layer of 300 nm (h1), followed by alternating layers of SiO₂ and Fe, with the SiO₂ layers measuring 150 nm (h2), 170 nm (h4), and 170 nm (h6), respectively. The intermediate Fe layers were strategically deposited at 10 nm thickness (h3 and h5). The structure was crowned with a Ti layer of 155 nm thickness (h7), featuring the carefully optimized h3 and h5 widths (w3 and w5) of 285 nm and 235 nm and an optimized h7 radius (r7) of 70 nm. In subsequent sections, we presented a comprehensive analysis of the rationale behind these specific dimensional parameters, which were carefully selected through rigorous electromagnetic simulations and theoretical calculations. The polarization direction of the light used to find the optimal geometric parameters of our designed optical absorber was normal to the top layer material in the negative z-direction. Comprehensive numerical simulations and structural analyses of the absorber device were conducted using COMSOL Multiphysics^® (version 6.0), a robust commercial finite element analysis software. The optimization process focused on determining the critical dimensional parameters of our proposed ultra-wideband absorber, specifically the thicknesses of all layers, with particular emphasis on the widths of layers h3 and h5, and the radius of layer h7.

Using a systematic parametric sweep approach, we varied the mentioned parameter of each layer individually while maintaining constant dimensions for the remaining layers. This methodical investigation allowed us to examine the impact of geometric variations on absorption performance and electromagnetic response characteristics. Our primary objective was to identify the optimal parameters’ configurations that would maximize absorption efficiency across the desired frequency range while maintaining structural feasibility. The investigated absorber incorporated a mesh structure with the following specifications: 23,113 grid nodes with grid lengths ranging from 0.80 nm (minimum grid length) to 1.60 nm (maximum grid length). The mesh consisted of 125,329 tetrahedral elements, 14,390 triangular elements, 60 endpoint elements, and 1212 edge finite elements. The mesh quality was characterized by an average element quality of 0.6598 and a minimum element quality of 0.1821. With an element volume ratio of 0.004687 and a total grid volume of 2.16 × 10⁸ nm³, the mesh structure ensured reliable and accurate simulation results.

3. Processes to Investigate the Optimal Parameter of Each Layer

Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 explore the absorptivity variations in different layers in the investigated absorber by systematically modifying specific structural parameters. Figure 2 examines the width of the h3 Fe layer, Figure 3 analyzes the width of the h5 Fe layer, Figure 4 investigates the radius of the h7 Ti layer, Figure 5 studies the thickness of the h2 SiO₂ layer, and Figure 6 assesses the thickness of the h3 Fe layer. The research methodology involved simulating the seven-layer structure illustrated in Figure 1, where one parameter was varied at a time while maintaining other parameters constant. These parameter optimizations were conducted across a wavelength range of 200 to 10,000 nm. By systematically adjusting each layer’s structural parameters, the study aimed to identify the optimal configuration for achieving ultra-broadband absorption with high absorptivity. The results, presented in Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, reveal significant absorptivity changes corresponding to the different structural parameter modifications. This approach not only provides insights into the absorption characteristics of each layer but also establishes a comprehensive method for parameter optimization in multi-layered absorber designs.

Figure 2 investigates the profound influence of varying the width of the h3 Fe layer (w3) on absorptivity across different wavelengths. The study systematically explores the absorptivity changes when modifying the layer width within the 245–295 nm range, while maintaining the remaining layer thicknesses and width constants. The results reveal a significant correlation between the h3 Fe layer width and absorption characteristics. Specifically, increasing the layer width from 245 nm to 295 nm produced two critical outcomes:

(1)

A notable expansion of the absorption bandwidth with absorptivity consistently above 0.900 is observed.

(2)

A wavelength bandwidth shifted from approximately 200–4800 nm at w3 = 245 nm to 200–5800 nm at w3 = 285 nm.

This observation suggests that enlarging the h3 Fe layer width effectively extends the upper wavelength limit for high absorptivity, demonstrating the layer’s crucial role in modulating the absorber’s spectral performance. The findings highlight the importance of precise structural parameter optimization in designing ultra-broadband optical absorbers. When studying the effect of different nanofilm thicknesses on optical absorption characteristics, Figure 2a,b present key spectroscopic observations. Specifically, significant optical property changes were observed as the width of the h3 layer varied:

(1)

Width effect on absorption characteristics: When the width of h3 exceeds 285 nm, a characteristic low-absorption region appears in the spectral range of approximately 5000–7000 nm. This finding suggests the sensitivity of optical absorption to the thickness of the nanofilm in the structure.

(2)

Redshift of the wavelength cutoff: As the width of h3 gradually increases from 245 nm to 285 nm, a noticeable redshift occurs in the long-wavelength cutoff of the absorption spectrum. This redshift results in a significant increase in the bandwidth where the absorptivity exceeds 0.900, indicating that the material exhibits high absorptivity over a broader wavelength range.

(3)

Critical width and decreased absorption: When the width of h3 further increases to 295 nm, the absorption rate in the 5000–7000 nm range sharply decreases. This phenomenon is in close agreement with the low absorption region observed in Figure 2a, revealing the complex mechanism of optical property regulation through film thickness.

For these reasons, we chose 285 nm as the width of the h3 layer. This specific width was selected because it provides the absorber with the maximum bandwidth for optimal performance. These results highlight the precise control of optical absorption via thickness parameters in nanoscale structures. By carefully tuning the width of the h3 layer, directional adjustments to the spectral properties can be achieved, which is of great significance for optoelectronic devices, solar cells, and related fields. Therefore, we will further explore the microscopic mechanisms underlying these spectral changes to gain a deeper understanding of the physical nature of this phenomenon.

Figure 3a shows the change in the wavelength range with an absorptivity greater than 0.900 as the width of h5 (w5) increases from 195 nm to 245 nm. Specifically, the wavelength range expands from the original 200–5200 nm to approximately 200–5880 nm. In Figure 3b, significant spectral changes are observed when the width of h5 increases from 195 nm to 245 nm. The absorption peak for wavelengths below 1000 nm shifts towards shorter wavelengths, while the absorption peak for wavelengths above 1000 nm shifts towards longer wavelengths. Although the width of h5 at 245 nm has the broadest bandwidth, the study ultimately selects 235 nm as the optimal width. This decision is based on two key considerations. When the width reaches 245 nm, the absorptivity within the 2400–5900 nm range significantly decreases. Additionally, there are two absorption valleys in the region below 1000 nm, both with absorptivity below 0.900. Taking these factors into account, 235 nm is considered the best choice for the width of h5, as it maintains good absorption properties while avoiding the unfavorable spectral features observed at 245 nm.

Figure 2b and Figure 3b reveal that variations in the third and fifth Fe layer widths significantly alter the absorption spectra’s peak wavelengths. Specifically, Figure 2b demonstrates a critical optical phenomenon: when the third Fe layer width increases from 285 nm to 295 nm, the absorption spectrum exhibits peak splitting and a concurrent decline in absorptivity. The underlying mechanisms can be used to explain why the Fe layer widths must be narrower than the SiO₂ layer width to achieve optimal absorption characteristics. The SPR effect, optical coupling and mode conversion, and interface modulation are related to optical impedance, which plays a physical role in enhancing absorption efficiency. On the other hand, the SPR effect, electromagnetic field distribution and penetration, edge effects and light scattering, and interface modulation are related to the absorption effects from structural changes, which also contribute to improving absorption efficiency. Therefore, we will provide a detailed analysis of optical impedance and structural changes to demonstrate the impact of these two effects on the absorption efficiency of the investigated absorbers.

(1)

Surface plasmon resonance (SPR) effect: Fe, as a metallic material with free electrons, can excite SPR when incident light is applied. Reducing the Fe layer width relative to the SiO₂ layer can modify the plasmon resonance conditions, enabling more efficient light absorption. Variations in layer width fundamentally alter the free electron oscillation patterns, consequently modulating light–material interactions.

(2)

Optical coupling and mode conversion: Width disparities significantly alter the optical coupling efficiency between different layers. Also, narrower Fe layers enhance light mode conversion across different media, thereby increasing energy absorption capabilities.

(3)

Electromagnetic field distribution and penetration: Decreasing Fe layer width induces substantial changes in electromagnetic field distribution within the structure. These modifications increase the effective light propagation pathways, prolonging the light’s residence time within the absorption layers.

(4)

Edge effects and light scattering: Reduced Fe layer width generates more pronounced edge effects and light scattering phenomena. These effects substantially increase light scattering and absorption probabilities, thereby improving overall light absorption efficiency.

(5)

Interface modulation: The interfacial characteristics between Fe and SiO₂ are intrinsically sensitive to width variations. Strategic width adjustments can optimize interface optical properties, consequently enhancing light absorption mechanisms.

In this study, we employed a discontinuous cylinder Ti matrix as the top-layer structure. As shown in Figure 4, we performed a scanning experiment over a wavelength range of 200–10,000 nm, using the radius (r7) of the h7 cylinder Ti as the variable. The r7 value varied between 60 nm and 100 nm, and our findings indicate that the r7 value has a significant impact on the absorption characteristics. Specifically, when the r7 value was less than 65 nm, the absorptivity in the 500–800 nm range was significantly lower than 0.900. When the r7 value exceeded 80 nm, a region of low absorptivity appeared around 1000 nm. However, when r7 was between 65 and 80 nm, there was virtually no region with low absorptivity below 1000 nm. Figure 4b demonstrates the changes in the absorption spectrum as r7 varies from 60 nm to 100 nm. As the r7 value increased, the absorptivity below 2000 nm decreased, and the bandwidth narrowed accordingly. Notably, when r7 was 60 nm, the low absorption valleys in the 500–800 nm range became more pronounced, which is consistent with the absorption distribution observed in Figure 4a. After conducting the comprehensive analyses, we selected r7 = 70 nm as the optimal parameter for our study.

The influence of the r7 variation on the absorption characteristics can be attributed to the interplay between the structure of the discontinuous cylinder Ti matrix and its interaction with incident light. Smaller r7 values lead to a more confined interaction between the material and the incident light, particularly in the shorter wavelength ranges, resulting in lower absorption efficiency. On the other hand, larger r7 values tend to create a more dispersed interaction with light, leading to regions of low absorptivity around specific wavelengths (e.g., near 1000 nm), which is undesirable for applications requiring uniform absorption. This study highlights the importance of optimizing the r7 value to balance the absorption performance across the entire wavelength range. The deeper mechanisms behind the effect of varying r7 on the absorption behavior will be discussed in the subsequent section, focusing on how the structural properties of the discontinuous cylinder Ti matrix influence light–matter interactions at different scales.

Figure 5a illustrates the change in the wavelength range with an absorptivity greater than 0.900 as the thickness of the h2 layer increases from 130 nm to 170 nm. As the h2 layer thickness increases, the high absorptivity (>0.900) wavelength range expands from the original 200–5400 nm to approximately 200–5950 nm. However, in the wavelength range below 1000 nm, the changes in absorptivity are less pronounced, making it difficult to directly discern any specific trend from the graph. To further investigate the effect of h2 layer thickness on the absorption spectrum, Figure 5b provides more detailed scan results, with measurements taken at 10 nm intervals within the 130 nm to 170 nm thickness range. The data clearly show that, as the thickness of the h2 layer increases, there is a noticeable redshift in the cutoff wavelength at the longer wavelength end. Notably, when the h2 layer thickness reaches 160 nm and 170 nm, a dip in the absorptivity below 0.900 appears in the wavelength range below 1000 nm. Based on these experimental findings, we ultimately selected a thickness of 150 nm for the h2 layer to achieve optimal optical performance. These results highlight the significant impact of h2 layer thickness on optical absorption properties, particularly in the longer wavelength range. Increasing the thickness of the h2 layer helps extend the absorption range to longer wavelengths. This redshift effect is crucial for designing high-performance optical materials or devices, as it can enhance the material’s ability to absorb light over a broader wavelength range. However, excessive thickening of the h2 layer may lead to a decrease in absorptivity in the shorter wavelength range, making the choice of optimal thickness critical.

Furthermore, we conducted a detailed analysis of the effect of the h3 Fe layer thickness on the optical absorption characteristics. As shown in Figure 6a, significant spectral response features emerge when the h3 Fe layer thickness changes. When the h3 Fe layer thickness was less than 6 nm, a region with low absorptivity (absorptivity < 0.900) appeared around 1000 nm. This indicates that an ultra-thin Fe layer significantly reduces the light absorption capability within a specific wavelength range. When the h3 Fe layer thickness exceeded 15 nm, a low absorption region appeared at wavelengths near 4000 nm. Notably, this low absorption region expanded as the h3 Fe layer thickness increased. To gain a more comprehensive and accurate understanding of the effect of h3 Fe layer thickness on light absorption properties, we conducted detailed spectral scanning analysis experiments. The spectral scanning range was from 200 nm to 10,000 nm, with h3 Fe layer thickness varying in 5 nm increments from 5 nm to 30 nm. The analysis results are presented in Figure 6b, which reveal the following key findings. When the h3 Fe layer thickness was 5 nm, a distinct low absorption valley appeared around 1000 nm. As the thickness increased to 15 nm, the low absorption valley at 1000 nm disappeared, and a new low absorption region emerged near 4000 nm. As the Fe layer thickness increased further, this region not only expanded but also showed a significant decrease in absorptivity. In conclusion, we determined that a thickness of 10 nm is optimal for the h3 Fe layer.

Additionally, we conducted a detailed analysis of the effect of the h5 Fe layer thickness on the light absorption properties. Similarly to the variation in the h3 Fe layer thickness, when the h5 Fe layer thickness was less than 6 nm, a region with low absorptivity (<0.900) appeared around 950 nm. This also demonstrates that the ultra-thin Fe layer significantly reduces light absorptivity in a specific wavelength range. When the h5 Fe layer thickness exceeded 15 nm, a low absorption region appeared around 3800 nm. This low absorption region also expanded as the h5 Fe layer thickness increased. As the thickness increased to 15 nm, the low absorption dip at 950 nm disappeared, and a new low absorption region emerged near 3800 nm. As the Fe layer thickness further increased, this region not only expanded but also experienced a significant decrease in absorptivity. Based on these observations, we determined that the optimal thickness for the h5 Fe layer is 10 nm. Similar results were observed for other layers’ thickness analyses, as seen in Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, and are therefore not repeated here.

The absorption spectroscopy results show that both discontinuous cylinder Ti matrix-containing (h7 layer) and discontinuous cylinder Ti matrix-free structures exhibit significant optical properties within the wavelength range of 600 to 5800 nm, as shown in Figure 7a. Specifically, both structures demonstrate excellent performance in terms of absorptivity within this range, with average absorptivity rates of 0.978 and 0.963, respectively. Additionally, both achieve a perfect absorption state with a maximum absorptivity of 0.999. This indicates that both structures possess extremely high light absorption capabilities in the near-infrared (NIR) and visible light regions. These findings suggest that these structures have broad potential for optical applications, particularly in fields such as photovoltaics and optical sensors. Upon closer examination, it is found that the discontinuous cylinder Ti matrix-free structure exhibits a significant reduction in absorptivity in the short wavelength range of 200 to 600 nm, as Figure 7b shows. This observation is noteworthy, as it implies that the presence of the discontinuous cylinder Ti matrix has a significant impact on the optical properties of the structure, particularly in the UV and visible light regions. Specifically, the introduction of the discontinuous cylinder Ti matrix likely plays a regulatory role in the optical absorption process of the structure, potentially involving multiple physical mechanisms.

Firstly, the discontinuous cylinder Ti matrix may affect the SPR behavior of the structure. In metal materials, especially those with nanoscale metal layers, SPR phenomena can significantly alter light absorption characteristics. The discontinuous cylinder Ti matrix can tune the SPR frequency of the material, enhancing light absorption within specific wavelength ranges. This effect is particularly prominent in the UV and visible light regions, especially in the shorter wavelength range, which may explain the observed decrease in absorptivity in the Ti-free structure in this range. Secondly, the introduction of the discontinuous cylinder Ti matrix can influence the optical interference effects within the material. Since the discontinuous cylinder Ti matrix may alter the refractive index distribution of the structure, it could affect the interference patterns of light within the material, thereby modifying the absorption characteristics. This optical interference effect could either enhance or diminish the absorptivity in the UV-to-visible light range, depending on the thickness and placement of the discontinuous cylinder Ti matrix. Additionally, we will analyze the effect of the discontinuous cylinder Ti matrix on light impedance, which may provide further insights into its influence on the optical properties of the structure.

When the real part of the impedance of an optical system approaches 1, it typically indicates that the system is matched with the medium, meaning that when light propagates through the system, there is minimal reflection and most of the energy is transmitted [20,21]. This suggests that the refractive index of the system’s surface is very close to that of air. Additionally, when the imaginary part is near zero, it signifies that the energy loss during the propagation of light is minimal, meaning that the system exhibits low absorption and scattering. When the real part of the optical impedance approaches 1 and the imaginary part approaches zero, light waves can be efficiently transmitted through the system with minimal energy loss. Figure 8 illustrates that the real part of the optical impedance for the discontinuous cylinder Ti matrix-free structure deviates from 1, and the imaginary part deviates from 0 within the 200–600 nm range. In this range, the optical impedance real part of the multi-layer planar optical absorber with the discontinuous cylinder Ti matrix-free structure designed in this study deviates from 1, and the imaginary part deviates from 0. This deviation may be caused by the inappropriate refractive index of the structure within the 200–600 nm range, leading to the real part deviating from 1 and causing light reflection. Additionally, the excessive absorption properties of the structure in this wavelength range make it difficult to effectively control light absorption.

However, by adding a discontinuous cylinder Ti matrix on top of the structure, the real part of the optical impedance can be brought closer to 1, and the imaginary part closer to 0 within the 200–600 nm range. The introduction of the discontinuous cylinder Ti matrix at the top layer of the optical absorber has a significant impact on the reflection and propagation characteristics of light. Ti, a material with a high refractive index, when arranged in a cylinder structure, may influence the behavior of incident light waves within the absorber, including phenomena such as local resonance and SPR. At the interface between the discontinuous cylinder Ti matrix and the original multi-layer planar absorber material, complex electromagnetic wave reflection, refraction, and absorption may also occur. These interface effects can significantly alter the propagation characteristics of light waves. Additionally, the discontinuous cylinder Ti matrix structure introduces more scattering and refraction phenomena, which complicate the propagation of light within the multi-layer structure. This, in turn, helps to effectively reduce reflection and improve light absorption. These scattering effects can also alter the phase and intensity distribution of light, making the impedance characteristics of light closer to the ideal values (with the real part approaching 1 and the imaginary part approaching 0). Figure 8a also shows that the real parts of the impedances were significantly increased around the wavelength of 9 μm for the structures with or without Ti. The reason for this result is that at specific wavelengths, the dielectric constant of materials undergoes significant changes. For SiO₂, phonon resonance occurs at 9 μm, which leads to changes in the dielectric constant. This results in a refractive index mismatch, causing increased reflectivity, and directly affects the optical impedance.

Figure 9a illustrates four distinct absorber structures, which include the primary research structure (structure 1), the primary structure with the top discontinuous cylindrical Ti matrix removed (structure 2), the primary structure with the h3 and h5 Fe layers converted to continuous planes (structure 3), and structure 3 with the top discontinuous cylindrical Ti matrix additionally removed (structure 4). Through a detailed comparative analysis of these four structures, we can derive the following key observations.

(1)

Compared to structure 1, structure 2 exhibits relatively lower absorptivity in the 1800–5800 nm range. More notably, distinct absorption valleys appear near 250 nm and 600 nm, characterized by extremely low absorptivity.

(2)

Structures 3 and 4, which modify the h3 and h5 Fe layers to continuous planes, demonstrate significantly inferior absorptivity and bandwidth compared to structure 1. Interestingly, structure 3 does not exhibit absorption valleys near 250 nm and 600 nm.

The most compelling discovery is that the absorptivity valleys in structure 4, occurring near 250 nm and 600 nm, almost completely overlap with those in structure 2. These comparative results conclusively establish two critical findings:

(1)

The high absorptivity across the 200–5800 nm range is collectively generated by the holistic features of structure 1.

(2)

The high absorptivity below 1000 nm primarily originates from the top discontinuous cylindrical Ti matrix in structure 1.

This diagram further validates that the structure we proposed exhibits remarkable ultra-broadband absorption characteristics under optimal parameter configurations. Specifically, the structure demonstrates outstanding absorption capabilities across an extremely wide spectral range, from visible to near-infrared.

In this study, the ultra-wideband optical absorber under investigation is a structure designed to absorb light over an extensive wavelength range, spanning from 200 nm (ultraviolet C) to 5800 nm (mid-infrared, MIR). Figure 10 presents the analysis results of the electric and magnetic fields, as shown in Figure 10a,b, for wavelengths of 720 nm, 1850 nm, 3250 nm, and 5180 nm. The electric field intensity distribution results in Figure 10a indicate that the structure exhibited relatively high absorptivity across all the measured wavelengths of 720 nm, 1850 nm, 3250 nm, and 5180 nm. Moreover, as the wavelength increased from 720 nm to 5180 nm, the structure showed a slight decrease in absorptivity, although this trend was not particularly pronounced. Figure 10b illustrates that as the wavelength increased from 720 nm to 5180 nm, the absorption region within the structure clearly diminished. Specifically, Figure 10b shows that as the wavelength increased from 720 nm to 5180 nm, not only did the high absorptivity region become shallower, but the extent of the high absorptivity area also decreased, as indicated by the increase in the yellow regions. This observation suggests that the structure’s ability to absorb light decreases across this broader range of wavelengths. Moreover, these results further confirm that the primary mechanism responsible for achieving high absorptivity in this structure across the entire 200 nm ultraviolet C to 5800 nm mid-infrared range is SPR. The decrease in absorptivity at longer wavelengths can be attributed to the changing resonance conditions, where the plasmonic effects become less effective as the wavelength extends into the mid-infrared region.

The absorption performance of the metamaterial absorber was comprehensively evaluated across the entire electromagnetic spectrum, spanning from 200 nm ultraviolet C to 5800 nm mid-infrared wavelengths. The assessment covered incident angles ranging from 0° to 90°, examining both transverse electric (TE) polarizations, as depicted in Figure 11a, and transverse magnetic (TM) polarizations, as depicted in Figure 11b. The results in Figure 11 demonstrate a notable disparity in absorption performance between TE and TM polarizations. The simulation reveals how absorptivity changes with varying incident wavelengths and angles across different experimental conditions. The optical absorber structure we designed consists of the following layers: the bottom layer is a continuous Fe plane, and the third and fifth layers are discontinuous square Fe structures (Fe squares), instead of continuous planes. The second, fourth, and sixth layers are continuous SiO₂ planes, while the top layer is a discontinuous cylinder Ti matrix structure. The absorptivity varies as the incident light angle changes from 0° to 90°. For both TE-polarized and TM-polarized lights, high absorptivity is observed across a broad range of wavelengths (200–5800 nm) at most angles. However, there are regions near 30° and 80° where the absorptivity significantly decreases with increasing wavelength. While the underlying mechanisms are complex, we can identify several key factors that collectively contribute to these results, which are explained as follows.

(1)

Surface structures and optical resonance effects: The absorber’s structure incorporates discontinuous metallic layers consisting of square Fe structures and cylindrical Ti structures. These structures can generate localized resonance at specific wavelengths and angles of incidence. Strong absorption occurs when the incident light’s wavelength and angle match the dimensions of these structures. However, at incident angles of 30° and 80°, the wavelengths of light may exceed the resonant range of these structures, resulting in a significant decrease in absorptivity.

(2)

Optical interference effects at interfaces and layered structures: In multilayer structures, multiple reflections and interference effects of light waves alter their phase relationships. This phenomenon is particularly pronounced when the wavelength of light approaches the structural dimensions. These interference effects can lead to reduced absorptivity at specific angles of incidence.

(3)

SPR effects: Metals such as Fe and Ti may exhibit SPR at specific angles. The metallic Fe and Ti used in this study exhibit SPR at specific angles. These resonances modify the light–matter interactions, resulting in reduced absorption efficiency at certain angles of incidence.

(4)

Incident angle effects on electromagnetic waves: Light waves with different polarization states (TE- and TM-polarized light) exhibit distinct transmission and reflection characteristics at varying angles of incidence. In multilayered heterogeneous structures, wave vector mismatching leads to decreased absorption efficiency at specific angles.

To highlight the advantages of this study,

Table 1. A comparison of the absorption characteristics of several recently published studies with the investigated ultra-wideband absorber. Bandwidth: defining the wavelength region with an absorptivity higher than 0.900.

	Used Technology	Bandwidth	Average Absorptivity
Ref. [22]	The use of gold as a substrate was utilized to block light transmission, and a double-layer gold multipattern swastika resonator is based on a SiO2 substrate.	314–2830 nm	0.940
Ref. [23]	The bottom layer consists of Ti metal, topped with a thin MgF2 layer. Above these, square pillars made of three layers of Ti/MgF2/Ti are formed, surrounded by a square hollow with cylindrical Ti structures on the outside.	276–2668 nm	0.965
Ref. [24]	The absorber consisted of a two-dimensional Ti grating in the shape of a Au substrate, a one-dimensional SiO2 grating, and a four-edged platform.	300–2400 nm	0.957
Ref. [25]	The absorber consisted of a Ti disk resonator and a stack of TiO2/Ti square resonators, supported by TiO2/Ti thin layers, forming a metal–insulator–metal configuration.	392–3901 nm	0.963
Ref. [26]	The design featured a top dielectric layer and a bottom metallic layer, with the dielectric layer consisting of a rectangular block with an etched cross groove.	287–1544 nm	~0.946
Ref. [27]	The bottom layer is made of a continuous flat Ti plane, the middle layer consists of a continuous flat SiO2 plane, and the top layer is formed by a square Ti layer, creating a metal–insulator–metal (MIM) structure.	250–1600 nm	~0.910
This study	The bottom (h1), h3, and h5 layers are made of metallic Fe, two middle Fe layers (h3 and h5) in a square structure, while the layers above them (h2, h4, and h6) use SiO2. The top layer is composed of a discontinuous cylinder Ti matrix.	200–5800 nm	0.978

compares the results of our research with those of several recently published studies on ultra-wideband absorbers spanning the ultraviolet and infrared ranges. As shown in Table 1, compared to these ultra-wideband absorbers, the bandwidth of our solar absorber extends to wavelengths of 200 nm, and our absorber achieves an absorptivity greater than 0.900 starting from 200 nm. Additionally, our absorber also reaches an absorption rate above 0.900 in the 5800 nm range, with an average absorption rate of 0.978 over the 200–5800 nm range. It is rare for an ultra-wideband absorber to simultaneously have an ultra-wide bandwidth of 5600 nm and a high absorption rate of 0.978.

4. Conclusions

In this study, the strategic combination of metallic (Fe and Ti) and dielectric (SiO₂) layers creates multiple resonant cavities and plasmonic effects, enabling the device to achieve exceptional absorption characteristics across this extensive wavelength range of ultraviolet C to middle wavelength infrared. The structure features precisely engineered layer thicknesses: a 300 nm Fe base layer (h1), alternating layers of silicon dioxide (SiO₂) and Fe with SiO₂ layers of 150 nm (h2), 170 nm (h4), and 170 nm (h6), and 10 nm Fe layers (h3 and h5). The structure is capped with a 155 nm Ti layer (h7), with optimized widths of 285 nm (w3) and 235 nm (w5) for the Fe layers, and a 70 nm radius (r7) for the Ti layer. This study’s absorber exhibited an ultra-broadband range of 200–5800 nm, with an average absorptivity of 0.978 and a maximum absorptivity of 0.999, demonstrating a state of nearly perfect absorption. This design’s absorber exhibited four absorption peaks at 720 nm, 1850 nm, 3250 nm, and 5180 nm. High absorptivity was observed across a broad range of wavelengths (200–5800 nm) for both TE- and TM-polarized light at most angles. However, at angles near 30° and 80°, the absorptivity significantly decreased as the wavelength increased. This design represents a significant advancement in broadband absorption technology, offering potential applications in solar energy harvesting, thermal emission control, and spectroscopic sensing.