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Abstract
In this study, we designed a novel ultra-wideband (UWB) absorber and numerically analyzed it to demonstrate that its light absorptivity was greater than 90% in the wavelength range of visible light and near-infrared (405-1505 nm). The structure of proposed novel UWB absorber consisted of four layers of films, including silica, titanium, magnesium fluoride, and aluminium, and the upper silica and titanium layers had rectangular cubes in them. For that, the excitations of propagating surface plasmon resonance (PSPR), local surface plasmon resonance (LSPR), and the resonance of Fabry-Perot (FP) cavity were generated at the same time and combined to reach the effect of perfect absorption and ultra-wideband. The proposed absorber had an average absorptivity of 95.14% in the wavelength range of 405 ∼ 1505 nm when the light was under normal incidence. In addition, the UWB absorber was large incident angle insensitive and polarization-independent. The absorber proposed in the paper had great prospects in the fields of thermal electronic equipment, solar power generation, and perfect cloaking.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since Landy et al. first proposed a nearly perfect absorber in 2008 [1], in the past decade, people pay more and more attentions to investigate nearly perfect absorbers because of their important applications in solar cells, sensors, and other fields [2–4]. The developments of metamaterial absorbers mainly have the following trends for different applications: narrower ideal absorption band for sensing [5–8], multi-band for perfect absorption [9–12], and higher absorptivity in solar band for solar power generation [13–19]. For that, many studies have tried to investigate different technologies for constructing the absorbers with high absorptivity in an ultra-wideband (UWB). For example, Cui et al. reported an infrared absorber made of saw-toothed anisotropic metamaterial, which could achieve more than 90% absorptivity in the continuous spectrum range of 3000 nm to 5000 nm [20]. Ding et al. proposed an UWB polarization-independent metamaterial absorber, which composed of a periodic array of multilayered quadrangular frustum pyramids with different side lengths for different absorption mode [21]. Graphene-based absorbers have also received extensive attention [22,23].
Chen et al. proposed an UWB metamaterial absorber composed of a periodic array of dielectric cylinder sandwiched by the Ni films, and the structure had an average absorptivity of 90% in the visible light range of 400-700 nm [24]. Qi et al. reported an UWB perfect absorber based on the connected cylindrical holes, and the designed structure could achieve more than 90% absorptivity in the wavelength of ranging from 300 to 1250 nm [25]. Tuan et al. reported an UWB absorber based on a periodic array of multilayered conical frustums, and the proposed structure also had an average absorptivity of 90% in the wavelength range of 480-1480 nm [26]. However, these proposed UWB absorbers still have many shortcomings, including that the absorption bandwidths (defined as absorptivity > 90%) are usually below 1000 nm [24–35], the maximum absorptivity peak or the average absorptivity in the whole broadband are lower [36], or the device configuration is very complicated to achieve the broadband [20–23].
However, investigation of an absorber with high absorptivity peak and average absorptivity in the UWB and with simple structure is very important for us to actually manufacture the device. In this paper, we investigated and designed a novel UWB absorber based on silica-titanium-magnesium fluoride-aluminium (SiO2-Ti-MgF2-Al) four-layer structure, and we numerically demonstrated which had the property of a nearly perfect absorber. The designed absorber had light absorptivity greater than 90% over the wavelength range from visible light to near infrared (405-1505 nm). The maximum absorptivity in the high absorption window was 99.9% and the average absorptivity in the UWB was 95.1%. We would show that the propagating surface plasmon resonance (PSPR), local surface plasmon resonance (LSPR), and the resonance of Fabry-Perot (FP) cavity could be importantly excited in proposed novel structure. We would also show that the proposed multi-layer metamaterial could present as a nearly perfect absorber because the high absorptivity was combined the excitations of PSPR, LSPR, and resonance of FP cavity. We analyzed in detail the influence of different materials and structural parameters on the absorption performance of the designed absorber. Furthermore, we would prove that the proposed absorber had the characteristics of wider incident angle insensitivity and polarization independence. The most important is that the proposed four-layer absorber can be achieved and fabricated easily by using the radio-frequency sputtering or E-beam to deposit the four-layer thin films and using lithography process and different etching process to construct the rectangular cubes in SiO2 and Ti films. The optical properties of the absorber can be measured by using a Fourier transform infrared (FTIR) spectrometer. At present, the preparation and measurement technologies of the absorbers with different nano-structures or metamaterials are becoming more and more mature [37–39]. Ultra-wideband, simple manufacturing method, angle insensitive, and polarization independence, thickness of 415 nm makes the proposed structure have a great application prospect.
2. Structural design with ultra-wideband absorption
The stereoscopic outward appearance of the designed absorber is shown in Fig. 1(a), and it was noteworthy that the absorber on x-y plane consisted of infinite number cells. The dotted red line in Fig. 1(b) represented a smallest unit consisting of four semi cubes with symmetric arrangements. The proposed structure had combined the excitations of PSPR, LSPR, and the resonance of FP cavity to reach the nearly perfect absorber in an UWB, which would be proven in this study. The length of the side marked with the dotted red line was 380 nm and the side view of the proposed absorber is shown Fig. 1(c). From top to bottom, the proposed structure consisted of a SiO2 anti-reflection cube layer, a Ti cube array layer, a MgF2 dielectric layer, and an Al plate layer, corresponding to r1 = 200 nm, r2 = 190 nm, h1 (SiO2) = 80 nm, h2 (Ti) = 45 nm, h3 (MgF2) = 90 nm, and h4 (Al) = 200 nm, respectively. The complex dielectric constants of Al and Ti metals were obtained by Drude-Lorentz fitting, the refractive indexes of SiO2 and MgF2 were set as 1.45 and 1.38 [40], and the surrounding material was assumed as air.
Fig. 1. (a) Stereogram image of the proposed UWB solar absorber. (b) Top view of the proposed absorber, the dotted red line represented a smallest unit consisting of four semi-cube symmetric arrangements with r1 = 200 nm and r2=190 nm. (c) Side view of the proposed structure unit cell, and the parameters of the unit cell are set as h1 = 80 nm, h2 = 45 nm, h3 = 90 nm, and h4 = 200 nm.
The finite difference time domain (FDTD) method, which is a numerical analysis technique and can be used to model and simulate the computational electromagnetic wave, was used to simulate the optical properties and absorptivity of the designed solar absorber [41]. FDTD is one of the most commonly used numerical techniques for calculating the optical properties. It can solve Maxwell's equations and many papers ensure the reliability and accuracy of numerical results [22–25]. We built the model shown in the red dotted line of Fig. 1(b) to find the FDTD Solutions. We set the periodic boundary conditions in the x and y directions and set the perfectly matching layer to eliminate the boundary scattering along the z direction. It should be noted that when the device would be illuminated by a plane wave propagating at an angle. For that, we used the Bloch boundary conditions in the x-direction and y-direction and the broadband fixed-angle source technique to ensure that that all the wavelengths would have exactly the same incident angle. The reflection ratio (R) can be set as the power monitor by above the light source. Because the bottom Al layer is too thick for light to pass through, the transmission rate (T) of the absorber can be approximately zero, the absorptivity (A) of the proposed absorber can be calculated by A = 1 - R.
3. Results and discussion
The absorption of the proposed absorber under normal incident light was numerically simulated and the result is shown in Fig. 2. It can be clearly seen that the absorber had a continuous high absorptivity (> 90%) in the wideband range of 405 ∼ 1505 nm, this result suggests that the designed absorber had a bandwidth (BW) up to 1100 nm. In the range of 405 ∼ 1505 nm, the maximum absorptivity was 99.9%. The average absorptivity was 95.1%, which could be calculated as $A = \mathop \smallint \nolimits_{\lambda 2}^{\lambda 1} A(\lambda )d\lambda /({\lambda 1 - \lambda 2} )$, where $\lambda 1$ and $\lambda 2$ were 1505 and 405 nm.
In order to explain the physical mechanism for the investigated absorber had the high absorptivity in an UWB, we calculated the distribution of electric field intensity (|E|) in the x-y plane and that of magnetic field intensity field (|H|) in the x-z plane. The TE-polarized light at the direction of normal incidence with different resonance wavelengths of 450, 900, and 1300 nm were used as the sources to excite the designed absorber. Propagating surface plasmon resonance (PSPR) can be generated by the resonant oscillation of conduction electrons at the interface between negative and positive permittivity materials when the incident light is used to stimulate on their surfaces. For that, as the normally incident light was used on the surface of the designed absorber. From the distribution of electric field intensity (|E|) under normal incident TE polarized lights of different wavelengths, as Figs. 3(a)–3(c) show, the light was coupled to the edge of the Ti cube and positioned in the gap of the adjacent Ti cube and the surface plasmon polarization was clearly excited in the absorber. In addition, the magnetic field distributions of the resonance wavelengths for the incident lights of 450, 900, and 1300 nm are shown in Fig. 3(d), 3(e), and 3(f), respectively. As the wavelength of incident light was 450 nm, we could really observe from Fig. 3(a) that in each period, the electric field was mainly distributed at the edge of the top Ti cube. Figure 3(d) shows that a strong magnetic field not only existed on the top of Ti cube and above the continuous Al film. These phenomena indicate that the perfect absorption at 450 nm can be attributed to the excitations of PSPR at two different films, one is the PSPR between the continuous Al film and MgF2 dielectric layer, and the other is the PSPR between the Ti cube and SiO2 anti-reflection cube layer.
Fig. 3. Distribution of electric field intensity (|E|) and magnetic field intensity (|H|) under normal incident TE polarized light. (a)-(c) The distribution of the electric field in the x-y plane. (d)-(f) The distribution of the magnetic field in the x-z plane.
A localized surface plasmon resonance (LSPR) can be generated by the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. For that, as the light wavelength is longer than 180 nm, the rectangular cubes in the silica and titanium layers can generate the LSPR. However, the excitation intensity of the LSPR is dependent on the wavelengths of incident light and the sizes and materials of the nanoparticle or the constructed nano-structure. As the wavelength of incident light was 900 nm, as shows in Fig. 3(b), strong electric field appeared in the area of adjacent Ti cubes. It can be clearly seen from Fig. 3(e) that the magnetic field was mainly distributed in the top of Ti cubes, and there was also a magnetic field distribution in the MgF2 dielectric layer. The distribution patterns indicate that the LSPR is really excited in the Ti cubes and on the surfaces of MgF2 dielectric layer.
A conventional FP cavity can be constructed by two mirrors (in general, they are the same or different metals) to be separated by a lossless dielectric spacer with at least a quarter-wavelength thick, for that a FP effect can be formed by the Ti and Al layers separated by MgF2 layer. Because the optical wavelength of 900 nm satisfies the resonance of the FP cavity condition, the normal incident light will conduct constructive or destructive interferences with the reflected light. Therefore, the intensities of reflected and transmitted electromagnetic waves can be controlled and the resonance of the FP cavity is excited. When the incident light was reflected between the Ti mirror and the Al mirror, there was also a strong magnetic field distribution between the Ti cube and SiO2 anti-reflection layer. The observation of magnetic field on MgF2 dielectric layer can be used to prove the existence of resonance of the FP cavity. For that, the perfect absorptivity at the wavelength of around 900 nm can be explained by the combinations of the PSPR, LSPR, and resonance of FP cavity, and the PSPR and LSPR play a leading role. Finally, it can be seen from the electromagnetic field distribution in Fig. 3(c) and 3(f) that electric field mainly appeared in the area between adjacent Ti cubes and magnetic field mainly appeared in the MgF2 dielectric layer. For that the absorption in this band of 1300 nm is the result of the combinations of the PSPR, LSPR, and resonance of FP cavity, and the LSPR and resonance of FP cavity play a leading role.
In order to further verify the advantages of the designed structure, we compared the absorption capacity of the proposed absorber with the four-layer plane SiO2-Ti-MgF2-Al continuous films, in which the layers’ thicknesses were the same with those of Fig. 1(c). The absorption for the devices with different structures are compared in Fig. 4. The absorption peak for the device of four-layer plane SiO2-Ti-MgF2-Al films was about 80% at ∼520 nm, which was thought as the effect of SPR on the SiO2-Ti films. An absorption peak located at ∼1280 nm was not observed in the device of the four-layer plane SiO2-Ti-MgF2-Al films, which further proves that the SiO2-Ti cube will cause the excitation of the LSPR and resonance of FP cavity. The absorption performance of the proposed absorber is much better than that of the continuous film due to the enhancements of the LSPR and resonance of FP cavity in the range of analyzed wavelength. In Fig. 5, the absorber with the SiO2 anti-reflection layer shows a superior absorption as compared to the absorber without anti-reflection in the region of visible light. This is because the topmost SiO2 cube can excite multiple electromagnetic resonance modes in the region of visible light, such as the cavity resonance and PSPR. Therefore, as expected, the absorption value of the absorber with the SiO2 anti-reflection layer is higher than that of without anti-reflection. The results further indicate that the strong absorption in the broadband region is caused by the combinations of the PSPR, LSPR, and resonance of FP cavity.
Fig. 4. Spectra of the proposed UWB absorber (black line) and the device with the SiO2-Ti-MgF2-Al multi-layer plane structure (red line).
Fig. 5. Absorber with SiO2 anti-reflection layer (black line) and without anti-reflection layer (red line)
For further detailed analysis, we studied the effects of different structure parameters on the absorption performance of the proposed absorber. As the side length of r1 increased from 180 to 220 nm and the other parameters shown Fig. 1 were unchanged, a groove wave appeared in the absorption, it reached the minimum value of ∼ 91.5% as r1 was equaled to 200 nm. However, an absorption wave peak also appeared in the range of 1050 ∼ 1280 nm as r1 value increased, and as r1 was equal to 200 nm, the absorption wave peak reached the maximum value of 99.9% at the longer wavelength of 1280 nm. As the r1 value further increased, both the values of absorption peak and wavelength to present the absorption peak decreased. The variations in absorption Fig. 6(a) show that the r1 value have large effect on the SPR and prove again that a suitable side length of the rectangular cubes can excite the strongest SPR. Figure 6(b) illustrates the variations of absorption performance as a function of the thickness of the SiO2 anti-reflection layer (h1). When h1 value increases from 0 nm to 120 nm, the absorption of the absorber first increase, reach an optimum value at 80 nm, and then decrease as the h1 value further increase.
Fig. 6. Effects of variations in structural parameters on the absorption performance of the designed absorber (a) length (r1) and width (r2) of the SiO2-Ti cubes, (b) thickness of anti-reflection layer (h1), (c) thickness of the top Ti metal layer (h2), and (d) thickness of the MgF2 dielectric layer (h3).
Figure 6(c) illustrates the variations of absorption performance as a function of the thickness of Ti layer (h2) and the other parameters shown Fig. 1 were unchanged. As shows in Fig. 6(c), when the thickness of top Ti layer was 45 nm, the proposed absorber had the largest absorption rate. Figure 6(d) shows the variation in absorption performance caused by only changing the thickness of the MgF2 dielectric layer (h3) revealed in Fig. 1. When h3 value increased from 0 nm to 200 nm, the absorption of the absorber first increased, reached an optimum value at 90 nm, and then decreased as the h3 value further increased.
The variations of the free space impedance can be used to explain these results. According to effective medium theory, the relation between the free space impedance Z and S parameters can be expressed as [42,43]:
In these formula, n, k, and d represent the refractive index, the wave vector, and the thickness of the proposed absorber, respectively. S11, S22, S12, and S21 all are S parameters, where reflectivity R = S11. Because our metal substrate is greater than the skin depth of the incident light, for that S12 = T = 0. The free space impedance of our absorber can be calculated by:
When the impedance of the absorber matches the free space impedance (Z = Z0), the absorber can achieve the maximum absorptivity [44,45]. As shown in Fig. 7, when the real part of the impedance is close to 1, and the imaginary part is close to 0, the structure has the highest absorption efficiency, which is basically consistent with the perfect absorption effect of wide bandwidth. When the thicknesses of different films of the absorber are too thin or too thick, the absorber cannot match the free space impedance, and the strong SPR cannot be excited in the designed structure.
Fig. 7. (a) Impedance and reflection curves of free space. (b) Impedance and reflection curves of the proposed structure
The effects of the different materials on the absorption performance of the proposed absorber are also analyzed here, and their geometric parameters are same with those of Fig. 1. Under the condition of invariable in the geometric parameters, as shown in Fig. 8(a), we through used different kinds of commonly metals instead of the top Ti metal and studies the effects of metal materials for absorption performance. One can clearly see from Fig. 8(a) that the top metal also had a great influence on the absorption performance of the proposed structure. When Ag or Al were used as the top metals of the structured cubes, the absorption showed a narrow bandwidth with lower absorption peak. The reason is that their metal loss is smaller, then the relatively large quality factor results in a narrow bandwidth. When Ni was used as the top metal of the structured cubes, the absorption band of the proposed absorber was wider, but the absorptivity was relatively low in the range of 440∼1400 nm, which is caused by the intrinsic dispersion property. On the other hand, as shown in Fig. 8(b), when different metals were used to replace the bottom Al film, the absorption performance changed slightly, because the bottom was mainly used as a back reflector rather than a layer for the SPR or LSPR. To sum up, the absorptivity is greatly affected by the top metal and less affected by the bottom metal. In addition, as the refractive index of SiO2 changed greatly with the change of the incident wavelength. We calculated the absorption with different refractive indexes of the SiO2 film, and the results are shown in Fig. 9. When the refractive index of the SiO2 film increased from 1.43 to 1.49, only a little change in absorption was observed. This result suggests that we can just incorporate 1.45 as the refractive index of the SiO2 film into the calculation.
Fig. 8. Effects of variations in used materials on the absorption performance of the designed absorber (a) using nickel (Ni), aluminum (Al), and silver (Ag) as top metal cap and (b) using silver (Ag), gold (Au), and copper (Cu) as bottom metal.
Figure 10(a) and 10(b) describe the absorption evolution spectra with the incident angles from 0° to 60° at TE and TM polarization. Under TE polarization, when the angle of incident light was 20°, it was clear that the absorber was able to absorb more than 90% in the spectral range between 404 nm and 1465 nm. When the larger angle of 60° was used for incident light, absorptivity of more than 90% ranged for wavelength of 372 ∼ 1024 nm, with a bandwidth exceeding 650 nm. Under TM polarization, when the angle of incident light was 20°, absorptivity of more than 90% ranged for wavelength of 395 ∼ 1134 nm, with a bandwidth exceeding 700 nm. When the larger angle of 60° was used for incident light, absorptivity of more than 90% ranged for wavelength of 366 ∼ 1358 nm, with a bandwidth exceeding 950 nm. These features prove that the designed absorber has good incident-angle insensitive. In addition, the absorption performance is also polarization-independence due to the high symmetrical structure of the proposed absorber.
Fig. 10. Absorption of (a) TE-polarized light and (b) TM-polarized light at different oblique incidence angles.
4. Conclusions
In summary, we innovatively designed and numerically simulated an UWB absorber, which composed of SiO2-Ti cube periodic array, MgF2 dielectric layer, and Al substrate. This structure revealed the properties of a near-perfect absorber in the range of visible to near-infrared (405-1505 nm). In this band, the average absorptivity was 95.1% and the absorption peak was 99.9%. The combinations of the SPR, LSPR, and resonance of FP cavity made the broadband and high absorptivity and absorption peak possible. We compared the influences of different metal materials and the parameters of structure geometry on the absorption performance and revealed the absorption physical mechanisms of the absorber through the electric field diagram and the magnetic field diagram. For TE and TM polarized light with incident angle of 60 °, the average absorptivity was higher than 90%. The proposed absorber had the advantages of simple structure, ultra-wideband, and polarization-insensitive. In addition, the absorber can be easily manufactured using electron beam lithography, making the low-cost manufacturing possible. The UWB absorber proposed in this paper have great prospects in the fields of thermal electronic equipment, solar power generation, and perfect cloaking.
Funding
National Natural Science Foundation of China (6207032307); Youth Talent Support Program of Jimei University (ZR2019002); Innovation Fund for Young Scientists of Xiamen (2020FCX012501010105); Fujian Provincial Department of Science and Technology (2019H0022); Science Fund for Distinguished Young Scholars of Fujian Province (2020J01311454); Youth Talent Support Program of Fujian Province (2020).
Acknowledgments
Thanks to Dr. Haoyuan Cai from Zhejiang University and Jing Wang from Hebei University for their support.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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