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Abstract
We propose a switchable perfect absorber with broadband and narrowband absorption based on alternating dielectric and metal nano-film structures in this paper. The lithography-free pattern is equipped with polarization insensitivity, good ductility and manufacturability, which has great significance in practical device development and applications. The quasi-complete selective absorption of incident light can be originated from asymmetric Fabry-Perot resonance, which combines the destructive interference in dielectric layers with inherent absorption in metal layers. When the light incidents on the surface covered with ultra-thin metal film of this structure, it acts as a narrowband absorber with over 99.90% absorption at 771 nm wavelength and a full wave at half maximum of 20 nm. When the light incidents on other surfaces covered with anti-reflective dielectric film, it achieves broadband perfect absorption with an average absorption exceeding 96.02% in a 500-1450 nm wavelength range. The absorption spectrum of oblique incidence shows that the broadband absorption behaves big angle range tolerance while the narrowband absorption exhibits angular dependence. The band-switchable performance of this absorber makes it valuable for energy harvesting/re-radiation applications in solar thermal photovoltaic systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical perfect absorbers have been widely investigated in the past years due to their quasi-complete absorption characteristics, integration capability and multi-domain application [1–4]. Most of their structures are based on meta-materials or meta-surfaces with periodic patterns [5,6], grating structures [7–9], etc. Compared with other traditional absorbers with complex structures [1–9], they have attracted the great research interests in light absorber areas due to their simple structures, good ductilities, fabrication convenience and production ability [10–14]. From a practical perspective, the selective absorption of incident electromagnetic waves may play a significant role in chemical sensing [7,15], photoelectric detection [16], color filters [17,18], photovoltaic devices [19,20], and thermal emitters [21,22].
Recently, switchable perfect absorbers with two modes of broadband absorption and narrowband absorption open a new research field due to their potential values in many fields such as multifunctional radar device [23], integrated device for seawater detection and desalination [24], and moreover, energy harvesting and re-radiation device in solar thermal photovoltaic systems [25]. Song et al. designed a double-sided perfect absorber with both heat absorption and radiation functions [26]. Through the periodic pyramid structure and double-layer nested structure composed of tungsten and silicon, 99% solar absorption in the range of 0.28µm-1.5µm and thermal radiation with 99.75% emissivity at 3.1µm are realized respectively. Wang et al. proposed an integrated device of narrowband infrared emission and broadband energy absorption using nano ultrathin molybdenum absorption layer and hafnium oxide as reflective phase adjuster. And it shows strong angle stability and good high temperature stability at 1373K [27]. In addition, there are many studies on switchable bidirectional absorbers by choosing materials whose electromagnetic properties can change with external conditions to achieve dynamic adjustment such as grapheme [28,29], VO2 and Ge2Sd2Te2 [30–32], liquid crystal [23,33] and the equivalent LC circuit [34,35]. However, most previous studies have adopted complex structures to achieve broader or narrower absorption but they are extremely unstable at high temperatures and not conducive to large-area preparation. Although the design of a unidirectional absorption device can simply device structure, in order to satisfy both narrowband absorption and broadband absorption, a part of the bandwidth of broadband absorption will inevitably be lost. In addition, dynamic adjustment of temperature and bias are difficult to adapt to the application scenarios of photovoltaic devices.
In this paper, we present a switchable bidirectional perfect absorber based on continuous metal-dielectric films stack. When the light is incident from two opposite directions that is perpendicular to structure surface respectively, accordingly, the system exhibits broadband absorption of 96.02% during 500-1450 nm wavelength range with 80° angle tolerance, narrowband absorption of 99.90% at wavelength 771 nm with a 20 nm full wave at half maximum (FWHM) and angular dependence. According to Kirchhoff’s law, the absorber’s absorbance in thermal equilibrium is equal to its emissivity, then one side of this bidirectional absorber can be used for broad-spectrum absorption, while the other side is for narrowband thermal radiation. These energy absorbing properties increases the absorption rate and preventing heat radiation escaping in solar photovoltaic devices.
2. Structural design and physical modeling
Figure 1 illustrates the schematic principle of this switchable bidirectional perfect absorber. The three-dimensional view in Fig. 1(a) shows that what we have proposed is a metal-insulator-metal-insulator-metal-insulator (MIMIMI) six-layer film system composed of silica (SiO2), chromium (Cr) and silver (Ag). Starting from the bottom, the optimized thicknesses of each layer are: h1 = 30 nm, h2 = 210 nm, h3 = 100 nm, h4 = 110 nm, h5 = 10 nm, h6 = 110 nm. Unlike the traditional metal-dielectric stacks designed as unidirectional absorbers, we make it a bidirectional perfect absorber by cleverly setting the materials of each layer and adjusting the thickness of the films. At present, the usual idea of designing a perfect absorber is mainly to make it meet the phase matching and zero transmission. Thus accordingly, the absorption rate can be expressed as A = 1 - R - T, Where the R and T represents the reflectance and transmittance respectively. Based on the above two aspects, when the reflectance and transmittance is extremely small or infinitely close to zero, the absorber can get almost perfect absorption. Here, the absorption rate A is infinitely close to 1.
Fig. 1. Schematic principle illustration of switchable bidirectional perfect absorber. (a) Structure illustration (b) Absorption spectrum under two incident directions.
We use commercial software Lumerical solutions for making detailed simulation and calculation of the absorption performance of bidirectional absorber through finite-difference time-domain method (FDTD). Planar electromagnetic wave is set to propagate along the z-axis direction, the six-layer film system can be infinitely extended in the x and y directions and arranged in the z direction in the simulation. Based on the above assumptions and settings, the periodic boundary conditions are applied to the x-axis and y-axis, and the z-axis direction is set to a perfectly matched layer (PML). Besides, the background refractive index is set of 1, the refractive index of SiO2 comes from the experimental data of Palik [36], and the optical constants of the metallic materials Ag and Cr is described by the Drude-Lorentz model, which is given by [37].
To ensure calculation correctness, we use transfer matrix method (TMM) to reproduce the simulation results that we achieved from FDTD. In TMM calculation, the reflectance and transmittance at the interface of each layer are given by Fresnel formula, for s- polarization:
In Eqs. (2) and (3), ri,i+1,ti,i+1 are reflection and transmission at the interface between the i-th layer and the i+1-th layer respectively, θi is refraction angle of i-th layer, and the propagation matrix Pi in i-th layer film is:
3. Results and discussion
3.1 Perfect absorption and performance analysis
We calculated the reflectance and transmittance of this absorber structure when the electromagnetic wave is normal incidence, and obtained its absorbance ratio as shown in Fig. 1(b). When the light is incident on the absorber along + z direction, it achieves 99.90% absorption at 771 nm as a quasi-perfect absorber. Partial enlarged view shows that the FWHM is 20 nm. When light is incident along -z direction, the perfect absorber has a stable absorption of more than 90% in a range from 500 nm to 1450 nm, FWHM as a broadband absorption reaches 950 nm. Then the absorber can work as a switchable device between narrowband absorption and broadband absorption. For sub-wavelength devices, even some slight dimensional changes will have an impact on the performance of the device. Therefore, through a large number of numerical simulations, we successively analyzed the absorption performance with the thickness of each film layer, and got a set of parameters to optimize performance.
First, we study the effect of material thickness on narrowband absorption as shown in Fig. 2. Figure 2(a) shows the change in absorbance with the thickness of the bottom Ag h1 (other parameters of the absorber remain unchanged) for the normal incidence from -z direction, it can be seen that the highest absorbance is obtained when the thickness of h1 is selected from 10 to 50 nm. At the same time, as shown in Fig. 2(b), the absorbance shows a trend of increasing at first and then decreasing. Especially, it is indicated from the legend that when h1 is 30 nm, it has the highest absorbance and the narrowest FWHM, so 30 nm is determined to be the thickness of h1 for the next parameter optimization. In the order from bottom to top, Fig. 2(c) shows the absorption rate at different thickness h2 of SiO2. It indicates that the absorption rate is basically not affected while the wavelength position of the absorption peak is red-shifted as the thickness of SiO2 increases. The specific spectral data (h2 = 190, 200, 210, 220, 230 nm) we plotted in Fig. 2(d) also proved this conclusion. It is because the loss mainly occurs in the metal layer, the dielectric thickness only changes the phase of electromagnetic wave propagating in the cavity, which means the absorption mainly depends on the metal property and thickness. Furthermore, the loss will increase as the metal layer become thicker within a certain range. When the thickness exceeds the certain value, the incident light cannot enter the absorber which will be reflected for a large amount. In this case, the increasing of metal layer thickness will reduce the absorption. The Ag layer corresponding to h3 is used to prevent transmission in narrowband and broadband absorption, its thickness is much greater than Ag skin depth. As a result, incident electromagnetic waves will propagate a short distance in this layer and are completely attenuated without penetration. Therefore, the thickness of h3 does not need to be optimized both in the proposed narrowband absorption and broadband absorption system.
Fig. 2. Effect of material thickness on narrowband absorption. (a)(b) Ag layer thickness h1 (c)(d) SiO2 layer thickness h2.
For the case where the incident light towards –z direction, the absorption within different thicknesses of each layer was calculated and shown in Fig. 3. We simultaneously scan the thickness of the two dielectric layers from 50 nm to 70 nm with h4 was equal to h6 (other parameters of absorber remain unchanged). As shown in Fig. 3(a), the scanned thickness can ensure the broadband absorption covers the entire working area except when it is less than 90 nm or greater than 130 nm. Therefore, we control the h4 and h6 size range between 90-130 nm in Fig. 3(b). It is shown that the absorption gradually increases at long wavelengths, while decreases at short wavelengths. This is caused by the dielectric layer thickness increases, which results in a red shift of the absorption peak at short wavelength. The thickness of h4 and h6 is set to be equal within 110 nm will ensure that strong destructive interference occurs at the same position to increase the absorption of metallic Cr, thereby maintaining high absorption in the entire working frequency band. Next, we explore the effect of metal Cr thickness h5 on broadband absorption. We first scan the absorption spectrum as the thickness of h5 increases from 0 to 60 nm. It can be seen from Fig. 3(c) that the absorber can obtain high absorption when the thickness of Cr is 5 to 25 nm, it will block the entry of incident light when h5 thickness is greater than the Cr skin depth. Similarly, we calculate the absorption spectra of five different thicknesses of h5 in Fig. 3(d). The spectrum shows that as the thickness of Cr increases, the overall absorption rate also increases first and then decreases. In order to obtain the maximum overall absorption, we choose the thickness of h5 to be 10 nm.
Fig. 3. Broadband absorptions change with material thickness. (a)(b) SiO2 layer thickness h4and h6. (c)(d) Cr layer thickness h5.
In addition, we investigate the effect of metal materials on the performance of bidirectional absorption to determine the most suitable metal. The result is shown in Fig. 4. For narrowband perfect absorption, we compared the absorption spectra of traditional narrowband plasmonic metals gold, silver, copper and aluminum for h1 and h3. As shown in Fig. 4(a), the gold, silver, copper structure can achieve higher absorption rate than aluminum. Compared with gold and copper, it can obtain a narrower FWHM when silver is selected, which makes silver the best choice for narrowband perfect absorption. Because the imaginary part of dielectric constant of aluminum is large, which is not suitable as a narrowband absorption material. While the imaginary part of silver is smoother in the working wavelength compared with gold and copper, which induced a narrower line width in the absorption peak. For broadband perfect absorption, four different materials such as chromium, aluminum, nickel, and tungsten are selected as the h5 metal layer. Comparing their absorption spectrum shown in Fig. 4(b), we can see that it has the best absorption effect when chromium was selected as the metal layer due to it has relatively high and stable absorption over the entire simulated wavelength range. In the calculated waveband, chromium and tungsten have large imaginary parts of their dielectric constants, and their real parts are less fluctuation with the wavelength, which allow them to achieve relatively stable and high absorption rates in this structure. However, the imaginary part of tungsten decreases at long wavelength, then the absorption rate decreases accordingly, here we select chromium as the metal responsible for broadband absorption. Moreover, due to the low chemical activity and thermal expansion coefficient of chromium, it can work stably in various environments even at high temperatures, which has significant advantages in the application of solar thermal photovoltaic devices.
3.2 Angle tolerance
Angular dependence is one of the important properties of a perfect absorber. Next, we conduct angle tests on narrowband and broadband absorption respectively. Figure 5 shows the absorption spectrum of s-polarized wave and p-polarized wave respectively when the incident light towards the + z direction. It can be seen that the resonance peak is blue-shifted in both cases as the incident angle increases, but still maintains a high absorption rate. For the Fabry-Perot (FP) cavity, the incident angle increases can decrease its optical path difference, which reduces the wavelength value that meets the destructive interference conditions, and induces the resonance peak blue-shifted. Obviously, this is not friendly to the absorption of the absorber at a single frequency, because the resonance peak moves with the increase of the incident angle. However, for the emitter in a solar thermal photovoltaic system, the angular dependence has irreplaceable advantages. At present, the unidirectional radiation of the radiator faces the problem of energy escape from the side. Many studies are devoted to solving this problem, such as adding side mirrors or using near-field radiation [35,38], but all require special design or control of the battery's operating temperature. In comparison, the narrowband absorber we designed naturally has the phenomenon that the absorption peak blue shift with the incident angle increases. The emitter only emits positive angle radiation, and the emissivity of other angles at the resonance wavelength is close to zero. In other words, it does not require special design to provide a solution for prevent lateral radiation.
Fig. 5. Incident angle effect on narrowband absorption under different polarized waves (a) s-polarized wave (b) p-polarized wave.
For broadband energy harvesting such as solar cells, thermal emitters, solar thermo photovoltaic, incident light usually enters the structure from all directions, which requires the broadband absorber have greater angular tolerance. Figure 6(a) is the absorption spectrum when the s-polarized wave is obliquely incident (incidence along -z). It shows that the absorption remains a high value at 500 -1450 nm as the angle increases. It is calculated that at a large incident angle of 60° for s-polarization, the absorber can maintain an average absorption rate of 74% in the entire working band. On the other hand, from the absorption spectrum of p-polarized wave at oblique incidence shown in Fig. 6(b), calculated result indicates that absorber can maintains 82% average absorption rates 70°incidence angle. Due to the accumulation of phase shifts, broadband absorption decreases with incident angle increasing. However, the absorbers under both polarizations can still maintain an average broadband absorption of 50% under 80° oblique incidence here, which indicates that the designed broadband absorber exhibits good angular insensitivity.
Fig. 6. Incident angle effect on ion broadband absorption under different polarized waves (a) s-polarized wave (b) p-polarized wave.
3.3 Physical mechanism of perfect absorption
We investigate the physical origin of proposed perfect absorption in narrowband and broadband: asymmetric FP resonance, which originates from combination of FP dielectric cavity and metal layers. Conventional lossless FP dielectric cavity usually induces strong destructive interference through the effective optical path of a quarter-wavelength to modulate the transmission and reflection spectra without any absorption. When the lossless dielectric cavity is combined with metal, however, the absorption of the metal will be modulated through destructive interference, resulting in asymmetric Fabry-Perot resonance. A notable feature of asymmetric Fabry-Perot resonance is that the electric field at the resonance wavelength is locally distributed in the dielectric cavity. Therefore, as shown in Fig. 7, we calculate and plot the electric field distribution diagram of narrowband absorption to prove this. Figure 7(a) shows the electric field intensity distribution from wavelength 200 nm to 2000 nm. It can be clearly seen that the electric field is strongly distributed at wavelength 771 nm, which corresponds to narrowband resonance wavelength, and there is no electric field distribution in the dielectric layer at the non-resonant wavelength. We further plot the electric field distribution at wavelengths of 600 nm, 771 nm, and 1000 nm, as shown in Fig. 7(b)-(d). It can be seen from the figure that there is almost no electric field distribution in the dielectric cavity at the non-resonant wavelengths of 600 nm and 1000 nm. In contrast, at the resonance wavelength of 771 nm, the electric field is mainly distributed inside the dielectric cavity (h2). This verifies the excitation of the asymmetric Fabry-Perot resonance. On the other hand, the narrowband absorption characteristics are determined by the nature of metallic silver (h1 and h3). The uppermost metal layer h1 serves as the main absorption layer, and the lower h2 and h3 serve as the reflector together. The role of h2 is to isolate the absorption film from the reflective bottom, and adjust the reflection phase to control the wavelength of perfect absorption.
Fig. 7. Electric field intensity distribution of switchable bidirectional absorber narrowband part (h1, h2 and h3) at different wavelengths. (a) From wavelength 200 to 2000 nm. (b) 600 nm. (c) 771 nm. (d) 1000 nm.
Based on the asymmetric Fabry-Perot resonance, the physical mechanism of broadband absorption is further analyzed. We prove that the broadband absorption of the bidirectional perfect absorber comes from the intrinsic absorption of metallic Cr (h5) and destructive interference in the SiO2 dielectric layer (h6 and h4), in which the intrinsic absorption can be improved through destructive interference. We have drawn the incidence optical path diagram in broadband structure in Fig. 8. As shown in the Fig. 8(a), the incident light first enters the medium layer h6 from the air, where h6 not only plays the role of anti-reflection, but also causes strong destructive interference through the effective light path of a quarter wavelength, which promotes the light absorption of h5. The incident light not absorbed by h5 will pass through and enter the dielectric layer h4. Similarly, h4 also promotes the absorption of h3 under destructive interference, although h3 is so thick that the absorption is relatively small. The more important role of h4 is to act as a reflecting cavity, so that the reflected light at the interface of h4 and h3 can be reflected through it and then incident on the metal Cr again. Incident light entering the Cr layer will increase the absorption rate through intrinsic absorption. Multiple reflections and transmissions of incident light result in the absorber exhibiting broad-spectrum absorption characteristics. In addition, the absorbed power distribution of bidirectional absorber is calculated and the result is shown in Fig. 8(b). As shown in the Fig. 8(b), the absorption covering the entire working band mainly comes from the Cr metal layer, and it is also distributed in the Ag metal layer, but only a small amount of distribution near the long wave. The absorption power distribution verifies our conclusion: the intrinsic absorption of Cr and the destructive interference of SiO2 together lead to broadband absorption behavior.
Fig. 8. Physical mechanism of broadband absorption. (a) Incident light path diagram. (b) Absorbed power distribution of bidirectional absorber narrowband part (h3, h4, h5 and h6) in the work region.
4. Conclusions
In summary, a switchable bidirectional nano-film perfect absorber is presented in this paper. The optimal parameters are determined by testing materials with different dielectric properties and optimizing their thickness. Numerical calculation results verify that for incident light from two opposite directions, a broadband perfect absorption rate more than 96.02% during 500-1450 nm spectral range and a narrowband absorption with a working wavelength of 771 nm with more than 99.90% absorption rate and a full wave at half maximum of 20 nm is achieved respectively. The quasi-complete selective absorption originates from the asymmetric Fabry-Perot resonance that produced by the combination of inherent absorption of metal and destructive interference of dielectric cavity. Further calculations show that broadband absorption has good angle tolerance, while the narrowband absorption is angle-dependent, and the band combination characteristics can be used to reducing radiation side leakages. This bidirectional absorber provides new application scenarios for energy harvesting/re-radiation equipment in solar thermal photo voltaic systems.
Funding
National Natural Science Foundation of China (61727816, 61705100, 61520106013); Fundamental Research Funds for the Central Universities (DUT18RC016, DUT20RC(3)008).
Disclosures
The authors declare no conflicts of interest.
References
1. N. Landy, S. Sajuyigbe, J. J. Mock, D. Smith, and W. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
2. L. Meng, D. Zhao, Z. Ruan, Q. Li, and M. Qiu, “Optimized grating as an ultra-narrow band absorber or plasmonicsensor,” Opt. Lett. 39(5), 1137–1140 (2014). [CrossRef]
3. C. Chen, G. Wang, and Z. Zhang, “Dual narrow-band absorber based on metal–insulator–metal configuration for refractive index sensing,” Opt. Lett. 43(15), 3630–3633 (2018). [CrossRef]
4. G. Deng, K. Lv, and H. Sun, “An ultrathin, triple-band metamaterial absorber with wide-incident-angle stability for conformal application at X and Ku frequency band,” Nano. Res. Lett. 15(1), 217 (2020). [CrossRef]
5. H. Gao, W. Peng, W. Cui, S. Chu, and X. Yang, “Ultraviolet to near infrared titaniumnitride broadband plasmonic absorber,” Opt. Mater. 97(2), 109377 (2019). [CrossRef]
6. Y. Cheng, H. Luo, and F. Chen, “Triple narrow-band plasmonic perfect absorber for refractive index sensing applications of optical frequency,” OSA Continuum 2(7), 2113 (2019). [CrossRef]
7. S. Luo, J. Zhao, and D. Zuo, “Perfect narrow band absorber for sensing applications,” Opt. Express 24(9), 9288–9294 (2016). [CrossRef]
8. Y. Li, Z. Liu, H. Zhang, P. Tang, B. Wu, and G. Liu, ,“Ultra-broadband perfect absorber utilizing refractory materials in metal-insulator composite multilayer stacks,”Opt. Express 27(8), 11809–11818 (2019). [CrossRef]
9. Y. Liao and Y. Zhao, “An ultra-narrowband absorber with a dielectric-dielectric-metal structure based on guide-mode resonance,” Opt. Commun. 382, 307–310 (2017). [CrossRef]
10. K. Lee, C. Ji, and L. Guo, “Wide-angle, polarization-independent ultrathin broadband visible absorbers,” Appl. Phys. Lett. 108(3), 031107 (2016). [CrossRef]
11. D. Wu, C. Liu, and Y. Liu, “Numerical study of an ultra-broadband near-perfect solar absorber in the visible and near-infrared region,” Opt. Lett. 42(3), 450–453 (2017). [CrossRef]
12. C. Yang, C. Ji, and W. Shen, “Compact multilayer film structures for ultrabroadband, omnidirectional, and efficient absorption,” ACS Photonics 3(4), 590–596 (2016). [CrossRef]
13. D. Zhao, L. Meng, H. Gong, X. Chen, Q. Li, and M. Qiu, “Ultra-narrow-band light dissipation by a stack of lamellar silver and alumina,” Appl. Phys. Lett. 104(22), 221107 (2014). [CrossRef]
14. C. Park and S. S. Lee, “Narrowband and flexible perfect absober based on a thin-film nano-resonator incorporating a dielectric overlay,” Sci. Rep. 10(1), 17727 (2020). [CrossRef]
15. K. V. Sreekanth, S. Han, and X. Chen, “Biosensing with the singular phase of an ultrathin metal-dielectric nanophotonic cavity,” Nat. Commun. 9(1), 369 (2018). [CrossRef]
16. Z. Xia, H. Song, and M. Kim, “Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities,” Sci. Adv. 3(7), e1602783 (2017). [CrossRef]
17. B. Zeng, Y. Gao, and F. J. Bartoli, “Ultrathin nanostructured metals for highly transmissive plasmonic subtractive color filters,” Sci. Rep. 3(1), 2840 (2013). [CrossRef]
18. Z. Li, S. Butun, and K. Aydin, “Large-Area,lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015). [CrossRef]
19. A. K. Azad, A. J. Taylor, and H. T. Chen, “Metasurfacebroadband solar absorber,” Sci. Rep. 6(1), 20347 (2016). [CrossRef]
20. R. Eden and S. Fan, “Absorber and emitter for solar thermophotovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit,” Opt. Express 17(17), 15145–15159 (2009). [CrossRef]
21. W. Zhou, Y. Shen, E. Hu, Y. Zhao, M. Sheng, Y. Zheng, and S. Wang, “Nano-Cr-film-based solar selective absorber with high photo-thermal conversion efficiency and good thermal stability,” Opt. Express 20(27), 28953–28962 (2012). [CrossRef]
22. C. Wu, B. Neuner, J. John, A. Milder, B. Zollars, S. Savoy, and G. Shvets, “Metamaterial-based integrated plasmonic absorber/emitter for solar thermo-photovoltaic systems,” J. Opt. 14(2), 024005 (2012). [CrossRef]
23. S. Zhou, Z. Shen, R. Kang, W. Hu, and S. Ge, “Liquid crystal tunable dielectric metamaterial absorber in the terahertz range,” Appl. Sci. Basel 8(11), 2211 (2018). [CrossRef]
24. C. Chen, L. Zhou, and J. Yu, “Dual functional asymmetric plasmonic structures for solar water purification and pollution detection,” Nano Energy 51, 451–456 (2018). [CrossRef]
25. H. Wang, J. Chang, Y. Yang, and L. Wang, “Performance analysis of solar thermophotovoltaic conversion enhanced by selective metamaterial absorbers and emitters,” Int. J. Heat Mass Transfer 98, 788–798 (2016). [CrossRef]
26. M. Song, H. Yu, C. Hu, M. Pu, Z. Zhang, and J. Luo, “Conversion of broadband energy to narrowband emission through double-sided metamaterials,” Opt. Express 21(26), 32207–32216 (2013). [CrossRef]
27. Y. Wang, L. Zhou, Z. Zhang, J. Yu, B. Huang, Y. Wang, and Y. Lai, “Hybridsolar absorber–emitter by coherence-enhanced absorption forimprovedsolar thermophotovoltaic conversion,” Adv. Opt. Mater. 6(24), 1800813 (2018). [CrossRef]
28. M. R. Soheilifar and B. Ferdow, “Reconfgurable metamaterial absorber as an optical switch based on organic graphene control,” Opt. Quant Electron 51(5), 155 (2019). [CrossRef]
29. H. Wang, Y. Yang, and L. Wang, “Switchable wavelength-selective and diffuse metamaterial absorber/emitter with a phase transition spacer layer,” Appl. Phys. Lett. 105(7), 071907 (2014). [CrossRef]
30. Y. Guo, Y. Zhang, X. Chai, L. Wu, Y. Cao, and L. Song, “Tunable broadband, wide angle and lithography-free absorber in the near-infrared using an ultrathin VO2 film,” Appl. Phys. Express 12(7), 071005 (2019). [CrossRef]
31. Z. Song, A. Chen, and J. Zhang, “Terahertz switching between broadband absorption and narrowband absorption,” Opt. Express 28(2), 2037–2044 (2020). [CrossRef]
32. X. Hua and G. Zheng, “Intensity switchable and wide-Angle mid-Infrared perfect absorber with lithography-free phase-change film of Ge2Sb2Te5,” Micromachines 10(6), 374 (2019). [CrossRef]
33. D. Shrekenhamer, W. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial absorber,” Phys. Rev. Lett. 110(17), 177403 (2013). [CrossRef]
34. B. Zhu, Y. Feng, J. Zhao, C. Huang, and T. Jiang, “Switchable metamaterial reflector/absorber for different polarized electromagnetic waves,” Appl. Phys. Lett. 97(5), 051906 (2010). [CrossRef]
35. N. Hu, J. Zhang, S. Zha, C. Liu, H. Liu, and P. Liu, “Design of a multilayer broadband switchable absorber based on semiconductor switch,” Antennas Wirel. Propag. Lett. 18(2), 373–377 (2019). [CrossRef]
36. E. D. Palik,“Handbook of optical constants of solids,” Academic Press, Orlando69–88 (1985)
37. S. G. Sodrigo, F. J. García-Vidal, and L. Martín-Moreno, “Influence of material properties on extraordinary optical transmission through hole arrays,” Phys. Rev. B 77(7), 075401 (2008). [CrossRef]
38. V. Rinnerbauer, A. Lenert, and D. M. Bierman, “Metallic photonic crystal absorber-emitter for efficient spectral control in high-temperature solar thermophotovoltaics,” Adv. Energy. Mater. 4(12), 1400334 (2014). [CrossRef]
