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Abstract
Achieving perfect absorption and controlling the absorption bandwidth are highly desirable for many applications. In this work, we design a narrowband almost-perfect absorber by using a metal–insulator–metal thin-film stack with absorption up to 99.67% at 0.58μm incident wavelength. The peak of absorption can be totally controlled by adjusting the thickness of the insulator layer. When the top metal layer is patterned by crossed grating nanostructure with optimized parameters, the absorber becomes broadband over 150nm bandwidth with average absorption exceeding 97% from 0.5μm to 0.65μm in the visible region. Both the narrowband and broadband absorbers are independent on polarization in specific incident angle range. This work opens up a promising new approach to control bandwidth of perfect absorption, which implicates many potential applications.
© 2017 Optical Society of America
1. Introduction
Controlling the spectral absorption property of optical structures is a great interest of both fundamental theory and practical applications in photonics. Particularly, the designs of perfect absorbers have been widely investigated since they are crucial in many promising applications, such as selective thermal emitters [1], sensitive detectors [2], photodetectors [3, 4], photovoltaic [5] and thermophotovoltaic devices [6], and solar thermoelectric generators [7]. Perfect absorbers are devices in which all incident electromagnetic wave is absorbed totally at the operating frequency (wavelength), which means that transmissivity, reflectivity, scattering and all other light propagation channels are disabled [8]. Several ways to achieve the almost-perfect absorption have been reported: a single layer in coherent perfect absorber (CPA) system [9, 10], functional multi-films with absorbing layer [11], gratings [12–14], metasurfaces [15, 16] and metamaterials [1, 2, 4, 8, 17]. These absorbers usually can be categorized into two types for different applications: narrowband and broadband absorbers [8], and most studies and researches were focused on only the former or the latter working on the polarized incident light [2, 12, 18–20] with less discussion about the bandwidth. We present the systematic designs for both narrowband and broadband absorbers with similar structures and polarization-independence. A narrowband almost-perfect absorber was designed working in visible spectrum with only two normal materials (a reflective metal and a lossless dielectric), and its absorption band was designed to become broad by changing nanostructure with optimized parameters. Polarization-independence design has been considered because polarization sensibility is not ideal for some applications. 99.67% peak absorption with narrowband and exceeding 97% average absorption with broadband have been achieved in our designs, and the relationship of absorption bandwidth has been discussed.
2. Narrowband design
Noble metals such as gold (Au), silver (Ag) and copper (Cu) are usually used as reflectors with low absorption at visible wavelengths. The dispersion relationship of refractive index (n) and extinction coefficient (k) of a single thin Au film was obtained with a variable-angle spectroscopic ellipsometer, and the absorption spectrum of the single Au film was indirectly calculated by A = 1-R-T using the one-dimensional rigorous coupled wave analysis (RCWA) [21] based on this dispersion as shown in Fig. 1(a), where R and T are the total reflectance and transmittance at normal incident condition.
Fig. 1 (a) Absorption spectrum (red line, left axis) and dispersion relationship of refractive index (blue line, right axis) and extinction coefficient (black line, right axis) of a single Au film. (b) Absorption spectrum of the three-layer MIM stack composed with a top 30nm Au layer, a 131nm SiO2 layer and a bottom 200nm Au layer coated on a silica substrate.
One way to achieve extremely high absorption of one single inorganic film is building a CPA optical system [9, 10], because absorption can be enhanced by two counter propagating incident fields in a cavity that contains a loss medium when the relative phases of the incident fields are coherently matched. A thick metal layer with high reflectance was used to reflect the incident light. So the reflected light will oscillate with the incident light. An insulator layer was added to match the phase for coherence enhancement. Another but thin metal layer was set on the top as an absorption layer, which acts like the loss medium of CPA system but only one incident light is needed. So our narrowband almost-perfect absorber consists of a three-layer metal–insulator–metal (MIM) thin-film stack. Gold was used as the top (30nm thickness) and bottom (200nm thickness) metal layer and dielectric SiO2 (131nm thickness) was chosen as the insulator layer. This stack led to extremely high absorption up to 99.67% at 0.58μm incident wavelength, shown in Fig. 1(b). This is a similar result with other reports in [13, 14, 20] Evgeny Popov et al’s work but no complicated pattern of fabrication is needed here, which means lower cost of large-area fabrication in our design.
Film-stack is naturally polarization-independent at normal incident situation, but differentiation is occurred during incident angle increasing. Absorption spectra for the designed narrowband absorber as a function of incident angle from 0° to 89° (no interaction at 90°) and incident wavelength from 0.5μm to 0.65μm in both TE and TM polarizations incident wave were calculated in Fig. 2. The narrowband absorber maintains polarization-independence with over 97% absorption when incident angle is from 0° to 10° and the wavelength corresponding to the peak absorption becomes differentiated when incident angle keeps increasing.
Fig. 2 Calculated absorption spectra for the designed MIM stack as a function of incident angle and incident wavelength in (a) TE mode and (b) TM mode.
The thickness of the top Au film was designed to be optically transparent (30 nm) in our narrowband absorber design, so that incident light could pass through a middle dielectric SiO2 layer and be reflected by the bottom Au film, resulting in a resonance that caused obvious enhancement of electric intensity in the dielectric layer at high-absorption wavelength as shown in Fig. 3(a), and the intelligible physical image of local absorption distribution is shown in Fig. 3(b) by directly multiplying the electric intensity and the imaginary part of the dielectric function:
where α is the normalized coefficient and the electric field E(x, z) was calculated by RCWA. We notice that the high electric intensity was “trapped” in the dielectric phase-matching layer which is similar to a Fabry–Pérot resonator and main absorption was located in the top Au film when the phase was matched at the perfect absorption wavelength while absorption is much lower at the other wavelength.Fig. 3 (a) Electric intensity and (b) local absorption distribution of the design MIM stack at different incident wavelength, 0.55μm and 0.58μm, respectively corresponding to the low and high absorption situation.
The phase-matching layer is significant for controlling the peak absorption at special wavelength. The relationship between incident wavelength and thickness of the phase-matching layer (SiO2) was calculated in Fig. 4 and the extremely high absorption will pseudoperiodly appear (corresponding to the 1st, 2nd and 3rd resonance modes) because of the guide-mode resonance effect when the thickness of SiO2 layer is increasing. More importantly, the wavelength corresponding to the peak of absorption shifts rapidly when the thickness of SiO2 layer changes slightly, so the peak of absorption can be totally controlled by fine adjusting the thickness of phase-matching layer, and this result portends a lot of potential applications such as bolometers and color filters.
Fig. 4 Calculated absorption spectra for the MIM stack as a function of thickness of SiO2 layer and incident wavelength with fixed thicknesses of top (30nm) and bottom (200nm) Au layers.
3. Broadband design
Metamaterials with metasurface and plasmonic grating structures are always promising candidates to enhance electromagnetic wave absorption because of resonant effects [8]. Our broadband absorber was based on the MIM stack where only the top metal (Au) layer is patterned with simple grating structure. Unlike the MIM films stack with natural polarization-independence at normal incident condition, the two-dimensional symmetry of the grating pattern should be considered to obtain polarization insensitivity. A biperiodic crossed grating with equal widths and periods in both direction (x and y directions) was carried out in our work and its schematic drawing is shown in Fig. 5(a). Such a symmetric configuration yields the same optical responses for TM and TE polarizations. The thicknesses of dielectric SiO2 layer td and bottom Au layer tm were respectively set to 0.131μm and 0.2μm as the same as the narrowband design because we focused on the design of the top metal crossed grating structure. Three parameters of the top Au grating structure (period Λ, duty cycle f and height of grating ridge hg) were optimized for broadband absorption by the following merit function:
where N is the dividing number of the wavelength range. The Genetic Algorithm (GA) [22] was used in our optimization for searching extremely high average of absorption Aave in broad spectrum range and the total reflectance and transmittance were calculated by two-dimensional RCWA [23]. The absorption and reflectance spectra of the broadband optimized result was shown in Fig. 5(a) with the values of period Λ = 0.5μm, duty cycle f = 0.115, height of grating ridge hg = 0.18μm and the dispersion relationship in Fig. 1(a) also had been used. The average absorption Aave exceeds 97% over 150nm bandwidth from 0.5um to 0.65um at normal incident, which is much broader than the narrowband design. The duty cycle f and thickness of grating ridge hg are critical to absorption, so we calculated the Aave around the spectra from 0.5μm to 0.65μm for the biperiodic crossed grating as a function of hg and f with fixed other parameters, shown in Fig. 5(b). It’s not easy to obtain extremely high Aave but only a few combinations of these two parameters can achieve that. Fortunately, there is a large range of hg for gaining extremely high Aave when f is around 0.11, which means a large tolerance for fabrication in our broadband absorption design. This continuous-variable multi-parameters analysis can be used to achieve different bandwidths of perfect absorption when different wavelength ranges of merit function are applied. So it allows us to design specific perfect absorbers for our needs and opens up a promising new approach to achieve controllable bandwidth of perfect absorption. Polarization divergence of incident angle is carried out in Fig. 5(c). The absorber maintains broadband polarization-independent over 95% average absorption when incident angle is below 10°, but this angle range extend to 20° when the cut-off average absorption is set to 85%, which is beneficial to practical application over a wide range of incident angle. Compared with the narrowband absorber, the broadband absorption spectra with different polarization also become differentiated when incident angle keeps increasing. Besides, because of the definition of incident angle in x0z plane, the asymmetric incident situation results that the performance of both the narrowband and the broadband absorber in TM mode is better than in TE mode.Fig. 5 (a) Schematic drawing of the biperiodic crossed grating, and the absorption and reflectance spectra of the broadband optimized results: period Λ = 0.5μm, duty cycle ƒ = 0.115, height of grating ridge hg = 0.18μm. Otherwise, thicknesses of SiO2 layer td and bottom Au layer tm are set to 0.131μm and 0.2μm. (b) Calculated average absorption around the spectra from 0.5μm to 0.65um for the biperiodic crossed grating as a function of height of crossed grating ridge and duty cycle with fixed other parameters. (c) Calculated absorption spectra for the designed broadband absorber as a function of incident angle and incident wavelength in different polarization incident wave with TE mode and TM mode.
We also calculated the local absorption of this broadband absorption structure at 0.58μm (corresponding to the narrowband peak) and three slice views are shown in Fig. 6. The top Au layer (structure) still contributes to the most absorption as same as the narrowband design, and interestingly main absorption is located at the surface between the top Au layer and both sides of it, which is attributed to cavity mode resonance caused by the coupled surface plasmon polaritons (SPPs) [14, 18]. Commonly, SPPs excited on metallic metasurfaces have applications in biosensors, thin-film photovoltaic systems, photoelectrochemical cells, and photodetectors [24] and the theoretic analyses have been widely investigated. Compared with the other reported broadband absorbers [8, 25], our design with simple crossed pattern can be fabricated by laser beam interference lithography method [26] instead of electron beam lithography, which means lower cost and higher efficiency of large-area fabrication.
Fig. 6 Sliced views of the calculated local absorption map of the optimized crossed grating at 0.58μm wavelength: (a) xy cross-section at z = 0.341μm, (b) yz plane at x = 0.25μm and (c) xz plane at y = 0.25μm.
4. Conclusion
We have presented a rigorous design of polarization-independent almost-perfect absorber from narrowband to broadband in the visible spectrum. Based on a three-layer MIM ultrathin film-stack, a narrowband absorber can achieve a peak of absorption up to 99.67% at 0.58μm incident wavelength. The peak of absorption can be totally controlled by adjusting the thickness of insulator layer. Main absorption was located in the top metal thin-film due to the guide-mode resonance. The absorber becomes broadband with average absorption exceeding 97% over 150nm bandwidth from 0.5μm to 0.65μm when the top metal layer is patterned by crossed nanostructure with appropriate parameters, which were optimized by continuous-variable multi-parameters optimization. Main absorption of the broadband absorber is located at the surface between the top Au layer and both sides of it, which is attributed to SPPs. Both the narrowband and broadband absorbers are independent on polarization when incident angle is below 10°. This quantitative analysis allows us to design specific perfect absorbers for our needs and opens up a promising new approach to achieve controllable bandwidth of perfect absorption.
Funding
This work is supported by the National Natural Science Foundation of China (Grant No. 11604352, 10704079, U1630140 and U1430121), and the Shanghai Science and Technology Committee Program (Grant No.16JC1420600).
References and links
1. X. Liu, T. Tyler, T. Starr, A. F. Starr, N. M. Jokerst, and W. J. Padilla, “Taming the blackbody with infrared metamaterials as selective thermal emitters,” Phys. Rev. Lett. 107(4), 045901 (2011). [CrossRef] [PubMed]
2. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef] [PubMed]
3. J. Rosenberg, R. V. Shenoi, T. E. Vandervelde, S. Krishna, and O. Painter, “A multispectral and polarization-selective surface-plasmon resonant midinfrared detector,” Appl. Phys. Lett. 95(16), 161101 (2009). [CrossRef]
4. W. Li and J. Valentine, “Metamaterial perfect absorber based hot electron photodetection,” Nano Lett. 14(6), 3510–3514 (2014). [CrossRef] [PubMed]
5. K. Nakayama, K. Tanabe, and H. A. Atwater, “Plasmonic nanoparticle enhanced light absorption in GaAs solar cells,” Appl. Phys. Lett. 93(12), 121904 (2008). [CrossRef]
6. H. Wang, Q. Chen, L. Wen, S. Song, X. Hu, and G. Xu, “Titanium-nitride-based integrated plasmonic absorber/emitter for solar thermophotovoltaic application,” Photonics Res. 3(6), 329–334 (2015). [CrossRef]
7. D. Kraemer, B. Poudel, H.-P. Feng, J. C. Caylor, B. Yu, X. Yan, Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. Ren, and G. Chen, “High-performance flat-panel solar thermoelectric generators with high thermal concentration,” Nat. Mater. 10(7), 532–538 (2011). [CrossRef] [PubMed]
8. C. M. Watts, X. Liu, and W. J. Padilla, “Metamaterial electromagnetic wave absorbers,” Adv. Mater. 24(23), OP98 (2012). [PubMed]
9. Y. D. Chong, L. Ge, H. Cao, and A. D. Stone, “Coherent perfect absorbers: time-reversed lasers,” Phys. Rev. Lett. 105(5), 053901 (2010). [CrossRef] [PubMed]
10. W. Wan, Y. Chong, L. Ge, H. Noh, A. D. Stone, and H. Cao, “Time-reversed lasing and interferometric control of absorption,” Science 331(6019), 889–892 (2011). [CrossRef] [PubMed]
11. J. R. Tischler, M. S. Bradley, and V. Bulović, “Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film,” Opt. Lett. 31(13), 2045–2047 (2006). [CrossRef] [PubMed]
12. B. C. P. Sturmberg, T. K. Chong, D.-Y. Choi, T. P. White, L. C. Botten, K. B. Dossou, C. G. Poulton, K. R. Catchpole, R. C. McPhedran, and C. Martijn de Sterke, “Total absorption of visible light in ultrathin weakly absorbing semiconductor gratings,” Optica 3(6), 556–562 (2016). [CrossRef]
13. E. Popov, D. Maystre, R. C. McPhedran, M. Nevière, M. C. Hutley, and G. H. Derrick, “Total absorption of unpolarized light by crossed gratings,” Opt. Express 16(9), 6146–6155 (2008). [CrossRef] [PubMed]
14. W. Zhou, K. Li, C. Song, P. Hao, M. Chi, M. Yu, and Y. Wu, “Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region,” Opt. Express 23(11), A413–A418 (2015). [CrossRef] [PubMed]
15. S. B. Glybovski, S. A. Tretyakov, P. A. Belov, Y. S. Kivshar, and C. R. Simovski, “Metasurfaces: From microwaves to visible,” Phys. Rep. 634, 1–72 (2016). [CrossRef]
16. G. M. Akselrod, J. Huang, T. B. Hoang, P. T. Bowen, L. Su, D. R. Smith, and M. H. Mikkelsen, “Large-Area Metasurface Perfect Absorbers from Visible to Near-Infrared,” Adv. Mater. 27(48), 8028–8034 (2015). [CrossRef] [PubMed]
17. X. Tian and Z.-Y. Li, “Visible-near infrared ultra-broadband polarization-independent metamaterial perfect absorber involving phase-change materials,” Photonics Res. 4(4), 146–152 (2016). [CrossRef]
18. J. Cong, Z. Zhou, B. Yun, L. Lv, H. Yao, Y. Fu, and N. Ren, “Broadband visible-light absorber via hybridization of propagating surface plasmon,” Opt. Lett. 41(9), 1965–1968 (2016). [CrossRef] [PubMed]
19. Y. K. Zhong, S. M. Fu, W. Huang, D. Rung, J. Y.-W. Huang, P. Parashar, and A. Lin, “Polarization-selective ultra-broadband super absorber,” Opt. Express 25(4), A124–A133 (2017). [CrossRef] [PubMed]
20. E. Popov, A.-L. Fehrembach, and R. C. McPhedran, “Almost-total absorption of light in thin, biperiodic, weakly-absorbing semiconductor gratings,” Opt. Express 24(15), 16410–16424 (2016). [CrossRef] [PubMed]
21. M. G. Moharam, T. K. Gaylord, E. B. Grann, and D. A. Pommet, “Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary gratings,” J. Opt. Soc. Am. A 12(5), 1068–1076 (1995). [CrossRef]
22. A. Conn, N. Gould, and P. Toint, “A globally convergent Lagrangian barrier algorithm for optimization with general inequality constraints and simple bounds,” Math. Computation Am. Math. Soc. 66(217), 261–288 (1997). [CrossRef]
23. L. Li, “New formulation of the Fourier modal method for crossed surface-relief gratings,” J. Opt. Soc. Am. A 14(10), 2758–2767 (1997). [CrossRef]
24. Q. Xiong, J. Wei, S. M. Mahpeykar, L. Meng, and X. Wang, “Observation of localized surface plasmons and hybridized surface plasmon polaritons on self-assembled two-dimensional nanocavities,” Opt. Lett. 41(7), 1506–1509 (2016). [CrossRef] [PubMed]
25. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat. Commun. 2, 517 (2011). [CrossRef] [PubMed]
26. C. Lu and R. Lipson, “Interference lithography: a powerful tool for fabricating periodic structures,” Laser Photonics Rev. 4(4), 568–580 (2010). [CrossRef]
