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Near-infrared multi-narrowband absorber based on plasmonic nanopillar metamaterial
Journal Pre-proof Near-infrared multi-narrowband absorber based on plasmonic nanopillar metamaterial Qingfang Zhong, Tao Wang, Xiaoyun Jiang, Le Cheng, Ruoqin Yan, Xing Huang
PII: DOI: Reference:
S0030-4018(19)30861-2 https://doi.org/10.1016/j.optcom.2019.124637 OPTICS 124637
To appear in:
Optics Communications
Received date : 31 March 2019 Revised date : 6 September 2019 Accepted date : 24 September 2019 Please cite this article as: Q. Zhong, T. Wang, X. Jiang et al., Near-infrared multi-narrowband absorber based on plasmonic nanopillar metamaterial, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124637. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Near-infrared Multi-narrowband Absorber Based on Plasmonic Nanopillar Metamaterial
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Qingfang Zhonga, Tao Wanga,* , Xiaoyun Jianga, Le Chenga, Ruoqin Yana, and Xing
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Huanga
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China
Corresponding author. E-mail address: [email protected]
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*
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Abstract
A multi-narrowband absorber composed of gold nanopillar metamaterial is numerically designed and investigated in this paper. Full-wave simulations indicate that five distinctive narrow-band absorption peaks with the maximum absorption rate up to 99.3% and the lowest above 80% can be achieved in the near-infrared regime at normal incidence. Polarization dependence over a incident angular range of ± 30° is also researched. We demonstrate the spectroscopic tunability of our absorber by adjusting the
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structural parameters of the gold nanopillar array. Physical mechanism of the multinarrowband absorption is construed as the vertical Fabry-Perot-like gap plasmonic resonances induced by electric and magnetic dipolar polaritons. Compared to the previous researches on multi-band absorbers, we realize no less than five near-infrared narrow
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absorption bands by utilizing just simple configuration with single pattern, which makes considerable application potential in the fields of absorption filtering and spectroscopic sensing.
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Keywords: Multi-band absorber; Metamaterial; Gap plasmonic resonance; Near-infrared
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1. Introduction
Over the past decade, plasmonic metamaterials based on noble metals have attracted
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extensive attention and research due to their unique properties including negative index of refraction [1], sub-wavelength imaging [2] and cloaking [3]. They have been broadly
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employed in fields such as optical filtering [4], modulation [5, 6], biosensing [7-10], thermoelectric acquisition [11] and photodetection [12], which mostly benefits from the localized or non-localized plasmonic resonance modes of metallic nanostructure arrays that forms the plasmonic metamaterials [13]. Plasmonic metamaterial-based absorbers (PMAs) perform well in light absorption thanks to the intrinsic loss of metals. They are generally classified into broadband absorbers and narrowband absorbers. Broadband absorbers are commonly applied for solar cells [14, 15], while narrowband ones are
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widely used in thermal emission manipulation [16, 17], refractive index sensing [18], resonators [19], nano-antennas [20]. In order to broaden the application fields of PMAs, much effort have been made to shift the operating band from the microwave [21-24] and terahertz regime [25-27] to the
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infrared regime [18, 28-32]. And many methods including magnetic dipolar resonance [33, 34], surface lattice resonance [35] and gap plasmon resonance [18, 30] have been employed to achieve single narrowband absorption. Nevertheless, absorbers with just
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single narrowband are still faced with certain limitations in applications such as spectroscopy and imaging [36, 37]. Therefore, dual-band or multi-band absorbers with
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high performance are more desirable. Mao et al. [38] devised a three-layer metalmedium-metal structured absorber composed of elliptical and circular disk arrays as the top-layer to achieve multi-band absorption in the near-infrared frequency. Feng et al. [31]
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reported a mid-infrared dual-band perfect absorber based on an asymmetric T-type plasmonic array. Liu et al. [39] introduced a multi-band absorber utilizing a triple-layer
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dielectric metamaterial structure coupled with metal substrate. Meng et al. [40] designed a five-band terahertz metamaterial absorber using two concentric metallic circular rings as a compact single particle. Wang [41] presented a dual-band absorber formed by only a single square metallic patch and a metallic board separated by a dielectric layer, which utilizes the overlapping of the fundamental resonance and highorder response. However, the vast majority of multi-band absorbers consist of either composite unit-cells or stacked multiple functional layers, which makes fabrication
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processes relatively complex and hence restrict their practical applications. Moreover, most of them operate in the terahertz and microwave range, while near-infrared multiband PMAs are rarely investigated. In this paper, we theoretically propose a near-infrared multi-narrowband absorber by
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employing the gold nanopillar metamaterial. Numerical simulation results demonstrate that this absorber has five distinctive narrowband absorption peaks with high efficiency over a relatively wide incident angular range of ± 30° under both TM and TE polarized
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light, which is attributed to the high-order Fabry-Perot-like gap plasmonic resonance induced by the strong electric and magnetic polaritons excited in the gold nanopillar array.
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The dependence of absorption performance on the height, radius and spacing of gold nanopillars is also investigated, confirming the spectroscopic tunability and selectivity of our structure. Moreover, it should require relatively easy fabrication processes of one-
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step electron-beam-lithography firstly and electrochemical deposition followed [42]. Given the impressive properties and simple composition, we believe that our design has
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potential application in the fields of absorption filtering and spectroscopic sensing.
2. Structure design and simulation model
Figure 1 is a schematic image showing the general structure of our multi-narrowband absorber designed in this work. The absorber appears as a three-layer structure comprised of a gold nanopillar array (GNPA) fully embedded in a dielectric layer and the glass
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substrate, separated by a t = 50 nm thick gold layer. The gold nanopillars with radius of R = 160 nm are orthogonally arranged in the period of p = 350 nm. The height of gold nanotubes, h = 1500 nm, is just equal to the thickness of the host dielectric. Dielectric materials such as SiO2, MgF2, Al2O3 or other polymers are ideal optional substitutes of
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the host medium for that they have relatively low permittivity, which is beneficial for absorbing [43, 44]. Considering the processes of nanofabrication techniques, here we take the photoresist PMMA as the host dielectric. Transverse magnetic (TM, magnetic field
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parallel to y-axis) polarized light is normally irradiated from above the structure as shown in Fig. 1. Full-wave numerical simulation is performed by employing the commercial
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software package of Lumerical FDTD solutions to calculate the reflectance R(λ) and transmittance T(λ) of our structure, hence the absorption rate A(λ) equals to 1 - R (λ) T(λ). In our model, the refractive index of PMMA is set to be 1.49, and the permittivity
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of gold determined by the Drude model [28] is given by ε(Au) = 1 − 𝜔𝑝2 ⁄𝜔(𝜔 + 𝑖𝜔𝑐 ), where 𝜔𝑝 = 1.37 × 1016 rad/s , 𝜔𝑐 = 4.08 × 1013 rad/s are the plasma frequency
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and collision frequency of gold, respectively. Additionally, we set the four boundary planes that are perpendicular to the x-y plane as periodic boundary conditions, while the other two parallel to the x-y plane are set to be PML boundary conditions.
3. Results and discussion
As can be seen from Fig. 2(a), the numerically calculated transmission of our structure
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is nearly zero, that is T(λ) ≈ 0, in the simulation wavelength range of 0.8 μm to 2 μm, for the reason that the thickness of middle gold layer is larger than the skin depth of gold in the near-infrared regime. Thus the absorption A(λ) is equal to 1 - R(λ) to some extent. Five distinctive sharp absorption peaks at the center wavelengths of λ1= 876 nm, λ2 = 998
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nm, λ3 = 1166 nm, λ4 = 1411 nm, λ5 = 1800 nm with efficient absorption rate of 97.6%, 99.3%, 94.9%, 88.7%, 82.3% are obtained, respectively. Moreover, it is worth noting that the five resonant absorption peaks have significantly narrow full-width at half-maximum
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(FWHM) of 10 nm, 15 nm, 22 nm, 35 nm, 56 nm, respectively. Hence the quality factors Q (ratio of resonance wavelength to FWHM) are 87.6, 66.5, 53, 40.3, 32.1, respectively.
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Such narrow absorption bandwidths are impressively smaller than that of the general plasmonic structure-based metamaterial absorbers [34, 35], which benefits a lot to spectroscopic detection and sensing area [34]. Additionally, to demonstrate the necessity
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of the middle gold film, we further calculate the reflectance and transmission of our structure with the 50 nm thick gold film removed. As depicted in Fig. 2(b), even though
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there still exist several absorption peaks, the absorption efficiencies decrease obviously at the near-infrared resonance wavelengths, for the reason that a considerable part of incident electromagnetic wave transmits through the structure in the absence of the gold film, instead of being reflected by the thin gold mirror. In order to intuitively know the working status of our structure at resonance wavelengths, we simulate the distributions of electric field for the five resonance modes. As shown in Figure 3, electric field is periodically confined in the dielectric gap between
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adjacent gold nanopillars along z-axis, which indicates that different orders of vertical Fabry-Perot-like gap plasmonic resonance (GPR) modes [18, 30, 45] are excited by the incident electromagnetic wave coupling with the GNPA and the middle gold film. Consequently, resonant absorption peaks with narrow bandwidths is obtained for that
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large amount of energy at the resonance wavelength is concentrated inside the structure with just little reflected. From the perspective of physical mechanism, the GPR mode mentioned above is
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attributed to the electric dipole resonance based on the localized surface plasmon polaritons of single gold nanopillar and the magnetic dipole resonance excited in the gold
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nanopillar array. Furthermore, to explicitly understand the principles of the electric and magnetic resonance, we numerically investigate the distributions of both surface charge density and displacement current density in the GNPA at resonance wavelength of λ5 =
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1800 nm as a representative. Surface charge density distributions on the cross section α (parallel to x-y plane) and longitudinal section β (parallel to x-z plane) are pictured in Fig.
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4(b) and Fig. 4(e), respectively. It is seen that vertically alternating arrangements of positive and negative charge along z-axis are formed on both opposite sides of gold nanopillars. These dipole-like positive-negative charge pairs are known as plasmon polaritons resulting from the localized surface plasmon resonance excited by the incident electromagnetic wave. Meanwhile, from the distributions of displacement current density depicted in Fig. 4(c) and Fig. 4(f), one can see that the surface currents induced by electric dipoles flow anti-parallelly on the opposite sides of the vertical dielectric gap at the same
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height. Consequently, several circulating current loops are formed along the gap, indicating a strong magnetic dipole resonance. Hence strong confinement and enhancement of electromagnetic field is drived in the gap due to the coupling between magnetic dipole induced moment and the incident light wave at resonance wavelength.
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We further investigate the structural parameter sensitivity of the designed multinarrowband absorber. Fig. 5(a) and Fig. 5(b) describe the simulated absorption spectra in the near-infrared regime with the period of GNPA increasing from 350 nm to 410 nm
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(with R = 160 nm, h = 1500 nm) and the radius of gold nanopillars decreasing from 160 nm to 130 nm (with p = 350 nm, h = 1500 nm), respectively. It is intuitively seen that all
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the five resonant absorption peaks are blue-shifted as p value increases or R value decreases, and there consequently appears just four absorption peaks in our simulation wavelength range when the period is larger than 390 nm or the radius less than 140 nm.
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Meanwhile, absorption efficiencies at resonance wavelengths decline apparently, which is related to the weaker gap plasmonic resonances in GNPA. More specifically, either
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larger period or smaller radius makes a wider vertical gap between the adjacent gold nanopillars, which weakens the magnetic dipole resonance. In other words, the coupling between magnetic moment and incident electromagnetic wave tends to be feebler at such status compared with the optimized one. For better understand, electric field distributions at mode λ5 on the longitudinal section parallel to x-z plane for the parameters of p = 410 nm (with R = 160 nm, h = 1500 nm) and R = 130 nm (with p = 350 nm, h = 1500 nm) are drawn in Fig. 6(a) and Fig. 6(b), respectively. It is obviously seen that the electric field
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confined in the wide gaps is much weaker than half of that confined in narrow gaps (with p = 350 nm, R = 160 nm, h = 1500 nm), thus more than half of the incident electromagnetic energy is reflected, resulting in undesirable absorption efficiencies. In addition, we also examine the influence of the height of gold nanopillars on the absorption
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spectrum. As can be seen in Figure 7, the five absorption peaks shift towards long wavelength with absorption efficiency basically unchanged as the h value increases from 1500 nm to 1600 nm while maintaining the R = 160 nm and p = 350 nm, indicating a
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performance of spectroscopic tunability. This can be explained by that higher vertical dielectric gaps come into being as the height h increases, which allows gap plasmonic
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resonance at longer wavelengths [16].
The incident angle dependence and polarization dependence of the designed multinarrowband absorber are also numerically investigated. As shown in Fig. 8(a) and Fig.
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8(b), five distinctive narrow red strips representing for the narrowband resonant absorption peaks are obtained over the incident angular range of ± 30° for both TM
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( magnetic field parallel to y-axis) and TE (electric field parallel to y-axis) polarized light. Unsurprisingly, the absorber has the same resonance wavelengths under TM and TE polarization modes at normal incidence due to the symmetrical arrangement of GNPA. From Fig. 8(a), one can see the perfect angle insensitivity of the absorber under TM mode for that the absorption strips go straightly as incident angle increases to 30°, with absorption efficiency nearly invariable. While the absorption strips under TE mode bend to short wavelengths slightly as depicted in Fig. 8(b), for the reason that the magnetic
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field component perpendicular to gold nanopillars decreases upon increasing incident angle, which makes the center wavelengths where magnetic resonances are excited shift slightly [22].
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4. Conclusion
In conclusion, a design of near-infrared multi-narrowband absorber comprised of gold
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nanopillar metamaterial has been investigated. Five distinct absorption peaks with the maximum absorption rate up to 99.3% and the lowest above 80% are obtained in the nearinfrared regime at normal light incidence. The narrowband light absorbing performance
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is demonstrated to be related to the high-order Fabry-Perot-like gap plasmonic resonance induced by the strong electric and magnetic dipole resonance excited in the gold
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nanopillar array. The overlapping of different orders of GPR modes makes the multinarrowband absorption. Moreover, simulation results imply that both the position and strength of absorption peaks are sensitive to structural parameters, indicating the spectroscopic tunability and selectivity of our structure. Research for the angle and polarization sensitivity shows that the absorption spectra remain nearly invariable over the incident angular range of ± 30° under TM polarized mode, and the expected polarization-independence at normal incidence is also proved. Considering the impressive
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absorption properties and simple structural composition, our design has potential application in the fields of absorption filtering and spectroscopic sensing.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC)
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(Grant NoS. 61775064) and the Fundamental Research Funds for the Central Universities (HUST: 2016YXMS024).
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Figure Captions
Fig. 1. Schematic image of the multi-narrowband absorber structure. Orthogonally
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arranged gold nanopillar array with period of p is embedded in PMMA. h, R represent the height and radius of gold nanopillars, respectively. t stands for the thickness of Au film.
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The optimized structural parameters are: h = 1500 nm, R = 160 nm, p = 350 nm, t = 50
Fig. 2. Numerically calculated reflectance, transmission and absorption spectra of the GNPA-based structure (a) containing or (b) not containing the 50-nm thick gold film under TM polarized light at normal incidence.
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Fig. 3. Numerically simulated distribution profiles of electric field (|𝑬|) on the section (a) parallel to x-y plane and (b-f) parallel to x-z plane for the five resonance wavelengths under TM polarized light at normal incidence.
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Fig. 4. Schematics of (a) the cross section α parallel to x-y plane and (d) the longitudinal section β parallel to x-z plane. Simulated distribution profiles of surface charge density on section (b) α and (e) β. Distribution of (c) the displacement current density component
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𝑱𝒛 on the cross section and (f) the current density magnitude (|𝑱|) on the longitudinal section. The cross sections for (b) and (c) are at z = 1.05 μm and z = 0.85 μm, respectively.
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Four circulating current loops ( black arrows) are formed along each vertical dielectric gap between the two adjacent gold nanopillars at resonance wavelength λ5 = 1800 nm.
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Fig. 5. Simulated absorption spectra of the designed GNPA-based absorber with various (a) p values while maintaining R = 160 nm, h = 1500 nm, t = 50 nm and various (b) R
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values while maintaining p = 350 nm, h = 1500 nm, t = 50 nm.
Fig. 6. Simulated electric field (|𝑬|) distributions of the designed GNPA-based absorber at mode λ5 on the longitudinal section parallel to x-z plane for different structural parameters of (a) p = 410 nm, R = 160 nm, h = 1500 nm, t = 50nm and (b) p = 350 nm, R = 130 nm, h = 1500 nm, t = 50nm.
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Fig. 7. The dependence of absorption spectra on the height of gold nanopillars. Absorption peaks are red-shifted with absorption rates basically unchanged as the h value increases from 1500 nm to 1600 nm.
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Fig. 8. The incident angle dependence of the multi-narrowband absorber for (a) TM polarized mode and (b) TE polarized mode. It's worth noting that the BFAST plane wave type is taken to do the angle sweeping jobs in our model to make the numerical simulation
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more efficient.
Fig. 1. Schematic image of the multi-narrowband absorber structure. Orthogonally arranged gold nanopillar array with period of p is embedded in PMMA. h, R represent the
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height and radius of gold nanopillars, respectively. t stands for the thickness of Au film. The optimized structural parameters are: h = 1500 nm, R = 160 nm, p = 350 nm, t = 50 nm.
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Fig. 2. Numerically calculated reflectance, transmission and absorption spectra of the
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GNPA-based structure (a) containing or (b) not containing the 50-nm thick gold film
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under TM polarized light at normal incidence.
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Fig. 3. Numerically simulated distribution profiles of electric field (|𝑬|) on the section (a) parallel to x-y plane and (b-f) parallel to x-z plane for the five resonance wavelengths under TM polarized light at normal incidence.
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Fig. 4. Schematics of (a) the cross section α parallel to x-y plane and (d) the longitudinal
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section β parallel to x-z plane. Simulated distribution profiles of surface charge density on section (b) α and (e) β. Distribution of (c) the displacement current density component 𝑱𝒛 on the cross section and (f) the current density magnitude (|𝑱|) on the longitudinal section. The cross sections for (b) and (c) are at z = 1.05 μm and z = 0.85 μm, respectively. Four circulating current loops (black arrows) are formed along each vertical dielectric
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gap between the two adjacent gold nanopillars at resonance wavelength λ5 = 1800 nm.
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Fig. 5. Simulated absorption spectra of the designed GNPA-based absorber with various
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(a) p values while maintaining R = 160 nm, h = 1500 nm, t = 50 nm and various (b) R
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values while maintaining p = 350 nm, h = 1500 nm, t = 50 nm.
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Fig. 6. Simulated electric field (|𝑬|) distributions of the designed GNPA-based absorber at mode λ5 on the longitudinal section parallel to x-z plane for different structural parameters of (a) p = 410 nm, R = 160 nm, h = 1500 nm, t = 50nm and (b) p = 350 nm, R = 130 nm, h = 1500 nm, t = 50nm.
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Fig. 7. The dependence of absorption spectra on the height of gold nanopillars. Absorption
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peaks are red-shifted with absorption rates basically unchanged as the h value increases
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from 1500 nm to 1600 nm.
Fig. 8. The incident angle dependence of the multi-narrowband absorber for (a) TM polarized mode and (b) TE polarized mode. It's worth noting that the BFAST plane wave
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type is taken to do the angle sweeping jobs in our model to make the numerical simulation
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PII: DOI: Reference:
S0030-4018(19)30861-2 https://doi.org/10.1016/j.optcom.2019.124637 OPTICS 124637
To appear in:
Optics Communications
Received date : 31 March 2019 Revised date : 6 September 2019 Accepted date : 24 September 2019 Please cite this article as: Q. Zhong, T. Wang, X. Jiang et al., Near-infrared multi-narrowband absorber based on plasmonic nanopillar metamaterial, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124637. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Near-infrared Multi-narrowband Absorber Based on Plasmonic Nanopillar Metamaterial
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Qingfang Zhonga, Tao Wanga,* , Xiaoyun Jianga, Le Chenga, Ruoqin Yana, and Xing
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Huanga
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China
Corresponding author. E-mail address: [email protected]
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*
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Abstract
A multi-narrowband absorber composed of gold nanopillar metamaterial is numerically designed and investigated in this paper. Full-wave simulations indicate that five distinctive narrow-band absorption peaks with the maximum absorption rate up to 99.3% and the lowest above 80% can be achieved in the near-infrared regime at normal incidence. Polarization dependence over a incident angular range of ± 30° is also researched. We demonstrate the spectroscopic tunability of our absorber by adjusting the
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structural parameters of the gold nanopillar array. Physical mechanism of the multinarrowband absorption is construed as the vertical Fabry-Perot-like gap plasmonic resonances induced by electric and magnetic dipolar polaritons. Compared to the previous researches on multi-band absorbers, we realize no less than five near-infrared narrow
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absorption bands by utilizing just simple configuration with single pattern, which makes considerable application potential in the fields of absorption filtering and spectroscopic sensing.
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Keywords: Multi-band absorber; Metamaterial; Gap plasmonic resonance; Near-infrared
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1. Introduction
Over the past decade, plasmonic metamaterials based on noble metals have attracted
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extensive attention and research due to their unique properties including negative index of refraction [1], sub-wavelength imaging [2] and cloaking [3]. They have been broadly
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employed in fields such as optical filtering [4], modulation [5, 6], biosensing [7-10], thermoelectric acquisition [11] and photodetection [12], which mostly benefits from the localized or non-localized plasmonic resonance modes of metallic nanostructure arrays that forms the plasmonic metamaterials [13]. Plasmonic metamaterial-based absorbers (PMAs) perform well in light absorption thanks to the intrinsic loss of metals. They are generally classified into broadband absorbers and narrowband absorbers. Broadband absorbers are commonly applied for solar cells [14, 15], while narrowband ones are
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widely used in thermal emission manipulation [16, 17], refractive index sensing [18], resonators [19], nano-antennas [20]. In order to broaden the application fields of PMAs, much effort have been made to shift the operating band from the microwave [21-24] and terahertz regime [25-27] to the
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infrared regime [18, 28-32]. And many methods including magnetic dipolar resonance [33, 34], surface lattice resonance [35] and gap plasmon resonance [18, 30] have been employed to achieve single narrowband absorption. Nevertheless, absorbers with just
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single narrowband are still faced with certain limitations in applications such as spectroscopy and imaging [36, 37]. Therefore, dual-band or multi-band absorbers with
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high performance are more desirable. Mao et al. [38] devised a three-layer metalmedium-metal structured absorber composed of elliptical and circular disk arrays as the top-layer to achieve multi-band absorption in the near-infrared frequency. Feng et al. [31]
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reported a mid-infrared dual-band perfect absorber based on an asymmetric T-type plasmonic array. Liu et al. [39] introduced a multi-band absorber utilizing a triple-layer
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dielectric metamaterial structure coupled with metal substrate. Meng et al. [40] designed a five-band terahertz metamaterial absorber using two concentric metallic circular rings as a compact single particle. Wang [41] presented a dual-band absorber formed by only a single square metallic patch and a metallic board separated by a dielectric layer, which utilizes the overlapping of the fundamental resonance and highorder response. However, the vast majority of multi-band absorbers consist of either composite unit-cells or stacked multiple functional layers, which makes fabrication
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processes relatively complex and hence restrict their practical applications. Moreover, most of them operate in the terahertz and microwave range, while near-infrared multiband PMAs are rarely investigated. In this paper, we theoretically propose a near-infrared multi-narrowband absorber by
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employing the gold nanopillar metamaterial. Numerical simulation results demonstrate that this absorber has five distinctive narrowband absorption peaks with high efficiency over a relatively wide incident angular range of ± 30° under both TM and TE polarized
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light, which is attributed to the high-order Fabry-Perot-like gap plasmonic resonance induced by the strong electric and magnetic polaritons excited in the gold nanopillar array.
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The dependence of absorption performance on the height, radius and spacing of gold nanopillars is also investigated, confirming the spectroscopic tunability and selectivity of our structure. Moreover, it should require relatively easy fabrication processes of one-
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step electron-beam-lithography firstly and electrochemical deposition followed [42]. Given the impressive properties and simple composition, we believe that our design has
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potential application in the fields of absorption filtering and spectroscopic sensing.
2. Structure design and simulation model
Figure 1 is a schematic image showing the general structure of our multi-narrowband absorber designed in this work. The absorber appears as a three-layer structure comprised of a gold nanopillar array (GNPA) fully embedded in a dielectric layer and the glass
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substrate, separated by a t = 50 nm thick gold layer. The gold nanopillars with radius of R = 160 nm are orthogonally arranged in the period of p = 350 nm. The height of gold nanotubes, h = 1500 nm, is just equal to the thickness of the host dielectric. Dielectric materials such as SiO2, MgF2, Al2O3 or other polymers are ideal optional substitutes of
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the host medium for that they have relatively low permittivity, which is beneficial for absorbing [43, 44]. Considering the processes of nanofabrication techniques, here we take the photoresist PMMA as the host dielectric. Transverse magnetic (TM, magnetic field
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parallel to y-axis) polarized light is normally irradiated from above the structure as shown in Fig. 1. Full-wave numerical simulation is performed by employing the commercial
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software package of Lumerical FDTD solutions to calculate the reflectance R(λ) and transmittance T(λ) of our structure, hence the absorption rate A(λ) equals to 1 - R (λ) T(λ). In our model, the refractive index of PMMA is set to be 1.49, and the permittivity
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of gold determined by the Drude model [28] is given by ε(Au) = 1 − 𝜔𝑝2 ⁄𝜔(𝜔 + 𝑖𝜔𝑐 ), where 𝜔𝑝 = 1.37 × 1016 rad/s , 𝜔𝑐 = 4.08 × 1013 rad/s are the plasma frequency
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and collision frequency of gold, respectively. Additionally, we set the four boundary planes that are perpendicular to the x-y plane as periodic boundary conditions, while the other two parallel to the x-y plane are set to be PML boundary conditions.
3. Results and discussion
As can be seen from Fig. 2(a), the numerically calculated transmission of our structure
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is nearly zero, that is T(λ) ≈ 0, in the simulation wavelength range of 0.8 μm to 2 μm, for the reason that the thickness of middle gold layer is larger than the skin depth of gold in the near-infrared regime. Thus the absorption A(λ) is equal to 1 - R(λ) to some extent. Five distinctive sharp absorption peaks at the center wavelengths of λ1= 876 nm, λ2 = 998
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nm, λ3 = 1166 nm, λ4 = 1411 nm, λ5 = 1800 nm with efficient absorption rate of 97.6%, 99.3%, 94.9%, 88.7%, 82.3% are obtained, respectively. Moreover, it is worth noting that the five resonant absorption peaks have significantly narrow full-width at half-maximum
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(FWHM) of 10 nm, 15 nm, 22 nm, 35 nm, 56 nm, respectively. Hence the quality factors Q (ratio of resonance wavelength to FWHM) are 87.6, 66.5, 53, 40.3, 32.1, respectively.
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Such narrow absorption bandwidths are impressively smaller than that of the general plasmonic structure-based metamaterial absorbers [34, 35], which benefits a lot to spectroscopic detection and sensing area [34]. Additionally, to demonstrate the necessity
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of the middle gold film, we further calculate the reflectance and transmission of our structure with the 50 nm thick gold film removed. As depicted in Fig. 2(b), even though
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there still exist several absorption peaks, the absorption efficiencies decrease obviously at the near-infrared resonance wavelengths, for the reason that a considerable part of incident electromagnetic wave transmits through the structure in the absence of the gold film, instead of being reflected by the thin gold mirror. In order to intuitively know the working status of our structure at resonance wavelengths, we simulate the distributions of electric field for the five resonance modes. As shown in Figure 3, electric field is periodically confined in the dielectric gap between
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adjacent gold nanopillars along z-axis, which indicates that different orders of vertical Fabry-Perot-like gap plasmonic resonance (GPR) modes [18, 30, 45] are excited by the incident electromagnetic wave coupling with the GNPA and the middle gold film. Consequently, resonant absorption peaks with narrow bandwidths is obtained for that
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large amount of energy at the resonance wavelength is concentrated inside the structure with just little reflected. From the perspective of physical mechanism, the GPR mode mentioned above is
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attributed to the electric dipole resonance based on the localized surface plasmon polaritons of single gold nanopillar and the magnetic dipole resonance excited in the gold
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nanopillar array. Furthermore, to explicitly understand the principles of the electric and magnetic resonance, we numerically investigate the distributions of both surface charge density and displacement current density in the GNPA at resonance wavelength of λ5 =
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1800 nm as a representative. Surface charge density distributions on the cross section α (parallel to x-y plane) and longitudinal section β (parallel to x-z plane) are pictured in Fig.
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4(b) and Fig. 4(e), respectively. It is seen that vertically alternating arrangements of positive and negative charge along z-axis are formed on both opposite sides of gold nanopillars. These dipole-like positive-negative charge pairs are known as plasmon polaritons resulting from the localized surface plasmon resonance excited by the incident electromagnetic wave. Meanwhile, from the distributions of displacement current density depicted in Fig. 4(c) and Fig. 4(f), one can see that the surface currents induced by electric dipoles flow anti-parallelly on the opposite sides of the vertical dielectric gap at the same
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height. Consequently, several circulating current loops are formed along the gap, indicating a strong magnetic dipole resonance. Hence strong confinement and enhancement of electromagnetic field is drived in the gap due to the coupling between magnetic dipole induced moment and the incident light wave at resonance wavelength.
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We further investigate the structural parameter sensitivity of the designed multinarrowband absorber. Fig. 5(a) and Fig. 5(b) describe the simulated absorption spectra in the near-infrared regime with the period of GNPA increasing from 350 nm to 410 nm
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(with R = 160 nm, h = 1500 nm) and the radius of gold nanopillars decreasing from 160 nm to 130 nm (with p = 350 nm, h = 1500 nm), respectively. It is intuitively seen that all
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the five resonant absorption peaks are blue-shifted as p value increases or R value decreases, and there consequently appears just four absorption peaks in our simulation wavelength range when the period is larger than 390 nm or the radius less than 140 nm.
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Meanwhile, absorption efficiencies at resonance wavelengths decline apparently, which is related to the weaker gap plasmonic resonances in GNPA. More specifically, either
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larger period or smaller radius makes a wider vertical gap between the adjacent gold nanopillars, which weakens the magnetic dipole resonance. In other words, the coupling between magnetic moment and incident electromagnetic wave tends to be feebler at such status compared with the optimized one. For better understand, electric field distributions at mode λ5 on the longitudinal section parallel to x-z plane for the parameters of p = 410 nm (with R = 160 nm, h = 1500 nm) and R = 130 nm (with p = 350 nm, h = 1500 nm) are drawn in Fig. 6(a) and Fig. 6(b), respectively. It is obviously seen that the electric field
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confined in the wide gaps is much weaker than half of that confined in narrow gaps (with p = 350 nm, R = 160 nm, h = 1500 nm), thus more than half of the incident electromagnetic energy is reflected, resulting in undesirable absorption efficiencies. In addition, we also examine the influence of the height of gold nanopillars on the absorption
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spectrum. As can be seen in Figure 7, the five absorption peaks shift towards long wavelength with absorption efficiency basically unchanged as the h value increases from 1500 nm to 1600 nm while maintaining the R = 160 nm and p = 350 nm, indicating a
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performance of spectroscopic tunability. This can be explained by that higher vertical dielectric gaps come into being as the height h increases, which allows gap plasmonic
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resonance at longer wavelengths [16].
The incident angle dependence and polarization dependence of the designed multinarrowband absorber are also numerically investigated. As shown in Fig. 8(a) and Fig.
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8(b), five distinctive narrow red strips representing for the narrowband resonant absorption peaks are obtained over the incident angular range of ± 30° for both TM
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( magnetic field parallel to y-axis) and TE (electric field parallel to y-axis) polarized light. Unsurprisingly, the absorber has the same resonance wavelengths under TM and TE polarization modes at normal incidence due to the symmetrical arrangement of GNPA. From Fig. 8(a), one can see the perfect angle insensitivity of the absorber under TM mode for that the absorption strips go straightly as incident angle increases to 30°, with absorption efficiency nearly invariable. While the absorption strips under TE mode bend to short wavelengths slightly as depicted in Fig. 8(b), for the reason that the magnetic
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field component perpendicular to gold nanopillars decreases upon increasing incident angle, which makes the center wavelengths where magnetic resonances are excited shift slightly [22].
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4. Conclusion
In conclusion, a design of near-infrared multi-narrowband absorber comprised of gold
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nanopillar metamaterial has been investigated. Five distinct absorption peaks with the maximum absorption rate up to 99.3% and the lowest above 80% are obtained in the nearinfrared regime at normal light incidence. The narrowband light absorbing performance
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is demonstrated to be related to the high-order Fabry-Perot-like gap plasmonic resonance induced by the strong electric and magnetic dipole resonance excited in the gold
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nanopillar array. The overlapping of different orders of GPR modes makes the multinarrowband absorption. Moreover, simulation results imply that both the position and strength of absorption peaks are sensitive to structural parameters, indicating the spectroscopic tunability and selectivity of our structure. Research for the angle and polarization sensitivity shows that the absorption spectra remain nearly invariable over the incident angular range of ± 30° under TM polarized mode, and the expected polarization-independence at normal incidence is also proved. Considering the impressive
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absorption properties and simple structural composition, our design has potential application in the fields of absorption filtering and spectroscopic sensing.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC)
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(Grant NoS. 61775064) and the Fundamental Research Funds for the Central Universities (HUST: 2016YXMS024).
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Figure Captions
Fig. 1. Schematic image of the multi-narrowband absorber structure. Orthogonally
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arranged gold nanopillar array with period of p is embedded in PMMA. h, R represent the height and radius of gold nanopillars, respectively. t stands for the thickness of Au film.
nm.
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The optimized structural parameters are: h = 1500 nm, R = 160 nm, p = 350 nm, t = 50
Fig. 2. Numerically calculated reflectance, transmission and absorption spectra of the GNPA-based structure (a) containing or (b) not containing the 50-nm thick gold film under TM polarized light at normal incidence.
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Fig. 3. Numerically simulated distribution profiles of electric field (|𝑬|) on the section (a) parallel to x-y plane and (b-f) parallel to x-z plane for the five resonance wavelengths under TM polarized light at normal incidence.
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Fig. 4. Schematics of (a) the cross section α parallel to x-y plane and (d) the longitudinal section β parallel to x-z plane. Simulated distribution profiles of surface charge density on section (b) α and (e) β. Distribution of (c) the displacement current density component
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𝑱𝒛 on the cross section and (f) the current density magnitude (|𝑱|) on the longitudinal section. The cross sections for (b) and (c) are at z = 1.05 μm and z = 0.85 μm, respectively.
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Four circulating current loops ( black arrows) are formed along each vertical dielectric gap between the two adjacent gold nanopillars at resonance wavelength λ5 = 1800 nm.
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Fig. 5. Simulated absorption spectra of the designed GNPA-based absorber with various (a) p values while maintaining R = 160 nm, h = 1500 nm, t = 50 nm and various (b) R
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values while maintaining p = 350 nm, h = 1500 nm, t = 50 nm.
Fig. 6. Simulated electric field (|𝑬|) distributions of the designed GNPA-based absorber at mode λ5 on the longitudinal section parallel to x-z plane for different structural parameters of (a) p = 410 nm, R = 160 nm, h = 1500 nm, t = 50nm and (b) p = 350 nm, R = 130 nm, h = 1500 nm, t = 50nm.
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Fig. 7. The dependence of absorption spectra on the height of gold nanopillars. Absorption peaks are red-shifted with absorption rates basically unchanged as the h value increases from 1500 nm to 1600 nm.
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Fig. 8. The incident angle dependence of the multi-narrowband absorber for (a) TM polarized mode and (b) TE polarized mode. It's worth noting that the BFAST plane wave type is taken to do the angle sweeping jobs in our model to make the numerical simulation
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Fig. 1. Schematic image of the multi-narrowband absorber structure. Orthogonally arranged gold nanopillar array with period of p is embedded in PMMA. h, R represent the
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height and radius of gold nanopillars, respectively. t stands for the thickness of Au film. The optimized structural parameters are: h = 1500 nm, R = 160 nm, p = 350 nm, t = 50 nm.
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Fig. 2. Numerically calculated reflectance, transmission and absorption spectra of the
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GNPA-based structure (a) containing or (b) not containing the 50-nm thick gold film
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under TM polarized light at normal incidence.
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Fig. 3. Numerically simulated distribution profiles of electric field (|𝑬|) on the section (a) parallel to x-y plane and (b-f) parallel to x-z plane for the five resonance wavelengths under TM polarized light at normal incidence.
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Fig. 4. Schematics of (a) the cross section α parallel to x-y plane and (d) the longitudinal
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section β parallel to x-z plane. Simulated distribution profiles of surface charge density on section (b) α and (e) β. Distribution of (c) the displacement current density component 𝑱𝒛 on the cross section and (f) the current density magnitude (|𝑱|) on the longitudinal section. The cross sections for (b) and (c) are at z = 1.05 μm and z = 0.85 μm, respectively. Four circulating current loops (black arrows) are formed along each vertical dielectric
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gap between the two adjacent gold nanopillars at resonance wavelength λ5 = 1800 nm.
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Fig. 5. Simulated absorption spectra of the designed GNPA-based absorber with various
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(a) p values while maintaining R = 160 nm, h = 1500 nm, t = 50 nm and various (b) R
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values while maintaining p = 350 nm, h = 1500 nm, t = 50 nm.
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Fig. 6. Simulated electric field (|𝑬|) distributions of the designed GNPA-based absorber at mode λ5 on the longitudinal section parallel to x-z plane for different structural parameters of (a) p = 410 nm, R = 160 nm, h = 1500 nm, t = 50nm and (b) p = 350 nm, R = 130 nm, h = 1500 nm, t = 50nm.
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Fig. 7. The dependence of absorption spectra on the height of gold nanopillars. Absorption
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peaks are red-shifted with absorption rates basically unchanged as the h value increases
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from 1500 nm to 1600 nm.
Fig. 8. The incident angle dependence of the multi-narrowband absorber for (a) TM polarized mode and (b) TE polarized mode. It's worth noting that the BFAST plane wave
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type is taken to do the angle sweeping jobs in our model to make the numerical simulation
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of
more efficient.
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