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Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings
Optics Communications xxx (xxxx) xxx
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Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings Wenqiang Wan a , Minghui Luo b , Yanfeng Su c ,∗ a
School of Science, East China Jiaotong University, Nanchang 330013, China SVG Optronics, Co., Ltd, Suzhou 215026, China c College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China b
ARTICLE
INFO
Keywords: Broadband absorber Polarization-insensitive Metagratings
ABSTRACT In this paper, we design and numerically investigate an ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings. The absorber, consisting of a dielectric grating grown in a metallic substrate with a vertically metallic covering, presents an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal incidence in the visible regime, respectively. For the incident angle ranges from 0◦ to 45◦ , the high absorption is almost unchanged for both polarizations. Moreover, the physical mechanism of broadband absorption is investigated. The strong absorption effect is derived from the excited SPR and CMR in the uniform nanocavity for TM polarization and TE polarization, respectively. Compared with those metasurface absorbers using a 2D array or 1D complicated structures, our design is efficient for both TM and TE polarizations using a straightforward 1D structure that provides excellent optical performance.
1. Introduction Since Landy et al. proposed a near-perfect absorber at 11.5 GHz in 2008, perfect absorber designs have attracted substantial attentions for various applications, such as sensors, photodetectors, solar cells, and thermal emitters [1–12]. Conventional absorbers are highly vulnerable to longstanding ultraviolet irradiation using absorbing materials, which results in significant performance degradation [13]. Absorbers based functional nanostructures, employing an interaction between light and structures, have been developed to realize near-perfect absorption in microwave to visible frequency that exhibit high efficiency and high stability. However, most of the previously structural absorbers reflect large amounts of incident light within a broad wavelength range, leading to a narrow-band absorption [14–19]. A broadband absorber in the visible regime is crucial for energy harvesting applications, such as thermophotovoltaic devices and solar cells. In recent years, the metasurfaces were utilized to realize broadband, polarization-independent, and omnidirectional near-perfect absorption by the intrinsic electronic and magnetic resonances, including surface plasmon resonance (SPR), magnetic resonance (MR), cavity-mode resonance (CMR), Mie resonance or hybrid resonance [20–29]. By integrating varied period into one unit, multiple resonant frequencies are excited simultaneously to permit broadband absorption. Wang et al. presented a perfect absorber with double-sized tungsten patch arrays assembled into one unit cell [20], which realized an average absorption of 88% in the visible and near-infrared region. Fountaine et al. proposed an absorber
consisting of multi-radii nano-wire arrays to realize 90% absorption in the spectrum ranges from 450 nm to 900 nm by mixing multiple resonators at different wavelengths [21]. However, these absorbers invariably involve two-dimensional (2D) elaborate patterns that require complicated fabricating methods to perform them at the subwavelength scale. One-dimensional (1D) metasurfaces have been demonstrated to provide a broadband absorption in the visible regime [30–32]. Nevertheless, the polarization-dependent and complex designs limit the practical applications. In this paper, we propose an ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, where grating ridges are covered with a metallic material. The broadband absorption effect of the proposed structure is numerically investigated. In the wavelength ranges from 400 nm to 700 nm, we achieve an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal incidence, respectively. To understand the physical mechanism of the broadband near-perfect absorption behind the proposed structure, we explore the electromagnetic field distributions at the resonant wavelength. The strong absorption effect is derived from the SPR and CMR in the nanocavity for TM polarization and TE polarization, respectively. Moreover, the proposed device remains high absorption with the angle of incident varying from 0 to 45◦ . Compared with those metasurface absorbers using a 2D array or 1D complicated structures, our design is efficient for both TM and TE polarizations using the straightforward 1D structure that provides excellent optical performance.
∗ Corresponding author. E-mail address: [email protected] (Y. Su).
https://doi.org/10.1016/j.optcom.2019.124857 Received 9 September 2019; Received in revised form 23 October 2019; Accepted 30 October 2019 Available online xxxx 0030-4018/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
W. Wan, M. Luo and Y. Su
Optics Communications xxx (xxxx) xxx
Fig. 1. (a) Schematic view of the proposed broadband absorber. (b) Front view of the absorber. (c) Calculated absorption spectra of the proposed absorber for TM and TE polarizations at normal incidence. (d) The absorption spectra of our design for different polarization angle ranging from 0◦ to 90◦ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Calculated H field and E field intensity distributions at normal incidence for (a) TM polarization at 476 nm and (b) TE polarization at 482 nm, respectively. Calculated Poynting vector distributions at normal incidence at wavelengths of (c) 476 nm for TM polarization and (d) 482 nm for TE polarization.
2. Structural design and results
field distributions at the peak of 476 nm for TM polarization. As plotted in Fig. 2(a), the light field is mainly localized at the bottom surface of the Si3 N4 gratings. The strong absorption is attributed to the excitation of the SPR, which results in decreasing the reflection and enhancing the absorption of the absorber. For TE polarization, the total electric field distributions at the wavelength of 482 nm are mapped in Fig. 2(b). It is obvious that the field is trapped in the Si3 N4 gratings, which means the CMR is excited by the interference effects of standing waves formed between the metallic Ni mirrors. The outstanding resonance is responsible for the weak reflection and high absorption at the peak wavelength. With the intrinsic absorption from the Ni, the near-perfect absorption can be achieved in the visible regime for TE polarization. To further understand the physical mechanism of the device, the Poynting vector distributions at the resonant wavelengths for both polarizations are investigated. As demonstrated in Fig. 2(c), for TM polarization at resonant wavelength of 476 nm, the incident flow bends and propagates toward the bottom surface of the Si3 N4 grating. Since the TM mode guided in the interface between the Si3 N4 grating and the Ni substrate does not have a cutoff frequency, the energy flow squeezes into the bottom surface of the grating where the SPR is excited. For TE polarization, Fig. 2(d) displays its Poynting vector distributions, where the incident flow squeezes into the Ni covering and the Si3 N4 cavity, which are attributed to the strongly attenuating properties of Ni and the excitation of CMR, respectively. Finally, the enhanced energy will be dissipated by the Ohmic losses of Ni for both of TM and TE polarizations. Therefore, the designed structure can permit high absorption with polarization-independent in the visible regime. Next, we explore the absorption spectra dependence on the metagrating period (p), duty cycle (f ), and the top layer thickness (h), as shown in Fig. 3. The absorption intensity as a function of p and f is plotted in Fig. 3(a)–(b) for TM polarization. It is obvious that the absorption intensity of the proposed structure strengthens first and then weakens over the entire visible wavelength when the p and f increase. For TE polarization, an adjustable spectral bandwidth can be achieved by gradually tuning the p while preserving the same resonant wavelength, as shown in Fig. 3(d). In Fig. 3(e), as the f increases, the peak absorption red-shifted. Moreover, the bandwidth broadens first and then narrows when the h increases for both polarizations, as shown in Fig. 3(c) and (f). From the above, it is found that the p, f and h are optimized as 220 nm, 0.42, and 13 nm, respectively, in order to achieve broadband near-perfect absorption.
Fig. 1(a)–(b) show the schematic view of the proposed broadband near-perfect absorber. The overall structure of the proposed absorber is a rectangular metagrating composed of a silicon nitride (Si3 N4 ) grating grown in a metallic substrate with a vertically metallic covering, which is characterized by the metagrating period p, the ridge width w, the Si3 N4 grating thickness d, and the metallic covering thickness h. The metal nickel (Ni) is selected as the metallic material since its high melting point and strongly attenuating properties. Here, simulations were completed by the finite difference time domain (FDTD) arithmetic [33]. In simulations, the index of the Ni is derived from the Palik [34], and the index of the Si3 N4 layer is set as 2. A plane beam is an incident from the top side of the device. In this structure, the thickness of the bottom Ni layer is set to be 100 nm which is large enough to block the transmission. Then, the total absorption is calculated by 1-R, where R is the total reflectance which can be obtained from the numerical simulation. The parameters of the proposed absorber are optimized to be 𝑝 = 220 nm, 𝑤 = 92 nm, 𝑑 = 70 nm, and ℎ = 13 nm. Fig. 1(c) presents the calculated absorption spectra at normal incidence for the wavelength ranging from 300 nm to 1000 nm. As we can see from Fig. 1(c), absorptions still reach above 0.6 at 1000 nm for both polarizations. Especially, an average absorption of 0.975 for TM polarization (red curve) is obtained across the wavelength range of 400 nm-700 nm, where the peak absorption reaches 0.987 at the resonant wavelength of 476 nm. For TE polarization, the blue curve represents an average absorption of 0.911 in the visible range of 400 nm–700 nm while the peak absorption is 0.996 at 482 nm. The broadband absorption over the visible regime is enhanced by the peak resonances for both TM and TE polarizations. The absorption spectra of our design for different polarization angle are shown in Fig. 1(d). With the polarization angle ranges from 0◦ to 90◦ , the absorption remains high efficiency (above 75%) over the entire visible wavelength. Those results indicate that the proposed absorber exhibits polarization-insensitive, broadband absorption employing a 1D structure. 3. Analysis and discussions To order to reveal the physical mechanism of the broadband nearperfect absorption, we investigate how the light is absorbed in the device based rectangular metagratings. We calculate the total magnetic 2
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
W. Wan, M. Luo and Y. Su
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Fig. 3. The dependence of the thickness of (a) the metagrating period, (b) duty cycle, and (c) the top layer thickness on the absorption for TM polarization. The dependence of the thickness of (d) the metagrating period, (e) duty cycle, and (f) the top layer thickness on the absorption for TE polarization.
To verify the selection of Si3 N4 as the dielectric layer, the relationship map between the absorption spectra and the refractive index of the dielectric grating is shown in Fig. 4(a)–(b). It is seen that the refractive index of Si3 N4 is the best choice for the proposed design, where the absorption is very high in the visible regime for both polarizations. Fig. 4(c)–(d) show the absorption spectra dependence on the Si3 N4 grating thickness (d). As the d increases, the absorption intensity strengthens first and then weakens in the visible wavelength for TM polarization. For TE polarization, the bandwidth broadens first and then narrows when the d increases. Therefore, the d is optimized as 70 nm for the proposed structure. In addition, Fig. 5 displays the calculated absorption spectra with various incident angle. As shown in Fig. 5(a), the proposed device remains high absorption (above 80%) with the angle of incident varying from 0 to 45◦ for TM polarization. As the incident angle increases, a significant red-shifted at the resonant peak (dotted circle) can be observed, which is consistent with the effect of the SPR. The SPR can be characterized by the following dispersion equation: √ 𝜀𝑑 𝜀𝑚 sin 𝜃𝑖 1 𝑤 𝑘𝑥 = 2𝜋( + )= (1) 𝜆 𝑝 𝑐 𝜀𝑑 + 𝜀𝑚
Fig. 4. The relationship map between the absorption spectra and the refractive index of the dielectric grating for (a) TM polarization and (b) TE polarization. The dependence of the thickness of the Si3 N4 grating on the absorption for (c) TM polarization and (d) TE polarization.
where 𝑘𝑥 is the light vector component along the metagrating surface; 𝜆 is the wavelength of incident beam; 𝜃𝑖 denotes the incident angle; p is the metagrating period; w and c denote the angular frequency and speed for the incident beam, respectively; 𝜀𝑑 and 𝜀𝑚 are the dielectric constants of the dielectric and metal, respectively [35]. For TE polarization, Fig. 5(b) shows that the absorption peak (dotted circle) is almost not affected by the incident angle of light, where is in accord with the angle insensitive deriving from the excitation of CMR. The average absorption still reaches 0.785 in the visible regime for the incident angle up to 45◦ . Thus, the absorber can present a wide angular tolerance that is essential for practical applications. To confirm the superiority of the proposed design, we propose a feasible fabrication procedure, as drawn in Fig. 6. Firstly, photolithography can be employed to produce grating patterns on the photoresist layer. Then, a Ni film will be deposited on the grating patterns using a magnetron sputtering tool. After lift-off processes, a Ni grating can be obtained. Finally, etching can be adopted to form the Si3 N4 grating with a Ni covering on the Ni substrate.
Fig. 5. Absorption spectra as a function of wavelength and angle of incidence (0◦ to 60◦ ) for (a) TM and (b) TE polarizations . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
wavelength ranges from 400 nm to 700 nm, an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal
4. Conclusions
incidence are realized. Moreover, the proposed design remains high In summary, an ultrathin polarization-insensitive, broadband visible absorber using rectangular metagratings has been numerically investigated. Different electromagnetic modes (SPR and CMR) are excited in a uniform nanocavity for both TM and TE polarizations. In the
absorption for the incident angle of light ranging from 0 to 45◦ . Finally, we propose a feasible fabrication procedure to confirm the superiority of the proposed design. It is believed that our device can provide useful 3
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
W. Wan, M. Luo and Y. Su
Optics Communications xxx (xxxx) xxx
Fig. 6. A schematic view of the proposed fabrication processes.
guidelines for the design of 1D broadband absorbers providing excellent optical performance.
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Acknowledgments The present study was supported by the Natural Science Foundation of China (NSFC) (61905077), Jiangxi Provincial Natural Science Foundation of China (No. 20192BAB217014), Foundation of jiangxi educational committee, China (No. GJJ180355), the Zhejiang Lab’s International Talent Fund, China for Young Professionals. We thank the School of Optoelectronics Science and Engineering, Soochow University and the SVG Optronics Corporation for the numerically simulated support. References [1] N.I. Landy, S. Sajuyigbe, J.J. Mock, D.R. Smith, W.J. Padilla, Perfect metamaterial absorber, Phys. Rev. Lett. 100 (20) (2008) 207402. [2] C. Sabah, F. Dincer, M. Karaaslan, E. Unal, O. Akgol, E. Demirel, Perfect metamaterial absorber with polarization and incident angle independencies based on ring and cross-wire resonators for shielding and a sensor application, Opt. Commun. 322 (2014) 137–142. [3] C. Qiu, J. Wu, R. Zhu, L. Shen, B. Zheng, Dual-band near-perfect metamaterial absorber based on cylinder MoS2-dielectric arrays for sensors, Opt. Commun. 451 (2019) 226–230. [4] M.K. Akhlaghi, E. Schelew, J.F. Young, Waveguide integrated superconducting single-photon detectors implemented as near-perfect absorbers of coherent radiation, Nature Commun. 6 (2015) 8233. [5] H. Kwon, J.B. You, Y. Jin, K. Yu, Ultra-compact silicon waveguide-integrated schottky photodetectors using perfect absorption from tapered metal nanobrick arrays, Opt. Express 27 (12) (2019) 16413–16424. [6] Y. Zheng, P. Chen, H. Yang, J. Ding, Y. Zhou, Z. Tang, X. Zhou, Z. Li, N. Li, X. Chen, W. Lu, High-responsivity and polarization-discriminating terahertz photodetector based on plasmonic resonance, Appl. Phys. Lett. 114 (9) (2019) 091105. [7] D. Katrodiya, C. Jani, V. Sorathiya, S.K. Patel, Metasurface based broadband solar absorber, Opt. Mater. 89 (2019) 34–41. [8] S. Agarwal, Y.K. Prajapati, Design of broadband absorber using 2-D materials for thermo-photovoltaic cell application, Opt. Commun. 413 (2018) 39–43. [9] L. Cao, P. Fan, A.P. Vasudev, J.S. White, Z. Yu, W. Cai, J.A. Schuller, S. Fan, M.L. Brongersma, Semiconductor nanowire optical antenna solar absorbers, Nano Lett. 10 (2) (2010) 439–445. [10] X. Liu, T. Tyler, T. Starr, A.F. Starr, N.M. Jokerst, W.J. Padilla, Taming the blackbody with infrared metamaterials as selective thermal emitters, Phys. Rev. Lett. 107 (4) (2011) 045901. [11] F.B. Barho, F. Gonzalez-Posada, M. Bomers, A. Mezy, L. Cerutti, T. Taliercio, Surface-enhanced thermal emission spectroscopy with perfect absorber metasurfaces, ACS Photon. 6 (6) (2019) 1506–1514. [12] J. Park, J.H. Kang, X. Liu, S.J. Maddox, K. Tang, P.C. McIntyre, S.R. Bank, M.L. Brongersma, Dynamic thermal emission control with inas-based plasmonic metasurfaces, Sci. Adv. 4 (12) (2018) eaat3163. [13] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. 7 (5) (2014) 1615–1627. 4
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings Wenqiang Wan a , Minghui Luo b , Yanfeng Su c ,∗ a
School of Science, East China Jiaotong University, Nanchang 330013, China SVG Optronics, Co., Ltd, Suzhou 215026, China c College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China b
ARTICLE
INFO
Keywords: Broadband absorber Polarization-insensitive Metagratings
ABSTRACT In this paper, we design and numerically investigate an ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings. The absorber, consisting of a dielectric grating grown in a metallic substrate with a vertically metallic covering, presents an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal incidence in the visible regime, respectively. For the incident angle ranges from 0◦ to 45◦ , the high absorption is almost unchanged for both polarizations. Moreover, the physical mechanism of broadband absorption is investigated. The strong absorption effect is derived from the excited SPR and CMR in the uniform nanocavity for TM polarization and TE polarization, respectively. Compared with those metasurface absorbers using a 2D array or 1D complicated structures, our design is efficient for both TM and TE polarizations using a straightforward 1D structure that provides excellent optical performance.
1. Introduction Since Landy et al. proposed a near-perfect absorber at 11.5 GHz in 2008, perfect absorber designs have attracted substantial attentions for various applications, such as sensors, photodetectors, solar cells, and thermal emitters [1–12]. Conventional absorbers are highly vulnerable to longstanding ultraviolet irradiation using absorbing materials, which results in significant performance degradation [13]. Absorbers based functional nanostructures, employing an interaction between light and structures, have been developed to realize near-perfect absorption in microwave to visible frequency that exhibit high efficiency and high stability. However, most of the previously structural absorbers reflect large amounts of incident light within a broad wavelength range, leading to a narrow-band absorption [14–19]. A broadband absorber in the visible regime is crucial for energy harvesting applications, such as thermophotovoltaic devices and solar cells. In recent years, the metasurfaces were utilized to realize broadband, polarization-independent, and omnidirectional near-perfect absorption by the intrinsic electronic and magnetic resonances, including surface plasmon resonance (SPR), magnetic resonance (MR), cavity-mode resonance (CMR), Mie resonance or hybrid resonance [20–29]. By integrating varied period into one unit, multiple resonant frequencies are excited simultaneously to permit broadband absorption. Wang et al. presented a perfect absorber with double-sized tungsten patch arrays assembled into one unit cell [20], which realized an average absorption of 88% in the visible and near-infrared region. Fountaine et al. proposed an absorber
consisting of multi-radii nano-wire arrays to realize 90% absorption in the spectrum ranges from 450 nm to 900 nm by mixing multiple resonators at different wavelengths [21]. However, these absorbers invariably involve two-dimensional (2D) elaborate patterns that require complicated fabricating methods to perform them at the subwavelength scale. One-dimensional (1D) metasurfaces have been demonstrated to provide a broadband absorption in the visible regime [30–32]. Nevertheless, the polarization-dependent and complex designs limit the practical applications. In this paper, we propose an ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, where grating ridges are covered with a metallic material. The broadband absorption effect of the proposed structure is numerically investigated. In the wavelength ranges from 400 nm to 700 nm, we achieve an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal incidence, respectively. To understand the physical mechanism of the broadband near-perfect absorption behind the proposed structure, we explore the electromagnetic field distributions at the resonant wavelength. The strong absorption effect is derived from the SPR and CMR in the nanocavity for TM polarization and TE polarization, respectively. Moreover, the proposed device remains high absorption with the angle of incident varying from 0 to 45◦ . Compared with those metasurface absorbers using a 2D array or 1D complicated structures, our design is efficient for both TM and TE polarizations using the straightforward 1D structure that provides excellent optical performance.
∗ Corresponding author. E-mail address: [email protected] (Y. Su).
https://doi.org/10.1016/j.optcom.2019.124857 Received 9 September 2019; Received in revised form 23 October 2019; Accepted 30 October 2019 Available online xxxx 0030-4018/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
W. Wan, M. Luo and Y. Su
Optics Communications xxx (xxxx) xxx
Fig. 1. (a) Schematic view of the proposed broadband absorber. (b) Front view of the absorber. (c) Calculated absorption spectra of the proposed absorber for TM and TE polarizations at normal incidence. (d) The absorption spectra of our design for different polarization angle ranging from 0◦ to 90◦ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Calculated H field and E field intensity distributions at normal incidence for (a) TM polarization at 476 nm and (b) TE polarization at 482 nm, respectively. Calculated Poynting vector distributions at normal incidence at wavelengths of (c) 476 nm for TM polarization and (d) 482 nm for TE polarization.
2. Structural design and results
field distributions at the peak of 476 nm for TM polarization. As plotted in Fig. 2(a), the light field is mainly localized at the bottom surface of the Si3 N4 gratings. The strong absorption is attributed to the excitation of the SPR, which results in decreasing the reflection and enhancing the absorption of the absorber. For TE polarization, the total electric field distributions at the wavelength of 482 nm are mapped in Fig. 2(b). It is obvious that the field is trapped in the Si3 N4 gratings, which means the CMR is excited by the interference effects of standing waves formed between the metallic Ni mirrors. The outstanding resonance is responsible for the weak reflection and high absorption at the peak wavelength. With the intrinsic absorption from the Ni, the near-perfect absorption can be achieved in the visible regime for TE polarization. To further understand the physical mechanism of the device, the Poynting vector distributions at the resonant wavelengths for both polarizations are investigated. As demonstrated in Fig. 2(c), for TM polarization at resonant wavelength of 476 nm, the incident flow bends and propagates toward the bottom surface of the Si3 N4 grating. Since the TM mode guided in the interface between the Si3 N4 grating and the Ni substrate does not have a cutoff frequency, the energy flow squeezes into the bottom surface of the grating where the SPR is excited. For TE polarization, Fig. 2(d) displays its Poynting vector distributions, where the incident flow squeezes into the Ni covering and the Si3 N4 cavity, which are attributed to the strongly attenuating properties of Ni and the excitation of CMR, respectively. Finally, the enhanced energy will be dissipated by the Ohmic losses of Ni for both of TM and TE polarizations. Therefore, the designed structure can permit high absorption with polarization-independent in the visible regime. Next, we explore the absorption spectra dependence on the metagrating period (p), duty cycle (f ), and the top layer thickness (h), as shown in Fig. 3. The absorption intensity as a function of p and f is plotted in Fig. 3(a)–(b) for TM polarization. It is obvious that the absorption intensity of the proposed structure strengthens first and then weakens over the entire visible wavelength when the p and f increase. For TE polarization, an adjustable spectral bandwidth can be achieved by gradually tuning the p while preserving the same resonant wavelength, as shown in Fig. 3(d). In Fig. 3(e), as the f increases, the peak absorption red-shifted. Moreover, the bandwidth broadens first and then narrows when the h increases for both polarizations, as shown in Fig. 3(c) and (f). From the above, it is found that the p, f and h are optimized as 220 nm, 0.42, and 13 nm, respectively, in order to achieve broadband near-perfect absorption.
Fig. 1(a)–(b) show the schematic view of the proposed broadband near-perfect absorber. The overall structure of the proposed absorber is a rectangular metagrating composed of a silicon nitride (Si3 N4 ) grating grown in a metallic substrate with a vertically metallic covering, which is characterized by the metagrating period p, the ridge width w, the Si3 N4 grating thickness d, and the metallic covering thickness h. The metal nickel (Ni) is selected as the metallic material since its high melting point and strongly attenuating properties. Here, simulations were completed by the finite difference time domain (FDTD) arithmetic [33]. In simulations, the index of the Ni is derived from the Palik [34], and the index of the Si3 N4 layer is set as 2. A plane beam is an incident from the top side of the device. In this structure, the thickness of the bottom Ni layer is set to be 100 nm which is large enough to block the transmission. Then, the total absorption is calculated by 1-R, where R is the total reflectance which can be obtained from the numerical simulation. The parameters of the proposed absorber are optimized to be 𝑝 = 220 nm, 𝑤 = 92 nm, 𝑑 = 70 nm, and ℎ = 13 nm. Fig. 1(c) presents the calculated absorption spectra at normal incidence for the wavelength ranging from 300 nm to 1000 nm. As we can see from Fig. 1(c), absorptions still reach above 0.6 at 1000 nm for both polarizations. Especially, an average absorption of 0.975 for TM polarization (red curve) is obtained across the wavelength range of 400 nm-700 nm, where the peak absorption reaches 0.987 at the resonant wavelength of 476 nm. For TE polarization, the blue curve represents an average absorption of 0.911 in the visible range of 400 nm–700 nm while the peak absorption is 0.996 at 482 nm. The broadband absorption over the visible regime is enhanced by the peak resonances for both TM and TE polarizations. The absorption spectra of our design for different polarization angle are shown in Fig. 1(d). With the polarization angle ranges from 0◦ to 90◦ , the absorption remains high efficiency (above 75%) over the entire visible wavelength. Those results indicate that the proposed absorber exhibits polarization-insensitive, broadband absorption employing a 1D structure. 3. Analysis and discussions To order to reveal the physical mechanism of the broadband nearperfect absorption, we investigate how the light is absorbed in the device based rectangular metagratings. We calculate the total magnetic 2
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
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Fig. 3. The dependence of the thickness of (a) the metagrating period, (b) duty cycle, and (c) the top layer thickness on the absorption for TM polarization. The dependence of the thickness of (d) the metagrating period, (e) duty cycle, and (f) the top layer thickness on the absorption for TE polarization.
To verify the selection of Si3 N4 as the dielectric layer, the relationship map between the absorption spectra and the refractive index of the dielectric grating is shown in Fig. 4(a)–(b). It is seen that the refractive index of Si3 N4 is the best choice for the proposed design, where the absorption is very high in the visible regime for both polarizations. Fig. 4(c)–(d) show the absorption spectra dependence on the Si3 N4 grating thickness (d). As the d increases, the absorption intensity strengthens first and then weakens in the visible wavelength for TM polarization. For TE polarization, the bandwidth broadens first and then narrows when the d increases. Therefore, the d is optimized as 70 nm for the proposed structure. In addition, Fig. 5 displays the calculated absorption spectra with various incident angle. As shown in Fig. 5(a), the proposed device remains high absorption (above 80%) with the angle of incident varying from 0 to 45◦ for TM polarization. As the incident angle increases, a significant red-shifted at the resonant peak (dotted circle) can be observed, which is consistent with the effect of the SPR. The SPR can be characterized by the following dispersion equation: √ 𝜀𝑑 𝜀𝑚 sin 𝜃𝑖 1 𝑤 𝑘𝑥 = 2𝜋( + )= (1) 𝜆 𝑝 𝑐 𝜀𝑑 + 𝜀𝑚
Fig. 4. The relationship map between the absorption spectra and the refractive index of the dielectric grating for (a) TM polarization and (b) TE polarization. The dependence of the thickness of the Si3 N4 grating on the absorption for (c) TM polarization and (d) TE polarization.
where 𝑘𝑥 is the light vector component along the metagrating surface; 𝜆 is the wavelength of incident beam; 𝜃𝑖 denotes the incident angle; p is the metagrating period; w and c denote the angular frequency and speed for the incident beam, respectively; 𝜀𝑑 and 𝜀𝑚 are the dielectric constants of the dielectric and metal, respectively [35]. For TE polarization, Fig. 5(b) shows that the absorption peak (dotted circle) is almost not affected by the incident angle of light, where is in accord with the angle insensitive deriving from the excitation of CMR. The average absorption still reaches 0.785 in the visible regime for the incident angle up to 45◦ . Thus, the absorber can present a wide angular tolerance that is essential for practical applications. To confirm the superiority of the proposed design, we propose a feasible fabrication procedure, as drawn in Fig. 6. Firstly, photolithography can be employed to produce grating patterns on the photoresist layer. Then, a Ni film will be deposited on the grating patterns using a magnetron sputtering tool. After lift-off processes, a Ni grating can be obtained. Finally, etching can be adopted to form the Si3 N4 grating with a Ni covering on the Ni substrate.
Fig. 5. Absorption spectra as a function of wavelength and angle of incidence (0◦ to 60◦ ) for (a) TM and (b) TE polarizations . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
wavelength ranges from 400 nm to 700 nm, an average absorption of 0.975 for TM polarization and 0.911 for TE polarization at normal
4. Conclusions
incidence are realized. Moreover, the proposed design remains high In summary, an ultrathin polarization-insensitive, broadband visible absorber using rectangular metagratings has been numerically investigated. Different electromagnetic modes (SPR and CMR) are excited in a uniform nanocavity for both TM and TE polarizations. In the
absorption for the incident angle of light ranging from 0 to 45◦ . Finally, we propose a feasible fabrication procedure to confirm the superiority of the proposed design. It is believed that our device can provide useful 3
Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.
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Fig. 6. A schematic view of the proposed fabrication processes.
guidelines for the design of 1D broadband absorbers providing excellent optical performance.
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Please cite this article as: W. Wan, M. Luo and Y. Su, Ultrathin polarization-insensitive, broadband visible absorber based rectangular metagratings, Optics Communications (2019) 124857, https://doi.org/10.1016/j.optcom.2019.124857.