Article Cite This: ACS Photonics 2018, 5, 2596−2601
Nonradiating Silicon Nanoantenna Metasurfaces as Narrowband Absorbers Chi-Yin Yang,† Jhen-Hong Yang,‡ Zih-Ying Yang,§ Zhong-Xing Zhou,‡ Mao-Guo Sun,† Viktoriia E. Babicheva,∥ and Kuo-Ping Chen*,† †
Institute of Imaging and Biomedical Photonics, National Chiao Tung University, 301 Gaofa 3rd Road, Tainan 711, Taiwan Institute of Photonic System, National Chiao Tung University, 301 Gaofa 3rd Road, Tainan 711, Taiwan § Institute of Lighting and Energy Photonics, National Chiao Tung University, 301 Gaofa 3rd Road, Tainan 711, Taiwan ∥ ITMO University, 49 Kronverksky Avenue, St. Petersburg, 197101, Russia ACS Photonics 2018.5:2596-2601. Downloaded from pubs.acs.org by DURHAM UNIV on 08/09/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: High-refractive-index (HRI) nanostructures support optically induced electric dipole (ED) and magnetic dipole (MD) modes that can be used to control scattering and achieve narrowband absorption. In this work, a highabsorptance device is proposed and realized by using amorphous silicon nanoantenna (a-Si NA) arrays that suppress backward and forward scattering with engineered structures and in particular periods. The overlap of ED and MD resonances, by designing an array with a specific period and exciting lattice resonances, is experimentally demonstrated. The absorptance of a-Si NA arrays increases 3-fold in the near-infrared range in comparison to unpatterned silicon films. Nonradiating a-Si NA arrays can achieve high absorptance with a small resonance bandwidth (Q = 11.89) at a wavelength of 785 nm. The effect is observed not only due to the intrinsic loss of material but by overlapping the ED and MD resonances. KEYWORDS: high-refractive-index (HRI), dielectric nanoantennas, metasurfaces, absorber
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Controlling the directional scattering and enhancing the absorption of silicon nanostructures have recently attracted attention, and the topics of particular interest and potential for successful applications include the cancellation of scattering in the predefined direction and anapole resonance.20,21,25,26 When the intensities of electric and toroidal dipoles are equal in magnitude and out of phase, the energy of scattering in silicon nanostructures would be transferred to absorption because of the destructive interference between the ED and the toroidal dipole; therefore, the absorption would be enhanced and the scattering would decrease, which is also called an anapole. The scattering signals of the anapole mode have been already investigated by dark-field microscopy and near-field scanning optical microscopy in the literature.25,27 However, the anapole mode is usually discussed in the single-particle resonance. Therefore, it would be difficult to apply the anapole resonance in dense arrays or in large-area samples. Large-area random silicon nanophotonic metasurfaces with polarization-independent absorption has been proposed recently, but the device requires prism coupling and total internal reflection, which might limit the application.28 Therefore, in our study, the silicon nanoantenna array with high
arrowband absorbers with high absorption efficiency and high quality factor have gained great attention in recent years, which could be potentially applied in sensors, thermal emitters, and thermophotovoltaics.1−6 In the literature, narrowband absorbers have been realized by metal−oxide−metal configuration,1−4 bilayer nanostructures,5 and Tamm plasmon polaritons,6−8 which are based on the energy confinement in the metals or at the metal−oxide interface. For the plasmonic narrowband absorbers, noble metals, such as gold or silver, are usually utilized to provide the localized electric field resonance. In the literature, plasmonic absorbers could provide high absorbance (>90%).3,9 However, their low melting points make noble metals unsuitable for applications with high temperature and strong light illumination, such as thermal radiation, thermophotovoltaics, or heat-assisted magnetic recording. To overcome the disadvantages of metallic absorbers, high-refractive-index (HRI) dielectric metasurfaces have attracted a lot of attention recently due to their advantages of low nonradiative losses and high melting temperatures. Silicon is a feasible HRI material that has been widely used in nanophotonics, in applications such as solar cells,10,11 photonic waveguides,12,13 semiconductor detectors,14 color filter,15 and metasurfaces.16 In an HRI dielectric nanostructure, electric dipole (ED) and magnetic dipole (MD) resonances have been studied because of the large scattering signals, including directional forward and backward scattering.17−24 © 2018 American Chemical Society
Special Issue: Recent Developments and Applications of Plasmonics Received: October 9, 2017 Published: January 24, 2018 2596
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resonance and to overlap it with the MD resonance by adjusting the lattice periods.37 As has been reported in the literature,26,28,38,39 for a lossless material, when the backward scattering decreases, it is accompanied by an increase in forward scattering. In order to create the perfect absorber, it is necessary to avoid the increasing of forward scattering (i.e., increasing of transmittance). Therefore, amorphous silicon is chosen as the material that has moderate loss to suppress the forward scattering and absorb the energy inside the a-Si NA arrays as shown in Figure 1. In this study, the narrowband absorber of a-Si NA arrays with effectively varying the resonance wavelength of ED and MD is experimentally demonstrated. By controlling the periods of NA arrays, absorption enhancement can be achieved by overlapping the ED and MD resonances. Then, the effect of material loss (refractive index imaginary part) is discussed to distinguish the difference in unidirectional scattering and a nonradiating device. It is concluded that utilizing amorphous silicon makes it possible to increase the absorptance in comparison to unpatterned silicon films and crystalline silicon nanoantenna (c-Si NA) arrays when ED and MD resonances overlap. Simulation and Experimental Results. Finite-difference time-domain (FDTD) simulations have been used to calculate the reflectance, transmittance, and absorptance spectra of a-Si NA arrays. The calculations are performed using Lumerical, a commercially available FDTD simulation software package. In the modeling, a-Si NA arrays are on a glass substrate (n = 1.52) with x-polarized light normally incident from the substrate, and the surrounding medium is immersion oil (n = 1.516). The width of the square a-Si NAs is 155 nm, and the thickness is 142 nm. From the simulation results in Figure 2, by fixing the longitudinal period (Px) at 400 nm and tuning the transverse period (Py), the ED lattice resonance wavelength could be modified significantly, but the resonance wavelength of MD changes slowly.37 The ED and MD resonance modes could be distinguished by Mie theory and also by observing the E-field distributions inside the nanoantenna. When the ED lattice resonance and MD are overlapping, the backward scattering would be suppressed because of the destructive interference of light scattered by ED and MD. This phenomenon has been first proposed by M. Kerker in 1983 (the so-called Kerker effect) and can also be observed in Figure 2(a) when the period is around
Figure 1. Schematic diagram of a silicon narrowband absorber. Directional scattering from a single nanoparticle and near-unit transmission from the nanoparticle array take place in the effectively lossless structure. In contrast, a lossy structure causes a dip in the transmittance.
In this work, the design of high-efficiency narrowband absorbers is proposed by applying the Kerker effect and amorphous silicon nanoantenna (a-Si NA) arrays’ lattice resonance mode.29,30 The destructive interference between ED and MD will result in scattering cancellation in the backward directionthe so-called Kerker effectwhen the magnitude and phase of ED and MD are equal to each other (i.e., the first Kerker’s condition), and in most cases it is achieved when ED and MD resonances are at the same or close wavelength. This phenomenon has been widely discussed in core−shell and HRI nanoparticles to produce cancellation of scattering in a certain direction.31−33 The theoretical prediction of magnetic and electric dipole resonances of silicon nanoparticles and the realization of directional light scattering due to the Kerker effect were first proposed in 2010 and verified experimentally in 2012.22,34 There are a couple of ways to realize the spectral overlap of electric and magnetic multipole resonances. One is by controlling the shapes of dielectric nanoparticles.23,24 The other is by arranging the periodic arrays. ED and MD will experience lattice resonances, which are excited at the wavelength close to the period of the structure.34−36 Recently, it has been theoretically proposed to control the position of the ED
Figure 2. Simulated silicon narrowband absorber (a) reflectance, (b) transmittance, and (c) absorptance spectra with different transverse periods (Py). The a-Si NA arrays are set on a glass substrate (n = 1.52), and the surrounding medium is immersion oil (n = 1.516). The width of the square a-Si NAs is 155 nm, the thickness is 142 nm, and the longitudinal period (Px) is 400 nm. Here, EDLR is the electric dipole lattice resonance. 2597
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Figure 3. SEM images, optical images, and the measured (solid line) results of reflectance (black line) and transmittance (red line) spectra with different transverse periods (a) Py = 380 nm, (b) Py = 480 nm, and (c) Py = 580 nm. The inset optical images are transmission images with a bandpass filter: 785 ± 10 nm.
480 nm.30 The suppression of backward scattering is present by the decrease in reflectance. However, the energy of backward scattering was not transferred into forward scattering (transmission) but absorbed because of the intrinsic loss of amorphous silicon and localized electric fields,40−42 resulting in high absorptance. In the experiment, a-Si NA arrays with three different transverse periods (Py) were fabricated on glass substrates. By changing the Py, the resonance wavelength of ED would be shifted to satisfy the following conditions: (i) the resonance wavelength of ED is shorter than that of MD (Py = 380 nm), (ii) the resonance wavelengths of ED and MD are overlapped (Py = 480 nm), (iii) the resonance wavelength of ED is longer than that of MD (Py = 580 nm). Conditions (i), (ii), and (iii) are shown in Figures 3(a), (b), and (c), respectively. The scanning electron microscopy (SEM) images, transmission images (with a bandpass filter: 785 ± 10 nm), and the measured results are shown in Figure 3. In Figure 3(a), it can be observed that at Py = 380 nm ED and MD resonances are separate and show two transmittance dips in the spectra. In Figure 3(b), ED and MD resonances are overlapped when Py is 480 nm. Due to destructive interference of ED and MD resonances, the backward scattering decreases, which can be observed in the reflectance spectrum. Furthermore, because losses in amorphous silicon are higher than in crystalline silicon,40−42 different from the results in the literature that use crystalline silicon for enhancing the forward scattering,21,26 amorphous silicon will result in low forward scattering.28,38,39 In other words, the a-Si NA arrays result in both low transmittance and reflectance at the resonance wavelength when ED and MD are overlapped. In Figure 3(c), if the Py continues to increase, without the overlapping of ED and MD resonances, the absorptance will decrease. Figure 3 shows a good match between the measurement results and the simulation results. The electric field distribution of a-Si NAs with x-polarized light incident from the glass substrate is shown in Figure 4. When ED and MD resonances overlap, the intensity of the reflected light and transmitted light are both reduced, and the energy is confined, with much stronger electric field enhancement gathering around the a-Si NAs. Comparing Figure 4(a) and
Figure 4. Electric field distributions of (a) nonoverlapping and (b) overlapping resonances. The electric field distribution of ED and MD resonances shown in (a) are related to condition (i) (Py = 380 nm), and those in (b) are related to condition (ii) (Py = 480 nm).
(b), the superposition of electric field distributions inside the particle of each ED and MD would be similar to ED and MD overlapping, which means the phenomenon is linear. The Effect of Material Loss. To discuss the intrinsic loss effect of the materials, in Figure 5, the Si NA arrays and Si films (thickness = 142 nm) are simulated and compared for the cases with different imaginary parts (k) of the refractive index. As mentioned in the previous discussion, the energy of backward scattering would be transferred to forward scattering for a lossless (low-k) particle, and this can be confirmed by Figure 5(a). However, when k gets larger, the energy of forward scattering can be stored inside the Si NAs, behaving like a cavity resonance instead of being scattered out, which results in low transmittance and high absorptance in the high-k area shown in Figure 5(a). The phenomenon that has been discussed above can also be observed in the electric field distributions shown in Figure 5(c). Therefore, because of possessing lower reflectance and trans2598
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Figure 5. Comparison of the reflectance (black line), transmittance (red line), and absorptance (blue) spectra versus different refractive index imaginary part of HRI material from k = 0 to 0.3 at a wavelength of 785 nm. The real part of the refractive index is set as 4.22. (a) Normalized spectra of HRI NA arrays, (b) normalized spectra of HRI films, and (c) total power dissipation density distributions for the cases with different imaginary parts of the refractive index (k).
mittance, Si NA arrays can have around 4 times higher absorptance than a-Si thin films (Figure 5(b)) when k ≈ 0.15. Compared with other surface texturing methods, such as KOH wet etching, Si NA arrays can be made by a-Si thin films, but KOH wet etching can only be applied on single-crystalline silicon.
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CONCLUSIONS The high quality factor absorption resonance has been demonstrated by overlapping ED and MD in this paper. It is worth noting that the strong absorption resonance is achieved not only due to the intrinsic loss of material but also due to the overlaps between ED and MD resonances for the proposed a-Si NAs. The strong localized electric fields inside the a-Si nanostructure cause the absorption enhancement. In addition, we also investigated the relation between the intrinsic material loss and optical responses by comparing the spectra for both a-Si and c-Si NA arrays. As shown in Figure 6, the a-Si NAs arrays show a higher absorptance than the following cases: c-Si NA arrays and a-Si and c-Si films, especially in the near-infrared wavelength range (700−1000 nm). The material loss as well as reflectance (R), transmittance (T), and absorptance (A) of NAs using both amorphous Si and crystalline Si are compared in Table 1. By observing T and A, crystalline silicon can be designed as a unidirectional scattering device because of the high transmittance caused by the low material intrinsic loss,26 but amorphous silicon can be used to effectively suppress both backward and forward scattering and achieve high absorption. The all-dielectric narrowband absorber shows a higher Q-factor than plasmonic absorbers,3 and the device can be further applied to selective narrowband thermal emitters, optical filters, and sensors.
Figure 6. Comparison of the absorptance spectra of a-Si NA arrays (blue solid line), c-Si NA arrays (blue dashed line), a-Si films (light gray area), and c-Si films (gray area). In order to compare the absorptance of a-Si and c-Si NA arrays, the width of square a-Si NAs is set as 155 nm with a thickness of 142 nm. Because the real parts of the refractive index in a-Si and c-Si are different, the width of square c-Si NAs is set as 175 nm with a thickness of 175 nm. The longitudinal periods (Px) of a-Si and c-Si NA arrays are both 400 nm, and the transverse periods (Py) are both 480 nm. Comparing these two cases, the absorptance of a-Si NA arrays is significantly larger than that of c-Si NA arrays. The detailed simulation results are shown in Figure S4.
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EXPERIMENTAL METHODS Fabrication. In the fabrication process, the amorphous silicon films are deposited on the glass substrate by using RF 2599
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Table 1. Comparison of the Material Intrinsic Loss and Optical Response of a-Si and c-Si NA Arrays at the Wavelength 785 nma
a
λ = 785 nm
k
R
T
A
amorphous Si crystalline Si
0.25 0.006
0.16 0.03
0.01 0.86
0.83 0.11
R is reflectance, T is transmittance, and A is absorptance.
sputter. The refractive index and thickness of deposited films are characterized by a spectroscopic ellipsometry apparatus (SENTECH SENDIR). In nanofabrication, the photoresist (PMMA A4) is coated and exposed by electron-beam lithography (ELIONIX, ELS-7500 EX). The 20 nm chromium film is deposited as an etching mask by an e-gun evaporator (ULVAC, VT1-10CE). Finally, an ICP etcher (ELIONIX, EIS700) with etching gas SF6 and C4F8 is used to produce the a-Si NA arrays. The etching mask will be removed by Cr etchant and covered by immersion oil (n = 1.516) after the etching process.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01186. Additional information (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zih-Ying Yang: 0000-0001-6601-1464 Viktoriia E. Babicheva: 0000-0002-0789-5738 Kuo-Ping Chen: 0000-0001-6256-9145 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology (MOST), Taiwan, ROC (MOST 104-2221-E-009130-MY3; MOST 105-2221-E-009-096-MY2).
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