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We experimentally demonstrated multiple Fano resonances in optical metamaterials. By combination of two different sized asymmetric-double-bar (ADB) structures, triple Fano resonance was observed in the near-infrared region. In addition to Fano resonance due to anti-phase modes in isolated ADB structures, an anti-phase mode due to coupling among different sized ADBs was observed. Dependence of characteristics of resonances on size difference was also investigated. At specific conditions of size difference, quality factors of three Fano resonances were improved compared with ADB metamaterials consisting of one kind of ADBs. The results will help to realize applications using metamaterial resonators with multiple functionalities and high performance.
© 2016 Optical Society of America
1. Introduction
Engineering Fano resonance in metamaterials is one of the hot topics in the metamaterial community [1,2]. Fano resonance in metamaterials is an interference effect between a bright mode and a dark trapped mode with small radiation loss, resulting in large spectral variation and phase shift with high quality factors (Q-factors) which can be applied to light emitting devices, absorbers, sensors, and so on [2]. Various metamaterial structures with Fano resonance have been proposed and demonstrated in a wide spectral range [2]. To realize Fano resonance in metamaterials, asymmetric structures are often employed [3–7]. Asymmetry of structures enables the excitation of a dark trapped mode by a free space electromagnetic wave, resulting in Fano interference with a bright mode.
Excitation of multiple resonances has particular importance for improvement and expansion of functionalities of metamaterials [8–10]. For example, in the case of sensor devices, by detection of wavelength shifts at multiple resonances, sensing accuracy can be improved. So far, various structures to realize multiple resonances in the range from GHz to the optical region have been proposed and demonstrated [11–18]. In terahertz region, multiple Fano resonances due to combination of two different sized unit cells have been demonstrated by Born et al [15]. By combining two different sized unit cells, Fano resonance occurs not only by trapped modes at individual unit cells but also another mode originates from collective excitation among unit cells.
Demonstration of metamaterials with multiple Fano resonances operating in the optical region is a crucial task for realistic applications. However, structure control of optical metamaterials is rather difficult due to requirement of ultrafine structures in the order of several tens of nanometers. Asymmetric double bar (ADB) metamaterials [19–25], which are composed of only two metal nano bars with slightly different bar lengths, are one of the promising candidates to realize Fano resonance in optical metamaterials due to their structural simplicity. ADB metamaterials exhibit Fano resonance due to interference between in-phase and anti-phase modes in coupled bars. In this study, by utilizing the structural simplicity of ADB metamaterials, we demonstrate multiple Fano resonances in the optical region. Effects of size difference between two kinds of unit cells are also experimentally investigated since size difference between unit cells is an important parameter for collective excitation among unit cells.
2. Design and fabrication
2.1 Design
The designed ADB metamaterials consist of large ADB (LADB) and small ADB (SADB) as shown in Fig. 1. The ADB structure consists of two gold bars having slightly different lengths. By length difference of the coupled bars, an anti-phase mode becomes excitable by a free space wave. ADB structures are arrayed on a silica substrate with a unit cell length of Lx, Ly = 700 nm along each axis. To investigate effects of size difference between the LADB and SADB, structural parameters of SADB are varied by changing size ratio r from 1.00 to 0.71while size of LADB is fixed. Here, r is defined as ratio of structural parameters of the SADB to those of LADB. The structural parameters of LADB and SADB are shown in Fig. 1. In the case of r = 1, structural parameters of the LADB and SADB are the same. Structural parameters of SADB at respective ratio r are summarized in Table 1. The thickness of the bars of all structures is 40 nm.
Fig. 1 Schematics of a designed structure consisting of LADBs and SADBs. Structural parameters of SADB are determined by structural ratio r.
Table 1. Structural parameters of the SADB structures for structural ratio r.
To design multiple Fano resonances in ADB metamaterials, transmission spectra and electric field distributions at the upper surfaces of the bars were simulated by finite difference time domain method (FDTD) [26] using commercial software (Fullwave, Synopsys). The dielectric function of metal determined by Johnson and Christy [27] and the refractive index of silica determined by Malitson [28] were used in the simulations. As shown in Fig. 2(a), in the case of ADB metamaterials with ratio r = 1.00, only one anti-phase mode [Fig. 2(e)] interferes with an in-phase mode [Fig. 2(d)] and exhibits single Fano resonance by the length difference between the coupled bars as indicated by a green dot since metamaterials consist of one kind of ADBs. On the other hand, even if the coupled bars have the same length, single Fano resonance can be observed in the case of r ≠ 1 as shown in Fig. 2(b). At this Fano resonance, phase relations of dipole resonances at the bars are in-phase in one ADB, but anti-phase among different sized ADBs [Fig. 2(g)]. This mode can be considered as anti-phase modes when each ADB is regarded as one dipole. Here, we call this mode as “an extended anti-phase mode.” The extended anti-phase mode has small radiative loss due to anti-phase oscillations among ADBs like anti-phase modes in normal ADBs. Since the extended anti-phase mode is generated by size difference between LADB and SADB, three Fano resonances can be achieved in the case of ADB metamaterials with r = 0.83 as shown in Fig. 2(c). These three modes correspond to the anti-phase mode at SADB [Fig. 2(i)], anti-phase mode at LADB [Fig. 2(j)], and extended anti-phase mode [Fig. 2(k)] indicated by blue, green, and red dots, respectively. The three trapped modes interfere with the broad in-phase mode [Fig. 2(h)], which results in multiple Fano resonances and rich spectral responses.
Fig. 2 (a-c) Simulated transmission spectra of ADB metamaterials. Dots indicate the resonant positions. (a) r = 1.00, l1 = l2. (b) r = 0.83, l1 ≠ l2. (c) r = 0.83, l1 ≠ l2. (d-k) z components of electric field distributions at the surface of the gold bars in resonant conditions. IP and AP stand for in-phase and anti-phase, respectively.
2.2 Fabrication
The ADB metamaterials were fabricated by a lift-off method. The ADB metamaterials were patterned on an electron beam (EB) resist (ZEP520A, Zeon) using an EB lithography system (ELS-G125S, Elionix). A detailed condition of EB lithography we used is the same as described in R. Hokari et al [29] to reduce structural imperfection of the fabricated structures. Each area of the ADB metamaterial array was 100 μm × 100 μm. Next, chromium and gold thin films with thicknesses of 1 and 40 nm, respectively, were deposited by an EB evaporator. The chromium layer was used as an adhesion layer. Finally, the EB resist was removed and structures were formed on a silica substrate.
3. Optical measurements
Figure 3 shows transmission spectra of the fabricated ADB metamaterials measured with a microspectroscope system. An input wave is incident normally to the surface of the substrate from the air side. Polarization of the incident light is along the bars (x-axis in Fig. 1). Scanning electron microscope (SEM) images of the fabricated ADB metamaterials are also shown in Fig. 3. In the case of r = 1.00, resonant dips of an in-phase mode (a black dot) and only one Fano resonance due to an anti-phase mode (a green dot) are observed at around 1200 and 1600 nm, respectively. On the other hand, in the case of r < 1.00, three Fano resonances are observed as indicated by blue, green, and red dots. The blue, green, and red dots correspond to an anti-phase mode at SADBs, an anti-phase mode at LADBs, and an extended anti-phase mode, respectively. Resonant wavelengths of Fano resonances shift with change in r, resulting in rich variations of spectra. Simulated spectra show good agreement with measurements as shown in Fig. 4. Simulated spectra also show the three Fano resonances in the case of r < 1.00 and reproduce the spectral shape and the shifts in the resonant wavelengths observed in measurements.
Fig. 3 Measured transmission spectra of the fabricated ADB metamaterials with multiple Fano resonances. Solid and dashed lines correspond to measured and fitted spectra. Dots indicate the resonant positions. Black, blue, green, and red dots correspond to in-phase modes, anti-phase modes at SADB, anti-phase modes at LADB, and extended anti-phase modes, respectively. SEM images of the fabricated ADB metamaterials for respective r are shown in the right hand side of the spectra.
Fig. 4 Simulated transmission spectra of the designed ADB metamaterials with multiple Fano resonances. Dots indicate the resonant positions. Black, blue, green, and red dots correspond to in-phase modes, anti-phase modes at SADB, anti-phase modes at LADB, and extended anti-phase modes, respectively. Designed structures of ADB metamaterials for respective r are shown in the right side of the spectra.
4. Discussion
To extract resonant wavelengths and Q-factors of Fano resonance, measured transmission spectra T were fitted by the expression using Lorentzian functions as follows,
Here, y0 and Ai are fitting constants. λi and wi are a resonant wavelength and a FWHM of resonant dips, respectively. Fitting were carried out by using fitting parameters that are y0 Ai, λi, and wi. Fitted spectra are indicated by dashed lines in Fig. 3.Dependence of characteristics extracted by fitting on size difference is shown in Fig. 5. Black, blue, green, red lines correspond to characteristics of in-phase modes, anti-phase modes at SADB, anti-phase modes at LADB, and extended anti-phase modes. To compare the characteristics of multiple Fano resonances with single Fano resonance, normal ADB metamaterials consisting of one kind of ADBs were also fabricated and characteristics were extracted by fitting as indicated by gray and orange lines in Fig. 5. The structural parameters of ADB structures in normal ADB metamaterials are the same as those of SADBs with structural ratio r listed in Table 1.
Fig. 5 (a) Resonant wavelengths and (b) Q-factors of resonances as a function of r. IP and AP stand for in-phase and anti-phase, respectively.
Figure 5(a) shows resonant wavelengths of resonances as a function of r. The resonant wavelengths of anti-phase modes at SADB shift to a shorter wavelength side with decrease in r due to change of the sizes of SADBs. In contrast, for anti-phase modes at LADB, resonant wavelengths are almost constant due to size invariance. In the case of the extended anti-phase mode, resonant wavelengths are almost constant as well as anti-phase modes at LADB. This constant behavior is interpreted by electric field concentration around the long bars in LADB at the extended anti-phase mode as shown in Fig. 2(k). Therefore, resonant wavelengths of the extended anti-phase modes are insensitive to change in r.
Q-factors shown in Fig. 5(b) are calculated by the expression λi / wi. In the case of r = 0.77, since anti-phase mode at LADB observed at 1465 nm is too small, resonance dip cannot be fitted and wi cannot be extracted. Q-factors of anti-phase modes show complicated behavior for change in size difference. Figure 5(b) shows that Q-factors of all anti-phase modes are larger than those of in-phase mode due to small radiative loss. Q-factors of anti-phase at SADB (Blue) increase with increase in r while Q-factors decrease in the case of anti-phase at LADB (Green). In the case of extended anti-phase mode, Q-factors are almost constant (Red). Important finding is that every Q-factor of three anti-phase resonances is larger than those of anti-phase mode in normal ADB metamaterials (Orange) when r = 0.83 ~0.91, which means improvement of sensor sensitivity can be achieved by multiple Fano resonances.
5. Conclusions
We experimentally demonstrated multiple Fano resonances in optical metamaterials. By combination of two different sized ADB structures, triple Fano resonance was observed in the near-infrared region. Three Fano resonances resulted from the excitation of anti-phase modes at SADB, antiphase modes at LADB, and extended anti-phase modes. The extended anti-phase mode originated from coupling among two ADBs due to size difference. Dependence of characteristics of resonances on size difference was also investigated. The resonant wavelengths and Q-factors were extracted from fitting by using Lorentzian functions. In the case of r = 0.77 ~0.83, Q-factors of three Fano resonances were larger than those of Fano resonance in normal ADB metamaterials with single Fano resonance, which is applicable to sensor devices using Fano type metamaterials with improved sensitivities operating in the optical region. The results will help to realize applications using metamaterial resonators with multiple functionalities and high performance.
Acknowledgment
A part of this work was supported by MEXT KAKENHI 25109702, JSPS KAKENHI 264080 and MEXT Nanotechnology Platform, and performed in the CINTS and MNC, Tohoku University, Japan.
References and links
1. N. Verellen, Y. Sonnefraud, H. Sobhani, F. Hao, V. V. Moshchalkov, P. Van Dorpe, P. Nordlander, and S. A. Maier, “Fano resonances in individual coherent plasmonic nanocavities,” Nano Lett. 9(4), 1663–1667 (2009). [CrossRef] [PubMed]
2. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef] [PubMed]
3. V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, “Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry,” Phys. Rev. Lett. 99(14), 147401 (2007). [CrossRef] [PubMed]
4. A. Christ, O. J. F. Martin, Y. Ekinci, N. A. Gippius, and S. G. Tikhodeev, “Symmetry breaking in a plasmonic metamaterial at optical wavelength,” Nano Lett. 8(8), 2171–2175 (2008). [CrossRef] [PubMed]
5. F. Hao, Y. Sonnefraud, P. Van Dorpe, S. A. Maier, N. J. Halas, and P. Nordlander, “Symmetry breaking in plasmonic nanocavities: subradiant LSPR sensing and a tunable Fano resonance,” Nano Lett. 8(11), 3983–3988 (2008). [CrossRef] [PubMed]
6. K. Aydin, I. M. Pryce, and H. A. Atwater, “Symmetry breaking and strong coupling in planar optical metamaterials,” Opt. Express 18(13), 13407–13417 (2010). [CrossRef] [PubMed]
7. C. Wu, A. B. Khanikaev, R. Adato, N. Arju, A. A. Yanik, H. Altug, and G. Shvets, “Fano-resonant asymmetric metamaterials for ultrasensitive spectroscopy and identification of molecular monolayers,” Nat. Mater. 11(1), 69–75 (2011). [CrossRef] [PubMed]
8. X. Liu, T. Tyler, T. Starr, A. F. Starr, N. M. Jokerst, and W. J. Padilla, “Taming the blackbody with infrared metamaterials as selective thermal emitters,” Phys. Rev. Lett. 107(4), 045901 (2011). [CrossRef] [PubMed]
9. M. G. Banaee and K. B. Crozier, “Mixed dimer double-resonance substrates for surface-enhanced raman spectroscopy,” ACS Nano 5(1), 307–314 (2011). [CrossRef] [PubMed]
10. Y. Yuan, C. Bingham, T. Tyler, S. Palit, T. H. Hand, W. J. Padilla, D. R. Smith, N. M. Jokerst, and S. A. Cummer, “Dual-band planar electric metamaterial in the terahertz regime,” Opt. Express 16(13), 9746–9752 (2008). [CrossRef] [PubMed]
11. J. Wang, C. Fan, J. He, P. Ding, E. Liang, and Q. Xue, “Double Fano resonances due to interplay of electric and magnetic plasmon modes in planar plasmonic structure with high sensing sensitivity,” Opt. Express 21(2), 2236–2244 (2013). [CrossRef] [PubMed]
12. A. Artar, A. A. Yanik, and H. Altug, “Multispectral plasmon induced transparency in coupled meta-atoms,” Nano Lett. 11(4), 1685–1689 (2011). [CrossRef] [PubMed]
13. A. Artar, A. A. Yanik, and H. Altug, “Directional double Fano resonances in plasmonic hetero-oligomers,” Nano Lett. 11(9), 3694–3700 (2011). [CrossRef] [PubMed]
14. D. Dregely, M. Hentschel, and H. Giessen, “Excitation and tuning of higher-order Fano resonances in plasmonic oligomer clusters,” ACS Nano 5(10), 8202–8211 (2011). [CrossRef] [PubMed]
15. N. Born, I. Al-Naib, C. Jansen, T. Ozaki, R. Morandotti, and M. Koch, “Excitation of multiple trapped-eigenmodes in terahertz metamolecule lattices,” Appl. Phys. Lett. 104(10), 101107 (2014). [CrossRef]
16. M. Miyata, J. Hirohata, Y. Nagasaki, and J. Takahara, “Multi-spectral plasmon induced transparency via in-plane dipole and dual-quadrupole coupling,” Opt. Express 22(10), 11399–11406 (2014). [PubMed]
17. Y. U. Lee, E. Y. Choi, E. S. Kim, J. H. Woo, B. Kang, J. Kim, and T. Y. Byung Cheol Park, “Double Fano resonances in a composite metamaterial possessing tripod plasmonic resonances,” J. Opt. 17(2), 025103 (2015). [CrossRef]
18. S. Hu, H. Yang, S. Han, X. Huang, and B. Xiao, “Tailoring dual-band electromagnetically induced transparency in planar metamaterials,” Appl. Phys. Lett. 117(4), 043107 (2015).
19. Z.-G. Dong, H. Liu, M. X. Xu, T. Li, S. M. Wang, S. N. Zhu, and X. Zhang, “Plasmonically induced transparent magnetic resonance in a metallic metamaterial composed of asymmetric double bars,” Opt. Express 18(17), 18229–18234 (2010). [CrossRef] [PubMed]
20. Z.-G. Dong, J. Zhu, J. Rho, J.-Q. Li, C. Lu, X. Yin, and X. Zhang, “Optical toroidal dipolar response by an asymmetric double-bar metamaterial,” Appl. Phys. Lett. 101(14), 144105 (2012). [CrossRef]
21. X. R. Jin, J. Park, H. Zheng, S. Lee, Y. Lee, J. Y. Rhee, K. W. Kim, H. S. Cheong, and W. H. Jang, “Highly-dispersive transparency at optical frequencies in planar metamaterials based on two-bright-mode coupling,” Opt. Express 19(22), 21652–21657 (2011). [CrossRef] [PubMed]
22. N. E. J. Omaghali, V. Tkachenko, A. Andreone, and G. Abbate, “Optical sensing using dark mode excitation in an asymmetric dimer metamaterial,” Sensors (Basel) 14(1), 272–282 (2013). [CrossRef] [PubMed]
23. F. Zhang, X. Huang, Q. Zhao, L. Chen, Y. Wang, Q. Li, X. He, C. Li, and K. Chen, “Fano resonance of an asymmetric dielectric wire pair,” Appl. Phys. Lett. 105(17), 172901 (2014). [CrossRef]
24. Y. Moritake, Y. Kanamori, and K. Hane, “Experimental demonstration of sharp Fano resonance in optical metamaterials composed of asymmetric double bars,” Opt. Lett. 39(13), 4057–4060 (2014). [CrossRef] [PubMed]
25. Y. Moritake, Y. Kanamori, and K. Hane, “Enhanced quality factor of fano resonance in optical metamaterials by manipulating configuration of unit cells,” Appl. Phys. Lett. 107(21), 211108 (2015). [CrossRef]
26. K. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antenn. Propag. 14(3), 302–307 (1966). [CrossRef]
27. P. B. Johnson and R. W. Christy, “Optical constant of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
28. H. Malitson, “Interspecimen comparison of the refractive index of fused silica,” J. Opt. Soc. Am. 55(10), 1205–1209 (1965). [CrossRef]
29. R. Hokari, Y. Kanamori, and K. Hane, “Fabrication of planar metamaterials with sharp and strong electromagnetically induced transparency-like characteristics at wavelengths around 820 nm,” J. Opt. Soc. Am. B 31(5), 1000–1005 (2014). [CrossRef]
