Multispectral Plasmon-Induced Transparency Based on Asymmetric Metallic Nanoslices Array Metasurface
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We propose a 3D metasurface structure with unsymmetrical metallic slices array. The tunable plasmon-induced transparency (PIT) effects and different electric field mode distributions could be realized by modulating the structure parameters and angle of incidence. The radiative and dark elements of the asymmetric metallic slices unit cell structure are analyzed. The transmission spectra and the electric fields distributions are studied by the finite element method (FEM). We demonstrate that PIT phenomena based on those metasurface array structures may have applications as tunable sensors and filters in nanophotonics and integrated optics.
KeywordsMetasurface Metallic slices Plasmon-induced transparency FEM
Nowadays, metamaterials and metasurfaces have attracted more attention in the nanophotonics and plasmonics fields in recent years. Metamaterials can achieve significant novel electromagnetic and optical characteristics such as and negative refraction [1, 2, 3, 4]. Metasurfaces are kinds of 2D metamaterials which thicknesses much smaller than the wavelength of the incident light, enabling flexible manipulation of the basic characteristics of optical waves [5, 6]. Therefore, metasurface has been extraordinarily paid attention by lots of scientists in recent years due to its novel optical properties and potential applications. The fantastic applications based on metasurface structures have been utilized in many fields such as optical holography , superlens [8, 9, 10], refraction and reflection , light propagating, and optical information processing . For each dedicated design metasurface unit cell, the tunable optical properties such as phase, amplitude, and polarization can be easily achieved by elaborately modulating structural geometry parameters [13, 14, 15].
Electromagnetically induced transparency (EIT) is a quantum interference effect that appears when arises from the coupling between coherent optical fields and the states of a material quantum system in three-level atomic systems [16, 17]. In recent years, many research results demonstrate that classical configurations can implement EIT-like effects, which is named plasmon-induced transparency (PIT). Furthermore, there are two different physical ways to achieve the PIT effect . First, PIT arises from the normal-mode splitting into a low-Q resonance (bright eigenmode) induced by its coupling with a high-Q resonance (dark eigenmode) . Second, PIT is achieved by a well-defined phase coupling that can be established between detuned resonances . The transparency window appearing from the phase transformation between the resonance modes and the PIT is achieved only when above two modes at the same frequency [18, 21]. In hence, this effect can be applied for optical storages and information processing, slow light, optical detection, and biosensors [22, 23, 24, 25, 26]. However, the PIT effects are seldom studied in the metasurface array structures which have great potentials in application.
We propose a 3D metasurface structure with unsymmetrical metallic slices array. The controllable PIT effects and different electric field mode distributions could be presented by modulating the structure parameters and angle of incidence. The radiative and dark elements of the asymmetric metallic slices unit cell structure are analyzed. The transmission spectra and the electric fields distributions are studied by the finite element method (FEM). We demonstrate that PIT phenomena based on those metasurface array structures may have applications as tunable sensors and filters in nanophotonics and integrated optics.
Model Design and Theoretical Method
Simulation and Results
Preliminary, we set incidence wavelength λ = 1380 nm, L 1 = 350 nm, W 1 = 100 nm, L 2 = 300 nm, W 2 = 50 nm, thickness of metallic unit cell t = 100 nm, and the shifting distance x is fixed at 120 nm. Figure 2 shows the transmission of single short nanorod (element 2) structure, single long nanorod (element 1) structure, and the hybrid structure of those two nanorods (the PIT phenomena), respectively. The transmission of the single longer nanorod shows that the Lorenz-type resonance  can be found at around λ = 1.16 nm, which result from the directly stimulate between the incident light and the single long nanorod (named the bright mode). However, the element 2 indicates little optical response corresponding to electromagnetic field because it could not be directly excited by incident light, which is named the dark mode. When those two nanorods are arrayed vertically within single unit cell, a PIT transparency window will be achieved in the transmission curve, which rises from the broad transmission valley. It can be indicated that the PIT transparency window results from the destructive interference between the bright mode and the dark mode . Moreover, due to the same values of ω 1 and ω 2, the transmission is noted that the resonant frequencies of those two modes (bright and dark) are approximately same.
Figure 4b shows the transmission spectra with different L 1. Obviously, the PIT responses can be achieved and with the increasing L 1, there are the red shiftings in spectra and lower transmission rates of PIT peaks. Similarly, Fig. 4c shows the transmission spectra of PIT responses and red shifting with increasing length L 2 of element 2. Subsequently, Fig. 4d shows the transmission spectra of various shifting distance x. It is obviously that there is no PIT phenomenon and only the filtering effect when x = 0 nm. Then, PIT responses occur and keep enlargement with increasing shifting distance x.
In conclusion, based on constructive interference between the bright eigenmode and the dark eigenmode, we demonstrate theoretically the PIT phenomena in unsymmetrical metallic slices array metasurface structures by using finite element method. The simulated transmission spectra of PIT peaks are separately and dynamically modulated by varying structure parameters and incident angle, which is in good accordance with the coupled mode theory method analysis. In order to study the transmittance spectra in different corresponding electronic resonant vibration models, the electric field distributions of the unit cell structures at different wavelengths are simulated. These electric field distributions indicate that the bright and dark modes are excited by the localized plasmon polarizations introduced at both ends of the nanorods which lead to the PIT phenomena. We consider that these PIT-based metasurface structures will have important application prospects as tunable sensors and filters in nanophotonics and integrated optics.
This work is supported by the National Natural Science Foundation of China (Grant No. 11504139), the Natural Science Foundation of Jiangsu Province (Grant No. BK20140167), the China Postdoctoral Science Foundation (2017M611693), and the Training Programs of Innovation and Entrepreneurship for Undergraduates of Jiangnan University (Grant No. 2016336Y).
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