Significantly increased surface plasmon polariton mode excitation using a multilayer insulation structure in a metal–insulator–metal plasmonic waveguide Hongyan Yang,1 Jianqing Li,1,* and Gongli Xiao2 1
Faculty of Information Technology, Macau University of Science and Technology, Macao, China
2
School of Information and Communication, Guilin University of Electronic Technology, Guilin 541004, China *Corresponding author:
[email protected] Received 20 February 2014; revised 29 April 2014; accepted 30 April 2014; posted 2 May 2014 (Doc. ID 206633); published 4 June 2014
In this paper, we propose a novel multilayer insulation structure in a metal–insulator–metal (MIM) plasmonic waveguide to explore the possibility of increasing surface plasmon polariton (SPP) mode excitation. Numerical investigations show that the effective refractive index of the multilayer insulation structure affects symmetric SPP mode excitation. The significant enhancement of electric field intensity in horizontal and vertical profiles with a dipole in SiO2 compared with in Al2 O3 is observed in the proposed MIM plasmonic waveguides due to a combination of the improved optical density and dipole radiation intensities under a low refractive index. The Au∕SiO2 ∕Al2 O3 ∕SiO2 ∕Au geometry shows the best enhancement performances, which can serve as an excellent guideline for designing and optimizing a high-performance SPP source using a multilayer insulation structure. © 2014 Optical Society of America OCIS codes: (230.7390) Waveguides, planar; (240.6680) Surface plasmons; (310.4165) Multilayer design; (160.6030) Silica. http://dx.doi.org/10.1364/AO.53.003642
1. Introduction
Surface plasmon polariton (SPP) modes, yielded by the coupling of electromagnetic waves with the electron oscillations in metal, are bound to a metal– dielectric interface [1]. The metal–insulator–metal (MIM) structure, as shown in Fig. 1(a), supporting coupled SPP modes has attracted a great deal of attention recently [2–5]. It serves as a plasmonic slot waveguide, “squeezing” the SPP field into the dielectric core [6]. SPP mode excitation is a real challenge due to the need of providing the missing momentum between the photon and SPP of the same frequency [7]. Most recently, the light-induced SPP excitation in 1559-128X/14/173642-05$15.00/0 © 2014 Optical Society of America 3642
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a MIM waveguide has been proposed [8]. An electrical excitation of SPP through coupling to an organic light-emitting diode structure was reported by Koller et al. [9]. A year later, Walters et al. proposed a silicon-based electrical SPP source [10]. In particular, Walters et al. theoretically testified that SPP mode excitation with a dipole can occur under a certain condition. These pioneering works have stimulated an intense search for the best possible SPP source. With high demand for SPP source applications in the plasmonic circuitry, the question that has arisen is how to increase SPP mode excitation efficiency. To explore the possibility of creating a new highperformance SPP source, in this paper, multilayer insulation structure response of symmetric SPP mode excitation in a MIM plasmonic waveguide is numerically modeled by using the finite-difference
Fig. 1. (a) Schematic of the MIM plasmonic waveguide. (b) The side structure of the Au∕SiO2 100 nm∕Au plasmonic waveguide and a dipole embedded in the SiO2 layer to excite a symmetric MIM SPP mode for forward propagating. (c) The simulated electric field distribution of the MIM SPP mode.
time-domain (FDTD) method. The size of the electric field intensity in horizontal and vertical profiles of excited MIM SPP modes for single-layer, doublelayer, and triple-layer insulation structures were compared. The enhanced symmetric SPP mode excitation with a dipole in SiO2 nSiO2 ≈ 1.4 compared with in Al2 O3 nAl2 O3 ≈ 1.7 is clearly observed. The Au∕SiO2 ∕Al2 O3 ∕SiO2 ∕Au geometry shows the best enhancement performances. The presented method can be valuable for quantitatively studying SPP mode excitation effects in various multilayer insulation structures, which can be applied in designing a new SPP source. 2. Theoretic Model and the FDTD Modeling Method
The proposed MIM plasmonic waveguide is schematically shown in Fig. 1(b), which is composed of two identical Au films (200 nm) separated by an insulator layers SiO2 (100 nm), and its horizontal length is infinite. The simulation lengths are x from −4 to 4 μm and y from −5 to 5 μm, respectively, taking into account the computation time and the amount of memory for the computer. When the width (W) of the SiO2 layer is reduced below the diffraction limit, conventional light-guiding modes cannot exist. In this case, a transverse electric (TE) 90° polarization incident light (electric dipole) being embedded in the SiO2 layer will be transformed into a symmetric MIM SPP (H z ) mode on the Au surface and propagates along the waveguide. The dispersion relation of symmetric SPP mode could be expressed as [11] pεd 1 − ekw ; kεm 1 ekw
(1)
where s βSPP 2 k k0 − εd ; k0
(2)
s βSPP 2 p k0 − εm ; k0
(3)
and βSPP neff k0 ;
neff
0 p@ εd 1
11 2 λ A ; pq πω −εm 1 −εεdm
(4)
(5)
where k0 is the wave vector in free space, βSPP and neff are the complex propagation constant and the effective refractive index [12] of SPP, respectively. λ is the wavelength in free space. εm and εd are the relative dielectric constant of the metal and the dielectric material, respectively. Gold for the metallic cladding layers is characterized by the Drude model, and the data is in [13]. εm 1 −
ω2p ; ωω iγ
(6)
where ωp 1.2 × 1016 Hz and γ 1.2 × 1014 Hz are the bulk plasma and damping frequencies, respectively. The Au∕SiO2 ∕Au waveguide structure is designed and modeled by a commercial FDTD package. The FDTD method with the perfectly matched layer (PML) as the boundary condition is used in this simulation. The fundamental TM mode Ex ; Ey ; H z is excited by a dipole source located in the SiO2 layer at distance of 10 nm from interface 1. The simulated electric field intensity distribution of the MIM SPP mode is shown in Fig. 1(c). For a view of experimental fabrication, the structure of our devices can be designed as the optically thick Au cladding layers surrounding the SiO2 layer that contains silicon quantum dots [10]. The power monitor (PD), 10 June 2014 / Vol. 53, No. 17 / APPLIED OPTICS
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vertically placed at interface 1 about 1 μm from the dipole source, is set to detect the incident power flow information. 3. Results and Discussion
The horizontal profile of electric field intensity (ixE ) at interface 1 along the x axis with changing nd is calculated for a Au/insulator(100 nm)/Au plasmonic waveguide, with the free space wavelength of the dipole from 0.6 to 1.8 μm, is shown in Fig. 2. It can be found that ixE decreases with increasing nd from 1 to 2 in steps of 0.2, due to the augmentation of optical distance leading to a reduced optical density, which makes the SPP excitation weaker. The MIM SPP resonance mode (peak), which results from the Fabry–Perot interference of SPP within the MIM
Fig. 2. ixE versus wavelength plots of the Au/insulator(100 nm)/Au plasmonic waveguide with changing nd . The inset shows the dependence of ixEpeak and λSPP on nd .
plasmonic waveguide, clearly appears. We have plotted two curves based on the data from the peak of ixE. The electric field peak intensity (ixEpeak ) and resonance wavelength λSPP as a function of nd are shown in the inset of Fig 2. λSPP is redshifted consistently and (ixEpeak ) decays exponentially. The redshift of λSPP results from the increase in neff . According to Eq. (5), neff is a function of εd, where p nd εd . With the augmentation of nd from 1 to 2, neff becomes larger. So, the redshift of peaks appear (λSPP ∝ neff ). We found that nd is more sensitive to the enhanced ixE . This is because nd is a key figure of merit for MIM multilayer structures, where electromagnetic properties of the medium are closely related to nd . In optics, nd of a material is conventionally taken to be a measure of the optical density [14]. The optical density is critical to the SPP coupling of dipole and MIM plasmonic waveguides [10]. When nd is increased, the optical density must be reduced regularly. As a result, a dipole in low optical density is not conducive to SPP mode excitation. In brief, we present an important approach to obtain large SPP mode excitation based on a dipole in low nd for a MIM plasmonic waveguide. In the following study, we vary composing the intermediate dielectric layer (W 100 nm) with different nd forming a multilayer insulation structure to investigate the influence of neff on SPP mode excitation. ixE at interface 1 are calculated for Au∕SiO2 100 nm∕Au and Au∕Al2 O3 50 nm∕SiO2 50 nm∕Au plasmonic waveguides, as shown in Fig. 3(a). Two λSPP are 0.74 and 0.79 μm, respectively, with the latter redshifted due to the increase in neff . The enhancement of ixE of up to 2 arbitrary units (∼9%) in the Au∕Al2 O3 50 nm∕SiO2 50 nm∕Au
Fig. 3. (a) ixE versus wavelength and (b) iyE versus y position plots of Au∕SiO2 100 nm∕Au and Au∕Al2 O3 50 nm∕SiO2 50 nm∕Au plasmonic waveguides. (c) ixE versus wavelength and (d) iyE versus y position plots of Au∕Al2 O3 100 nm∕Au and Au∕SiO2 50 nm∕Al2 O3 50 nm∕Au plasmonic waveguides. 3644
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plasmonic waveguide compared with the Au∕ SiO2 100 nm∕Au plasmonic waveguide with a dipole in SiO2 is clearly observed. In order to gain a deeper understanding, we analyzed the vertical profiles of the electric field intensity (iyE ) along the y axis of the two MIM plasmonic waveguides corresponding to λSPP as 0.74 and 0.79 μm as a function of the y position, as shown in Fig. 3(b). The positions of the peaks in iyE are centered around 0.044 μm. An increase in ixE of up to 10 arbitrary units (∼9%) is observed. Thus, the Al2 O3 ∕SiO2 structure supports an SPP mode with electromagnetic energy mostly confined to the SiO2 layer. This helps SPP excitation due to a combination of the improved optical density and dipole radiation intensities under low nd. In addition, ixE at interface 1 are calculated for Au∕Al2 O3 100 nm∕Au and Au∕SiO2 50 nm∕Al2 O3 50 nm∕Au plasmonic waveguides, as shown in Fig. 3(c). Two λSPP are 0.76 and 0.72 μm, respectively, with the latter blueshifted due to the decrease in neff . The enhancement of ixE of up to 1 arbitrary unit (∼7%) in the plasmonic waveguide compared with the Au∕Al2 O3 100 nm∕Au plasmonic waveguide with a dipole in Al2 O3 is clearly observed. iyE of two MIM plasmonic waveguides corresponding to λSPP as 0.76 and 0.72 μm are shown in Fig. 3(d). The enhancement of iyE of up to 16 arbitrary units (∼27%) is observed. What is interesting here is that a remarkable enhancement of ixE and iyE with a dipole in SiO2 for the Au∕Al2 O3 50 nm∕SiO2 50 nm∕Au plasmonic waveguide compared with that with a dipole in Al2 O3 for the Au∕SiO2 50 nm∕Al2 O3 50 nm∕ Au plasmonic waveguide is clearly observed. The enhancement of ixE and iyE of up to 10 arbitrary units (∼67%) and 60 arbitrary units (∼100%) are observed, respectively. It is thought to result from a combination of the improved optical density and dipole radiation intensities with the dipole in low nd. To further verify the above findings, we have evaluated ixE at interface 1 and iyE along the y axis for Au∕Al2 O3 40 nm∕SiO2 20 nm∕Al2 O3 40 nm∕ Au and Au∕SiO2 40nm∕Al2 O3 20nm∕SiO2 40nm∕ Au plasmonic waveguides, as shown in Fig. 4. Two λSPP are 0.76 and 0.75 μm, respectively, with the latter slightly blueshifted due to the small decrease in neff . The enhancement of ixE of up to 15 arbitrary units (∼125%) in the Au∕SiO2 40 nm∕Al2 O3 20 nm∕SiO2 40 nm∕Au plasmonic waveguide with a dipole in SiO2 is clearly observed. Moreover, it can be found that the peak magnitude of iyE with a dipole in SiO2 is significantly larger than that with a dipole in Al2 O3 . The notable enhancement of iyE of up to 76 arbitrary units (∼146%) is clearly observed. For this reason, the excited SPP mode can be strongly confined to the SiO2 ∕Al2 O3 ∕SiO2 structure consisting of a high nd dielectric (Al2 O3 ) embedded in a low nd dielectric (SiO2 ) near the Au surface [15]. Evidently, the SiO2 layer provides the means to store electromagnetic energy (as shown in Fig. 4, upper inset), leading to a highly efficient SPP excitation. It arises from the continuity of the displacement field
Fig. 4. ixE versus wavelength and (inset) iyE versus y position plots of Au∕SiO2 40 nm∕Al2 O3 20 nm∕SiO2 40 nm∕Au and Au∕ Al2 O3 40 nm∕SiO2 20 nm∕Al2 O3 40 nm∕Au plasmonic waveguides.
at the different nd dielectric material interfaces, which leads to a strong normal electric field component in the low nd dielectric [16]. So, the proposed Au∕SiO2 ∕Al2 O3 ∕SiO2 ∕Au geometry shows the best enhancement performances. On the contrary, the Al2 O3 ∕SiO2 ∕Al2 O3 slot-waveguide-like structure [6], which allows light to be strongly confined inside the low nd (SiO2 ) dielectric layer (as shown in Fig. 4, lower inset) far from the Au surface, does not contribute to the SPP excitation. This structure should be avoided in the design of an SPP source based on the MIM plasmonic waveguide. 4. Conclusion
In conclusion, we have addressed a new SPP source designing technology using multilayer insulation structure in a MIM plasmonic waveguide, which significantly increased SPP mode excitation. We found that the neff of a multilayer insulation structure played a critical role in SPP excitation. Corresponding to a single-layer insulation structure, a multilayer insulation structure can significantly increase SPP excitation. The Au∕SiO2 ∕Al2 O3 ∕ SiO2 ∕Au geometry has shown the best enhancement performances. The present findings can provide possibilities for designing a high-performance SPP source in applications such as plasmonic circuitry. Future works will be focused on the investigation of the possible SPP coupling mechanisms to individually modify the effective gaps of the proposed SiO2 ∕Al2 O3 ∕SiO2 configuration. This work is partially supported by the Science and Technology Development Fund, Macao SAR (No. 082/2012/A3), the Science and Technology Research Key Project of the Guangxi Department of Education (No. 2013ZD026), and the Guangxi Natural Science Foundation (No. 2013GXNSFAA019338), China. References 1. H. Raether, Surface Plasmons on Smooth Surfaces (Springer, 1988). 2. K. Tanaka and M. Tanaka, “Simulations of nanometric optical circuits based on surface plasmon polariton gap waveguide,” Appl. Phys. Lett. 82, 1158–1160 (2003). 10 June 2014 / Vol. 53, No. 17 / APPLIED OPTICS
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