Localized surface plasmons, surface plasmon polaritons, and their coupling in 2D metallic
array for SERS
Luping Du,2 Xuejin Zhang,2 Ting Mei,1, 2* and Xiaocong Yuan 3
1Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, China 2Photonics Rearch Center, School of Electric & Electronic Engineering, Nanyang Technological University,
Nanyang Avenue, Singapore 639798, Singapore 3Institute of Modern Optics, Key Laboratory of Optoelectronic Information Science & Technology, Ministry of
Education of China, Nankai University, Tianjin, 300071, China
*i@ieee
Abstract: A substrate with ea for fabrication is propod
for surface enhanced Raman spectroscopy (SERS). A two-dimensional dielectric grating covered by a thin silver film enables the excitation of both localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). The finite-difference time-domain simulation results show that the coupling between LSPs and SPPs is able to highly improve the Raman enhancement (2 × 109 as obtained by simulation). In addition, the near-field distribution at the top of cubic bumps along the transver plane prents a highly regular hotspots pattern, which is required for an ideal SERS substrate.
©2009 Optical Society of America
OCIS codes: (050.2770) Gratings; (240.0310) Thin films; (240.6680) Surface plasmons; (240.6695) Surface-enhanced Raman scattering.
References and links
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创业语录
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1. Introduction
Surface enhanced Raman spectroscopy (SERS) is a powerful analytical tool for chemical and
biological nsing application which provides detailed material information at molecular页面紧急升级中
level, e.g. at single-molecule nsitivity [1], for which a Raman enhancement factor at an
order of 1014 is required. As a basic requirement for nsitive SERS, an ideal substrate must guarantee a high enhancement effect for SERS and a reproducible uniform respon, and thus
needs a large area with regular hotspots, whereas easiness for fabrication is desired [2]. Since
the discovery of SERS in 1970s, rearchers have made great effort theoretically and experimentally to develop robust substrates for SERS, including single nanoparticles [1, 3–5], nanoparticle dimers [6], clusters [7], nanorods [8] and nanowire arrays [9]. For the dimer configuration with extremely small gap, which provides a high Raman enhancement factor to
perform the single molecule SERS, the reported electromagnetic enhancement is at an order
of 1011[10]. This electromagnetic enhancement effect mainly comes from the excitation of
localized surface plasmons (LSPs) with strong interaction among them. Metallic periodic structures are another type of configuration which plays a significant role as SERS substrate
in bionsing. Many works, either theoretically or experimentally, have been conducted in
recent years [11–15]. The Raman enhancement reported is mainly among 105– 108, and is
only considering the effect of LSPs, or surface plasmon polaritons (SPPs) parately. A recent theoretical work shows that structures comprising metal nanoparticles within periodic arrays
can produce highly regular hotspots owing to the excitation of LSPs and SPPs [16]. The
coupling between them leads to a high electric field enhancement.
In this article, we propo a structure consisting of a two-dimensional dielectric grating
covered by a silver film with a thickness of tens of nanometers for a SERS substrate. Our
structure is easy for fabrication and able to provide highly regular hotspots with much high
electric field enhancement.
2. Plasmonic Respon
Figure 1 shows the propod structure for SERS in this study. A two-dimensional array of
SiO2 cuboids with fixed size of 50 × 50 × 100 nm3 is patterned on the Si substrate, and a layer
of 40-nm-thick silver film is covered on the surface. An array of cubic bumps is formed as
viewed from the top. Obviously, such a structure can be easily fabricated using the conventional silic
on process technology. To obtain the plasmonic respon of such a structure, the DiffractMod package bad on rigorous coupled wave analysis (RCWA) in the
大学生简历怎么写严格耦合波分析commercial software R-Soft was applied. For water (as the environment), Si, and SiO2, the
refractive indices are t to be 1.3364, 3.99, and 1.5458, respectively; while for silver, which
is a lossy material posssing wavelength-dependent dielectric constant, the refractive index is
obtained from Ref [17]. in the visible range (400 nm-800 nm).
#116455 - $15.00 USD Received 31 Aug 2009; revid 29 Oct 2009; accepted 6 Nov 2009; published 19 Jan 2010
(C) 2010 OSA 1 February 2010 / Vol. 18, No. 3 / OPTICS EXPRESS 1960
Fig. 1. The propod SERS structure and its cross ctional view along the dashed line.
Figure 2 shows the absorption characteristics bad on various configurations. Using the mi-infinite water-silver flat interface model (dashed line), the reflectance at the interface for normal incidence is: R = ((n0 – n r)2 + k2) / ((n0 + n r)2 + k2). Where n0 is the refractive index of environment, n r is the real part of the refractive index of silver and k is the imaginary part which gives ri to the absorption in the silver. For a 40-nm-thick silver film sandwiched between mi-infinite water and Si (dotted line), the absorption will decrea slightly and transmission appears due
to the finite thickness of the silver film. Further, if we produce the two-dimensional infinite dielectric grating and cover the whole surface conformally by a 40-nm-thick silver film, multiple absorption peaks will prent in the absorption spectra due to the excitation of LSPs and SPPs (solid line). The broad ones denoted as peaks 2 and 3 in Fig.
2 correspond to the first-order LSPs and high order LSPs, respectively, whereas the sharp one denoted as peak 1 corresponds to the SPPs.
Fig. 2. Absorption spectra bad on: mi-infinite water-silver flat interface configuration
(dashed line), 40-nm-thick silver film sandwiched between water and Si configuration (dotted
line), and dielectric grating covered by 40-nm-thick silver film configuration (solid line).
At normal incidence, the SPPs can be excited in a periodic structure satisfying
πΛ=(1)
2/k
where Λ is the structural period, k0 is the wavenumber of light in vacuum, and εm and εe are the permittivity of metal and environment, respectively. The term on the right hand side of Eq. (1) reprents the wavenumber of the SPPs propagating along the single interface. Equation (1) predicts an SPP excitation wavelength of 670 nm for Λ= 475 nm, which is slightly longer than the numerical simulation result (637 nm). The difference is due to the
标音#116455 - $15.00 USD Received 31 Aug 2009; revid 29 Oct 2009; accepted 6 Nov 2009; published 19 Jan 2010 (C) 2010 OSA 1 February 2010 / Vol. 18, No. 3 / OPTICS EXPRESS 1961
modification of Eq [1]. for periodic bump structures. Figure 3 gives the near-field distributions at different resonance conditions in the solid-line curve in Fig. 2, which is obtained by applying the FullWave Package bad on the finite-difference time-domain (FDTD) method in R-Soft. In the simulation, the grid size is t to be 2 or 1 nm and the
periodic boundary conditions are t for X and Y directions and the perfectly matched layer for Z direction.
If we define E out (λ) as the local electric field at a wavelength λ, the enhancement factor for SERS is: EF SERS (λs ) = |E out (λ)2 ||E out (λs )2| / |E 0|4, where λs is the Stokes-shifted wavelength [18]. The wavelength difference, ∆λ = λs - λ, between the incoming and scattered photons in general is much smaller than the linewidth of a surface plasmon mode, and thus E out (λ) is approximately equal to E out (λs ), resulting in the commonly ud expression for the enhancement of the stocks beam: EF SERS (λs ) = |E out (λ)4| / |E 0|4. Since EF SERS (λs ) = EF SERS (λs , x , y , z ) is spatia
lly varying over the particle surface, we focus on the largest value of EF SERS in this article, like what was done in many previous literatures,. For the ca of LSPs and SPPs in Fig. 3, the Raman enhancements are 3 × 107, 4 × 108 and 3 × 108 for the high order LSPs, the first-order LSPs and the SPPs, respectively. Apart from the high Raman enhancement, the highly regular hotspots are obviously produced. In the following ction, we take the structural period, the film thickness and the environment refractive index (RI) as variables and investigate their effects on the Raman enhancement.
Fig. 3. |E | distributions at the top of cubic bumps along XY plane at various resonance
conditions, i.e., the incident wavelength of (a) 450 nm corresponding to the high order LSPs,
(b) 637 nm corresponding to the SPPs, and (c) 670 nm corresponding to the first-order LSPs.
The colorbar scale is t as ln(|E |).
3. Influence of period, bump size and environment RI on Raman enhancement
Figure 4(a) shows a t of absorption spectra for the period ranging from 402 nm to 546 nm with a step of 24 nm. The peak absorption wavelengths are denoted as λmax and their plots against the st
ructural period for both the LSPs and SPPs are shown in Fig. 4(b). The red-shift trend of the resonance wavelength for SPPs for increasing period can be predicted from the excitation condition [Eq. (1)]. As the structural period increas, the right hand side value in Eq. (1) should be lower accordingly to satisfy the condition. Becau of the dispersion property of silver refractive index in visible range, a red shift of excitation wavelength is required from calculation. For the LSPs, the resonance wavelength is affected by the distance between bumps. For large structural period, meaning that bumps are parated far away, the plasmon resonance wavelength shift results from long-range interactions. From Fig. 4(b), LSPs resonance wavelength shows slightly blue shift as bumps distance decreas, which is in consistence with previous works [19, 20].
#116455 - $15.00 USD Received 31 Aug 2009; revid 29 Oct 2009; accepted 6 Nov 2009; published 19 Jan 2010
(C) 2010 OSA 1 February 2010 / Vol. 18, No. 3 / OPTICS EXPRESS 1962入射波长丗450•C 高阶局域SPs 670nm 一阶局域SPs 折射率
Fig. 4. (a) Absorption spectra for various periods from 426 nm to 546 nm with a 24 nm
increment; (b) absorption peak positions versus different periods for LSPs (circles) and SPPs
(squares), with corresponding maximum local |E| at resonance wavelength of SPPs (triangles).
美国南北战争时间The silver film thickness is 40 nm, the bump height is 100 nm and the environment is water.
The Raman enhancement shows a great dependence on the structural period becau of the interaction between the LSPs and SPPs. In Fig. 4(b), two peaks can be found from the |E max(λ)| pl
ot for the resonance wavelength of SPPs at each period (green curve with triangles). The red (with circles) and black (with squares) curves clearly show that there is an interction between them, implying the occurrence of LSPs and SPPs at the same wavelength. In this ca, strong coupling between the SPPs and LSPs will occur, leading to considerable enhancement of the local electric field responsible for the right peak in the |E max(λ)| curve. As for the left peak at smaller wavelength, the analysis of the |E(λ)| distribution shows that it is from the coupling between the SPPs and the cond-order LSPs.
Fig. 5. Absorption peak positions at various silver film thickness for LSPs (circles) and SPPs
(squares), with corresponding maximum local |E| at resonance wavelengths of SPPs (triangles).
The structural period is fixed at 475 nm, the bump height is 100 nm and the environment is
water.
Next, we consider the influence of the silver film thickness to the resonance wavelength and Raman enhancement. For a fixed structural period, the resonance wavelength of SPPs remains constant approximately. For the LSPs, as the film thickness increas, three aspects should be considered for contribution to the wavelength shift. Firstly, the distance between the bumps decreas for increasing film thickness. As stated above, this caus slightly blue shifts of the resonance wavelength. Secondly, since the nanoparticle (bump here) is not pure metal, as the film thickness increas, the effective refractive index of nanoparticles will change accordingly, which induces slightly blue shifts as well. Thirdly, the size and shape of the #116455 - $15.00 USD Received 31 Aug 2009; revid 29 Oct 2009; accepted 6 Nov 2009; published 19 Jan 2010 (C) 2010 OSA 1 February 2010 / Vol. 18, No. 3 / OPTICS EXPRESS 1963
