Giant Electric Field Enhancement and Localized Surface Plasmon

Nov 4, 2013 - The surface plasmon resonances of gold contour bowtie nanostructures were simulated in the present study. The local electromagnetic fiel...
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Giant Electric Field Enhancement and Localized Surface Plasmon Resonance by Optimizing Contour Bowtie Nanoantennas Li-Wei Nien,† Shih-Che Lin,† Bo-Kai Chao,† Miin-Jang Chen,† Jia-Han Li,‡ and Chun-Hway Hsueh*,† †

Department of Materials Science and Engineering and ‡Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: The surface plasmon resonances of gold contour bowtie nanostructures were simulated in the present study. The local electromagnetic field enhancement and the resonance wavelength for different dimensions of contour bowtie antennas with various contour thicknesses were investigated to find the critical conditions to induce additional enhancement compared to the solid bowtie antenna. Both the phase of the electric field and the bound surface charge distribution on the surface of the contour bowtie were studied to characterize the coupled plasmon configurations of the contour bowtie antenna. Also, a model was proposed to explain the resonance and hybridization behavior in the contour bowtie nanoantenna, and it was verified by examining the phase of the electric field in the polarization direction.

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INTRODUCTION The discovery of surface-enhanced Raman scattering (SERS) helped to improve the efficiency of Raman spectroscopy.1 The SERS technique refers to the phenomenon of the Raman signal from the probed molecules on the metallic nanostructure substrate being enhanced by a factor of 104−106 because of the local enhanced electromagnetic field resulting from the excitation of the localized surface plasmon resonance. It has been shown that a large electromagnetic field can be produced and confined in the gap region of a bowtie-shaped antenna in which two metallic triangular prisms face tip-to-tip and are separated by a small gap.2−7 This field enhancement has been applied to the detection of single molecules via SERS.8−12 In addition, a large number of investigations of nanoshells, tunable plasmonic nanoparticles consisting of a dielectric core and a thin metallic shell, exhibit better sensitivity while varying the local dielectric environment as compared to solid spherical nanoparticles.13−16 However, there exist few studies17 on the advantages of combining the large electromagnetic field enhancement confined within the bowtie antenna gap with the enhanced sensitivity characteristic of the nanoshell structure. Although a single contour bowtie antenna model with different contour thicknesses has been proposed and showed a 28% increase of the electromagnetic field enhancement factor,18 the critical dimension of the contour bowtie antenna to induce the optimum enhancement is still unknown. The purpose of the present work is to perform simulations on a model system of contour bowtie antenna to examine the resonance wavelength and the local electromagnetic field enhancement as functions of the bowtie size and contour thickness and use the plasmon hybridization model to explain the simulation results. © 2013 American Chemical Society

THEORETICAL METHODS A model system of the single gold contour bowtie nanostructure was used for simulations. The dimension of the contour bowtie was defined by the circumradius of the equilateral triangle, R, and three circumradii, NCB150, NCB100, and NCB60, which corresponded to the side-length of the triangle of about 259.8, 173.2, and 103.9 nm, respectively, were simulated with various contour thicknesses, t. The Au contour bowtie of 40 nm thickness (in the zdirection) was placed on the silica substrate. A schematic drawing of the top view of the contour bowtie structure on the xy-plane is shown in Figure 1a, and the cross section on the xzplane is shown in Figure 1b. While a perfectly sharp tip at the apex of the triangle cannot be achieved because of the

Figure 1. Schematics showing (a) a rounded-corner contour bowtie on the xy-plane and (b) the cross section of SiO2/Au bowtie nanostructure on the xz-plane. A polarized plane wave is illuminated from the bowtie side. Received: August 28, 2013 Revised: November 2, 2013 Published: November 4, 2013 25004

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Figure 2. Maximum E intensity enhancement at the apex and on the contour bowtie surface for (a) NCB150, (b) NCB100, and (c) NCB60 with different contour thicknesses. (d) Gap enhancements of NCB150, NCB100, and NCB60 normalized by the maximum E intensity enhancement of each corresponding solid bowtie antenna as functions of normalized contour thickness, t/R.

perfectly matched layers were used in the simulation domain to absorb waves leaving the simulation domain in all wave propagation directions. The dielectric property of Au used in the simulations was taken from the Johnson and Christy report,22 and the SiO2 information was obtained from Palik’s handbook.23 The mesh sizes in the bowtie region (including the gap) varied from 1 to 1.5 nm, and automatic graded mesh was used in the region outside the contour bowtie structure to ensure the numerical accuracy in consideration of reasonable computation time.

limitations of using electron beam lithography followed by vapor deposition of gold or similar fabrication techniques, simulations of imperfect bowtie structures have been performed by either truncating the tips of the triangle19,20 or assuming a curvature at the apex.4 In the present study, the contour bowtie with a rounded corner radius of 5 nm (Figure 1a) was adopted as a simplified structure in simulations, and both the gap between apexes and the interbowtie distance remained constant at 30 nm. By systematically varying the contour thickness t at a fixed antenna dimension, the essential trends of how the contour thickness affects the plasmon resonance could be elucidated. It is worth mentioning that the conventional (solid) bowtie antenna could evolve from the contour bowtie antenna when the contour fully occupies the bowtie region. First, the local electromagnetic field enhancement and the resonance wavelength of NCB150, NCB100, and NCB60 bowtie antennas with various contour thicknesses were examined. Then, the mechanism of the contour thickness dependence of the field enhancement was explored. Finally, we developed a plasmon hybridization model to analyze the coupling effect in the contour bowtie antenna, and the model was verified by the simulation results. Lumerical FDTD Solutions,21 a commercial electromagnetic software based on the finite-difference time-domain method, was used to perform the simulation. A plane wave (of 500− 2000 nm wavelength) polarized across the junction between triangular prisms (i.e., along the x-direction in Figure 1) was illuminated in the negative z-direction from above the bowtie (see Figure 1b). Several simulation domains in the z-direction were simulated to ensure the convergence of simulation results. The adopted simulation domain is the region of 2500 nm × 2500 nm on the xy-plane, 240 nm above the Au contour bowtie, and 1000 nm below the Au contour bowtie structure in the z-direction such that the total z-dimension is 1240 nm. The

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RESULTS AND DISCUSSION The simulation results of the near-field (i.e., the local field) and the electric field distribution (including the amplitude and the phase) on the surface of the contour bowtie antenna were recorded. For the local electromagnetic field, E intensity (i.e., | E|2) is greatest on the plane containing the bowtie surface among all the constant-z planes, and it is located at the junction of the contour bowtie nanostructure. Critical Dimension of Contour Bowtie and Local Electromagnetic Field Enhancement. We first studied the local electromagnetic field enhancement and the resonance wavelength of different dimensions of bowtie antennas with various contour thicknesses. For the three circumradii of the equilateral triangle in the contour bowtie structures, NCB150, NCB100, and NCB60, the corresponding maximum E intensity enhancements (i.e., |E|2/|E0|2 where E0 is the electric field of the illumination wave) are partially shown in Figures 2a, 2b, and 2c, respectively, as functions of the illumination wavelength λ at different contour thicknesses t. There are two resonance peaks in each solid bowtie antenna in Figure 2a−c. These two plasmon resonances have also been identified elsewhere by using discrete dipole approximation as dipole and quadrupole resonances, respectively, at the longer and the shorter 25005

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Figure 3. (a) Maximum plasmon resonance wavelength, λmax, as a function of the normalized contour thickness t/R for NCB150, NCB100, and NCB60 and (b) the fractional shift Δλ/λ0 of the maximum plasmon resonance wavelength of the contour bowtie antenna with respect to the plasmon resonance of the solid gold bowtie antenna as a function of the normalized contour thickness t/R for NCB150, NCB100, and NCB60. The data for the three contour bowtie dimensions can be fitted by a single curve.

wavelengths.24,25 Compared to the solid bowtie antenna, the contour bowtie antenna exhibits a red shift of the maximum resonance wavelength regardless of the contour thickness. However, it should be noted that the plasmon resonances of the contour bowtie antenna are not purely dipole or quadrupole resonances like the case of the solid bowtie structure, and the resonance modes are more complex because of the presence of the cavity in the center region of the contour bowtie structure. It is shown in Figure 2a for NCB150 that the maximum E intensity enhancement initially becomes larger than that of the solid bowtie antenna as the contour thickness decreases. This is in agreement with the previous report that the gap enhancement of the antenna could be increased with decreasing contour thickness.18 However, as the contour thickness decreases to a critical value, the E intensity enhancement reaches a maximum and then decreases to a value that is even smaller than the E enhancement of the solid bowtie antenna. On the other hand, there are no obvious E intensity enhancements for NCB100 and NCB60 with decreasing contour thickness as shown in Figure 2b and 2c, respectively. For NCB100 and NCB60, the maximum E intensity enhancement initially drops with decreasing contour thickness while the characteristic plasmon resonance wavelength splits into two peaks which signifies the possible excitation of two different plasmon modes under the condition of these contour thicknesses.26 The maximum E intensity enhancements of NCB150, NCB100, and NCB60 contour bowtie antennas normalized by the maximum E intensity enhancement of the corresponding solid bowtie antenna are shown in Figure 2d as functions of the normalized contour thickness, t/R. It can be seen that the maximum E intensity enhancement of NCB150 is enhanced compared to the solid bowtie antenna when the t/R ratio is between 0.2 and 0.4, and the largest enhancement factor is ∼16%. However, when the circumradius of the equilateral triangle is decreased to NCB100, the additional E intensity enhancement effect resulting from the contour structure does not exist. Also, a transition fluctuation in the maximum E intensity enhancement can be observed in Figure 2d for NCB100 when the t/R ratio is between 0.25 and 0.4, which corresponds to the distinguishable split of plasmon resonance modes shown in Figure 2b. Similar results occur for NCB60. On the basis of a previous study18 and the present simulations, the resultant E intensity enhancement of the

contour bowtie antenna can be determined by two competing factors: geometry and polarizability. For the geometry factor, both the solid bowtie and the cavity bowtie of the contour structure could induce additional E intensity enhancement18 and result in the split of plasmon resonance modes because of the coupling effect.27 However, when the dimension of contour bowtie antenna is sufficiently large (e.g., NCB150), the split of plasmon resonances could be neglected. For the polarizability factor, the E intensity is lower for a thinner contour thickness because of the smaller surface area of the antenna exposed to the incident light.28 Other than the geometry and the polarizability factors, it is worth noting that the resonance and the maximum E intensity enhancement shown in Figure 2 can also be affected by the dispersive properties of Au and the dielectric constant of the surrounding medium, εm. The maximum electromagnetic field enhancement in the gap region of the contour bowtie can be obtained when the dielectric constant of Au (which is wavelength-dependent) and εm satisfy a certain relationship. As the size of the contour bowtie antenna or the contour thickness changes, the corresponding εm changes, which in turn would result in the shift of the resonance wavelength and the change of the E intensity enhancement. The maximum resonance wavelengths λmax of NCB150, NCB100, and NCB60 contour bowtie antennas as functions of the normalized contour thickness t/R are shown in Figure 3a. Red shift of the plasmon resonance wavelengths with decreasing t/R ratio can be observed for the three different sizes of contour bowtie antennas. This result is similar to the behavior of nanoshell plasmon resonance in previous studies.29,30 Both the experiment and the simulation show that the plasmon resonance of nanoshell depends on the shell thickness and the core material. The plasmon resonance of nanoshell red-shifts with the increasing radius of the core part (like the increasing dimension of the contour bowtie antenna in our work), while it blue-shifts with the increasing nanoshell thickness (like the increasing t/R ratio in our simulation). However, as the dimension of the contour bowtie antenna decreases to a critical value (e.g., NCB100), there is a discontinuity in the resonance wavelength versus t/R relation when t/R ∼ 0.3, which corresponds to the perceptible split of plasmon resonance modes at the t/R ratio between 0.25 and 0.4 shown in Figure 2b. The plasmon fractional shift Δλ/λ0 has been defined previously31 where λ0 is the plasmon resonance wavelength of 25006

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a solid structure and Δλ = λmax − λ0 is the shift of the maximum resonance wavelength of the shell structure from its corresponding solid structure. Figure 3b shows the plasmon fractional shift Δλ/λ0 of the contour bowtie antenna with respect to the solid bowtie antenna resonance wavelength versus the normalized contour thickness t/R for NCB150, NCB100, and NCB60. The trend of Δλ/λ0 versus t/R shown in Figure 3b is insensitive to the dimension of the contour bowtie antenna, and an empirical equation can be used to describe the relation between Δλ/λ0 and t/R. From previous studies,31,32 universal scaling equations have been applied to the particle pair system32 and the nanoshell structure31 and could successfully fit the plasmon fractional shift behavior obtained from the experiments. Adopting the similar analysis used for the particle pair system and the nanoshell structure, it was found that the plasmon fractional shift of contour bowtie antenna showed a functional dependence of (t/R + 1)−3, which is like the behavior of dipole resonance in the previous studies. Therefore, the plasmon fractional shift Δλ/λ0 versus t/R relation can be viewed as a near-exponential decay. Fitting the universal trend of NCB150, NCB100, and NCB60 contour bowtie antennas shown in Figure 3b by the least-squares fitting in the form of exponential decay, we obtained ⎡ −t /R ⎤ Δλ = 3.561 exp⎢ ⎣ 0.089 ⎥⎦ λ0

Figure 4. Maximum E intensity enhancement of NCB100 with 30 nm contour thickness (t/R = 0.3). The resonances occur at λ = 1156, 911, 690, and 530 nm.

where |Ex| and φ are the magnitude and the phase of Ex, respectively, such that (3a)

φ = tan−1(Im[Ex]/Re[Ex])

(3b)

The Ex intensity, i.e., |Ex| , enhancement profiles are shown in Figure 5a−d, the corresponding phase profiles of the dominate component of the electric field Ex, φ, are shown in Figure 5e−h, for the four resonance wavelengths shown in Figure 4. Only Ex is discussed here to avoid the complexity of the three components of the electric field. For λ = 1156 nm (dipolelike resonance), the phase is mainly approximately −π in the contour bowtie regions except for the end of the cavity bowtie region where the phase is about zero. Similarly, for λ = 911 nm (dipolelike resonance), the phase is slightly less than −π/2 in the contour bowtie regions except for the end of the cavity bowtie region where the phase is slightly larger than π/2. Hence, the phases of these two resonances, λ = 1156 and 911 nm, are relatively uniform on the contour bowtie surface and the phase difference between these two resonances is ∼π/2. There is phase retardation for resonances at λ = 690 and 530 nm (quadrupole-like resonances); the phase is about zero in the opposing apex region and progressively shifts toward −π/2 as the position moves toward the side edge of the contour bowtie. In addition, the phases at the end of the cavity bowtie are slightly higher than −π/2 and −π for λ = 530 and 690 nm, respectively, as shown in Figure 5e,f. The bound surface charge density can be derived from the electric field as mentioned in a previous study.34 The bound surface charge density ρb is related to the electric field, which comes from the alignment of polarization dipoles, and can be expressed as 2

(1)

It can be observed that excellent fitting is obtained between the data and the fitting curve for 0.3 < t/R < 0.5, and data scattering occurs when t/R < 0.3. This scattering could result from the splitting of plasmon resonance modes that are hybridized from an outer shell−surface solid bowtie mode and an inner shell−surface cavity bowtie mode in the plasmon hybridization model developed by Prodan et al.33 On the other hand, the strength of the near-field, which arises from the interaction of the dipole, accounts for the plasmon coupling in the particle pair system,32 and the higher particle polarizability from larger particle size could result in stronger coupling. Similarly, for the contour bowtie antenna, a larger antenna dimension has a larger polarizability and a thinner contour thickness can produce greater near-field coupling. As a result, a larger dimension of antenna with a thinner contour thickness could result in a greater near-field coupling and a larger shift of the fractional plasmon resonance. Phase of Electric Field and Bound Surface Charge. Discontinuity and deviation exist in both the maximum E intensity enhancement and the maximum resonance wavelength shown in Figures 2d and 3a, respectively, as the dimension of the contour bowtie antenna decreases to NCB100 with the t/R ratio of 0.3 (i.e., t = 30 nm). The corresponding resonances occur at λ = 1156, 911, 690, and 530 nm, as shown in Figure 4. It is worth exploring the phase of the electric field and the bound surface charge on the contour bowtie surface to understand the interaction between the electromagnetic wave and the metallic nanostructure. The dominant component of the electric field is Ex in most regions because of the polarization direction of the illumination wave (in the xdirection). The field has a harmonic time dependence, and the solution of the time-independent component Ex from FDTD simulations is a complex number, such that Ex = Re[Ex] + iIm[Ex] = |Ex|exp(iφ)

Re[Ex]2 + Im[Ex]2

|Ex| =

ρb = (εAu − ε0)E·n

(4)

where E is the electric field and n is the unit vector normal to the surface; εAu and ε0 are the electric permittivities of the gold and vacuum, respectively. Because of the characteristics of Au, Re[εAu] is negative in the visible light and near-infrared regions and has small dielectric loss; i.e., Im[εAu] is small and εAu − ε0 is negative. Also, taking the left triangle in Figure 1a as an example, the normal vectors n at the opposing apex and the base side of the cavity are x⃗, and at the end of bowtie and the side edge of the cavity are −x⃗. Therefore, we can obtain the

(2) 25007

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Figure 5. The E intensity enhancement profiles on the plane containing the contour bowtie surface for (a) λ = 530 nm, (b) λ = 690 nm, (c) λ = 911 nm, and (d) λ = 1156 nm. The color bar is a logarithmic scale and limited to −1 to 3. The corresponding phase profiles of Ex for (e) λ = 530 nm, (f) λ = 690 nm, (g) λ = 911 nm, and (h) λ = 1156 nm.

slightly larger than −π/2. At the end of the cavity bowtie, ρb is negative and positive, respectively, for λ = 530 and 690 nm, for which the corresponding phase of electric field are slightly higher than −π/2 and −π, respectively. Because the field enhancements for λ = 911 and 1156 nm are more confined in the gap region (Figure 5c,d), the bound surface charges are most likely to exist in the opposing apex region and in the end of the bowtie region. Furthermore, the cavity bowtie has little effect on the contour bowtie antenna except at the end of the cavity bowtie region, and the E intensity enhancements are dominated by the solid bowtie structure. In addition, the E intensity enhancement for λ = 1156 nm is larger than that for 911 nm because there are more bound surface charges accumulated at the opposing apex region and in the end of

bound surface charge at different resonance wavelengths from eq 4 and Figure 5, and the schematic drawings are shown in Figure 6. For both λ = 911 and 1156 nm, ρb is positive in the opposing apex region but is negative in the end of the contour bowtie region (Figure 6c,d). However, at the end of the cavity bowtie, ρb for λ = 911 and 1156 nm is positive and negative, respectively, for which the corresponding phase of the electric field are slightly larger than π/2 and zero, respectively. For λ = 530 and 690 nm, φ approaches 0 and −π/2 (i.e., Ex is positive and positive but very small), respectively, in the opposing apex and the corner of the bowtie regions (Figure 5e,f). As a result, ρb is negative in the opposing apex region and slightly negative in the side edge of the bowtie region. In addition, ρb is positive at the base side of the contour bowtie because the phase of Ex is 25008

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contour layer, the solid and the cavity bowtie structure plasmons would interact with each other, and the strength of the interaction is determined by the thickness of the metal contour layer. Moreover, this interaction could result in the splitting of the plasmon resonances into two new modes. In this model, four linearly independent plasmon resonance modes interact and result in four hybridized plasmon resonances as shown in Figure 7. Also, for simplicity, only the plasmon resonance modes of the same angular momentum l are allowed to interact with each other (i.e., l′ − l = 0), which is similar to the spherical symmetry in the particle pair system or the nanoshell structure.36 Additionally, the strength of coupling and the energy between the plasmons on the inner and outer contour structures determine the shift of the hybridization plasmon energy. As the contour thickness increases to a critical value, the strength of coupling would be too weak to be observed because of the weak interaction of the electric field. Verification of the above model for the plasmon energies of the contour bowtie antenna shown in Figure 7 can be achieved by examining the phase of Ex. For instance, the maximum E intensity enhancements for NCB100 with 30 nm contour thickness, the solid bowtie, and the cavity bowtie as functions of the wavelength are shown in Figure 8. The corresponding

Figure 6. Schematics showing the bound surface charge distribution on the contour bowtie surface for (a) λ = 530 nm, (b) λ = 690 nm, (c) λ = 911 nm, and (d) λ = 1156 nm.

the bowtie region. On the other hand, for the phase of Ex at λ = 530 and 690 nm (Figure 5e,f), the influence of the cavity bowtie becomes significant and comparable to that of the solid bowtie structure. The charge distributions can also be obtained from FDTD simulations, and the results for the four resonance wavelengths of NCB100 with t/R = 0.3 are shown in Figure S1 in the Supporting Information. Plasmon Hybridization Model of Contour Bowtie Antenna. Similar to the nanoshell system,35 the contour bowtie antenna has the characteristics of plasmon resonance frequencies sensitive to the thickness of the contour metal structure. Using the plasmon hybridization model developed by Prodan et al.,33 the fundamental insight of the tunable plasmon resonance in the particle pair system and the nanoshell structure has been successfully explained. Similarly, the contour bowtie antenna can be analyzed using the plasmon hybridization model, and we propose the following model to describe the hybridization for the case of NCB100 with the t/R ratio of 0.3 (i.e., contour thickness of 30 nm). The hybridization model is schematically depicted in Figure 7, and the thickness-dependent plasmon response of the

Figure 7. An energy-level diagram depicting the plasmon hybridization model in a metal contour bowtie antenna resulting from the plasmon interaction between the solid bowtie antenna and the cavity bowtie structure.

Figure 8. Maximum E intensity enhancement for NCB100 with 30 nm contour thickness (middle), which is hybridized from the interaction between solid NCB100 (left) and cavity bowtie structure (right), and the corresponding phase profiles of Ex for each resonance mode. All phase profiles of Ex are limited to −π to π and use the same scale color bar shown in Figure 5

contour bowtie antenna (NCB) at a fixed frequency can be viewed as an interaction between the solid bowtie antenna (NB) and the structure of a cavity bowtie (CB). On the basis of the mechanism described by Prodan et al.,33 the induced surface charges of the contour bowtie antenna result from both the solid and the cavity bowtie antenna plasmons excited by the electromagnetic wave. Because of the finite thickness of the

phase profiles of Ex for each resonance wavelength are also shown. For the plasmon resonances of the contour bowtie antenna at λ = 1156 and 911 nm, which are hybridized by the resonances of the solid bowtie antenna at λ = 921 nm and the cavity bowtie structure at λ = 886 nm, both have similar phase configurations of Ex and can be deduced from the original hybridization modes. In the metallic contour bowtie region, 25009

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resonances of the contour bowtie antenna at λ = 1156 and 911 nm are both in phase and have the same configuration as the solid bowtie (dipolelike resonance) and the cavity structure except at the end of the cavity bowtie region, which is slightly influenced by the cavity plasmon resonance. Moreover, the phases of Ex outside the contour bowtie antenna mostly concentrate at the junction of the bowtie and have similar distributions as well. In the same way, for the plasmon resonances of contour bowtie antenna at λ = 690 and 530 nm, they are hybridized from the original resonance modes of the solid bowtie antenna at λ = 617 nm and the cavity bowtie structure at λ = 620 nm. In the metallic contour bowtie region, the resonances of the contour bowtie antenna at λ = 690 and 530 nm both are no longer in phase. Instead, they vary with the combination of the gradual change phase of the solid bowtie antenna hybridized with the quadrupole-like phase of the cavity bowtie structure. Alternatively, the plasmon hybridization model can also be verified using the charge distributions obtained from FDTD simulations; this is shown in Figure S2 in the Supporting Information.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was jointly supported by the National Science Council, Taiwan under Contract NSC 100-2221-E-002-128 and Excellent Research Projects of National Taiwan University under Project 102R8918.

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REFERENCES

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CONCLUSIONS In conclusion, we used FDTD methods to simulate the plasmon resonance of gold bowtie nanostructures with different contour thicknesses. The illumination light was polarized along the direction connecting the two opposing apexes of the two triangles of a contour bowtie. Bowties with different dimensions and contour thicknesses were considered to systematically examine (i) the critical dimension of contour bowtie to produce the electromagnetic field enhancement compared to the original (solid) bowtie antenna and (ii) how the thickness of the contour structure affected the local electromagnetic field enhancement and the resonance wavelength. As the dimension of the contour bowtie antenna decreases to the critical case of NCB100 (bowtie triangle side-length of about 173 nm) or the contour thickness diminishes to a certain value, the contour bowtie antenna can no longer exhibit the additional local electromagnetic field enhancement in the gap region. In the meantime, the maximum plasmon resonance red-shifts with decreasing contour thickness and an exponential decay equation can be used to describe the universal trend of the plasmon fractional shift as a function of t/R where t is the contour thickness and R is the circumradius of the equilateral triangle. Both the phase of the electric field and the bound surface charge were examined for the contour bowtie antenna case of NCB100 with the t/R ratio of 0.3 (contour thickness of 30 nm) to understand the dipolelike and quadrupole-like resonance behaviors in the metallic contour structure. Finally, we proposed a hybridization model, which originated from the interaction between the solid bowtie antenna and the cavity bowtie structure, to explain the resonance in the contour bowtie antenna and successfully verified our proposed model by examining the phase of Ex.

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ASSOCIATED CONTENT

* Supporting Information S

Simulated charge distributions on the contour bowtie surface at different resonance wavelengths (Figure S1) and verification of the plasmon hybridization model using the simulated charge distributions (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. 25010

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp408610q | J. Phys. Chem. C 2013, 117, 25004−25011