Discrete Dipole Approximation Simulation of the Surface Plasmon

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Discrete Dipole Approximation Simulation of the Surface Plasmon Resonance of Core/Shell Nanostructure and the Study of Resonance Cavity Effect Xi-bin Xu,†,‡,§ Zao Yi,†,‡ Xi-bo Li,‡ Yu-ying Wang,†,‡,§ Xing Geng,∥ Jiang-shan Luo,‡ Bing-chi Luo,‡ You-gen Yi,*,† and Yong-jian Tang*,‡ †

College of Physical Science and Technology, Central South University, Changsha 410083, China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China § Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China ∥ Electronics and Information Engineering College, Tongji University, Shanghai 201804, China ‡

ABSTRACT: Nanoshells have been previously shown to tune absorption frequencies efficiently. What will happen when a nanoshell embeds on a small core system? A theoretical model that a core composed of gold is embedded within a nanoshell of Au/Ag is constructed to answer this question. The calculations based on the discrete dipole approximation (DDA) method are performed and proved to converge accurately by satisfying the usual criteria related to the applicability of the DDA. The results show that the nanoshells in the core/shell system greatly influence the surface plasmon resonance (SPR). Indeed, the shell frequency is tuned to match the optical properties of the absorbing core leading to hybridization/mixing and possibly overall enhancement of absorption crosssection. The calculation of the field enhancement also shows that the location of the field enhancement is specified by the different resonance patterns.

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INTRODUCTION Gold or silver nanostructures have attracted huge attention for decades because of their special optical behavior resulting from the interaction of their free conduction electrons with the incident light. The surface plasmon resonance (SPR) emerging with an intense band in the extinction spectrum arises when the oscillating electric field of the incident light resonantly couples to the conduction electrons making them collectively oscillate at the same frequency.1 Gold colloidal nanoparticles are responsible for the brilliant reds seen in stained glass windows, whereas silver particles are typically yellow, and so is the wave range of their SPR. The SPR spectra markedly depend on the structural characteristics such as their size, shape, composition, and deposited substrate as well as their external dielectric environment.2,3 Knowledge about the influence of these parameters on the optical properties of nanocrystals is crucial to use the SPR for various applications in optics, surfaceenhanced Raman spectroscopy (SERS), biosensor, and medical diagnostics.4,5 However, by the nature of their size, the nanoparticles have a small absorption cross-section compared with bulk (large) materials; therefore, large absorption could not be expected for a very small particle.2,6 It is conceivable that nanostructuring of materials coupled to band gap/molecular orbital engineering could considerably enhance the absorption of a nanoparticle at particular regions of the electromagnetic spectrum.7 The modification of structure and composition on the nanoscale could offer interesting possibilities.8 For example, © 2012 American Chemical Society

strong electronic transitions of dye molecules or semiconductor nanocrystals could be coupled to plasmon resonances in small metallic particles. Such an enhancement could be very useful in also increasing the cross-section for emission of fluorescing nanoparticles or increasing heat delivery and photoablation for therapeutic purposes. The main motivation is the increase in absorption cross-section while maintaining smallest size particles possible.7 In recent years, experimental and theoretical investigations on metals have revealed the applicability of nanostructure of metal core/shell system.9−12 Core/shell system consist of a dielectric core surrounded by a metallic shell of nanometric thickness. The plasmon absorption frequencies will be tuned from the high bulk value down to zero. The SPR of the core/ shell system is tuned to match the optical property of the metal core, leading to hybridization/mixing and possibly overall enhancement of absorption cross-section of the composite nanoparticle.7 Several groups have synthesized Au/Ag core− shell nanostructures with various compositions.7,13 The SPR properties of Au−Ag core/shell nanoparticles (with Au or Ag as the core and Ag or Au as the shell) are different from those of solid nanoparticles made of pure Ag or Au. The former will show two extinction peaks, whereas the latter exhibits one SPR Received: June 25, 2012 Revised: October 28, 2012 Published: November 2, 2012 24046

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peak.14−17 It has been shown that the SPR band of gold nanoshells supported on dielectric cores could be readily swept from 500 to 1200 nm by varying their diameters, shell thickness, or both.13−16 Theoretical work has also predicted that the optical extinction peak of a gold core encapsulated in a shell could be greatly enhanced due to the resonance cavity effect.7,13 In the present work, we use a DDA code to study theoretically the SPR properties of core/shell nanospheres with different size and take dielectric environment into account. With different diameter of core, thickness of shell, and dielectric environment, the SPR properties of Au/Ag core/shell nanospheres change obviously and regularly. The results show that the SPR spectra of these nanoparticles are highly sensitive to the parameters previously mentioned. Despite the previous studies, it is worth noting the current lack of results dealing with the study of their optical properties to provide a complete description and better knowledge of the resonance cavity effect. To realize the resonance cavity effect, we calculated and discussed the system that the Au core and Ag shell are separated by a cavity.

E loc(ri) = E loc, i = E inc, i + Eself, i = E0 exp(iK ·ri) −

3d3 εi − 1 4π εi + 1

E0 and K are the amplitude and wave vector of the incident wave, respectively, and the interaction matrix A has the following form Aij ·Pj =

exp(ikrij) ⎧ (1 − ikrij) ⎪ ⎨k 2rij × (rij × Pj) + 3 ⎪ rij rij2 ⎩ ⎫ ⎪ × [rij2Pj − 3rij(rij·Pj)]⎬ ⎪ ⎭

(4)

α−1 j

where k = ω/c. Defining Ajj = reduces the scattering problem to finding the polarizations Pj that satisfy a system of 3N complex linear equations N

∑ Ajk Pk = Einc,j k=1

(5)

Once eq 5 has been solved for the unknown polarizations Pj, the extinction and absorption cross-sections Cext may be evaluated Cext =

4πk |E0|2

N

∑ Im(Einc,j*·Pj) j=1

(6)

2.2. Target Geometry. The DDA method treats each object approximately with a cubic array of polarizable dipoles points in response to both the incident electric field and the fields created by all other dipoles in the target.29 The target built to mimic AU/Ag core/shell nanocrystal is shown in Figure 1. The center of the AU/Ag core/shell nanosphere is set at the

(1)

Figure 1. Half-sectional drawing of the AU/Ag core/shell system simulated: (A) solid core/shell sphere and (B) separate core/shell sphere.

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Purcell and Pennypacker used the Clausius−Mossotti polarizabilities to relate to the dielectric function, where d is the interdipole spacing and εi is the complex dielectric function at location ri. The dielectric function of the Au and Ag is taken to be the bulk experimental value in the article. Analytical modification in eq 1 has been reportedly implemented in DDSCAT, the open source code of the DDA method.22−25 The polarization induced in each dipole as a result of interaction with a local electric field Eloc

Pi = αi·E loc(ri)

(3)

j≠i

2. COMPUTATIONAL APPROACH FOR THE SIMULATIONS: DISCRETE DIPOLE APPROXIMATION Numerical techniques such as the boundary element method (BEM), the discrete dipole approximation (DDA), the finitedifference time-domain (FDTD) method, and the finite element method (FEM) are now available to calculate the optical properties of nanostructures with different shapes by solving the electromagnetic scattering problem.18−24 Among all of these techniques, the DDA, which is also known as the coupled dipole approximation, is most frequently used numerical methods for the calculation of the SPR in metal nanoparticles of arbitrary geometry.19−22 In this article, the DDA method is explored to tune largely the resonance peak of AU/Ag core/shell nanocrystals by taking both their size, geometrical parameters, and dielectric environment into account. This article will not consider particle−particle interaction effects to facilitate the study of their resonance cavity effect. 2.1. Computational Method. The DDA method is reviewed in refs 22−28. The DDA starts by dividing the object of interest into a cubic array of N-point dipoles, whose positions are denoted ri with polarizabilities αi, where αi =

∑ Aij ·Pj

origin of the coordinate. The wave vector of the incident electromagnetic radiation is considered here to propagate along the x-axis direction and to be linearly polarized with its electric field being oriented along the y- or z-axis direction. Taking computing times into account, we keep the same geometric incidence configuration for all of our calculations. The dielectric function of the Au and Ag is taken to be the bulk experimental value, as published in Palik’s handbook.30 The work done in ref 31 not only reveals that the shape and the assembly of nanoparticles strongly affect the SPR but also suggests that morphological differences are important only above a certain

(2)

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size threshold of ∼5 nm. Because the geometry of target calculated in this article is larger than 10 nm, the size-corrected effect of dielectric constant is ignorable. The Au/Ag core/shell nanospheres are simulated in vacuum with the refractive index of the surrounding medium fixed at unity. The typical dimensions of the core range from 10 to 40 nm, and the shell thickness ranges from 1 to 11 nm. The polarization of incident light is along the y direction in the followed calculations.

remains constant at 10 nm while the thickness of the shell increases from 1 to 15 nm. The resonance peak shown in the Figure 3 is at ∼505 nm with no shift while the thickness changes. In this stage, the amplitude of the absorption becomes stronger with changing shell thickness. In the stage, whatever changes the radius of the core or the shell thickness, the absorption cross-section becomes larger; it is reasonable that the location of the resonance peak has no shift, not only the red shift but also the blue shift, or a slightly red shift shown in the Figure 2, and the absorption intensity shows obvious enhancement. Also, through the absorption comparison of the bare core, the empty shell, and the solid Au/Au core/shell system, it can be concluded that the absorption intensity of the empty shell is the highest with the same size. The empty shell with a cavity provides two surfaces, that is, the inner surface and the outer surface. The existence of the two surfaces makes it possible that the plasmon of the inner and outer surfaces interact with each other, but apparently there is no obvious change in the spectrum shown in the Figure 2. The same composition of the surfaces makes the effect of cavity looks like an amplifier. The absorption intensity is enhanced, and the location of the resonance peak of the empty Au shell has a red shift of ∼20 nm compared with the spectra of the bare Au core and Au/Au solid core/shell system. 3.1.2. Au/Ag Solid Core/Shell System. The photoabsorption cross section of the bare core, the empty shell, and the core− shell systems are shown in the Figure 4. A strong absorption is

3. RESULTS AND DISCUSSION 3.1. Size Dependence of the Extinction Spectrum. 3.1.1. Au/Au Solid Core/Shell System. In Figures 2 and 3,

Figure 2. Spectrum calculated for the bare core, the empty shell, and the core−shell systems. The radius of the core increases from 10 to 30 nm at intervals of 5 nm, and the shell thickness is fixed at 5 nm.

Figure 4. Spectrum calculated for the bare core, the empty shell, and the core−shell systems. The radius of the core increases from 10 to 40 nm at intervals of 5 nm.

Figure 3. Spectrum calculated for the core−shell systems with different shell thickness and fixed core radius.

seen for the system that the shell embedded on the core at 360 nm, the same plasmon frequency range with Ag. The wavelength of the SPR for the bare core and the empty shell is at 520 and 360 nm, respectively. The shell, too, has a peak at the 360 nm, the same wavelength for the core/shell system; however, it is stronger by a factor of about three. That is to say, the shell embedded on the metal core could considerably enhance the absorption of a nanoparticle at particular regions of the electromagnetic spectrum. In this stage, the dependence of the SPR spectrum on the nanosphere size is through two procedures, that is to say, keeping the radius of the core constant at 10 nm while the thickness of the shell ranges from 1 to 5 nm successively and 5 to 20 nm at intervals of 5 nm or keeping the thickness of the shell at 5 nm by changing the

spectra show the photoabsorption cross section of the bare core with radius of 10 nm, the empty shell with the thickness of 5 nm and inner radius of 10 nm, and the Au/Au core−shell systems. In the first stage, the radius of the core varies from 10 to 30 nm, whereas the thickness of the shell remains constant at 5 nm, and the absorption spectrum is shown in the Figure 2. It can be seen that the bare Au core has a resonance peak at 510 nm, and the empty Au shell has a resonance peak at 530 nm; the amplitude of the latter is higher. Also, the spectra in Figure 2 reveal that while the radius of the core increases from 10 to 30 nm, the resonance peak of the solid Au/Au core/shell system red shift from 508 to 520 nm and the amplitude becomes higher. In the second stage, the radius of the core 24048

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radius of the core from 10 to 40 nm at intervals of 5 nm. Figure 4 shows the extinction spectra calculated for nanospheres having a different radius of core with the fixed thickness of the shell of 5 nm. In increasing the radius of the core, we note an evolution of the spectra plotted in Figure 4 with the band being shifted toward the longer wavelength slightly. The intensity of the SPR increases when the radius of the core changes from 10 to 30 nm. From the data obtained previously and the data we calculated here, the band at 400 nm is contributed by Ag and the amplitude increases when the radius of the core becomes bigger. There are very weak peaks between 500 and 900 nm in these diagrams for the radius of core from 10 to 30 nm. Compared with the spectrum of the bare Au core and the unassisted Ag shell, the amplitude of the spectrum for solid Au/ Ag core/shell nanosphere is enhanced. The increase in the band amplitude with the radius of core at constant can obviously be explained by the fact that the effective radius of the Au/Ag core/shell solid sphere also increases. Although the Au core added with the Ag shell has a bigger absorption crosssection and effective radius, the absorption intensity attributed to Au core is small. A conclusion can be obtained that the noble-metal shell added to a core, forming a solid core/shell system, strongly shields the electromagnetic property of the metal core, and the SPR property of the metal shell plays the decisive role in this system, revealed by the spectrum shown in Figure 4. When the radius of the core reaches higher than 30 nm, two peaks begin to appear at 525 and 360 nm, respectively. According to the spectrum calculated for the bare Au core and precious data, the wave range of the SPR for Au is in the red field; that is to say, it is reasonable that we attribute the band appearing at 525 nm to the effect of the Au core. Also, a peak at 425 nm is obtained whether or not the spectrum of Au core or Ag shell has no peak around this wavelength. This peak is located at the area between the SPR peak of the Au core and the Ag shell, and the amplitude is far smaller than that of the peak attributed to Ag shell, whereas it is stronger than that of the peak attributed to Au core. Although the metal shell plays a major role for the SPR in the solid metal core/shell system and the SPR of metal core is strongly shielded by the shell added to it, with the radius of the metal core becomes bigger and bigger, the effect of plasmon−plasmon coupling between the metal core and shell appears and can not be neglected. The peak at 425 nm is attributed to this effect, and in the solid metal core/ shell system, we think the deduction that the intensity of the plasmon−plasmon coupling is stronger than the effect of the SPR of Au core is reasonable. Furthermore, shielded by the metal shell, the SPR of the Au core reveals its existence in the solid metal core/shell system. We keep the radius of core as a constant and change the thickness of the metal shell from 1 nm to 5 nm at intervals of 1 nm and from 5 to 15 nm at intervals of 5 nm. In this stage, the amplitude of the spectrum becomes bigger and so do the discrepancy of the amplitude between the wave range from 350 to 400 nm and the area from 500 to 900 nm. When the thickness of the shell increases, the spectrum in Figure 5 shows a slight red shift from 360 to 410 nm, and the intensity around 400 nm is obviously enhanced. The fact that the absorption cross-section and the effective radius of the core/shell system become bigger while the shell thickness increases can explain this phenomenon. The immense discrepancy of the amplitude between the wave range from 350 to 400 nm and the area from 500 to 900 nm implies further that the metal shell in the solid metal core/shell system shields the SPR of the core so strongly

Figure 5. Spectrum calculated for the core−shell systems with different shell thickness. The shell thickness increases from 1 to 15 nm.

that the SPR of shell plays a dominant role in this system. With the increase in the shell thickness, the shielding effect to the SPR of the core becomes bigger, and the SPR of the solid core/ shell system is tuned to match the optical property of the metal shell. At the same time, the amplitude of the SPR of the core/ shell system obviously enhances. Through the two stages above, we can conclude that whatever increases the radius of the ball core or the thickness of the shell will make the SPR of the solid core/shell system red shift slightly and the amplitude enhanced. The shielding effect of the shell dominates the optical property of the system with the metal core combined with the metal shell to be a solid core/shell sphere. To make the metal core and the metal shell separate by several nanometers is a pertinent pathway to further study the core/shell system and the effect of the metal shell embedded on the metal core. The work about this study is written in Section 3.2, and the resonance cavity effect is completely discussed. 3.2. Calculation and Discussion of Resonance Cavity Effect. 3.2.1. Calculation of the Separate Au/Au Core/Shell System. The calculation of Au/Au solid core/shell systems in Section 3.1.1 shows one resonance peak in the spectrum, and the trend is the same with the Au nanosphere. The plasmon− plasmon interaction is not observed in Section 3.1.1. To further study the core/shell system, we will calculate the spectrum of the separate core/shell system, and the results are revealed in the Figures 6 and 7. The simulated system calculated here is shown in Figure 1B. Compared with the system shown in the Figure 1A calculated in Section 3.1, the Au core and the Au shell are separated by several nanometers, leading to the formation of a cavity. Figure 6 shows the system keeping the radius of core 10 nm and shell thickness 5 nm while changing the distance between the core and shell from 2 to 10 nm. The spectrum of system with the core radius 10 nm and the distance remaining at 10 nm while shell thickness increases is shown in the Figure 7. Apparently, the spectra show two different absorption peaks. Also, the intensity of the resonance is enhanced compared with the solid core/shell system, and the enhancement factor is even high to 4, shown in the Figure 7. In the Figure 6, one peak is located at 515 nm and one peak is located at ∼700 nm which will blue shift from 775 to 680 nm while the separate distance ranges from 2 to 10 nm. The peak at 515 nm is the resonance peak of Au with no cavity between the 24049

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Figure 6. Spectrum calculated for the core/shell system separated by 2, 4, 6, 8, and 10 nm, respectively. Figure 8. Spectrum calculated for the core/shell system separated by 2, 4, 6, 8, and 10 nm, respectively. The intensity of the spectrum with the separate distance is amplified by a factor of 2, 3, and 4, respectively.

leading to the formation of a cavity here. First, with the same radius of core and shell thickness, it can be seen that the amplitude is enhanced by a factor of about four in this system. Second, one new resonance peak appears in the spectrum calculated for this separate system. In the Figure 8, one new resonance peak is obtained compared with the spectrum calculated for the solid Au/Ag core/shell nanospheres. Also, Figure 8 clearly displays that the enhancement of the amplitude increases with the bigger distance. We think the resonance peaks at 330, 400, and 470 nm in Figure 8 result from the blue shift of the resonance peaks at 360, 410, and 520 nm shown in Figure 4. Figure 9 also shows four resonance peaks at about Figure 7. Spectrum calculated for the core/shell system separated by 10 nm; the thickness of the shell is 3, 5, 7, 9, and 11 nm, respectively.

core and shell and does not change with the different separate distance. The peak at ∼700 nm shows the trend that is tuned to match the spectrum line of the core and the absorption intensity becomes stronger with bigger separate distance. It is reasonable to attribute the resonance peak to the plasmon− plasmon interaction and the existence of the cavity. The spectra in the Figure 7 show the same trend as Figure 6. The peak at 700 nm blue shifts while the shell thickness increases, but absorption intensity generally remains constant. It can be seen the absorption intensity is strongest when the separate distance is 10 nm and the shell thickness is 5 nm, which is two times smaller than the core radius. Through the study above, it can be concluded that the shell plasmon frequency is tuned to the core absorption line due to the existence of the cavity 3.2.2. Calculation of the Separate Au/Ag Core/Shell System. In Figure 8, the calculated spectrum reveals four significant resonance peaks; they are located at 330, 400, 470, and 600 nm, respectively. The resonance peaks at 330, 400, and 470 nm in the spectrum have been attributed to the Ag shell, the plasmom−plasmon coupling effect, and the Au core in the study above. The simulated system calculated here is shown in Figure 1B. The radius of the core and the shell thickness remains as 10 and 5 nm, respectively. Compared with the system shown in the Figure 1A calculated in Section 3.1, the Au core and the Ag shell are separated by several nanometers,

Figure 9. Spectrum calculated for the core/shell system separated by 10 nm and the shell thickness ranges from 3 to 11 nm at intervals of 2 nm.

330, 400, 476, and 600 nm in the spectrum. The trend that the resonance peak blue shifts from 630 to 480 nm can be obtained with the increased shell thickness. Here the separate distance and the radius of the core remain at constant 10 nm. With the increasing shell thickness, two resonance peaks at about 360 and 500 nm are left in the spectrum. It is logical to think that the shielding effect of the shell becomes stronger when the size 24050

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Figure 10. (A) Enhancement of separate Au/Au core/shell system. (B) Enhancement of separate Au/Ag core/shell system.

Figure 11. Calculated electric field |E|2/|E0|2 of (A) the bare core, (B) the empty shell, and (C,D) two different resonance wavelengths for the separate core/shell system.

of the shell is equivalent to the size of the core and the separate distance. Furthermore, the highest intensity reveals that the shell size is equivalent to the core size or two times smaller than the core size and the separate distance. In Figures 8 and 9, a strong blue shift happens and the resonance peak is tuned to the line of the core varying the shell thickness or separate distance. All of these changes that happened here result from the cavity between the Au core and the Ag shell. A cavity resonator uses resonance to amplify an electromagnetic wave. The cavity has interior surfaces that supply the condition for a wave of a specific frequency to bounce back and forth within it, with low loss. The system with separate core and shell undoubtedly forms a cavity between the core and shell. Here the cavity as a bridge between the Au core and Ag shell joins or connects them. Obviously, the existence of the cavity makes the blue shift happen and enhances the plasmon−

plasmon coupling effect. In ref 7, the calculation shows that the sum of the positive charge density of the separate core and shell system is to a high degree of accuracy equal to the density of the combined system. This is an indication of the weak initial coupling effect between the metal core and shell. A strong magnetic field in a resonance cavity is an ideal circumstance for initiating orbital motions of electrons and nuclei in solids. Because the SPR results from the collective oscillating when the electric field of the incident light resonantly couples to the conduction electrons at the same frequency, the motion of the electrons will enhance the collective oscillating anyway and obtain higher absorption intensity. Both of these effects change the absorption property of the core/shell system (Figure 10). 3.3. Computional Study of Electric Field. Figure 11 shows a plot of the electric field intensity |E|2/|E0|2 for the plane that the polarization direction is perpendicular to the 24051

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propagation direction, as shown in the graph. The calculated four systems are as follows: (A) the bare Au core with radius of 10 nm, (B) the empty Au shell with thickness of 5 nm and internal radius of 10 nm, and (C,D) the separate core/shell system with core radius of 10 nm, shell internal radius of 15 nm, and shell thickness of 5 nm. The four resonance wavelengths are 505, 530, 514, and 680 nm, respectively. As shown in the Figure 8, the strong field enhancement in the core and shell is along the polarization, whereas the shell has the higher field enhancement. Also, it can be obviously seen that the field enhancement is located at on the surface of the core and the shell. C and D are two resonance patterns of separate core/shell system located at 514 and 680 nm, respectively. When the resonance wavelength is 514 nm, the field enhancement is along the polarization, whereas that of the resonance wavelength 680 nm is perpendicular to the polarization. The field enhancement of the two patterns is strongly localized on the surface of the core and shell, respectively. The fact above further ensures that the resonance peak at 514 nm results from the SPR of core. Also, for the resonance wavelength at 680 nm, the surface of the core has very high field enhancement along the polarization compared with that of the shell, and the field enhancement of the latter resonance pattern is far higher than that of the former. The results in Section 3.2 show that the resonance of the core/shell system matches the extinction line of the core and the resonance peak at ∼600 nm blue shifts while the shell thickness or the separation distance increases. This implies that the field enhancement of the separate core/shell system will transform from the shell to the surface of the core. The field enhancement at 680 nm confirms it with higher localized field enhancement on the surface of the core. From the previous results and Section 3, it can be concluded that the higher field enhancement induces stronger absorption intensity and the location of the field enhancement of the separate core/shell system is determined by the specific resonance wavelength.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86 0816 2480827; Fax: +86 0816 2480830 (Y.Y.). E-mail: [email protected]; Tel: +86 0816 2480827; Fax: +86 0816 2480830 (Y.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (no. 11075143), Foundation of Science and Technology on Plasma Physics Laboratory (grant no. 9140C682501110C6803), Open-End Fund for the Valuable and Precision Instruments of Central South University, and the Developing Foundation of China Academy of Engineering Physics (grant no. 2010B0401056).

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4. CONCLUSIONS Indeed, the response of the shell-enclosed core to the incident wave grows, and large mixing coupling effects and enhancement of core absorption are obtained. The response of a core system can be profoundly changed when enclosed in a metallic nanoshell. The spectrum of the core added to a separate metal shell reveals different characteristics compared with the solid core/shell system. The mixing coupling effect changes the SPR of the core/shell system greatly. No matter how the shell thickness and the separate distance are changed, the resonance peaks blue shift. The shell frequency is tuned to match the optical properties of the absorbing core, leading to hybridization/mixing and possibly overall enhancement of the absorption cross-section of the composite nanoparticle or its individual components and shows excellent ability to adjust the resonance peaks. Furthermore, the highest intensity reveals when the shell size is equivalent to the core size or two times smaller than the core size and the separate distance. The calculation of the field enhancement also shows that the location of the field enhancement is specified by the resonance pattern. 24052

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