- Email: [email protected]
Fabrication and infrared-transmission properties of monolayer hexagonal-close-packed metallic nanoshells
Optics Communications 297 (2013) 194–197
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Fabrication and infrared-transmission properties of monolayer hexagonal-close-packed metallic nanoshells Jing Chen a,n, Rongqing Xu a, Zhengqi Liu b, Chaojun Tang b, Zhuo Chen b, Zhenlin Wang b a b
College of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2012 Received in revised form 23 January 2013 Accepted 26 January 2013 Available online 22 February 2013
This paper presents a novel method for fabricating a monolayer of hexagonal-close-packed metallic nanoshells with a small opening, based on a combination of a porous polymer template and a nanocrystal-seeded electroless plating technique. Light transmission spectra of the metallic nanoshell arrays are measured, which show that light can transmit through the dense particle assemblies via excitations of a variety of surface-plasmons (SPs). Further numerical simulations confirm these transmission resonances and reveal that they are attributed to the excitations of localized quadrupolar spherelike and Fano-type hybridized SP modes supported by the specific structure. The present metallic microstructure could find applications in plasmonics. & 2013 Elsevier B.V. All rights reserved.
Keywords: Metallic nanoshell array Surface plasmons Transmission resonance
1. Introduction Metallic nanoparticles exhibit vivid optical properties [1] mainly because they can support localized surface plasmons (LSPs), i.e., collective oscillations of conduction-band electrons confined in the particles [2]. Optical behavior of metallic particles can be tailored over a broad range of spectrum by changing the particle size, shape, composition or surrounding medium [3]. For example, it has been reported experimentally that when an individual gold nanoshell is reshaped into a symmetry-reduced nanoegg or nanocup, splitting of plasmon modes at the single nanostructure level is formed [4]. A large amplification of the local optical fields around these particles associated with LSP excitations has lead to important applications in plasmonic nanolasing [5], sensing [6], surface-enhanced Raman scattering [7,8], optical antennas [9], and optical tweezers [10]. When metallic nanoparticles are further patterned into one- or twodimensional (2D) arrays, they exhibit optical responses dramatically different from those of an individual nanoparticle [11–16]. For example, narrow-band plasmon modes caused by the interaction between LSP modes and diffracted lattice modes occurring near the Wood anomaly have been predicted [11,12] and demonstrated experimentally [13–16], whose physical mechanism has been revealed to be the far-field interparticle LSPs coupling. Due to the characteristics of narrow bandwidth, such a hybridized
n
Corresponding author. Tel.: þ86 159 519 97978. E-mail address: [email protected] (J. Chen).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.01.074
plasmon mode has been used to enhance the fluorescent emission in plasmonic nanoantenna arrays [17]. However, less is known about the optical properties of dense metallic nanoparticle arrays [18], where higher-order SP modes and their near-field interactions could become extremely important for controlling the collective optical properties [19–22]. Among near-field interparticle LSPs coupling, the classical Fano effect, which occur in the form of a asymmetric profile in the spectra, arise from the interference between a spectrally overlapping broad resonance or a continuum with a narrow discrete resonance [23]. Recent works have shown that in some carefully engineered plasmonic nanostructures, highly localized optical fields can be generated at subradiant plasmon resonance frequencies [24]. In this letter, we reported a novel method to prepare a monolayer of hexagonal-close-packing (HCP) gold nanoshells with hollow interiors by electroless deposition within a templating organic porous mold. Light transmission resonances (TRs) are observed experimentally through the metallic periodic microstructure and their physical origins as a result of localized quadrupolar spherelike and Fano-type hybridized SP modes excitations are identified through our numerical simulations.
2. Sample fabrication procedure The process for the fabrication of hollow ordered gold nanoshells involves several distinct steps [25], which are illustrated schematically in Fig. 1. In the experiment, the starting material was a well-ordered monolayer colloidal crystal consisting of silica beads (1.58 mm in diameter with a standard deviation of 3%) on a
J. Chen et al. / Optics Communications 297 (2013) 194–197
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Fig. 2. SEM image (top view) of the 2D HCP array of hollow gold nanoshells. The upper inset shows the SEM image of the bottom-side of the same sample. The gold shell thickness is about 60 75 nm.
that the replicated gold nanoshells (radius 738 nm, thickness 60 75 nm) have a continuous and uniform wall and form a highly ordered structure (period 1476 nm). Shown in the upper inset of Fig. 2 is the SEM image of the other side of the same sample, from which it can be seen that each gold nanoshell in fact has a small opening (the inverted cone opening spanning an azimuth angle of 401 relative to the shell center), as do the voids in the PMMA matrix [25,26]. Thus, the hollow gold nanospheres in the 2D metal networks are not complete shells but looks like nanocups that are interconnected at their equator.
3. Results and discussion
Fig. 1. Schematic illustration of preparing gold nanoshell arrays.
quartz substrate. A thin layer of gold nanoparticles of about 5–8 nm in diameter was sputtered onto the surfaces of the colloidal crystal template using a Precision Etching and Coating System (PECS, Gatan Corp., Model 682). Then, the chlorobenzene solution of polymethyl methacrylate (PMMA) was dripped into this treated silica colloidal template. After the evaporation of chlorobenzene, the void spaces among the silica spheres covered with metal nanoparticles were filled with solid PMMA polymer. The subsequent removal of the silica beads by using 1 wt% HF solution overnight produced the freestanding and porous membrane with a complex architecture of air balls that replicated the structure of the starting silica colloidal template [26]. A key feature in the step was that, after etching silica spheres, Au nanoseeds were transferred to the inner wall of the polymer membrane. These seeds were accessible from the openings on the surface of the PMMA film and acted as reactive sites for the growth of the following metallic shells during electroless plating. In the next step, the polymer template was immersed in a gold plating solution to initiate a chemical reaction. In this process, the metallic seeds increased in size and coalesced, leading to the formation of continuous gold nanoshells inside the polymer cavities where adequate seeds have existed. Finally, the polymer was dissolved with chloroform solution, leaving behind a freestanding ordered array of metallic nanoshells [25,26]. Fig. 2 shows the scanning electron microscopy (SEM) image of the resulting 2D HCP array of hollow gold nanoshells. It is seen
The near-infrared transmission spectrum under normal incidence of light for the metallic shell array is measured using a Fourier-transform infrared spectrometer with a polarizer. The incident light polarization configuration in the present studies with respect to the ordered array of metallic shells is schematically shown in the left upper inset of Fig. 2. The coordinate is chosen such that the metallic shells lie on the xy-plane. Light is incident along z-axis with its polarization along x-axis. As shown in Fig. 3(a), multiple TRs indicated using the arrows, which are, respectively, located at 2012 nm, 1640 nm, 1324 nm, and 1204 nm, are observed in the normal optical transmission spectrum. In order to understand these TRs, the transmission spectrum of the gold nanoshell array embedded in PMMA matrix is measured under normal incidence and is shown in Fig. 3(b), in which the transmission spectrum of the gold nanoshell array (the dashed line) is also exhibited for comparison. More importantly, comparing the transmission spectra of the gold nanoshell array with the gold nanoshell array embedded in PMMA matrix, it can be found that the positions of the TRs for l2 and l4 indicated on Fig. 3(a) remain unchanged when the surrounding environment changes from air to PMMA matrix, which implies that the TRs of l2 and l4 are independent of the surrounding medium. As changing the surrounding medium, however, there exist a larger and slightly redshift for the TRs of l1 and l3, respectively. Based on the above discussion and our previous theoretical work [20,27], a conclusion can be made that the TRs of l2 and l4 are ascribed to the excitations of LSPs trapped inside the gold nanoshells (also referred to as voidlike plasmon modes), and for the TRs of l1 and l3, they may arise from the excitations of LSPs trapped outside the nanoshells (also referred to as spherelike plasmon modes) or delocalized
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J. Chen et al. / Optics Communications 297 (2013) 194–197
Fig. 3. Measured transmission spectra for the gold nanoshell array (a) and the gold nanoshell array in PMMA matrix (b) under normal incidence of light.
Fig. 4. xz-plane and xy-plane view of the gold nanoshell array. The dashed-line indicates the calculation domain, consisting of one complete and four quarter metallic shells.
surface-plasmon polaritons (SPPs) (also referred to as Bragg-type SPP modes) of the 2D HCP array [20,27]. To further reveal the physics of these TRs, three-dimensional numerical simulations are carried out using a commercial finiteelement method based software package (COMSOL Multiphysics). The calculation domain, indicated as a dashed-line box in the xy-plane view in Fig. 4, constitutes one complete and four quarter metallic shells. Periodic boundary conditions are applied to the four sides of the rectangular calculation domain to fit the periodicity of the whole structure [28]. As schematically shown in Fig. 4 [xz-plane view], each metallic shell is characterized by three geometry parameters: the radius of the outer shell (R), the thickness and the opening of the metallic shell (t) and (y). The values of R¼738 nm and t ¼60 nm are estimated from the SEM image shown in Fig. 2, and each shell with an inverted cone opening spanning an azimuth angle of y relative to the shell center, which is estimated to be y ¼ 401 from the inset SEM of Fig. 2. The lattice period of the metallic shell array is equal to the diameter of shell p ¼1476 nm. The permittivity of gold is described by a Drude model egold ¼1 o2p/[o(o þioc)] with plasma frequency op ¼1.367 1016 rad/s and collision frequency oc ¼4.084 1013 rad/s [29]. In the simulations, the effect of the substrate (transparent silica substrate with a refractive index of 1.45) on the resonances has been taken into account. The zero-order transmission spectrum is calculated for the metallic shell array and is shown in Fig. 5(a). As is seen, the experimentally observed transmission features for the metallic shell array (Fig. 5(b)) are well reproduced in the calculated transmission spectrum. The remaining discrepancies, such as peak narrowing and shifting, most likely arise from our simplified model for the nanostructure and the fabrication tolerances in the experiment. It can be clearly seen from Fig. 5(a) that the simulated spectra at the wavelengths of l2, l3 and l4 are very
different from l1 and show well defined asymmetric line shapes, which are the common character for Fano-type resonances [24]. We now turn our attention to the nature of these TRs in the metallic shell array. For the broad transmission peak at the wavelength of l1, the fields plotted in the xz-plane are seen to be tightly localized around the outer surface of the shell (Fig. 5(c)). It is further seen that the whole field distributions is separated into four regions by four field nodes indicated by the dashed lines [28]. This implies that the broad longer wavelength transmission peak l1 is mainly relevant to the excitation of the quadrupolar spherelike plasmons on the outer surface of single metallic shell [28]. In addition, the transmission peak at the wavelength of l3, with its field distributions shown in Fig. 5(e), is also predicted in the calculation, which could be explained as contribution from a delocalized SPP mode excited in the structure because the corresponding enhanced fields is no longer tightly confined within the nanogaps, but instead extends into the surrounding medium from the particle surface with multiple nodes [20,27]. Meanwhile, it could be seen from both Fig. 5(d) and (f) that the fields plotted in the xz-plane show strong enhancement within the void of the metallic shell, which coincides with the typical feature of the dipole and quadrupolar voidlike plasmon modes, respectively [20,27,28]. However, it is immediately found from Fig. 5(d–f) that for the TRs at the wavelengths of l2, l3 and l4, a large portion of the fields are at the same time found to be localized around the outer surface of the shell with six field nodes indicated by the dashed lines, which is the typical feature of the hexapolar spherelike plasmons on the outer surface of single metallic shell [28]. Therefore, the Fanotype resonances at the wavelengths of l2, l3 and l4 are the results of the overlap of the broader hexapolar spherelike plasmon mode with a relatively narrower dipolar voidlike, delocalized SPP and quadrupolar voidlike plasmon modes, respectively [20,27,28].
4. Summary In summary, a novel electroless deposition method was developed to fabricate large-scale 2D array of metallic nanoshells. Light transmission resonances through the metallic structure were studied both experimentally and theoretically, and these transmission resonances are confirmed to be mediated due to the excitations of localized quadrupolar spherelike and Fano-type hybridized SP modes supported by the specific structure. The plasmonic crystal could find important applications in biosensing and nanolasing.
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Fig. 5. Calculated (a) and measured (b) normal incidence transmission spectra for the metallic nanoshell array. (c)–(f) Calculated normalized electric filed intensity distributions along the xz-plane at the transmission resonances for the Au shell array. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements The authors would acknowledge financial supports from the State Key Program for Basic Research of China under Grant no. 2012CB921501 and the National Science Foundation of China under Grant Nos. 11174137, 11021403, 11104136. References ¨ [1] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995. ¨ ¨ [2] H. Hovel, S. Fritz, A. Hilger, U. Kreibig, M. Vollmer, Physical Review B: Condensed Matter 48 (1993) 18178. [3] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, Journal of Physical Chemistry B 107 (2003) 668. [4] J.B. Lassiter, M.W. Knight, N.A. Mirin, N.J. Halas, Nano Letters 9 (2009) 4326. [5] M.A. Noginov, G. Zhu, A.M. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner, Nature 460 (2009) 1110. [6] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Nature Materials 7 (2008) 442. [7] S. Nie, S.R. Emory, Science 275 (1997) 1102. ¨ L. Borjesson, ¨ [8] H. Xu, E.J. Bjerneld, M. Kall, Physical Review Letters 83 (1999) 4357. ¨ [9] P. Muhlschlegel, H.J. Eisler, O.J.F. Martin, B. Hecht, D.W. Pohl, Science 308 (2005) 1607. [10] M.L. Juan, M. Righini, R. Quidant, Nature Photonics 5 (2011) 349. [11] S. Zou, N. Janel, G.C. Schatz, Journal of Chemical Physics 120 (2004) 10871. [12] F.J.Garcı´a de Abajo, Reviews of Modern Physics 79 (2007) 1267.
[13] B. Lamprecht, G. Schider, R.T. Lechner, H. Ditlbacher, J.R. Krenn, A. Leitner, F.R. Aussenegg, Physical Review Letters 84 (2000) 4721. [14] E.M. Hicks, S. Zou, G.C. Schatz, K.G. Spears, R.P. Van Duyne, L. Gunnarsson, ¨ Nano Letters 5 (2005) 1065. T. Rindzevicius, B. Kasemo, M. Kall, [15] V.G. Kravets, F. Schedin, A.N. Grigorenko, Physical Review Letters 101 (2008) 087403. [16] B. Auguie´, W.L. Barnes, Physical Review Letters 101 (2008) 143902. [17] G. Vecchi, V. Giannini, J.G. Rivas, Physical Review Letters 102 (2009) 146807. [18] Z. Chen, P. Zhan, Z.L. Wang, J.H. Zhang, W.Y. Zhang, N.B. Ming, C.T. Chan, P. Sheng, Advanced Materials 16 (2004) 417. [19] B.N. Khlebtsov, V.A. Khanadeyev, J. Ye, D.W. Mackowski, G. Borghs, N.G. Khlebtsov, Physical Review B: Condensed Matter 77 (2008) 035440. [20] C.J. Tang, Z.L. Wang, W.Y. Zhang, S.N. Zhu, N.B. Ming, G. Sun, P. Sheng, Physical Review B: Condensed Matter 80 (2009) 165401. [21] S.A. Maier, P.G. Kik, H.A. Atwater, S. Meltzer, E. Harel, B.E. Koel, A.G. Requicha, Nature Materials 2 (2003) 229. [22] F. Le, D.W. Brandl, Y.A. Urzhumov, H. Wang, J. Kundu, N.J. Halas, J. Aizpurua, P. Nordlander, ACS Nano 2 (2008) 707. [23] U. Fano, Physical Review 124 (1961) 1866. [24] B. Luk’yanchuk, N.I. Zheludev, S.A. Maier, N.J. Halas, P. Nordlander, H. Giessen, C.T. Chong, Nature Materials 9 (2010) 707. [25] W. Dong, H. Dong, Z.L. Wang, P. Zhan, Z.Q. Yu, X.N. Zhao, Y.Y. Zhu, N.B. Ming, Advanced Materials 18 (2006) 755. [26] Z. Chen, H. Dong, J. Pan, P. Zhan, C.J. Tang, Z.L. Wang, Applied Physics Letters 96 (2010) 051904. [27] Q.G. Wang, C.J. Tang, J. Chen, P. Zhan, Z.L. Wang, Optics Express 19 (2011) 23889. [28] J. Chen, Q. Shen, Z. Chen, Q.G. Wang, C.J. Tang, Z.L. Wang, Journal of Chemical Physics 136 (2012) 214703. [29] M.A. Ordal, L.L. Long, R.J. Bell, S.E. Bell, R.R. Bell, R.W. Alexander Jr, C.A. Ward, Applied Optics 22 (1983) 1099.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Fabrication and infrared-transmission properties of monolayer hexagonal-close-packed metallic nanoshells Jing Chen a,n, Rongqing Xu a, Zhengqi Liu b, Chaojun Tang b, Zhuo Chen b, Zhenlin Wang b a b
College of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2012 Received in revised form 23 January 2013 Accepted 26 January 2013 Available online 22 February 2013
This paper presents a novel method for fabricating a monolayer of hexagonal-close-packed metallic nanoshells with a small opening, based on a combination of a porous polymer template and a nanocrystal-seeded electroless plating technique. Light transmission spectra of the metallic nanoshell arrays are measured, which show that light can transmit through the dense particle assemblies via excitations of a variety of surface-plasmons (SPs). Further numerical simulations confirm these transmission resonances and reveal that they are attributed to the excitations of localized quadrupolar spherelike and Fano-type hybridized SP modes supported by the specific structure. The present metallic microstructure could find applications in plasmonics. & 2013 Elsevier B.V. All rights reserved.
Keywords: Metallic nanoshell array Surface plasmons Transmission resonance
1. Introduction Metallic nanoparticles exhibit vivid optical properties [1] mainly because they can support localized surface plasmons (LSPs), i.e., collective oscillations of conduction-band electrons confined in the particles [2]. Optical behavior of metallic particles can be tailored over a broad range of spectrum by changing the particle size, shape, composition or surrounding medium [3]. For example, it has been reported experimentally that when an individual gold nanoshell is reshaped into a symmetry-reduced nanoegg or nanocup, splitting of plasmon modes at the single nanostructure level is formed [4]. A large amplification of the local optical fields around these particles associated with LSP excitations has lead to important applications in plasmonic nanolasing [5], sensing [6], surface-enhanced Raman scattering [7,8], optical antennas [9], and optical tweezers [10]. When metallic nanoparticles are further patterned into one- or twodimensional (2D) arrays, they exhibit optical responses dramatically different from those of an individual nanoparticle [11–16]. For example, narrow-band plasmon modes caused by the interaction between LSP modes and diffracted lattice modes occurring near the Wood anomaly have been predicted [11,12] and demonstrated experimentally [13–16], whose physical mechanism has been revealed to be the far-field interparticle LSPs coupling. Due to the characteristics of narrow bandwidth, such a hybridized
n
Corresponding author. Tel.: þ86 159 519 97978. E-mail address: [email protected] (J. Chen).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.01.074
plasmon mode has been used to enhance the fluorescent emission in plasmonic nanoantenna arrays [17]. However, less is known about the optical properties of dense metallic nanoparticle arrays [18], where higher-order SP modes and their near-field interactions could become extremely important for controlling the collective optical properties [19–22]. Among near-field interparticle LSPs coupling, the classical Fano effect, which occur in the form of a asymmetric profile in the spectra, arise from the interference between a spectrally overlapping broad resonance or a continuum with a narrow discrete resonance [23]. Recent works have shown that in some carefully engineered plasmonic nanostructures, highly localized optical fields can be generated at subradiant plasmon resonance frequencies [24]. In this letter, we reported a novel method to prepare a monolayer of hexagonal-close-packing (HCP) gold nanoshells with hollow interiors by electroless deposition within a templating organic porous mold. Light transmission resonances (TRs) are observed experimentally through the metallic periodic microstructure and their physical origins as a result of localized quadrupolar spherelike and Fano-type hybridized SP modes excitations are identified through our numerical simulations.
2. Sample fabrication procedure The process for the fabrication of hollow ordered gold nanoshells involves several distinct steps [25], which are illustrated schematically in Fig. 1. In the experiment, the starting material was a well-ordered monolayer colloidal crystal consisting of silica beads (1.58 mm in diameter with a standard deviation of 3%) on a
J. Chen et al. / Optics Communications 297 (2013) 194–197
195
Fig. 2. SEM image (top view) of the 2D HCP array of hollow gold nanoshells. The upper inset shows the SEM image of the bottom-side of the same sample. The gold shell thickness is about 60 75 nm.
that the replicated gold nanoshells (radius 738 nm, thickness 60 75 nm) have a continuous and uniform wall and form a highly ordered structure (period 1476 nm). Shown in the upper inset of Fig. 2 is the SEM image of the other side of the same sample, from which it can be seen that each gold nanoshell in fact has a small opening (the inverted cone opening spanning an azimuth angle of 401 relative to the shell center), as do the voids in the PMMA matrix [25,26]. Thus, the hollow gold nanospheres in the 2D metal networks are not complete shells but looks like nanocups that are interconnected at their equator.
3. Results and discussion
Fig. 1. Schematic illustration of preparing gold nanoshell arrays.
quartz substrate. A thin layer of gold nanoparticles of about 5–8 nm in diameter was sputtered onto the surfaces of the colloidal crystal template using a Precision Etching and Coating System (PECS, Gatan Corp., Model 682). Then, the chlorobenzene solution of polymethyl methacrylate (PMMA) was dripped into this treated silica colloidal template. After the evaporation of chlorobenzene, the void spaces among the silica spheres covered with metal nanoparticles were filled with solid PMMA polymer. The subsequent removal of the silica beads by using 1 wt% HF solution overnight produced the freestanding and porous membrane with a complex architecture of air balls that replicated the structure of the starting silica colloidal template [26]. A key feature in the step was that, after etching silica spheres, Au nanoseeds were transferred to the inner wall of the polymer membrane. These seeds were accessible from the openings on the surface of the PMMA film and acted as reactive sites for the growth of the following metallic shells during electroless plating. In the next step, the polymer template was immersed in a gold plating solution to initiate a chemical reaction. In this process, the metallic seeds increased in size and coalesced, leading to the formation of continuous gold nanoshells inside the polymer cavities where adequate seeds have existed. Finally, the polymer was dissolved with chloroform solution, leaving behind a freestanding ordered array of metallic nanoshells [25,26]. Fig. 2 shows the scanning electron microscopy (SEM) image of the resulting 2D HCP array of hollow gold nanoshells. It is seen
The near-infrared transmission spectrum under normal incidence of light for the metallic shell array is measured using a Fourier-transform infrared spectrometer with a polarizer. The incident light polarization configuration in the present studies with respect to the ordered array of metallic shells is schematically shown in the left upper inset of Fig. 2. The coordinate is chosen such that the metallic shells lie on the xy-plane. Light is incident along z-axis with its polarization along x-axis. As shown in Fig. 3(a), multiple TRs indicated using the arrows, which are, respectively, located at 2012 nm, 1640 nm, 1324 nm, and 1204 nm, are observed in the normal optical transmission spectrum. In order to understand these TRs, the transmission spectrum of the gold nanoshell array embedded in PMMA matrix is measured under normal incidence and is shown in Fig. 3(b), in which the transmission spectrum of the gold nanoshell array (the dashed line) is also exhibited for comparison. More importantly, comparing the transmission spectra of the gold nanoshell array with the gold nanoshell array embedded in PMMA matrix, it can be found that the positions of the TRs for l2 and l4 indicated on Fig. 3(a) remain unchanged when the surrounding environment changes from air to PMMA matrix, which implies that the TRs of l2 and l4 are independent of the surrounding medium. As changing the surrounding medium, however, there exist a larger and slightly redshift for the TRs of l1 and l3, respectively. Based on the above discussion and our previous theoretical work [20,27], a conclusion can be made that the TRs of l2 and l4 are ascribed to the excitations of LSPs trapped inside the gold nanoshells (also referred to as voidlike plasmon modes), and for the TRs of l1 and l3, they may arise from the excitations of LSPs trapped outside the nanoshells (also referred to as spherelike plasmon modes) or delocalized
196
J. Chen et al. / Optics Communications 297 (2013) 194–197
Fig. 3. Measured transmission spectra for the gold nanoshell array (a) and the gold nanoshell array in PMMA matrix (b) under normal incidence of light.
Fig. 4. xz-plane and xy-plane view of the gold nanoshell array. The dashed-line indicates the calculation domain, consisting of one complete and four quarter metallic shells.
surface-plasmon polaritons (SPPs) (also referred to as Bragg-type SPP modes) of the 2D HCP array [20,27]. To further reveal the physics of these TRs, three-dimensional numerical simulations are carried out using a commercial finiteelement method based software package (COMSOL Multiphysics). The calculation domain, indicated as a dashed-line box in the xy-plane view in Fig. 4, constitutes one complete and four quarter metallic shells. Periodic boundary conditions are applied to the four sides of the rectangular calculation domain to fit the periodicity of the whole structure [28]. As schematically shown in Fig. 4 [xz-plane view], each metallic shell is characterized by three geometry parameters: the radius of the outer shell (R), the thickness and the opening of the metallic shell (t) and (y). The values of R¼738 nm and t ¼60 nm are estimated from the SEM image shown in Fig. 2, and each shell with an inverted cone opening spanning an azimuth angle of y relative to the shell center, which is estimated to be y ¼ 401 from the inset SEM of Fig. 2. The lattice period of the metallic shell array is equal to the diameter of shell p ¼1476 nm. The permittivity of gold is described by a Drude model egold ¼1 o2p/[o(o þioc)] with plasma frequency op ¼1.367 1016 rad/s and collision frequency oc ¼4.084 1013 rad/s [29]. In the simulations, the effect of the substrate (transparent silica substrate with a refractive index of 1.45) on the resonances has been taken into account. The zero-order transmission spectrum is calculated for the metallic shell array and is shown in Fig. 5(a). As is seen, the experimentally observed transmission features for the metallic shell array (Fig. 5(b)) are well reproduced in the calculated transmission spectrum. The remaining discrepancies, such as peak narrowing and shifting, most likely arise from our simplified model for the nanostructure and the fabrication tolerances in the experiment. It can be clearly seen from Fig. 5(a) that the simulated spectra at the wavelengths of l2, l3 and l4 are very
different from l1 and show well defined asymmetric line shapes, which are the common character for Fano-type resonances [24]. We now turn our attention to the nature of these TRs in the metallic shell array. For the broad transmission peak at the wavelength of l1, the fields plotted in the xz-plane are seen to be tightly localized around the outer surface of the shell (Fig. 5(c)). It is further seen that the whole field distributions is separated into four regions by four field nodes indicated by the dashed lines [28]. This implies that the broad longer wavelength transmission peak l1 is mainly relevant to the excitation of the quadrupolar spherelike plasmons on the outer surface of single metallic shell [28]. In addition, the transmission peak at the wavelength of l3, with its field distributions shown in Fig. 5(e), is also predicted in the calculation, which could be explained as contribution from a delocalized SPP mode excited in the structure because the corresponding enhanced fields is no longer tightly confined within the nanogaps, but instead extends into the surrounding medium from the particle surface with multiple nodes [20,27]. Meanwhile, it could be seen from both Fig. 5(d) and (f) that the fields plotted in the xz-plane show strong enhancement within the void of the metallic shell, which coincides with the typical feature of the dipole and quadrupolar voidlike plasmon modes, respectively [20,27,28]. However, it is immediately found from Fig. 5(d–f) that for the TRs at the wavelengths of l2, l3 and l4, a large portion of the fields are at the same time found to be localized around the outer surface of the shell with six field nodes indicated by the dashed lines, which is the typical feature of the hexapolar spherelike plasmons on the outer surface of single metallic shell [28]. Therefore, the Fanotype resonances at the wavelengths of l2, l3 and l4 are the results of the overlap of the broader hexapolar spherelike plasmon mode with a relatively narrower dipolar voidlike, delocalized SPP and quadrupolar voidlike plasmon modes, respectively [20,27,28].
4. Summary In summary, a novel electroless deposition method was developed to fabricate large-scale 2D array of metallic nanoshells. Light transmission resonances through the metallic structure were studied both experimentally and theoretically, and these transmission resonances are confirmed to be mediated due to the excitations of localized quadrupolar spherelike and Fano-type hybridized SP modes supported by the specific structure. The plasmonic crystal could find important applications in biosensing and nanolasing.
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Fig. 5. Calculated (a) and measured (b) normal incidence transmission spectra for the metallic nanoshell array. (c)–(f) Calculated normalized electric filed intensity distributions along the xz-plane at the transmission resonances for the Au shell array. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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