Onset of Bonding Plasmon Hybridization Preceded by Gap Modes in

Nov 12, 2013 - Tunable, hybridized dark bonding modes were seen with first-order gap modes preceding the appearance of bonding dipole–dipole disk mo...
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Letter pubs.acs.org/NanoLett

Onset of Bonding Plasmon Hybridization Preceded by Gap Modes in Dielectric Splitting of Metal Disks Maj Frederiksen,† Vladimir E. Bochenkov,†,‡ Ryosuke Ogaki,† and Duncan S. Sutherland*,† †

Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus, Denmark Chemistry Department, Lomonosov Moscow State University, Moscow 119991, Russia

‡

S Supporting Information *

ABSTRACT: Dielectric splitting of nanoscale disks was studied experimentally and via finite-difference time-domain (FDTD) simulations through systematic introduction of multiple ultrathin dielectric layers. Tunable, hybridized dark bonding modes were seen with first-order gap modes preceding the appearance of bonding dipole−dipole disk modes. The observed bright dipolar mode did not show the energy shift expected from plasmon hybridization but activated dark higher order gap modes. Introducing lateral asymmetry was shown to remodel the field distribution resulting in 3D asymmetry that reoriented the dipole orientation away from the dipole of the elementary disk modes. KEYWORDS: Localized surface plasmon resonance, plasmon hybridization, MIM, nanodisk, symmetry breaking

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hybridization showing strong distance dependent coupling. A dimer of vertically stacked nanodisks can be generated conceptually from a metallic nanodisk by inserting a dielectric spacer layer, thus producing metal−insulator−metal (MIM) nanodisks. Currently, there is a strong interest in studying strongly coupled systems with small gap distances, which can be achieved through self-assembly of chemically synthesized structures.24 The self-assembly process, however, only offers limited control over interstructural alignment, orientation, and gap distances. The benefit of the MIM nanodisk system is the high level of control that present day fabrication techniques allow us to exert on the thickness of the insulator layer and hence the distance and coupling between the two plasmonic disks. Coupling in MIM nanodisks are often discussed in terms of the plasmon hybridization of the elemental dipole plasmon modes of the individual disks resulting in a bonding and an antibonding mode corresponding to the asymmetric (out of phase) and symmetric (in phase) charge distribution on the two disks, respectively.23,25 It is possible to experimentally isolate and study the response of plasmonic gap modes in the dielectric layer between the metal surfaces, as shown by Kuttge et al. who studied Ag/SiO2/Ag nanodisk resonators through cathodoluminescence imaging spectroscopy. The MIM structures were produced with capping 10 nm chromium layers between the silver structure and the substrate and on top of the upper silver disk to damp out SPPs on the silver/substrate and silver/air interfaces. A range of higher order modes, identified as cavity modes in a Fabry−Perot resonator were detected in

lasmonic nanostructures can provide an important contribution to the development and design of a wide range of functional materials and devices. Application fields of strong current interest for the plasmonics community include metamaterials,1,2 bio- and chemical sensors,3,4 photovoltaics,5 and biomedical applications such as drug delivery6,7 and photothermal cancer treatment.8 Future design and optimization of current devices rests on an in-depth understanding of the plasmonic response of complex nanostructures. Great advances in conceptual understanding were made with the introduction of the plasmon hybridization model by Nordlander and co-workers.9 The model considers the coupling of LSPRs supported by neighboring structures or metal surfaces in complex geometries. Complex nanostructure geometries are deconstructed into simpler, more elementary geometries, and the elemental LSPRs of the individual constituents are determined. The hybridized modes of the complex structures are subsequently generated by considering the interaction of the elemental resonances with one another. A new mode with the overall energy lowered by the interaction is referred to as a bonding mode while the one with increased overall energy is an antibonding mode, analogues to the bonding and antibonding splitting in molecular orbital theory. The model has been used to explain and predict the plasmonic response of a multitude of structures such as rings,10 shells,9,11 and dimers12,13 and more complex structures including star shapes14 and ring/disk cavities,15,16 underlining the versatility of the model. A much-studied geometry in plasmonics is the nanodisk. Single layer plasmonic nanodisks have been extensively studied and are found to support a single dipole plasmon mode when illuminated at normal incidence.17−21 Dimers of nanodisks arranged laterally22 or vertically23 have been used to study © 2013 American Chemical Society

Received: August 31, 2013 Revised: November 9, 2013 Published: November 12, 2013 6033

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colloidal lithography (HCL) method26 was used with 0.16 μm polystyrene particles to generate the mask utilized in the fabrication of both symmetrically and asymmetrically stacked disks. The bottom gold disk (20 nm height) and the spacer layers were prepared identically for both types of samples, and the upper gold (20 nm height) disk was evaporated with no further modification for the symmetrically stacked samples. The spacer layer thickness was controlled by performing a number of consecutive aluminum depositions interrupted by oxidation inducing procedures. In each deposition 1.5 nm of aluminum was evaporated, and the samples were subsequently treated for 30 s in oxygen plasma to oxidize the aluminum. XPS data presented in Figure S1 in the Supporting Information confirms that the aluminum is completely oxidized after the plasma treatment. A single evaporation and oxidation process resulted in an aluminum oxide spacer layer of 3 nm, and for each successive evaporation and oxidation step the spacer layer thickness grew by 3 nm (see AFM data in Figure S2 in the Supporting Information). Samples with zero to nine spacer layers (i.e., 0−27 nm spacer) were fabricated. For the asymmetrically stacked disks, the fabrication involved a maskmodification step before the evaporation of the upper, laterally offset gold disk (15 nm height), as illustrated in the right-hand panel of Figure 1A. An oblique angle deposition of titanium while the substrate was rotated was used to shrink the holes in the mask, and the upper gold disk was subsequently evaporated at a small tilt angle relative to the normal to generate the lateral offset of the gold disks evaporated through the colloidal hole masks. A more detailed description of the fabrication can be found in the Supporting Information. Scanning electron microscope (SEM) images of a sample of symmetric and a sample of asymmetric structures are shown in Figure 1B. For imaging a 5 nm titanium layer was evaporated to avoid charging effects. A symmetrically stacked sample with nine-spacer layer (i.e., a spacer layer thickness of 27 nm) is depicted on the left, and an asymmetrically stacked sample with only a single spacer layer (3 nm) is depicted on the right. It is noticed that the top gold disk in the asymmetric sample is laterally offset compared to lower gold disk and the spacer layers. Optical extinction measurements were done on a set of 10 samples with spacer layers varying in steps of 3 nm from 0 nm (a solid 40 nm gold disk) to 27 nm. The experimental data are shown in Figure 2A. For the solid, 40 nm gold disks a single extinction peak is identified at 665 nm. Upon splitting the solid disk by introducing a thin spacer layer an additional peak in the extinction spectrum emerges. Thus each spectrum of stacked, split disks shows two peaks: a high intensity, high energy (HE) peak at approximately 660 nm that is insensitive to the dielectric splitting and a low energy (LE), low intensity peak that shifts to shorter wavelengths with increasing spacer layer thickness. The HE peak is at higher energy than the elemental plasmon of an individual 20 nm Au disk but coincides with the energy of a 40 nm Au disk. As the LE peak blue shifts it narrows and increases in intensity. Finite-difference time-domain (FDTD) simulations of the extinction were performed, and the resulting spectra are plotted in Figure 2B. The structures were simulated as two 20 nm truncated gold cones separated by an alumina spacer layer of 0−27 nm thickness placed on a glass substrate. The structures were simulated with sidewall angles of 15°. Illumination was at normal incidence and from the glass side corresponding to the illumination configuration used in the experiments. A good agreement between the experimental and

the cavity of big (2000 nm diameter with Ag/SiO2/Ag thicknesses of 100 nm/50 nm/100 nm) nanodisk resonators, while only a first order, dipolar-like mode was detected in small (65−140 nm diameter with Ag/SiO2/Ag thicknesses of 100 nm/10 nm/100 nm) nanodisk resonators. The cavity mode detected for the small structures was noted to be identical in nature to modes in hot-spot geometries seen in plasmonic particle dimers, thus establishing a connection between traveling and localized plasmon cavities. Here we study the optical properties of small Au/Al2O3/Au nanodisk resonators fabricated by hole-mask colloidal lithography (HCL)26 in the limit of small gap thickness revealing a sequential onset of gap modes then bonding dipolar disk−disk coupling. With a starting point of a 40 nm thick gold disk we investigate by experiment and by FDTD calculations how two different modes emerge and evolve as a single disk is split into two separate metallic disks, and the separation is increased by the introduction of a dielectric spacer layer of increasing thickness. We observe low energy first-order, dipolar-like gap modes but not the low energy hybridized dipolar disk modes for very small spacer thicknesses, while for larger spacers a mixing of the two modes is seen. We identify dark, higher order gap modes which are activated by coupling to a high energy bright dipolar disk mode. The introduction of lateral asymmetry in MIM disk structure by offsetting one metal disk with respect to the other modified the field distribution and offset the dipole orientation of the high energy dipolar disks mode out of the plane of the substrate. Stacked disks were fabricated according to the process schematically illustrated in Figure 1A. A standard hole-mask

Figure 1. (A) Schematic representation of the fabrication process for symmetrically (left) and asymmetrically stacked (right) Au−Al2O3−Au disks. (B) SEM images of symmetrically (left) and asymmetrically (right) stacked disks. 6034

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disagreement between the peak position in the experimental and simulated data increases with decreasing gap sizes. The blue shift is potentially due to an overestimation of the refractive index of the alumina layer in the simulations. Similar effects (i.e., blue-shifted gap modes) have been observed before in systems with small gaps, and the effect has been related to nonlocal responses.27,28 The small differences in the relative intensity of the peaks can be ascribed to differences in the geometry and dielectric properties of the experimental and simulated structures and materials. A peak/dip feature is identified on the low energy side of the HE peak in the simulated spectrum of disks separated by 6 nm (third spectrum from the bottom in Figure 2B), and it will be discussed in more detail later in this Letter. Simulated charge distributions and electric field enhancement plots were extracted on-resonance for the LE and HE mode to investigate how the modes evolve as the physical separation of the two disks increases and thereby shed light on the nature of the two different modes. Figure 3A shows schematic representations of four of the structures investigated and the illumination configuration. The structures have spacer layers of 27 nm (top), 12 nm (second from the top), and 3 nm (third from the top) and 0 nm corresponding to a single 40 nm gold disk (bottom). The near-field plots and charge distributions for the LE mode are plotted in Figure 3B for the split disks. The near-field plots are extracted in a horizontal plane placed in the middle of the spacer layer and in a vertical plane down through the center of the structures. For the disks separated by a thin spacer layer of 3 nm the electric field enhancement of the LE mode is mainly confined to the alumina spacer. The charge plot reveals a dipolar charge distribution on both disks in a 180° out of phase configuration rendering the mode a bonding mode, according to the plasmon hybridization

Figure 2. Experimental (A) and FDTD simulated (B) extinction spectra of symmetrically stacked gold disks with Al2O3 spacer thickness varying in steps of 3 nm from 0 to 27 nm.

FDTD simulated data is observed. The simulations reproduce the overall spectral features of a LE mode that shifts to shorter wavelengths and increases in intensity with increasing spacer thickness and a high intensity HE mode that is constant in intensity and spectral position irrespective of the dielectric splitting. The LE peak is systematically blue-shifted in the experimental data compared to the FDTD simulated data. The

Figure 3. (A) Schematic illustration of the structures. (B) Field plots from the center of the dielectric spacer layer gap and a vertical cross section and 3D charge plots of the low energy mode of the three different structures (9, 4, 1 layers). (C) Same as in B but for the high energy mode and including a vertical cross section and 3D charge plot for the mode supported by an unsplit, solid 40 nm gold disk. 6035

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increasing the radial mode number to n = 2. Further decreasing the separation between the two disks results in three radial antinodes, and thus n = 3. This is most clearly seen in the nearfield plot at the center of the gap (Figure 3C) where the field enhancement pattern shows three antinodes along the radial axis. A mixing of the bright dipolar disk mode and gap modes of different orders, dependent on the disk separation, is observed. The gap mode order increases as the separation is made small. The higher order gap modes (n ≥ 2) are only observed when they overlap in energy and hence mix with the HE mode. This is presumably because the HE mode can supply the dipole moment necessary to make the mode dipole active and hence able to couple to the incident light. Thus when splitting a single disk into two separate disks by introducing a thin dielectric spacer layer a high energy mode arises that at small separations shows a radially propagating SPP mode character on the metal surfaces facing the gap. Not until larger separations (between 6 and 9 nm in the system investigated here) do simple dipolar charge distributions appear on the gap lining surfaces. This coupling with the bright dipolar disk mode appears to activate the spectrally narrower gap modes, but this does not appear to alter the bright dipolar mode overall since the energy of the HE peak does not change as the higher order gap modes are scanned across the peak position (seen by the travel of the small line-shape associated with the second order (n = 2) gap mode across the HE peak for 3, 6, and 9 nm dielectric spacers in Figure 2B). Thus in the system investigated here the HE mode cannot be explained simply as the antibonding combination of the elemental dipolar modes of the two disks as suggested in the available literature. The physical interpretation of energy shifts of such in phase charge distributions predicted for antibonding modes from plasmon hybridization theory has been repulsive electrostatic interactions and hence an increase in the mode energy.22 Such an antibonding hybridized mode is expected to be shifted in energy also in a distance-dependent manner through this electrostatic destabilization. Sphere dimers12 and rod dimers13,24 investigated in a similar illumination configuration experimentally and though simulations have shown modes that blue shift as the separation is decreased. The blue shift has been interpreted in terms of “anti-bonding” plasmon hybridization. Here no shift was observed in the experimental or simulated spectra of the system for the bright dipolar mode. The substrate and the angled sidewalls induce vertical asymmetry in the experimental and simulated disk systems. The vertical asymmetry provides a potential explanation for the lack of peak position shift for the HE mode upon disk splitting and separation. The presence of a substrate results in a large proportion of the enhanced electric field being buried in the high refractive index material of the substrate.30 As the two disks are moved further apart by introducing a thicker spacer layer, a red shift is expected due to dehybridization of the mode. At the same time the increased separation leads to a redistribution of the local field enhancement pattern that may decrease the amount of the enhanced field buried in the substrate and hence a blue shift of the mode. In the structures studied here, the expected red shift of the HE mode with increasing disk separation is apparently completely counteracted by a blue shift and the peak appears stationary. To test our assumptions regarding the influence of vertical asymmetry FDTD simulations were performed on structures of varying disk separations (3, 12, and 27 nm and a solid 40 nm gold disk)

theory. The charges are practically restricted to the metallic surfaces adjoining the alumina spacer, and only hints of charge density are seen on the exterior surfaces. The charge distribution combined with the confinement of the electric field enhancement indicates a gap mode. The mode is similar to the mode identified by Kuttge and co-workers29 in their cathodoluminescence imaging spectroscopy study of small Ag− SiO2−Ag MIMs. The difference between the system under investigation here and the Ag−SiO2−Ag MIMs are the vertical dimensions of the structures. Kuttge et al. studied chromium capped silver disks of 100 nm height separated by a 10 nm SiO2. In that work the chromium layer and the height of the silver disks appeared to isolate the gap modes. In the system investigated here we systematically increase the separation of the two disks by increasing the spacer thickness in units of 3 nm up to 27 nm and observe a corresponding change in the charge distribution (Figure 3B middle and top show 12 and 27 nm spacing, respectively). The field is less restricted to the gap, and charge distributions corresponding to the bonding mode of the elemental dipolar modes identified for single layer disks emerge. In theory the LE mode is a subradiant mode, as the out-of-phase charge distributions result in a partial cancellation of the net dipole moment of the mode and hence only weak coupling to electromagnetic radiation. The cancellation is only partial (i.e., the mode is not completely dark and is still observed in extinction measurements) mainly due to the difference in size of the upper and lower disk. The difference in disk sizes increases with the spacer layer thickness effectively resulting in a bigger dipole moment of the mode and hence an increase in the intensity of the LE peak in the extinction spectra. Near-field and charge distribution data for the HE mode resonances are plotted for the three different separations and for a solid 40 nm disk in Figure 3C. The vast majority of the high electric field enhancement sites occur outside the gap region for all three separations. Ignoring the charge densities on the gap lining surfaces of the split disks, the charges on the upper and lower disks are in phase resulting in high dipole moments in agreement with the high intensity of the HE mode observed both experimentally and in simulated extinction spectra. Considering the charges on the gap lining surfaces a different picture emerges. The charges on the two disks in regions directly opposite each other are of opposite sign and thus a bonding gap mode is superimposed on the bright disk mode. This is true for all separations investigated. For separations of 9 nm and more the charges are arranged so an additional change in the sign of the charge density is observed on the top of the lower disk while the upper disk exhibits the charge pattern of a perfect dipolar disk mode. At separations below 9 nm, higher order gap modes with near-field distributions, revealing radially propagating SPP mode character, appear. The nomenclature from Kuttge et al. where the gap modes are named according to their azimuthal (m) and radial (n) mode numbers, with the radial mode number corresponding to the number of radial antinodes (in the emission pattern) is adopted to describe the gap modes. In the illumination geometry used here only modes with m = 1 are exited. For separations of 9 nm and above the radial mode number is n = 1. Decreasing the gap size to 6 nm the charge pattern on each of the gap facing surfaces and the field distribution in the gap (Figure S3 in the Supporting Information) change from a dipolar (single radial antinode) to a quadrupolar-like (two radial antinodes) arrangement thus 6036

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and varying degrees of vertical asymmetry. Removing the substrate (i.e., replacing the glass with vacuum) but retaining the angled sidewalls resulted in the HE peak position being sensitive to the dielectric splitting, and the HE mode red shifts as the gold disk separation is increased due to the dehybridization (Figure S4A in the Supporting Information). Changing the sidewall angle to vertical but keeping the glass substrate in the simulations also results in the HE mode being sensitive to dielectric splitting and gold disk separation (Figure S4B). From the simulations we can conclude that the observed lack of HE peak shifts in the experimental and original simulated extinction spectra is due to a combined effect of the substrate and the sidewall angle. Making the structures vertically symmetric by both removing the substrate and imposing vertical sidewalls (Figure S4C) results in the HE mode being more sensitive to dielectric splitting (i.e., larger peak shifts as a function of disk separation) than the samples, eliminating only one of the asymmetry-causing effects. A combination of lateral and additional structural asymmetry was introduced in the stacked disks by shrinking the upper disk and displacing it laterally with respect to the lower disk. The shrinking of the upper disk increases the structural asymmetry that was already present in the unmodified stacked disks due to the shrinking of the holes in the hole-mask and hence the disk diameters during material deposition. A set of three stacked disk samples with the same size upper disk but varying lateral offset (from no offset to almost full offset without the upper disk spilling out over the lower disk) were fabricated. Here the thickness of the shrunk upper disk was set to 15 nm to obtain an aspect ratio (and thus dipole resonance energy) close to the aspect ratio of the lower disk. The alumina spacer layer was kept at 3 nm to promote the dominance of gap modes. Experimental extinction spectra of the structures are shown in Figure 4A along with FDTD simulated spectra. The polarization of the exciting electric field becomes important for laterally asymmetric structures. The FDTD simulations were performed with the electric field polarized along the direction of lateral offset, while the experimental data are obtained from plane polarized light. In spite of this experimental and simulated data for all three samples are in fairly good agreement. A high energy (HE), high intensity, and low energy (LE), low intensity peak are observed for all samples in both experiments and simulations. However, there are a few discrepancies between the experiments and the simulations. The experimentally obtained peak positions are not accurately reproduced by the FDTD simulations, and a feature seen on the low energy side of the HE peak in the simulations is not observed in the experimental data. The dip/peak feature will be discussed in more detail later. The intensity of the LE peak is substantially lower in the experimental data, and the signal is significantly affected by noise in that region, making reliable peak position determination difficult, but there appears to be a trend dictating a blue shift of the LE resonance as the asymmetry evolves. The plasmonic response of complex nanostructures is very sensitive to the geometry of the structure and the dielectric properties of the materials used. The blue shift trend is not reproduced in the simulations, potentially because the FDTD model does not exactly mimic the geometry of the experimentally obtained structures. The small peak/dip feature observed on the low energy side of the HE peak in the simulated data is similar to a feature identified in the simulated spectra of the symmetrically stacked disks with alumina spacer layers of 6 nm. This sharp feature is

Figure 4. (A) Experimental and FDTD extinction spectra for stacked disks with varying degrees of lateral asymmetry. The upper disk is deliberately shrunk to make room for the lateral offset, and its height is reduced to 15 nm to ensure an aspect ratio comparable to the aspect ratio of the lower disk. The black, blue, and red curves represent the laterally symmetric, the slightly laterally asymmetric, and the full laterally asymmetric arrangement, respectively. (B) Schematics of the laterally symmetrical (top, far left) and laterally asymmetric configuration (bottom, far left) and near-field plots for the LE (middle) and the HE mode (right) for the two configurations. (C) 3D field distribution plot of the HE mode of the laterally symmetric (top) and asymmetric structure (bottom) shown in B.

not observed in the experimental data likely because of inhomogeneous broadening of such a feature through structure to structure differences in diameter. The near-field distributions extracted on resonance for the HE peak, the small dip, and the small peak for both the symmetrically stacked disks with alumina spacer layers of 6 nm and the asymmetrically stacked disks (Figure S5 and Figure S6, respectively) indicate that the resonance responsible for the small dip and peak are dominated 6037

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in the field distribution leads to a difference in the vertical position of the net charge on the two sides of the structure moving the dipole orientation away from the dipoles of the elementary disk modes. The possibility to tune the dipole resonance out of the plane of the substrate via the introduction of structural asymmetry opens a route to tune far-field scattering. No asymmetry is induced in the near field distribution of the symmetrical structure. In conclusion we have experimentally and via FDTD simulations investigated the evolution of two plasmon modes as a gold disk is split, and the separation of the two resulting disks is increased. Upon splitting two plasmon modes arise of which the low energy one shows a transition from a gap mode, with charges and field distributions predominantly restricted to the gap, to a regular dipolar bonding mode as the splitting separation is increased. At small separations, the nature of the high energy mode is revealed to be a combination of higher order radially propagating SPP modes in the gap region and an in phase arrangement of the dipolar disk modes on the exterior surfaces of the two disks. An interesting observation was made regarding the high energy mode: the pure dipolar antibonding mode, predicted by the plasmon hybridization theory, was never observed. The spectral position of the HE mode did not change as the separation of the two disks was increased. The expected red shift of the HE mode upon increasing disk separation was apparently precisely counteracted by a blue shift related to the asymmetric lateral geometry. Changing the lateral asymmetry by removing the substrate or altering the sidewall angle resulted in the expected HE peak shifts in simulations. Lateral asymmetry was introduced to investigate the response of the two modes as the geometry of the supporting structure was gradually changed. Asymmetric structures with small separations (3 nm) were investigated to preserve the large influence of the gap modes observed in symmetrically stacked disks of small separations. The low energy mode showed little alteration of the field distribution apart from a shrinking and linear shift along with the upper disk, ensuring that the field enhancement was confined between the two metal surfaces. The field distribution of the high energy mode was remodeled upon introduction of the lateral asymmetry, though, resulting in an asymmetric 3D field distribution pushing the dipole orientation out of plane.

by the n = 2 gap mode. In comparison the HE mode is less dominated by the gap mode and has a stronger contribution from the outer dipolar disk mode. The interference of an n = 2 gap mode with a broader dipolar mode resulting in a peak/dip feature in the extinction spectra points to the existence of a Fano resonance. Since the feature is not seen in the experimental data, no further inspection of the mode will be performed here. Figure 4B shows the near-field plots extracted on-resonance for the LE and HE mode for the laterally symmetric (top) and laterally asymmetric (bottom) configuration. The field distribution of the LE mode of both the laterally symmetric and asymmetric stacked disks resemble the mode observed at low energies for the disks of approximately the same size and small separations (Figure 3B bottom). But whereas the nearfield pattern of the LE modes of disks of approximately the same size occupy the entire alumina layer, the near-field enhancements of the LE modes depicted in Figure 4 are basically restricted to the part of the alumina layer that is sandwiched between the two gold disks and is hence limited by the size of the upper disk. Offsetting the upper disk laterally with respect to the lower disk results in an equivalent lateral shift of the field enhancement so the upper disk and field distribution are aligned but no significant change in the enhancement pattern is observed. Thus the induced asymmetry results in little change in the spectral position or near-field pattern of the LE apart from the lateral shift in the field distribution that correlates with the lateral shift of the upper disk. The near-field distribution of the HE mode of the laterally symmetric and asymmetric stacked shrunk disks resembles the distribution seen for the HE mode of unshrunk disks separated by a 3 nm spacer layer. The HE peak is at slightly longer wavelength compared to the HE peak in the spectra of Figure 2, as expected from the smaller overall volume of gold due to thinner and smaller upper disks. An n = 3 order field distribution is seen in the gap region, and the field distribution indicative of a bright dipolar mode on the other surfaces of the disks is seen. The field enhancements observed in the gaps in Figure 4B are higher than the enhancements observed in Figure 3C for unshrunk stacked disks, suggesting that the HE mode is more dominated by the gap mode when the upper disk is shrunk. As in the case of the LE modes of structurally asymmetric stacked disks, the field distributions in the gaps are limited by the extent of the smaller upper disks, and the field distribution shifts laterally with the upper disk. Unlike the LE mode the shift in the field distribution of the HE mode is not simply a lateral translation, though. The lateral shift of the field distribution lags behind compared to the lateral shift in the upper disks. As the upper disk and hence also the field distribution is laterally displaced, an asymmetry is induced in the near-field pattern. 3D plots of the near-field distributions of the HE resonances of the laterally symmetric and asymmetric structures were generated (Figure 4C top and bottom, respectively) to help reveal the field distribution in three dimensions. The 3D plot of the asymmetric structure clarifies that the induced asymmetric in the near-field distribution is most pronounced at the top edges of the two disks. The field enhancement on the right-hand side, top edge of the lower disk increases, while the field enhancement on the left-hand side, top edge of the lower disk decreases. At the same time the intensity increases on the left-hand side, top edge and decreases on the right-hand side, top edge of the upper disk. The change

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ASSOCIATED CONTENT

S Supporting Information *

Materials and methods including sample fabrication, characterization, optical measurements, and FDTD simulations. XPS data of alumina films, disk height AFM data, additional FDTD generated near field plots, and FDTD generated extinction spectra of structures with varying degrees of vertical asymmetry. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was funded through the Danish research council FNU-grant (Sags. no. 09-065929). 6038

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dx.doi.org/10.1021/nl4032567 | Nano Lett. 2013, 13, 6033−6039