Hybridized Plasmonic Gap Mode of Gold Nanorod on Mirror

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Hybridized Plasmonic Gap Mode of Gold Nanorod on Mirror Nanoantenna for Spectrally Tailored Fluorescence Enhancement Hiroshi Sugimoto, Shiho Yashima, and Minoru Fujii ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00693 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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Hybridized Plasmonic Gap Mode of Gold Nanorod on Mirror Nanoantenna for Spectrally Tailored Fluorescence Enhancement Hiroshi Sugimoto*, Shiho Yashima, Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Plasmonic nanoparticle on mirror antennas with sub-10 nm gaps have shown the great potential in nanophotonic applications because they offer tightly confined electric field in the gap and resultant large Purcell factors. However, in a nanosphere on mirror (NSoM) structure being studied experimentally, the degree of freedom of the antennas in terms of spectral and polarization control is limited. In this work, we report spectral shaping and polarization control of Purcell-enhanced fluorescence by the gap plasmon modes of an anisotropic gold (Au) nanorod on a mirror (NRoM) antenna. Systematic numerical calculations demonstrate the richer resonance behaviors of a NRoM antenna than a NSoM antenna due to the hybridization of the bright and dark modes. We fabricate a NRoM antenna by placing a Au NR on an ultra-flat Au film via a mono-, double- or quadruple-layers of light emitting quantum dots (QDs) (3 nm in diameter). The scattering spectra of single NRoM antennas coincide very well with those of the

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numerical simulations. We demonstrate large enhancement (>900-fold) and strong shaping of the luminescence from QDs in the gap due to the coupling with the hybridized mode of a NRoM antenna. We also show that the polarization property of the emission is controlled by that of the mode coupled.

Keywords: Plasmonics, Nanorods, Nanonantenna, Surface enhanced fluorescence, Nanocavity

Plasmonic nanoantenna offers the ultimate spacial control over electromagnetic waves by localizing the optical energy in the nanoscale and has significant potential for the nanophotonic applications utilizing enhanced light matter interactions.1 In addition to the enhanced signal intensity of fluorescence, Raman scattering and nonlinear optical responses, the role of plasmonic a nanoantenna has been expanding to the enhancement of the spontaneous emission rate (i.e., Purcell effect) for ultrafast signal processing,2,3 radiation pattern control,4,5 shaping of an emission spectrum6,7 and polarization control.8,9 Among a variety of plasmonic antennas, a nanogap structure such as a bow-tie antenna10 is known to have an extremely small mode volume and a large field enhancement, and a resultant very large Purcell factor. However, formation of a sub-10 nm gap with the accuracy of ~1 nm in an exquisitely engineered nanoantenna structure is still challenging even by the state-of-the-art electron beam lithography (EBL) technology.1,2,11,12 More importantly, these EBL engineered structures have difficulty in placing active nanomaterials such as fluorophores and Raman molecules in the nanogap in a controlled manner. Recently, the metal nanoparticle on mirror (NPoM) structure in which a metal nanoparticle is placed on a metal film via dielectric spacer has been widely studied.6,13,14 The structure can be


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fabricated by a bottom-up process using colloidal gold (Au) and silver (Ag) nanoparticles (NPs) with an accurately controlled nanogap at the sub-10 nm scale.15,16 Thanks to the large Purcell factor of NPoM antennas, ultrafast single photon emission17 and strong-coupling between an emitter and the resonant mode at room temperature18,19 have been demonstrated. The simplest NPoM is a nanosphere on mirror (NSoM) structure, in which an isotropic metal nanosphere is placed on flat metal surface.13,18,20 In NSoM, a dipole oriented perpendicular to the surface couples with its image dipole in phase, which induces strongly enhanced electric field at the nanogap. The coupled mode is radiative in nature and the scattering spectrum of a NSoM antenna is relatively broad (~ 200 meV)6 with the Q-factor smaller than 10. Recently, the effect of the NP shape, i.e., sphere, rod and cube, on the performance of a NPoM antenna was studied theoretically.21 It was demonstrated that a nanorod on mirror (NRoM) antenna possesses an ultrasmall effective mode volume of 0.001(λ/n)3 and a higher Purcell factor compared to that of a NSoM. The theoretical study suggests that NRoM is a promising nanoantenna for controlling the spectral shape and polarization of fluorescence of light-emitters in the gap.21 However, in contrast to the large number of experimental studied on the NSoM13,18,20,22–24 and nanocube on mirror (NCoM)2,16 structures, that of NRoM is scarce. Only recently, Chen et al. experimentally studied the scattering properties of a single NRoM structure and demonstrated the polarized scattering due to the presence of longitudinal and transverse localized surface plasmon resonances (LSPR) in a Au NR.25 However, the measured spectra were quite different from those predicted by theory. In particular, higher order gap modes and hybridized modes predicted by theory were not observed. In order to take full advantage of the NRoM antenna, careful design of the geometry, an atomically flat metal mirror and a highly uniform dielectric spacer are indispensable. Last, but not least, the Purcell-enhanced polarization-controlled light emission


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predicted by the theoretical work21 has not been experimentally demonstrated in a NRoM antenna. In this work, we scrutinize the radiative property of a NRoM antenna by systematically changing the structural parameters such as the NR length and the spacer thickness and experimentally demonstrate the Purcell enhancement and the strong spectral shaping. We first analyze the property of the hybridized resonant modes under different excitation conditions (i.e., directions and polarizations) by the numerical simulation. We then fabricate the structure by a bottom-up process using an ultra-flat Au film and layers of luminescent colloidal silicon (Si) quantum dots (QDs) as a dielectric spacer. The formation of an almost ideal NRoM antenna allows us to systematically study the light scattering properties of a single NRoM antenna and its dependence on the spacer thickness. Finally, we demonstrate for the first time a large Purcell enhancement and strong spectral shaping of luminescence from QDs integrated in the gap.

Results and discussion The scattering spectrum and the electric field distribution of a NRoM structure are calculated using the MNPBEM code,26,27 which is based on the rigorous boundary element method (BEM). Figure 1a shows the geometry for the simulation. We fix the diameter of a Au NR to 45 nm and change the aspect ratio from 1 (i.e., sphere) to 3.5. The long-axis of a NR is parallel to the x direction. The thickness on a Au substrate is infinite and that of a dielectric spacer is changed from 3 to 12 nm. As will be shown later, the QD layers are employed as the spacer in the experiments and thus we set the refractive index to 1.6.28,29 A NRoM structure is excited by TE ሬറ in the y-z plane (TE1 and TM1 in Figure 1a, and TM polarized light with the wave-vector ݇


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respectively) and in the x-z plane (TE2 and TM2 in Figure 1a, respectively). The incident angle is fixed to 65˚. Figure 1b shows the scattering spectra of a NSoM antenna, i.e., NRoM with the aspect ratio of 1. Under the TM excitation, a prominent peak appears at 600 nm. The resonance induces strong enhancement of the electric field (a factor of ~60) in the gap region (Figure 1d). This gap mode arises from the coupling of the vertical dipolar mode of a Au NS with its image dipole in a Au film. The mode is radiative in nature and is called “antenna mode”.

13,16,20

On the other hand,

scattering is negligibly small under TE excitation. The TE polarized light excites only the horizontal dipole and the presence of its image dipole with anti-parallel orientation cancels the scattering. The scattering spectrum is strongly modified when the aspect ratio is larger than 1. Figure 1c shows the scattering spectra when the aspect ratio is 1.8 (45 nm in diameter and 80 nm in length). Under TM1 excitation, a resonance appears around 620 nm. In Figure 1e, the field distribution is shown in Figure 1e. The charge distribution is estimated from the direction of the electric field (see Figure S1 in Supporting Information). The field distribution shows a hotspot at the center of a gap, similarly to the case of the vertically-coupled dipolar mode in the NSoM.13,16,20 The resonance is due to the coupling of the transverse LSPR of a NR with its image dipole. We refer the mode to the vertically-coupled transverse mode (VTM). Under TE1 excitation, a peak appears at 700 nm. This mode has two hotspots with an opposite charge distribution (Figure 1f) and can be assigned to the longitudinal LSPR mode coupled with the image dipole. We refer this mode to the horizontally-coupled longitudinal mode (HLM). The HLM has dipoles with antiparallel orientations each other The cancellation reduces the radiative loss and narrows the scattering spectrum.30 Indeed, the scattering intensity of the HLM is about 30% of that of the


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longitudinal LSPR mode of the same size nanorod on silica. In both VTM and HLM, the field enhancement factor reaches ~60. The TM2 excitation can excite both the VTM and HLM and the spectrum exhibits two resonance peaks at 640 and 720 nm.

Figure 1. (a) Schematic illustration of the geometry of numerical simulation of a NRoM structure. Calculated scattering cross-section of (b) NSoM and (c) NRoM for polarization conditions designated in (a). Electric field (|E|/|E0|) distributions of (d) NSoM and (e, f) NRoM at different wavelengths; (d) TM at 600 nm, (e) TM1 at 620 nm and (f) TE1 at 700 nm.

We securitize the evolution of the scattering spectrum under TM2 excitation by systematically changing the aspect ratio from 1.8 to 3.6, i.e., the diameter is fixed to 45 nm and the length is varied from 80 to 160 nm. In Figure 2a, the scattering cross section is plotted as a function of the length and the wavelength. We can see strong modification of the spectral shape with increasing the aspect ratio. The HLM exhibits significant red shift (green dashed line) and the VTM splits into two modes (blue dashed-dotted line). In Figure 2b-e, we show the electric field distributions


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of a NRoM with a NR length of 125 nm at wavelengths indicated by circle, star, triangle and square in Figure 2a, respectively. The electric field distribution of HLM in Figure 2e is very similar to that in Figure 1f as expected. The maximum field enhancement reaches almost 100 times. At 700 nm (triangle) (Figure 2d), the field pattern is similar to the VLM in Figure 1e, i.e., a hotspot at the center of a gap. On the other hand, at 600 nm (circle) (Figure 2b), two hotspots are seen at both sides of the gap. In between the two modes (a dip at 660 nm (star) (Figure 2c), three hotspots with opposite vertical field components appear. From the field pattern, this mode can be assigned to a waveguide mode as schematically shown in the right side of Figure 2f. Similar waveguide mode is observed in one or two dimensional metal-insulator-metal (MIM) waveguides with sub-10 nm gaps,31,32 and also in cropped NSoM33 and NR dimers.30 The waveguide mode propagating in the gap is nonradiative in nature and observed as a scattering dip. Since the resonance wavelength is determined by the cavity length,32,34 the dip moves to longer wavelength with increasing the NR length (gray dashed line in Figure 2a). The waveguide mode can confine the field tightly and the enhancement factor in Figure 2c is at maximum about 65. The splitting of the VTM in the 600-750 nm range can be explained by the hybridization model in Figure 2f. When the VTM (closed circle in Figure 2a) and the dark waveguide mode (gray dashed line) hybridize, the mode splits into bonding (in-phase coupling) and antibonding (anti-phase coupling) modes (see blue dashed lines). The bonding mode has a strong vertical dipole at the center which corresponds to the resonance in Figure 2d. On the other hand, in the anti-bonding mode, the vertical electric field component at the center of a NR is cancelled whereas the two dipoles still present at NR edges. This results in the field distribution in Figure 2b. Both the bonding and anti-bonding modes are radiative in nature and observed as scattering peaks.


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Figure 2.(a) Calculated scattering cross-sections of NRoM with different NR lengths under TM2 geometry. (b-e) Electric field (|E|/|E0|) distributions at different excitation wavelengths indicated in (a). (f) Energy diagram showing hybridization of VTM and waveguide mode.

We fabricate the NRoM structure schematically shown in Figure 3a and study the scattering behavior by systematically changing of the gap length in the combination with calculation. Since the gap length is at minimum a few nanometers, perfect flatness of the metal surface is required to produce an ideal NRoM antenna. We employ a template-stripped method to obtain an ultraflat Au film (200 nm in thickness).35 The atomic force microscope (AFM) topographic image and the height profile in the Figure 3b demonstrates the formation of ultra-flat Au surface with the roughness well below 1 nm. To control the gap length precisely, we employ a monolayer of a


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colloidal solution of silicon (Si) QDs of 3.0 nm in diameter.6,28,36 By controlling the number of QD monolayers from 0 to 4, the gap length is varied from 0 to 12 nm.28 In this work, the QD monolayers work not only as the dielectric spacer but also as a stable fluorophore to monitor the Purcell effect in the gap resonator.37 An advantage to use Si QDs as a fluorophore is the very broad luminescence band with a full width at half maximum (FWHM) more than 300 meV,37 which allows us to monitor the Purcell effect in a wide wavelength range. Finally, a diluted colloidal solution of Au nanorods (45 nm in diameter and 125 nm in length) (transmission electron microscope image in Figure 3a) is dropped to finalize the NRoM structure. In Figure 3c, the scattering spectra of a single NR on silica and a NRoM antenna with the spacer thickness of 3 nm are shown. The spectra are obtained by illuminating unpolarized light through a dark-field objective with an incident angle of 65˚. In this configuration, the contribution of TM polarized light is enhanced, while that of TE polarized light is partly cancelled.38,39 As expected, the scattering spectrum of the NRoM antenna is totally different from that of a NR on silica. Figure 3d show the calculated spectra for TM2 excitation with the incident angle of 65˚. The experimentally obtained spectral shape coincides almost perfectly with the calculated one. This confirms the successful formation of a designed NRoM structure. As a reference, the scattering spectra of NRoM made on a vacuum evaporated Au films are shown in the Supporting Information (Figure S2). Due to the roughness on the Au surface, the spectra are much broader and the agreement with the calculated spectra are poor.


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Figure 3.(a) Schematic illustration of fabricated NRoM and transmission electron microscope image of a Au NR. (b) AFM topographic image and height profile of template-stripped Au substrate. (c) Measured and (d) calculated scattering spectra of NRoM with the spacer thickness of 3 nm (red solid curves) and NR on silica (black dashed lines).

Figure 4a and b display the measured and calculated scattering spectra of NRoM antennas with 4 different spacer thicknesses (0, 3, 6 and 12 nm). The overall agreement between the measured and calculated spectra is again obtained, confirming successful precise control of the spacer thickness. In the Supporting Information (Figure S3), we summarize scattering spectra obtained for different single NRoM antennas. The variation of the spectral shape between the NRoM antennas is small, further confirming the robustness of our procedure for the formation of NRoM antennas. In Figure 4c, the calculated scattering intensity is shown by a color gradient as functions of the wavelength and the spacer thickness. In the same figure, the scattering peaks of experimental spectra are shown by symbols. We can see good agreements between the bright


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regions in the calculation and the experimental scattering peaks. When the spacer thickness is the largest, the spectrum is very simple, suggesting negligibly small interaction between a NR and a flat surface. The interaction appears with decreasing the spacer thickness. The longest wavelength mode (i.e., HLM) red-shifts and becomes weak. This is due to cancellation of the scattering by anti-parallel coupling of the dipolar mode with its image dipole. On the other hand, the short wavelength mode (i.e., VTM) emerges around 12 nm and splits into two peaks with further decreasing the spacer thickness. This is consistent with the model suggested in Figure 2f. The smaller gap allows the co-existence of the bright VTM and the dark waveguide mode at the same wavelength and the coupling results in the sharp dip.

Figure 4. (a) Measured and (b) calculated scattering spectra of NRoM with different spacer thicknesses (0, 3, 6 and 12 nm). (c) Color map of calculated scattering intensity as functions of


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wavelength and spacer thickness. Scattering peak wavelengths of measured scattering spectra are plotted on the color map.

Figure 5a shows the photoluminescence (PL) spectrum of a QD monolayer (3 nm in thickness) in the gap of a NRoM structure. The excitation wavelength is 405 nm. This wavelength can excite the QD layer efficiently (see Figure S5 in the Supporting Information), while it is out of the resonance of a NRoM structure. Compared to the reference spectrum, i.e., PL spectrum of a QD monolayer on a flat Au film, the intensity is enhanced significantly. The maximum enhancement factor reaches approximately 8. Moreover, the spectral shape is strongly modified; a broad featureless spectrum is changed to that with two peaks. In Figure 5b, the PL enhancement factor spectrum obtained by dividing the PL spectrum of the QD monolayer in NRoM by that on a Au film is shown. The scattering spectrum of the same NRoM is also shown in the same graph. The features in the enhancement factors agree very well with those in the scattering spectrum. This is a direct evidence that the PL enhancement and the spectral shaping are caused by the coupling of the QD emission with the resonant modes in NRoM. It should be stressed here that the PL enhancement is due to the enhancement of quantum efficiency of QDs by the Purcell effect because the excitation wavelength (405 nm) is out of the NRoM resonance. We performed the same experiments for 11 NRoM structures. The enhancement factor taken at the PL maximum is distributed from 4 to 8 (Figure 5c), and that of the integral intensity from 2 to 5 (Figure 5d). The average enhancement factor of the integrated PL intensity is 3.4. The enhancement factor is, however, largely underestimated, because we collect luminescence from a much wider region than the actual size of NRoM. To estimate the net enhancement factor by NRoM, we take into account the detection area of 2 µm2 and the area of NRoM, which is


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approximated to a projection area of a NR, of 5.2 ൈ 10-3 µm2. The estimated average net enhancement factor is 920-fold. Note that the enhancement factor is with respect to the PL of QDs on a Au film, where the quantum efficiency is severely degraded due to nonradiative energy dissipation to a Au film.

Figure 5. (a) PL spectra of QD-monolayer on a Au film without (reference, black curve) and with a Au NR (red curve). (b) PL enhancement factor spectrum obtained from the data in (a) (black dots) and scattering spectrum of the same NRoM (blue dashed curve). (c, d) Enhancement factors at PL maximum (c) and of integral PL intensity (d) for 11 different NRoM.


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The coupling of the QD emission with the gap resonant modes affects not only the intensity and the spectra shape, but also the polarization patterns. Figure 6a shows a color map of the scattering intensity of a NRoM antenna as functions of the analyzer angle and the wavelength (see Method section for the measurement procedure). The incident light is non-polarized. The polarization pattern of the shorter wavelength antibonding mode shifts 90o from that of the longer wavelength bonding mode. Figure 6b and c show the scattering spectra (blue dashed curve) obtained for the analyzer angles of 60 and 150o, respectively. The anti-phase behavior can clearly be seen. This behavior can be explained by the different charge distributions of these modes. As can be seen in Figure 2f, the antibonding mode has two vertical dipoles with parallel orientation and thus the electric field component parallel to the long axis of a NR in the far field is negligibly small. On the other hand, the bonding mode has a weak dipole moment parallel to the NR long axis. In Figure 6b and c, polarization resolved luminescence spectra of the same NRoM antenna are also shown. We can see a clear correlation between the PL and scattering spectral shape. This is another evidence that the luminescence is coupled to the resonant modes of NRoM. In Figure 6c, the linewidth of the PL coupled to the long wavelength antibonding mode is very narrow (FWHM: ~0.1 eV). This value is much smaller than that observed for NSoM.6 NRoM is thus superior to NSoM for the capability of not only the emission intensity enhancement but also the spectral shaping.


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Figure 6. (a) Color plot of polarization-resolved scattering spectra of a single NRoM antenna. The analyzer angle is changed from 0 to 360o. (b and c) PL (red solid curves) and scattering spectra (blue dashed curves) and obtained at the analyzer angle of 60 and 150.

In summary, we have studied the optical responses of a NRoM structure by systematically changing the aspect ratio and the gap length. Our analyses revealed that the hybridization of a bright antenna mode and a dark waveguide mode results in the rich resonance spectra. We have succeeded in producing a high quality NRoM antenna with a light-emitting QD layer in the gap by controlling the gap length from 3 to 12 nm with very high accuracy. The scattering spectra of single NRoM antennas agreed very well with the calculated spectra. We demonstrated for the


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first time the strong enhancement of the intensity and spectral shaping of the emission from QDs in a gap with the coupling of the resonance modes of a NRoM antenna. The estimated enhancement factor with respect to the luminescence from a QDs monolayer on a flat Au film reached 900. These findings provide novel insights for the design of spectroscopy-based sensing and imaging at the single nanometer scale using the nanoantenna-enhanced emission enabled by hybridized plasmonic modes.

Methods Preparation of NRoM First, an ultra-flat Au film was prepared by the template-stripping method.35 Briefly, a Au film 200 nm in thickness was deposited by thermal evaporation on a Si wafer. A two-part quick-set epoxy (Loctite) was used as an adhesive layer to transfer the Au film to another Si wafer. On the flat Au film peeled off from the original Si wafer, a monolayer of Si QDs was deposited by dropcasting the methanol solution (QD concentration : 0.15 mg/ml) and dried in air at room temperature.6,29,40 For the formation of a multilayer, the deposition and drying processes were repeated. The formation of mono- and multi-layers was confirmed by spectroscopic ellipsometry.29,36 Finally, a diluted solution of Au nanorods (

Hybridized Plasmonic Gap Mode of Gold Nanorod on Mirror

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