Exploring Coupled Plasmonic Nanostructures in the Near Field by

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Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy Han Yu,† Quan Sun,† Kosei Ueno,† Tomoya Oshikiri,† Atsushi Kubo,‡ Yasutaka Matsuo,† and Hiroaki Misawa*,†,§ †

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan § Department of Applied Chemistry & Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan ‡

S Supporting Information *

ABSTRACT: The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelengthtunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field properties of coupled plasmonic nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed. KEYWORDS: near-field mapping, plasmon hybridization, Fano resonance, photoemission electron microscopy (PEEM), femtosecond laser pulses ization,15,16 Fano resonance,17−27 electromagnetically induced transparency (EIT),28 and plasmonic waveguiding.29,30 Many of the proposed optical applications of plasmonic nanostructures rely on the near-field properties of nanostructures. Specifically, plasmonic modes with a large field enhancement are particularly beneficial for plasmon-enhanced nonlinear optical effects1,2,31−34 and plasmon-enhanced photochemical reactions.9−11 Compared to a simple individual plasmonic nanoparticle, complex coupled nanostructures have more complicated resonance line shapes in the far field (e.g., Fano resonance) and much richer near-field properties. In particular, the spatially resolved spectral response of coupled plasmonic nanostructures is in high demand. However, such

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he optical properties of localized surface plasmon resonances (LSPRs) that occur on metallic nanoparticles (NPs) have been the subject of intense study for the past few decades. Under resonant light excitation, metallic NPs exhibit intense light absorption and scattering, and enhance local electromagnetic fields. As a result of these properties, metallic NPs that exhibit LSPRs have potential applications in many fields, including surface-enhanced Raman scattering,1,2 sensing,3−5 imaging,3,6 plasmon-assisted photocurrent generation,7,8 photochemical reactions,9−11 and artificial photosynthesis.12−14 Recent advances in synthesis and nanofabrication allow for the preparation of metallic NPs with nanometer accuracy and complex shapes. In addition, complex plasmonic NPs that resemble NPs with small gap distance can be readily fabricated. Complex plasmonic NPs can induce coupling that may result in greater field enhancement; they also exhibit some striking properties, such as plasmon hybrid© 2016 American Chemical Society

Received: September 14, 2016 Accepted: October 24, 2016 Published: October 24, 2016 10373

DOI: 10.1021/acsnano.6b06206 ACS Nano 2016, 10, 10373−10381

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RESULTS AND DISCUSSION Characterization of Topography and Far-Field Spectra. The metallic nanoparticles investigated in this study were Au dolmen structures consisting of a planar nanorod monomer and a planar nanorod dimer for each dolmen, as schematically shown in Figure 1a. The dolmen structures were fabricated

experiments remain challenging because accessing the near-field spectral response in coupled plasmonic systems requires wavelength-tunable excitation and near-field imaging techniques with a high spatial resolution. To date, only a few attempts using scanning near-field optical microscopy,21 electron energy loss spectroscopy, and cathodoluminescence (CL)27,35,36 have been reported to detect the near field of complex plasmonic systems. Recently, nonlinear photoemission electron microscopy (PEEM), using femtosecond laser pulses as the excitation light source, has been found to be promising for pinpointing the near-field properties of plasmonic modes.37−40 PEEM records the electrons emitted from the surface of a sample in response to the absorption of incident photons. When the photon energy is lower than the work function of the material, nonlinear photoemission (PE), especially multiphoton PE, becomes possible when ultrafast laser pulses (typically femtosecond lasers) are used as excitation sources. In general, the probability of multiphoton excitation is low. However, when plasmonic nanostructures are irradiated by ultrafast laser pulses at resonance wavelengths of LSPRs, the near-field enhancement effect can significantly promote the multiphoton PE process. Because the PE intensity is correlated with the near-field electric field intensity on the sample surface in a nonlinear manner, the PEEM images of plasmonic nanostructures upon resonant excitation can be regarded as nonlinear maps of the near-field distribution. Similarly, the wavelengthdependent PE curves obtained by integrating the PE signal against the incidence wavelength can be treated as the nonlinear near-field spectra of the plasmonic nanostructures. Previously, we obtained the near-field mapping and near-field intensity enhancement spectra of simple aluminum (Al) nanorods and gold (Au) nanoblocks using this PEEM technique.40−42 In this study, we employ PEEM to experimentally investigate the near-field spectral response and spatial evolution of the near-field intensity distribution on more complex Au dolmen structures that consist of a planar nanorod monomer and two parallel planar nanorod dimers. The spectral properties of dolmen structures have been intensively studied, and on the basis of far-field spectroscopic measurements, the spectral properties of dolmen structures have been primarily explained by the Fano resonance as the result of interference between a spectral wide bright dipole plasmon mode and a narrow dark quadrupole mode.17,22,27 However, in this study, we clarify that the spectral response of dolmen structures in both the far field and near field is primarily attributable to the bonding and antibonding plasmon modes that are thought to result from plasmon hybridization.15 This attribution is supported by nearfield spectra measured by PEEM. In particular, we obtained the spatially resolved near-field spectral response of dolmen structures and observed that the maximum near-field enhancement is dominated by bonding and anti-bonding modes. The high spatial resolution of PEEM allowed us to identify a quadrupole mode in the dimer part of the dolmen structure. This quadrupole mode was previously thought to account for the Fano resonance via the interference with the dipole mode and dominates the near-field enhancement; in our experimental work, it contributes only little to the near-field enhancement. Our near-field spectroscopic and mapping results can be reproduced very well using the finite-difference time-domain (FDTD) method.

Figure 1. Characterization of topography and far-field spectra. (a) Sketch map of the Au dolmen structure that consists of a planar nanorod monomer and a planar nanorod dimer. Design parameters of the structure are as follows: L1 = 150 nm, W1 = 100 nm, G = 25 nm, L2 = 140 nm, W2 = 80 nm, D = 70 nm, H = 30 nm. (b) SEM image of the Au dolmen array, with a pitch size of 1 μm; the scale bar is 200 nm. (c) Far-field extinction spectra collected using different linearly polarized light: vertical light irradiation (parallel to the symmetry axis (black curve)) and horizontal polarized light irradiation (perpendicular to the symmetry axis (red curve)).

using standard electron-beam lithography followed by metal sputtering. The planar nanorod monomer and each nanorod in the planar nanorod dimer had dimensions of 100 × 150 and 80 × 140 nm2, respectively. The edge-to-edge distance between two nanorods in the dimer part was 70 nm, and the gap size between the monomer and dimer was chosen to be 25 nm, which guaranteed a strong interaction between the monomer and dimer. The thickness of Au dolmen structures was 30 nm, with an additional 2 nm thick titanium layer as the adhesion layer. The structures were arranged in a two-dimensional square array in a 100 × 100 μm2 area, with a pitch size of 1 μm; this pitch size was thought to be sufficiently large to avoid nearfield interaction between the adjacent unit structures.22 Figure 1b shows a scanning electron microscopy (SEM) image of Au dolmen structures. As evident in this figure, the Au dolmen structures are uniform, with good quality. Figure 1c shows the far-field extinction spectra collected using a Fourier transform infrared spectrometer (FTIR) equipped with an infrared microscope. In the case of vertical polarization (black curve) (i.e., where the electric field vector is parallel to the symmetry axis of the dolmen structures), a broad extinction band centered at approximately 800 nm is observed. However, in the case of horizontal polarization (red curve) (i.e., where the electric field vector is perpendicular to the symmetry axis), two extinction 10374

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Figure 2. Near-field spectra from PEEM measurements. (a) Schematic of the PEEM setup. (b) Near-field spectra of the Au dolmen under horizontal polarization (red curve) and vertical polarization (black curve) using wavelength-dependent PEEM measurements with a wavelength-tunable laser. (c,d) Comparison between the near-field spectrum and far-field extinction spectrum under horizontal polarization and vertical polarization, respectively.

peaks, centered at approximately 770 and 860 nm, are observed. This observation is consistent with previous reports on the far-field optical properties of Au dolmen structures. When polarization is along the long axis of dimer nanorods, a broad dipole LSPR mode of the entire structure can be excited. For horizontal polarization, a dipole LSPR in the monomer nanorod can be excited. Due to the near-field interaction with the nearby monomer nanorod, two dipole LSPRs can also be excited, with π out of phase, in the two constituent nanorods of the dimer, forming a quadrupole-like mode. In the frame of plasmonic Fano resonance theory, the dipole mode in the monomer and the quadrupole mode in the dimer can interfere with each other, leading to the plasmonic Fano dip in the extinction spectrum.17,22,27 Here, a dip at 810 nm was clearly observed. The quadrupole LSPR mode is usually thought to be located close to this dip position, as demonstrated through calculations of the charge distribution.20 The radiant loss of the quadrupole mode is smaller; thus, it may exhibit a longer dephasing time and a stronger local field enhancement in the near field. Next, using wavelength-dependent PEEM measurements, we will determine whether the maximum near-field enhancement occurs at the so-called Fano resonance wavelength. Near-Field Spectra. The near-field properties of Au dolmen structures were investigated using PEEM, with femtosecond laser pulses as the excitation source. Assisted by LSPRs, femtosecond laser pulses provide substantial photoemission via a nonlinear photoemission process mainly including multiphoton PE, especially when the laser wavelength was at or approached the LSPR wavelengths. As discussed in the introduction, the PEEM images of plasmonic nanostructures upon femtosecond laser irradiation at resonance condition can be treated as the nonlinear near-field intensity mapping of the structures, and a near-field PE intensity spectrum can be

regarded as the nonlinear near-field LSPR response spectrum of plasmonic nanostructures. In our experiments, we employed a wavelength-tunable (720−920 nm) femtosecond laser with a pulse duration of ∼100 fs and a repetition rate of 77 MHz as the excitation source for the PEEM measurements. A schematic of the PEEM setup is shown in Figure 2a. The laser beam was focused onto the sample under normal incidence using a lens with a focal length of 600 mm. The focal spot on the sample surface was estimated to have a diameter of 120 μm. We primarily used a nominal field of view (FOV) of 1.25 μm for the PEEM measurements to ensure that only a single dolmen structure was imaged, considering that the pitch size of the nanostructure array was 1 μm. To obtain the near-field spectra, we imaged and integrated the near-field photoemission intensity over the entire FOV for the illumination wavelength range of 720 to 920 nm in 10 nm increments. Figure 2b shows the near-field spectra of a Au dolmen structure under horizontal (red curve) and vertical (black curve) polarization, as shown in the inset sketch map. The nearfield spectrum under the horizontally polarized excitation exhibits two peaks, located at approximately 760 and 850 nm. Additionally, the two peak wavelengths are approximately identical to those observed in the far-field extinction spectrum, as evident in Figure 2c. Unlike the results in a previous report on Fano resonant plasmonic heptamer structures,20 the maximum near-field intensity is not observed around the dip position. For the vertically polarized excitation, the near-field spectrum also gives one broad band centered at approximately 820 nm, which is remarkably similar to the far-field extinction spectrum in Figure 2d. However, our study focuses only on the horizontally polarized excitation case. Near-Field Mapping. The far-field and near-field optical properties were investigated, and we found that the near-field and far-field spectra of Au dolmen structures under horizontal 10375

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Figure 3. Near-field mapping obtained from PEEM measurements. (a) Near-field spectrum of a single Au dolmen structure, obtained using wavelength-dependent PEEM measurements. The inset shows an SEM image of a single dolmen structure investigated using PEEM. (b) PEEM image of Au dolmen structures simultaneously irradiated with UV light and femtosecond laser pulses, with a central wavelength of 800 nm. The PEEM image of a Au dolmen structure under UV light excitation reveals the morphology of the structure. Using PEEM images collected under simultaneous irradiation with UV light and laser pulses, the exact locations of hot spots were confirmed. (c−f) PEEM images collected under irradiation with femtosecond laser pulses at four different wavelengths: the shorter-peak wavelength of 760 nm (c), the dip wavelength of 800 nm (e), and the longer-peak wavelength of 850 nm (f) and additional 780 nm (d) with the maximum enhancement from the dimer part. These wavelengths are also marked as 1−4 in (a). The red dashed lines in (b−f) plot the dolmen structure geometry. The scale bar in all images is 100 nm.

simultaneously irradiated by the femtosecond laser pulses and UV light. This simultaneous irradiation helps to outline Au dolmen structures, which are shown as red dashed lines. Note that the size of the nanostructure shown in the UV-PEEM image (panel b) seems relatively smaller than that shown in the SEM image (inset in panel a), and it mostly results from the topographical contrast rendering the slightly low collection efficiency of the photoemission from edges. This approach allowed us to investigate different near-field intensity distributions at four excitation wavelengths: 760, 780, 800, and 850 nm. First, at the two peak wavelengths (760 and 850 nm), the photoemission (near-field enhancement) from the top monomer nanorod dominates the total near-field enhancement. At the shorter-wavelength peak of 760 nm, the two upper corners are stronger. However, at the longer-wavelength peak of 850 nm, two lower corners are stronger. This interesting observation will be discussed later. Second, at the dip wavelength of 800 nm, four hot spots from the dimer nanorods (two nearest lower corners and two outside upper corners) become clear. However, the near-field enhancement at the dip wavelength is much weaker. The hot spots from the dimer are found to be strongest at the wavelength of 780 nm, which is not located at but close to the dip position. Numerical Simulations and Discussion. To further explain our experimental observations, we performed numerical calculations of far-field extinction spectra, the near-field intensity enhancement spectra, the near-field intensity distributions, and charge distributions using the FDTD simulation

polarization excitation have approximately the same shape. The near-field intensity distributions at various wavelengths are a matter of particular interest. As previously mentioned, the wavelength-dependent PEEM measurements can give the spatially resolved near-field intensity distribution directly; however, the photoemission intensity is correlated with the local electric field intensity in a nonlinear manner. We created an animation that represents the spatial evolution of near-field patterns on the Au dolmen structure under horizontal polarization excitation at various laser wavelengths ranging from 720 to 920 nm. The animation is provided in the supplementary animation. Additionally, most of the frames are displayed in Figure S1 in the Supporting Information. We observed that hot spot distributions (i.e., the near-field intensity distribution) evolve dramatically, especially when the wavelength changes within two peak wavelengths. To present the evolution of the near-field intensity distribution more clearly, in Figure 3, we show typical PEEM images for four characterized wavelengths, as marked in panel a. An additional UV light source (i.e., a mercury lamp) was used for the PEEM measurements. Within the FOV of the PEEM images, the UV light is uniform and the UV-PEEM images can provide details of the morphology of the Au nanostructures, being an important reference for determining the locations of plasmonic hot spots irradiated by femtosecond laser pulses.40 The morphology of the Au nanostructure can be observed because of the work function contrast between Au and the substrate. Panel b shows the PEEM image when the sample was 10376

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Figure 4. Spectra and mapping calculated by FDTD simulations. (a) Far-field extinction spectrum (black) and near-field PE intensity spectrum (red), plotting the integral of the (I/I0)4 for a 320 nm × 320 nm area on the interface between the dolmen structure and the substrate. (b) FDTD calculated charge distribution at two characterized peak wavelengths, indicating two hybridized modes, namely, antibonding and bonding modes. (c) Calculated near-field intensity distribution under four characterized wavelengths: the shorter-peak wavelength of 759 nm (1), 788 nm (2) that gives the maximum enhancement for the dimer part, the dip wavelength of 802 nm (3), and the longer-peak wavelength of 853 nm (4), which are also marked as 1−4 in (a).

monomer and the quadrupole mode in the dimer results in two new hybridized states.15,22 The shorter-wavelength (higherenergy) peak is assumed to result from the anti-bonding state, whereas the longer-wavelength (lower-energy) peak results from the bonding state. This assumption is confirmed by the calculated surface charge distribution in Figure 4b. The charge distributions at the shorter-wavelength peak and longerwavelength peak match very well with the proposed antibonding and bonding coupled modes, respectively. In the case of the anti-bonding state, the charge distribution in the monomer and dimer part leads to a repulsive Coulomb force between the monomer and dimer. Thus, the charges in the monomer concentrate more at the two upper corners and exhibit stronger near-field enhancement there. By contrast, the bonding state leads to an attractive Coulomb force between the monomer and dimer. Thus, under the longer-peak wavelength irradiation, the near-field enhancement is stronger at the two lower corners of the monomer. Our results provide the nearfield experimental observation of the bonding and anti-bonding modes in coupled plasmonic nanostructures. Additionally, the phase in the dimer part undergoes a phase jump of π around the extinction dip wavelength, as can be found from the Figure S2 in the Supporting Information, where the calculated charge distribution at more wavelengths are presented. It is also worth noting that the plasmon hybridization between the nanorod monomer and the nanorod dimer part is very sensitive to the gap distances between the two parts, as shown in Figure S3 (experiments) and Figure S4 (simulations) in the Supporting Information, where the energy splitting between the two hybridized modes becomes smaller as the gap distances increase. In most coupled plasmonic systems, plasmon hybridization and Fano resonance are mixed; furthermore, they are hard to be distinguished from the far-field extinction or scattering spectra.

method. The FDTD simulation results for both the far-field extinction spectrum and the near-field intensity spectrum upon horizontal polarization excitation are shown in Figure 4a. In this figure, the normalized integrated value (I/I0)4 over an area of 320 nm × 320 nm at the interface between the dolmen structure and the substrate is shown for better comparison with the PEEM measurement results; we assumed an average fourphoton photoemission process. Here, I and I0 represent the local electromagnetic field intensity on the plane and the incident electromagnetic field intensity, respectively. We observed that, in the near field, the maximum intensity was not induced at the extinction dip wavelength from the near-field PE intensity spectrum (red curve). Instead, the peak wavelengths almost coincided with the far-field extinction spectrum (black curve), which is in good agreement with the experimental results. Figure 4b shows the calculated charge distribution at the two peak wavelengths, indicating the formation of an anti-bonding mode and a bonding mode that resulted from plasmon hybridization, which will be discussed later. Figure 4c presents the near-field electric field intensity distribution under the corresponding four characteristic wavelengths. The simulated near-field intensity distribution reproduces the experimental PEEM measurements qualitatively. In particular, the redistribution of the near-field enhancement between the upper and lower corners of the monomer rod is clearly observed when the incident wavelength is switched between the two peak wavelengths. At wavelength positions of 2 and 3, the near-field intensity distribution on the dimer rods is also reproduced well. The wavelength-dependent near-field patterns observed in both the experiments and simulations are worthy of discussion. We find that it is reasonable to interpret the results in the frame of the plasmon hybridization model, according to which the strong near-field coupling between the dipole mode in the 10377

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Figure 5. Spatially resolved PE intensity spectra. (a) Spatially resolved PE spectrum of the dolmen, obtained for four different regions by integrating the PE signal from selected areas. (b) Contour part of the FDTD simulation results. The corresponding selected areas are indicated by the dashed lines on the right side. From these analyses, different modes, including the quadrupole mode on the dimer part, can be identified.

near-field intensity enhancement at either the extinction/ scattering dip position or the Fano resonance position in the case of the dolmen structures investigated here. Instead, the anti-bonding and bonding modes dominated the near-field enhancement. In the plasmonic dolmen structure, the Fano resonance was previously thought to result from the interference between the super-radiant dipole mode in the monomer and the subradiant quadrupole mode in the dimer part. In the near field, we did not find a significant contribution from Fano resonance; thus, it is reasonable to state that plasmon hybridization is dominant in the dolmen structure. Furthermore, the high spatial resolution of PEEM measurements allows us to plot the near-field PE intensity spectra from different regions, as shown in Figure 5. In Figure 5a, the total PE intensity curve can be decomposed into three bands that dominate the PE signal in three different regions. Areas 1 and 2 contribute to the shorter-wavelength and longer-wavelength peaks in the total spectrum, respectively. However, for area 3, a small peak is found at 780 nm, corresponding to the wavelength 2 in Figure 3 and Figure 4. The simulation results agree well with their experimental counterparts. This indicates that three different modes can be identified and spatially separated. In particular, the quadrupole mode in the dimer rods is revealed. This quadrupole mode may contribute to the formation of the Fano resonance via the interference with the dipole mode in the monomer rod. However, the near-field enhancement at the Fano resonance wavelength or the quadrupole mode is lower than the two hybridized modes. From both our experimental and simulation results, it is reasonable to conclude that in the plasmonic dolmen structures, the contribution to near-field enhancement is dominated by plasmon hybridization rather than Fano resonance.

This problem is similar to the crossover between the EIT phenomena and Rabi splitting in a plasmon molecular-coupled system; a recent theoretical effort was used to distinguish EIT and Rabi splitting via excitation spectra that reflect molecular absorption spectra.43 On the basis of the aforementioned results, we propose that near-field spectral properties can be applied to distinguish the difference of the contribution to the near-field enhancement between a Fano resonance and plasmon hybridization. Halas and co-workers investigated the near-field properties of individual Fano resonant plasmonic heptamer structures using surface-enhanced Raman scattering (SERS) and numerical calculations.20 They observed that SERS signals are most enhanced when the wavelength of the pump laser and the Raman Stokes mode of interest overlap the Fano dip wavelength. Additionally, their calculations revealed that the wavelength of the Fano dip in the far-field scattering spectrum is very close to the wavelength of the maximum in the near-field enhancement spectrum. In a plasmonic heptamer, plasmon hybridization and Fano resonance coexist. The dipole LSPR of the central NP and the LSPRs of outer NPs are hybridized to a bonding bright (super-radiant) mode and an anti-bonding dark (subradiant) mode. Then, the interference between the bonding mode and the anti-bonding mode induces the Fano resonance.44,45 In such plasmonic Fano nanostructures, the spectrally narrow plasmon mode around the extinction/ scattering dip position should dominate the near-field enhancement. This finding was also reported by Frimmer et al., using the CL technique.35 However, a recent theoretical work reported that, for the plasmonic dolmen structure, the Fano resonance wavelength does not necessarily correspond to a specific point of the reflectance spectrum (for instance, a local minimum or maximum), but the maximum near-field intensity enhancement was reported at the Fano resonance position.46 Nevertheless, experimentally, we did not observe the strongest 10378

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was first deposited via sputtering (MPS-4000, ULVAC) as the adhesive layer, followed by deposition of a 30 nm thick Au film. Lift-off was performed by successively immersing the sample in anisole, acetone, methanol, and ultrapure water in an ultrasonic bath. Morphologies were analyzed using field-emission scanning electron microscopy (JSM-6700FT, JEOL). Far-field spectral properties were analyzed using FTIR spectroscopy (FT/IR-6000TM-M, JASCO). PEEM Measurements. The photoemission electron microscope used in this study was a PEEM-III equipped with an energy analyzer (Elmitec GmbH). We used two light beams as excitation sources for PEEM. One beam was UV light from a mercury lamp (unpolarized continuous wave light with a cutoff energy of 4.9 eV), which was used to characterize the morphologies of metallic nanoparticles. The other one was a Ti:sapphire femtosecond laser that delivers approximately 100 fs laser pulses with a tunable central wavelength (720−920 nm) at a repetition rate of 77 MHz. The laser beam was focused onto the sample from the Au nanostructure side under normal incidence using a lens with a focal length of 600 mm. We used this laser for wavelengthdependent PEEM measurements to obtain the near-field PE intensity spectra, as well as for near-field mapping at various wavelengths. By changing the irradiation wavelengths from 720 to 920 nm step-by-step in increments of 10 nm, a series of PEEM images can be obtained. The PE signal from the entire field of view or the selected area can be integrated and plotted against the incidence wavelength; thus, the socalled near-field PE spectra can be obtained. Thus, the near-field properties, such as mapping and spectra, can be investigated via this method. PEEM images in Figure 3 and Figure S1 were taken under the field of view of 1.25 μm, with an exposure time of 2 s. The laser power was kept as 120 mW before the PEEM window and the focal lens for the wavelength-dependent measurements. The laser intensity on the sample was estimated to be ∼120 MW/cm2, and considering a nearfield intensity enhancement factor of 102−103, the maximum local electric intensity could reach 12−120 GW/cm2, which is sufficient to generate multiphoton photoemission. Numerical Simulations. Numerical simulation of the near-field properties of Au dolmen structures was performed using the FDTD Solutions software package (Lumerical, Inc.). The ITO-covered glass substrate was assumed to behave as a dielectric material with an average refractive index n = 1.55. The optical properties of Au were obtained using the data from Johnson and Christy.49 The FDTD simulations were performed on a discrete, uniformly spaced mesh with a mesh size of 3 nm. The plane wave light source was injected onto Au dolmen structures from the structure side. In the light propagation direction, the perfectly matched layer boundary conditions were imposed, and in the plane perpendicular to the light propagation direction, the periodic boundary conditions were applied on each boundary. The simulation region in this plane is 1000 nm × 1000 nm, corresponding to one unit of the dolmen structure in the array. The extinction spectra were obtained by a transmission power monitor located at 230 nm from below the ITO surface. A power and profile monitor located at the interface of the Au nanostructure and ITO was used to calculate near-field spectra and near-field intensity distribution.

It is also important to remark about the link between plasmon hybridization and Fano resonance from two aspects. On the one hand, both phenomena can be explained in the frame of the interaction or coupling between different (typically two) plasmon modes with different interaction/coupling strength. In the strong plasmon interaction/coupling region, plasmon hybridization is strong such that the hybridized modes dominate the near-field enhancement. In the weak interaction/ coupling region, Fano resonance becomes strong and thus induces the largest near-field enhancement at the Fano resonance position due to its subradiant nature. On the other hand, when the widths of two plasmon resonance modes are significantly different (one is narrow and the other is broad), the plasmon interaction/coupling can be described by Fano models. When the widths of the two resonances are comparable, the plasmon interaction/coupling can be described by plasmon hybridization theory.47 As seen from the near-field spectra shown in Figure 5a, the quadrupole mode in the dolmen structure becomes broad due to the strong near-field interaction between the monomer rod and the dimer rods. We argue that in the dolmen structure, the broadening of the quadrupole mode leads to the attenuation and even disabling of the Fano resonance, thus strengthening the plasmon hybridization, resulting in the dominant near-field enhancement in the two hybridized modes, which can be regarded as two new plasmon eigenmodes of the whole dolmen nanostructure.48

CONCLUSIONS In summary, we experimentally investigated the near-field properties of a coupled Au dolmen structure using PEEM. The near-field plasmon coupling was revealed by the spatial evolution of the near-field patterns. In particular, the evolution of near-field mapping at high spatial resolution with the excitation wavelength revealed that the coupled bonding and anti-bonding plasmon modes were hybridized from a dipole mode in the monomer part and the quadrupole mode in the dimer part. We observed that the hybridized anti-bonding and bonding modes dominated the near-field enhancement, although the quadrupole mode in the dimer part can be identified via near-field mapping and spatially resolved nearfield spectra. Furthermore, the measurement of near-field spectral properties allows for distinguishing the difference of their contribution to the near-field enhancement between plasmon hybridization and Fano resonances in plasmonic dolmen structures. More generally, this methodology can be applied in evaluating other coupled plasmonic nanostructures. The results deepen our understanding of plasmon hybridization and plasmonic Fano resonances and are expected to facilitate further development of potential applications.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06206. Caption for the supplementary animation; PEEM images of the single Au dolmen excited by femtosecond laser pulses with different central wavelengths, charge distribution calculated by FDTD simulations, experimental results of far-field and near-field spectra of Au dolmen structures with different gap distances, and FDTD simulation results of far-field and near-field spectra of Au dolmen structures with different gap distances (PDF) Supplementary animation of the spatial evolution of the near-field patterns on the Au dolmen structure (AVI)

METHODS Sample Fabrication and Characterization. Planar Au dolmen structures were fabricated on indium tin oxide (ITO)-coated glass substrates with an approximately 150 nm thick ITO layer using electron-beam lithography (EBL) and metal evaporation techniques. The substrate was then sequentially cleaned with acetone, methanol, and ultrapure water in an ultrasonic bath (each step was 5 min). A conventional copolymer resist (ZEP520, Zeon Chemicals) diluted with a ZEP thinner (1:1) was then spin-coated onto the substrate at 1000 rpm for 10 s and at 4000 rpm for 90 s. The substrate was subsequently prebaked on a hot plate for 2 min at 150 °C. In this study, a high-resolution EBL system (ELS-F130HM, Elionix) operated at 130 kV was used for sample fabrication. The EBL was conducted at a current of 50 pA. After the development, a 2 nm thick titanium layer 10379

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by KAKENHI Grant Nos. JP23225006, JP26870014, JP15H00856, JP15H01073, JP15K04589, and JP15K17438, the Nanotechnology Platform (Hokkaido University), and Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT. REFERENCES (1) Zhang, Y.; Zhen, Y. R.; Neumann, O.; Day, J. K.; Nordlander, P.; Halas, N. J. Coherent Anti-Stokes Raman Scattering with SingleMolecule Sensitivity Using a Plasmonic Fano Resonance. Nat. Commun. 2014, 5, 4424. (2) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. Metallic Nanoparticle Arrays: A Common Substrate for Both Surface-Enhanced Raman Scattering and Surface-Enhanced Infrared Absorption. ACS Nano 2008, 2, 707− 718. (3) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (4) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (5) Lal, S.; Link, S.; Halas, N. J. Nano-Optics from Sensing to Waveguiding. Nat. Photonics 2007, 1, 641−648. (6) Kawata, S.; Inouye, Y.; Verma, P. Plasmonics for Near-Field Nano-Imaging and Superlensing. Nat. Photonics 2009, 3, 388−394. (7) Ueno, K.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation from Visible to Near-Infrared Wavelengths. NPG Asia Mater. 2013, 5, e61. (8) Shi, X.; Ueno, K.; Takabayashi, N.; Misawa, H. PlasmonEnhanced Photocurrent Generation and Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode. J. Phys. Chem. C 2013, 117, 2494−2499. (9) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source. J. Am. Chem. Soc. 2008, 130, 6928−6929. (10) Gao, S. Y.; Ueno, K.; Misawa, H. Plasmonic Antenna Effects on Photochemical Reactions. Acc. Chem. Res. 2011, 44, 251−260. (11) Brongersma, M. L.; Halas, N. J.; Nordlander, P. PlasmonInduced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10, 25−34. (12) Zhong, Y. Q.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy. Angew. Chem., Int. Ed. 2014, 53, 10350−10354. (13) Oshikiri, T.; Ueno, K.; Misawa, H. Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation. Angew. Chem., Int. Ed. 2014, 53, 9802−9805. (14) Oshikiri, T.; Ueno, K.; Misawa, H. Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light via Plasmon-Induced Charge Separation. Angew. Chem., Int. Ed. 2016, 55, 3942−3946. (15) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302, 419−422. 10380

DOI: 10.1021/acsnano.6b06206 ACS Nano 2016, 10, 10373−10381

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DOI: 10.1021/acsnano.6b06206 ACS Nano 2016, 10, 10373−10381