Exploring the Near-Field of Strongly Coupled Waveguide-Plasmon

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Exploring the Near-Field of Strongly Coupled Waveguide-Plasmon Modes by Plasmon-Induced Photocurrent Generation Using a Gold Nanograting-Loaded Titanium Dioxide Photoelectrode Jingchun Guo, Kosei Ueno, Jinghuan Yang, Xu Shi, Jie Li, Quan Sun, Tomoya Oshikiri, and Hiroaki Misawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07707 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Exploring the Near-Field of Strongly Coupled Waveguide-Plasmon Modes by Plasmon-Induced Photocurrent Generation Using a Gold Nanograting-Loaded Titanium Dioxide Photoelectrode

Jingchun Guo,† Kosei Ueno,† Jinghuan Yang,†,‡ Xu Shi,† Jie Li,† Quan Sun,† Tomoya Oshikiri,† and Hiroaki Misawa*,†,§

†

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021,

Japan ‡

State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking

University, Beijing 100871, China §

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010,

Taiwan

1

ACS Paragon Plus Environment


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ABSTRACT Near-field spectrum measurement techniques, including near-field scanning optical microscopy, electron energy loss spectroscopy, and multi-photon photoemission electron microscopy, are powerful means to investigate near-field interactions directly on closely spaced metallic nanoparticles or a metallic nanostructure coupled with optical modes, such as whispering gallery mode and waveguide mode, which are called coupled plasmonic systems. In the present study, we have successfully measured the near-field spectra of coupled plasmonic systems using a simple photoelectrochemical measurement based on plasmon-induced water oxidation. Coupling was explored between the localized surface plasmon resonance (LSPR) mode and waveguide mode of periodic gold nanogratings patterned on a thin titanium dioxide waveguide film. It is known that the far-field reflection spectrum of this waveguide-LSPR coupling system shows a complicated shape with multiple peaks, and the coupling has been explored by numerical electromagnetic simulations so far. However, in this study, it was clearly elucidated that an internal quantum efficiency (IQE) spectrum observed in the plasmon-induced photocurrent generation has successfully reproduced the near-field spectrum predicted by electromagnetic simulations under the coupling conditions. The photocurrent generation based on the plasmon-induced charge separation is due to the near-field effect, and it can be considered that the IQE spectrum corresponds to the near-field spectrum. This study paves a new way to indirectly measure the near-field spectrum of plasmonic systems. 2


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INTRODUCTION Metallic nanostructures exhibit various colors that are derived from localized surface plasmon resonances (LSPRs). LSPRs are collective oscillations of conduction electrons on metallic nanostructures, which induce large local electromagnetic field enhancements.1-3 The electromagnetic field enhancement effects are known to induce various optical effects, including surface-enhanced Raman scattering,4-8 two-photon induced photoluminescence,9-12 and higher-order harmonic generations.13-17 Recently, closely spaced metallic nanoparticles (e.g., nanogap dimer,8,17 dolmen,18-20 and oligomers.21-23) or metallic nanostructures coupled with optical modes (e.g., whispering gallery mode,24 and waveguide mode.25-31), which are called coupled plasmon systems, have been fabricated with nanometric accuracy due to the rapid development of nanofabrication technologies. Strongly coupled plasmonic systems such as nanogap dimers exhibit spectrum shifts and strong near-field intensity enhancement up to a factor of 105.32 In comparison to individual nanoparticles, some coupled plasmonic systems such as heptamers show tunable narrower line width in the reflection spectrum based on Fano resonance and better performance in refractive index chemical sensing.20,22 The spectrum modulations of coupled plasmonic systems are derived from near-field interactions between different modes. Therefore, near-field spectrum measurement techniques are crucial to investigate near-field interactions in coupled plasmonic systems. To date, various types of near-field spectrum measurement techniques have been 3



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proposed, such as near-field scanning optical microscopy,33 electron energy loss spectroscopy,34 cathodoluminescence,35,36 and multi-photon photoemission electron microscopy.37,38 In the present study, we propose a new way to investigate the near-field spectra of coupled plasmonic systems using plasmon-induced chemical reactions based on photoelectrochemistry. Gold (Au) nanostructure-loaded thin titanium dioxide (TiO2) photoelectrodes have been extensively used in visible light-induced photocurrent generation and water oxidation.39-42 The plasmonically induced near-field promotes the inter- and intra-band transitions of Au, resulting in the formation of electron-hole pairs.42 The excited electrons are transferred to the conduction band of TiO2, and the remaining holes have to receive electrons from electron sources to obtain a stable photocurrent. If an electron donor is not included in the photocurrent generation system, water molecules should work as an electron source.40,41 Actually, we have successfully confirmed a stoichiometric evolution of oxygen gas as a result of water oxidation using oxygen-18 isotopically labeled water with an irradiation of visible to near-infrared light on the Au nanostructured TiO2 photoelectrode.40,41 Water oxidation, which involves four electronic transitions, proceeds even with low energy light because the electron transfer reaction is induced by the near-field, and multiple holes are trapped in the surface states of TiO2 in the restricted nanospace near the Au/TiO2/water interface.42 In general, the internal quantum efficiency (IQE) in a photocurrent generation system such as a dye-sensitized solar cell is not dependent on the wavelength because the efficiency is normalized by the absorbed photons if the photocurrent generation is triggered only by 4


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the light absorption by organic dye molecules.43 However, the plasmon-induced water oxidation efficiency should be influenced not only by the light absorption but also by the local near-field intensity because the reaction proceeds via multi-electron transfers only in the restricted nanospace. Thus, the IQE spectrum in the plasmon-induced photocurrent generation using water as an electron source should correspond to the near-field spectrum. In the present study, the near-field spectra of coupled plasmonic systems were collected

by

measuring

simple

photoelectrochemical

responses

based

on

plasmon-induced water oxidation. Coupling was explored between the LSPR mode and waveguide mode for periodic gold nanogratings (Au-NGs) patterned on thin titanium dioxide waveguide film (Au-NGs/TiO2). The far-field reflection spectrum and near-field spectrum of this waveguide-LSPR coupling system showed a complicated shape with multiple peaks due to spectrum modulations based on the coupling and the oblique incidence (later described in detail). Therefore, this system is suitable for demonstrating the performance of the near-field spectrum measurement for challenging structures using the proposed chemical method. Here, Au-NGs/TiO2 was employed as the photoelectrode of a plasmon-induced photocurrent generation system, and the IQE spectra were explored to elucidate the near-field spectra of Au-NGs/TiO2, and finally compared with near-field spectra predicted by electromagnetic simulations. This system provides a new approach for the indirect measurement of the near-field spectra of metallic nanostructures in coupled plasmonic systems. 5



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EXPERIMENTAL SECTION Preparation and Characterization of Au-NGs/TiO2 structure. The glass substrate was first rinsed with acetone, methanol, and deionized water in an ultrasonic bath for 5 minutes and dried with compressed N2. After that, a 250-nm TiO2 thin film was deposited on one side of the glass substrate using a commercial hot-wall flow-type atomic layer deposition (ALD) reactor (SUNALETM R series, Picosun, Finland). The deposition of the TiO2 thin film was processed by alternating exposures to TiCl4 and subsequent deionized water vapor at a process temperature of 300°C with nitrogen as a precursor carrier and purge gas at a pressure of 1.6 kPa layer by layer. The silica glass substrate with deposited TiO2 was then rinsed with acetone, methanol, and deionized water in an ultrasonic bath for 5 minutes separately again and dried with compressed N2. After that, a conventional copolymer resist (ZEP-520A; Zeon Chemicals, Louisville, USA) diluted with ZEP-A (Zeon Chemicals) thinner (volumetric ratio 1:1) was spin-coated on the 250 nm TiO2 thin film (1000 rpm for 10 s, and 4000 rpm for 90 s), and prebaked on a hot plate for 2 min at 150°C. Electron beam lithography (EBL, ELS-F125; Elionix, Tokyo, Japan) was conducted at an acceleration voltage of 125 kV, a beam current of 3 nA and a dose of 512 µC·cm-2 by a skip scan method. After development with 4-methyl-2-pentanome (methyl isobutyl ketone, Wako) for 60 s, 1 nm of titanium as an adhesion layer was deposited onto the substrate followed by the deposition of a 50 nm-thick Au film via sputtering (MPS-4000, ULVAC). Finally, the

6


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residual resist was removed by a lift-off process in an ultrasonic bath of anisole (methoxybenzene, Wako) for 5 minutes, and subsequently rinsed with acetone, methanol, and deionized water in an ultrasonic bath for 5 minutes, respectively. The structural geometries of the Au-NGs/TiO2 structures were observed by field-emission scanning electron microscopy (FE-SEM, JSM-6700FT, JEOL) with a maximum resolution of 1 nm at an electron acceleration voltage of 15 kV. To investigate the far-field spectrum properties of the Au-NGs/TiO2, the extinction spectra were measured using a spectrometer (PMA-11, Hamamatsu Photonics) equipped with an optical microscope (BX-51, Olympus) via an optical fiber. Photoelectrochemical Measurement. An In−Ga alloy (4:1 in weight ratio) was adhered to the backside and sidewalls of the Au-NGs/TiO2 structure to obtain Ohmic contact and connected to an electrochemical analyzer (ALS/CH Instruments 852C, ALS) with a copper lead wire. A conventional photoelectrochemical measurement system based on three electrodes was employed in this study. The Au-NGs/TiO2 photoelectrode, a platinum wire and a saturated calomel electrode (SCE) were employed as working, counter and reference electrodes, respectively. Monochromatic light with a full width at half-maximum (FWHM) band width of less than 7 nm was employed to obtain the external quantum efficiency (EQE) spectrum, and the applied potential to the working electrode was set at +0.3 V versus the reference electrode during the photocurrent measurement. The incident light was polarized perpendicular to the long axis of Au-NGs during the photocurrent measurement. An aqueous Ar-gas-bubbled KClO4 (0.1 7



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mol/L) solution was used as a supporting electrolyte solution. Numerical Simulations. The far- and near-field spectra of Au-NGs/TiO2 structures were calculated using the finite-difference time-domain (FDTD) Solutions software package (Lumerical, Inc.). The dielectric function determined by Palik was used for the glass.44 The TiO2 substrate was assumed to behave as a dielectric material with an average refractive index n = 2.4. The optical constants of Au were obtained using the data from Johnson and Christy.45 A discrete, uniformly spaced mesh with a mesh size of 2.5 nm was performed during the FDTD simulations. The background index of FDTD was set to be 1.33. The plane wave light source was projected onto the Au-NGs/TiO2 structures from the structure side polarized perpendicular to the long axis of Au-NGs at normal incidence and oblique incidence with an incidence angle of 1.5°, respectively. In the light propagation direction, the perfectly matched layer boundary conditions were imposed, and in the plane perpendicular to the light propagation direction, the Bloch boundary conditions were applied at each boundary. The extinction spectra were obtained by a transmission power monitor located 400 nm below the TiO2 surface. Monitoring of the power and profile was performed twice to explore the near-field properties. One was located at the interface of the Au-NGs and TiO2 to monitor the near-field enhancement. Another one covered the whole cross-section of Au-NGs/TiO2 structures to monitor the electric and magnetic fields with various pitch sizes.

RESULTS AND DISCUSSION 8


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Characterization of Topography of Au-NGs/TiO2. Figure 1a shows a schematic illustration of periodic Au-NGs/TiO2 structure. The LSPR mode of the Au-NGs is excited (with resonance energy of Ep) and TiO2 film supported waveguide mode is induced (with resonance energy of Ew) when the incident light is polarized perpendicular to the long axis of Au-NGs. The waveguide-LSPR coupling modes, such as a strong coupling between exciton and optical modes, are induced. and the hybrid states (upper branch (P+) and lower branch (P－), separated by ℏΩ in energy) are formed, as indicated in the diagram of Figure 1a. Figure 1b shows the typical SEM image of the periodic arrays of the Au-NGs/TiO2 structure with a 300 nm pitch size.

Figure 1. (a) A schematic illustration of periodic Au-NGs 100 nm wide and 50 nm tall deposited on a 250 nm TiO2 film. The LSPR mode of the Au-NGs was excited, and the TiO2 film supports the waveguide mode is induced when the incident light is polarized perpendicular to the long axis of Au-NGs. The top diagram depicts the formation of the 9



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waveguide-LSPR coupling modes P+ and P‒ with a splitting energy of ℏΩ derived from uncoupled waveguide (Ew) and LSPR (Ep) modes, respectively. (b) A typical SEM image of the Au-NGs/TiO2 structure with a 300 nm pitch size. The scale bar represents 300 nm.

Far-Field Spectrum Properties and Near-Field Distributions of Au-NGs/TiO2. Figure 2a shows the extinction spectra of the periodic Au-NGs/TiO2 structures in water with increasing pitch sizes from 200 nm to 400 nm. The incident light was polarized perpendicular to the long axis of Au-NGs. There is only one peak corresponding to LSPR mode of periodic Au-NGs with 200 nm and 225 nm pitch sizes. Starting with a pitch size of 250 nm, three peaks can be observed, and the peaks show a spectral shift with increasing pitch size. We performed FDTD simulations to assign the origin of each peak of the far-field extinction spectra to understand the near-field distributions and spectra of Au-NGs/TiO2 with different pitch sizes. The simulated extinction spectra of such structures with various pitch sizes at normal incidence are shown in Figure S1. There is only one peak for 200 nm and 225 nm pitch sizes, which is in good accordance with Figure 2a. However, only two peaks can be observed with pitch size larger than 250 nm due to the excitation of the waveguide mode. It is inevitable for oblique incidence to occur because we employed a non-collimated incoherent light source for the extinction measurement. Therefore, we introduced incidence degrees to optimize the simulations. The simulated 10


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extinction spectra of such structures with various pitch sizes with an incidence angle of 1.5° are shown in Figure 2b, which satisfactorily reproduce the experimentally measured data shown in Figure 2a. A third peak appeared between the two peaks at normal incidence with pitch sizes larger than 250 nm due to the oblique incidence. It is known that two waveguide modes, namely a symmetric mode and an antisymmetric mode, are excited by the oblique incidence.25,26 The third peak could be assigned as the antisymmetric mode, which only weakly interacts with the LSPR mode.25 The electric field distributions of the characterized wavelengths of selected three pitch sizes of 200 nm, 300 nm and 350 nm are shown in Figures 2c, 2d and 2e, respectively. Only the LSPR mode was excited with the 200 nm pitch size, as shown in Figure 2c. In the case of the 300 nm pitch size, both shorter-wavelength and longer-wavelength peaks exhibited strong spatial confinement of the electric fields at the interface between the Au-NGs and supported TiO2 substrate, which are derived from the excitation of the LSPR mode of the Au-NGs, as shown in Figure 2d2 (shorter-wavelength peak) and 2d4 (longer-wavelength peak), respectively. Moreover, the electric field distributions inside the TiO2 thin film also show the waveguide patterns, especially at the shorter-wavelength peak, as shown in Figure 2d2. As the pitch size increased to 350 nm, the plasmon characteristics of the shorter-wavelength peak were clearer than those of the 300 nm pitch size, while the waveguide characteristics were depressed, as shown in Figure 2e5. Therefore, the shorter wavelength peak was a more plasmon-like mode with a 350 nm pitch size. In contrast, the longer-wavelength peak 11



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was a more waveguide-like mode, as shown in Figure 2e7. The two diagrams depict the waveguide-LSPR coupling system deriving from the symmetric waveguide mode Ew and LSPR mode Ep with 300 nm (left panel in Figure 2d) and 350 nm (right panel in Figure 2e) pitch sizes, respectively. The line widths of P+ and P‒ became broader and narrower, respectively, with increasing pitch size, also demonstrating the formation of waveguide-LSPR coupling modes. Such behavior is much clearer in the magnetic field distributions, as shown in Figure S2. Importantly, the electric field distributions of the middle-peak, which were induced by oblique incidence, show even higher enhancement ability of electric fields, as shown in Figures 2d3 and 2e6. It is also worth noting that the electric field distribution of the middle-peak (3 and 6) mostly remains the characteristic of the waveguide mode, since this antisymmetric mode only weakly interacts with the LSPR mode.

12


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Figure 2. (a) Measured extinction spectra of the Au-NGs/TiO2 structures with different pitch sizes in water (P200 denotes the Au-NGs/TiO2 structure with 200 nm pitch size). (b) Simulated extinction spectra corresponding to (a). (c) Calculated electric field |E| distributions under the characterized wavelengths with 200 nm pitch size. Only the LSPR mode is excited with the 200 nm pitch size. Calculated electric field |E| distributions under the characterized wavelengths with 300 nm (d) and 350 nm (e) pitch sizes. The three characterized wavelengths of the waveguide-LSPR coupling system with 300 nm pitch size: the shorter-peak wavelength (d2, P+), the middle-peak

13



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wavelength (d3), and the longer-peak wavelength of (d4, P‒) as well as the three characterized wavelengths (e5, P+), (e6), and (e7, P‒) with 350 nm pitch size. These seven wavelengths are also marked as 1−7 in (b). The two diagrams depict a waveguide-LSPR coupling system deriving from the waveguide mode Ew and LSPR mode Ep with 300 nm (left panel in Figure 2d) and 350 nm (right panel in Figure 2e) pitch sizes, respectively.

Dispersion Curves of Waveguide-LSPR Coupling Modes. In Figure 3, the peak energies of P＋ and P－ are plotted as a function of momentum. The horizontal green line shows the extinction maximum of the uncoupled bare LSPR mode (200 nm pitch size). The black line is the uncoupled bare waveguide mode estimated by numerical simulations using TiO2 nanogratings instead of Au-NGs. The interaction between the LSPR mode and the symmetric waveguide mode could result in hybrid states, which exhibit typical anticrossing behavior. The energies of P+ and P‒ are calculated using a coupled harmonic oscillator model:46

EUB,LB =

Ep +Ew 2

1 2 2 ± （Ep -Ew） +（hΩ） 2

(1)

where Ep and Ew are the resonance energies of the uncoupled bare LSPR mode and waveguide mode, respectively. ℏΩ is the splitting energy. The red and blue curves in Figure 3 are the fitting results calculated using Equation 1. ℏΩ is estimated to be approximately 250 meV. The simulated dispersion curve is also in very good agreement

14


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with the experimental results (Figure 3), as shown in Figure S3.

Figure 3. The dispersion curves of the waveguide-LSPR coupling system. The black and green lines correspond to the uncoupled bare waveguide and LSPR modes, respectively. The red and blue dots were obtained from the maxima of the extinction spectrum shown in Figure 2 (a). The blue and red curves are the fitting results calculated from Equation 1. The black and red dotted curves represent 300 nm and 350 nm pitch sizes corresponding to Figure 2 (d) and (e), respectively.

Photoelectrochemical and Near-Field Spectra of Au-NGs/TiO2. To elucidate whether the waveguide-LSPR coupling modes are induced or not, the near-field spectra of Au-NGs/TiO2 structures were measured by utilizing plasmon-induced photocurrent generation. The photocurrent response of the Au-NGs/TiO2 photoelectrode under 15



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irradiation with different wavelengths of light was explored. Linear sweep voltammograms and I-t curves of a 300 nm pitch size Au-NGs/TiO2 photoelectrode were measured under dark and irradiated conditions with monochromatic light and are shown in Figure 4a and Figure 4b, respectively. Two monochromatic wavelengths at 650 and 700 nm were employed for these measurements. Anodic photocurrents were clearly observed. Therefore, this indicates that the electron was transferred to the conduction band of TiO2, and the remaining hole might oxidize water to evolve oxygen, as analogous to our previous study.39-42 Importantly, the photocurrent value changes with the irradiation wavelength. Therefore, we measured the EQE spectrum, as shown in Figure 4c.

Figure 4. Linear sweep voltammograms (a) and I−t curves (b) measured using a 300 nm pitch size Au-NGs/TiO2 photoelectrode under dark and irradiated conditions with monochromatic light at wavelengths of 650 nm, and 700 nm, respectively. The sweep rate for the measurement of the linear sweep voltammograms was set at 5 mV/s, and the applied potential was set at +0.3 V versus SCE during the I−t curve measurements. (c) EQE spectrum of the Au-NGs/TiO2 photoelectrode with a 300 nm pitch size.

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The IQE, which is the number of electrons observed in the photocurrent generation system in relation to the number of photons absorbed by Au-NGs, can be estimated from the generated photocurrents from a monochromatic photon flux. The IQE is normally calculated by correcting the EQE for photons absorbed by the Au-NGs as: IQE = EQE/η

(2)

where η is the ratio of absorbed photons by Au-NGs to the irradiated photons.39 Therefore, η can be expressed by the following equation: absorbed photon flux/total photon flux. However, many photons in the total extinction are scattering in the case of the Au-NGs. Therefore, it is necessary to estimate η by taking into consideration the absorption ratio in total extinction (γ). γ can be expressed by the ratio of the absorbed photons among all the interacting photons. To obtain the γ values of each wavelength, the scattering, absorption and extinction cross-sections of Au-NGs/TiO2 structures with different pitch sizes are calculated by FDTD simulations. γ can be expressed by the following equation: γ = absorption cross-section/extinction cross-section

(3)

The calculated scattering, absorption and extinction cross-sections of the Au-NGs/TiO2 structures with 300 nm and 350 nm pitch sizes are shown in Figure S3. η was estimated by multiplying experimental extinction and γ. Thus, the IQE spectrum was obtained from Equation 2. The IQE spectra plotted as a function of illuminated wavelength with 300 nm and 17



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350 nm pitch sizes are shown in Figure 5a and Figure 5b (blue dots), respectively. The near-field intensity monitored at the interface between the Au-NGs and TiO2 substrate calculated by the FDTD simulations was also investigated. The simulated near-field spectra were obtained by normalizing the integrated |E|2 values at the interface as a function of incident wavelength. The near-field spectrum shows a spectrum shift to longer wavelength compared to the far-field extinction spectrum, which is known to result from intrinsic and radiative damping as inferred from the width of the plasmon resonance in the far-ﬁeld spectrum.47 The simulated near-field spectra (black curve) are compared with IQE spectra (blue dots), as shown in Figure 5a and 5b for 300 nm and 350 nm pitch sizes, respectively. The higher IQE values at the shorter-wavelength region are attributed to the defect-induced optical absorption of the TiO2 waveguide film by ALD. Both the simulated near-field and IQE spectra of the 300 nm and 350 nm pitch sizes show distinct three peaks. It is clearly demonstrated that the IQE spectra observed in the plasmon-induced photocurrent generation have successfully reproduced the near-field spectra of coupled plasmonic systems even coupling with optical modes by utilizing coupling with LSPR. Furthermore, the middle-wavelength peak with 300 nm and 350 nm pitch sizes could be confirmed by both simulated near-field spectra and IQE spectra. In the case of the 300 nm pitch size, importantly, P＋ (shorter-wavelength) and P－ (longer-wavelength) modes have comparable IQE values, as shown in Figure 5a. However, in the case of the 350 nm pitch size, as shown in Figure 5b, the P+ 18


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(shorter-wavelength)

has much

higher

IQE

value

in comparison

with

P‒

(longer-wavelength). It can be considered that the photocurrent generation is induced by the plasmon-induced charge separation between the Au nanostructures and TiO2 and subsequent water oxidation. However, the waveguide mode, which is confined inside the TiO2 waveguide film, cannot induce photocurrent generation because TiO2 does not absorb in the visible wavelength region. In the case of the 300 nm pitch size, both of the coupled P+ and P‒ have comparable plasmon characteristics and obtain comparable IQE values. However, in the case of the 350 nm pitch size, the P+ is a more plasmon-like mode and the P－ is a more waveguide-like mode based on detuning in the coupling system. Therefore, the P＋ has a much higher IQE value than the P‒ with the 350 nm pitch size.

Figure 5. IQE spectra (blue) and simulated near-field spectra (black) of the Au-NGs/TiO2 structures with 300 nm (a) and 350 nm (b) pitch sizes, respectively. The near-field intensity in the spectra was calculated by monitoring at the interface between the Au-NGs and the TiO2 film by the FDTD simulations. 19



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CONCLUSIONS Near-field spectra of the waveguide-LSPR coupling system have been successfully measured by a simple photoelectrochemical measurement method based on plasmon-induced water oxidation. Under coupling conditions, three peaks were observed in the IQE spectrum based on the waveguide-LSPR coupling modes, and the photocurrent responses were found to be highly sensitive to the pitch size change of the Au-NGs because of detuning in the coupling system. In other words, the near-field spectra in optical modes even without gold nanostructures, such as microcavities, photonic crystals, waveguides, and plasmonic nanostructures of different metal species (e.g., silver and aluminum), can be measured using the coupling with the LSPR mode of Au nanostructures. This study paves a new way to indirectly measure the near-field spectrum of optical modes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07707. This includes the simulated extinction spectra at normal incidence, calculated magnetic field |H| distributions under the characterized wavelengths with different pitch sizes as well as the simulated scattering, absorption and extinction cross-sections with 300 nm 20


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and 350 nm pitch sizes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81-11-706-9358. Fax: +81-11-706-9359. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge ﬁnancial support from JSPS KAKENHI (Grant Nos. JP17H01041, JP17H05245 JP17H05459, and JP15K04589), the Nanotechnology Platform (Hokkaido University), and the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT.

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Exploring the Near-Field of Strongly Coupled Waveguide-Plasmon

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