Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Direct Visualization of Near-Field Distributions on a TwoDimensional Plasmonic Chip by Scanning Near-Field Optical Microscopy Keisuke Imaeda,† Wataru Minoshima,§ Keiko Tawa,§ and Kohei Imura*,†,‡
J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/14/19. For personal use only.
†
Research Institute for Science and Engineering and ‡Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan § School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan S Supporting Information *
ABSTRACT: A precise understanding of the near-field distributions of plasmonic nanostructures is indispensable for their practical applications. In this study, we directly visualized near-field distributions on two-dimensional nanohole arrays covered with a gold film (plasmonic chip) by scanning near-field optical microscopy. In the near-field images, strong extinction spots were observed not only inside the nanoholes but also on the outside. We also found that the spatial characteristics outside the nanoholes are strongly dependent on the excitation wavelength. From the electromagnetic simulations, we revealed that the positions of the extinction spots in the near-field images are well correlated with the locations for the enhanced electric fields. We also found that the excitation wavelength dependency on the near-field distributions are rationalized with the classical grating coupling conditions for the plasmonic chip. These findings provide new physical insights into the near-field characteristics of the metallic nanohole arrays and are essential not only for optimizing the performances in the plasmonic nanohole arrays but also for boosting their practical applications.
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INTRODUCTION Surface plasmon polaritons are collective charge oscillations that arise from the resonant coupling of light fields with conduction electrons on a metal surface.1,2 Surface plasmons confine propagating light at a metal−dielectric interface and enable light manipulation on a subwavelength scale. These plasmonic properties have potential applications in various research fields, such as nanoscale waveguides,3 photonic circuits,4 and optoelectronic devices.5 Because far-field light cannot directly excite surface plasmons because of their momentum mismatch, nanoobjects on a metal surface, such as nanoslits and nanoholes, have often been utilized as a convenient way to excite surface plasmons with far-field light. In particular, nanohole arrays on thin metal films have been extensively investigated because they exhibit extraordinary light transmission at specific wavelengths.6−13 Because the resonance of this extraordinary transmission can be tuned by adjusting the dimensions or periodicity of an array, nanohole arrays can be applied in nanoscale optical elements such as color filters14,15 and lenses.16 Nanohole arrays can also be applied in biosensors because the plasmon resonance peaks © XXXX American Chemical Society
show a significant shift depending on the refractive index on the surface.17−20 In addition to these unique characteristics, the intense electric fields on nanohole arrays have attracted much attention in the context of nonlinear optical phenomena, including second harmonic generation21 and two-photon induced photoluminescence.22 Moreover, these enhanced fields strongly interact with molecules in the vicinity of the nanoholes and consequently amplify fluorescence23,24 and Raman scattering25−28 from these molecules; thus, nanohole arrays can also be applied for purposes such as molecular sensing29 and bioimaging.30,31 In most of the proposed applications of nanohole arrays, the near-field characteristics of the nanoholes play a dominant role, and therefore, a detailed knowledge of the near-field distributions of these arrays is fundamentally important not only for a deeper understanding of their optical characteristics but also for further development of their potential applications. Received: December 28, 2018 Revised: March 25, 2019
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DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. (a) AFM image of the 2D plasmonic chip. The scale bar is 500 nm. (b) Surface height profile along the dotted line in the AFM image shown in (a).
Figure 2. (a) Scanning electron micrograph of the 2D plasmonic chip. The scale bar is 400 nm. (b) Near-field extinction spectra of the 2D plasmonic chip. The red and blue curves represent the spectra measured at the red and blue points in (a), respectively.
local field enhancement is induced not only inside the nanoholes but also on the outside.
However, most previous works have been devoted to the optical characterization of nanohole arrays by far-field spectroscopy and have provided little information on their near-field distributions.32,33 To reveal spatially resolved nearfield distributions of nanohole arrays, advanced microscopic methods with high spatial resolution, such as scanning transmission electron microscopy with electron energy loss spectroscopy34,35 and scanning near-field optical microscopy (SNOM),36−39 are of great use. By utilizing these imaging tools, several research groups have reported the direct visualization of the near-field spatial features of single nanoholes or several nanohole arrays in thin metal films.40−46 However, the near-field spectroscopic properties on periodical nanohole arrays are not yet fully understood because the weak near-field signal is seriously overlapped with a large background and limits the spectral range under investigation. Because near-field characteristics on the nanohole array strongly depend on the excitation wavelength, nearfield microspectroscopy is essential to understand the optical functions of the arrays in detail. In this study, we fabricated a two-dimensional (2D) periodic nanohole array covered with a thin gold film (called as “plasmonic chip”) and visualized the near-field spatial and spectral characteristics of the plasmonic chip from the visible to near-infrared spectral region by using an aperture-type SNOM. We also performed electromagnetic simulations to explore the physical origins of the observed near-field images. From a comparison between the near-field observations and theoretical simulations, we revealed that the
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EXPERIMENTAL METHODS A replica of a 2D nanohole array with a 480 nm pitch was fabricated with an ultraviolet (UV)-curable resin (PAK-02A, Toyo Gosei Corp.) on a coverslip using the UV-nanoimprint method. The resin was dropped onto the coverslip, and a nanohole array mold (specially made by NTT-AT) was overlaid on the resin under UV light illumination. The replica was coated with Ti/Au/Ti/SiO2 thin films by the radiofrequency sputtering method. The thicknesses of the Ti, Au, and SiO2 layers were controlled to be 0.5, 40, and 20 nm, respectively. Here, the SiO2 film is used as a spacer to suppress possible perturbation due to the near-field probe tip. Figure 1a shows an atomic force microscopy (AFM) image of the fabricated plasmonic chip. A 2D pattern of periodic nanoholes can be clearly observed in Figure 1a. From the surface profile image shown in Figure 1b, the diameter and depth of the nanoholes were estimated to be approximately 240 and 40 nm, respectively. The optical properties of the 2D plasmonic chip were investigated with a homemade aperture-type SNOM instrument.47 An apertured near-field optical fiber probe fabricated by chemical etching was utilized for near-field local illumination. The diameter of the near-field fiber probe, which determines the spatial resolution, was determined to be 50−100 nm from a scanning electron microscopy (SEM, Hitachi, S-3400N) image. The sample substrate was mounted on a closed-loop piezo-driven stage for lateral scanning. The B
DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 3. (a,b) Simulation model of the 2D plasmonic chip: (a) top view and (b) side view. (c) Absorption spectrum of the 2D plasmonic chip calculated by the FIT simulation.
probe−sample distance was regulated at ∼10 nm by a shearforce feedback method. White light from a halogen lamp was used as a light source for near-field transmission measurements. The incident light was coupled to the other end of the optical fiber to create the near-field light at the aperture of the near-field fiber probe. The sample was locally illuminated through the aperture of the near-field fiber probe, and the transmitted light was collected by an objective lens (Nikon, CFI Plan-Fluor 60×, N.A. = 0.85) below the sample. The transmitted intensity spectrum was measured at each point within the scanned area by a polychromator equipped with a charge-coupled device (CCD, Princeton Instruments, PIXIS 256E) detector. Near-field transmission images were acquired by mapping the transmitted light intensity in a specific spectral region.
respectively assignable to the diffraction modes induced on the top and bottom surfaces of the plasmonic chip, as discussed later. To clarify the origin of the extinction peaks in the near-field extinction spectra, we simulated the absorption spectrum of the 2D plasmonic chip using the finite integration technique (FIT) method.48 Figure 3a,b shows the schematic illustrations of the simulation model. For simplicity and to reduce the computational cost, we assumed the thickness of the glass substrate (n = 1.5) to be 300 nm. From the AFM and SEM images, the diameter, depth, and periodicity of the nanoholes were determined to be 240, 40, and 480 nm, respectively. The thicknesses of the gold and SiO2 films were set to 40 and 20 nm, respectively. The surrounding medium was considered to be air (n = 1), and the dielectric constants of gold were adapted from ref 49. We used periodic boundary conditions in both x- and y-directions in the electromagnetic simulation. The perfect matching layer boundary condition was adopted in the z-direction. A linearly polarized plane wave was normally incident from the top of the plasmonic chip. The polarization direction of the plane wave is defined in terms of the angle θ between the x-axis and the incident electric field. We performed absorption spectral simulations with linear as well as circularly polarized lights and found that the spectral features are essentially the same regardless of the incident polarization characteristics. Figure 3c shows the simulated absorption spectrum of the plasmonic chip. Three resonance peaks (570, 620, and 770 nm) are observable in the simulated spectrum. The spectral features agree qualitatively with those of the observed near-field extinction spectra shown in Figure 2b. This result indicates that the extinction peaks observed in the near-field extinction spectra are attributable to plasmon resonances excited in the plasmonic chip. The minute differences between the observed and simulated resonance wavelengths could arise from the differences between the simulation model and the actual sample used. Here, we emphasize that the simulated spectrum provides the spatially
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RESULTS AND DISCUSSION Figure 2a shows a SEM image of the fabricated 2D plasmonic chip. As is evident in this figure, the gold film was deposited on not only the outside but also the inside of each nanohole. The red and blue curves in Figure 2b show the near-field extinction spectra observed at the red and blue points in Figure 2a, respectively. The vertical axis of the figure is defined as −(I − I0)/I0, where I and I0 represent the transmitted light intensities measured on the plasmonic chip and an unpatterned smooth gold film, respectively. Thus, positive and negative values on the vertical axis indicate reduction and enhancement of the transmitted light at the plasmonic chip with respect to that at a smooth gold film, respectively. As evident in Figure 2b, the transmitted light intensity observed on the plasmonic chip exceeds that on the smooth gold film in a spectral region longer than 590 nm, and extinction bands were observed near 770 nm. We also found that the extinction bands observed in the visible region strongly depend on the excitation position on the plasmonic chip; the band at 530 nm is observed only at the red point (red curve), whereas the one at 570 nm is only observed at the blue point (blue curve); these bands are C
DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. (a) Unpolarized near-field transmission image observed near 530 nm. (b,c) Polarized near-field transmission images observed near 530 nm. White arrows indicate the detected polarization directions. Black dotted circles represent the approximate shapes of the nanoholes. The scale bar is 400 nm. (d) Superposition of the electric field amplitudes simulated at 570 nm with horizontal, vertical, and diagonal polarizations. (e,f) Electric field amplitude distributions simulated at 570 nm with horizontal and vertical polarizations. Black arrows represent the excitation polarization directions.
the nanoholes are considered to be induced by the lightning rod effect at the sharp edges of the nanoholes in the simulation model. The lightning rod effect arises at the sharp edges; however, in the actual sample, the edges are truncated and smooth compared to those in the simulation model shown in Figure 3. Therefore, the corresponding field enhancement should be low in the actual sample, and consequently, the lightning rod effect was not observed in Figure 4a. In the near-field image taken near 530 nm (Figure 4a), strong extinction spots were periodically observed along the horizontal and vertical directions, indicating that the near-field distributions are attributed to plasmons propagating along the horizontal and vertical directions. We performed polarized near-field transmission measurements by placing a polarizer in front of the CCD detector. As previously reported, the detected polarization direction strongly reflects the plasmon propagation direction observed in the near-field image.50 Figure 4b,c shows the polarized near-field transmission images taken near 530 nm. The white arrows indicate the polarization directions of the detected light. The spatial patterns in Figure 4b,c are partially different from that of the unpolarized image in Figure 4a. We also found that the locations of the extinction spots in the polarized near-field images are different from each other; the extinction spots observed in Figure 4b are aligned along the horizontal direction between the nanoholes, whereas the extinction spots in Figure 4c are aligned along the vertical direction between the nanoholes. The polarization-dependent spatial characteristics are well reproduced by the electric field distributions simulated with polarized plane waves (see Figure 4e,f). These results strongly support that near-field distributions induced near 530 nm are attributed to surface plasmons propagating along the horizontal and vertical directions on the sample. Figure 5a shows a near-field transmission image taken near 570 nm. In this image, strong extinction spots are observed not only inside the nanoholes but also along the diagonal direction
averaged spectral features over the entire array, while the nearfield spectroscopy provides local spectral properties of the sample. To visualize the near-field spatial characteristics on the plasmonic chip, we obtained near-field transmission images by mapping the transmitted light intensity over the sample surface. Figure 4a shows a near-field transmission image taken near 530 nm. The black dotted circles represent the approximate shapes of the nanoholes. The dark regions in the image show the reduction of the transmitted light intensity because of the plasmon excitation. As shown in Figure 4a, the excitation probability varies depending on the excitation position on the sample, and consequently, a unique spatial pattern is observed in the near-field transmission image. We found from the near-field image that strong extinction spots are observed between the nanoholes aligned horizontally and vertically. To reveal the physical origin of the spatial features in the near-field image, we calculated the total electric field amplitude on the plasmonic chip by the FIT simulation. Figure 4d shows the electric field amplitude distribution on the plasmonic chip simulated at 570 nm. This image was obtained by superimposing the electric field amplitudes calculated for incident polarization angles of θ = 0, 45, 90, and 135° to facilitate comparison with the unpolarized near-field image shown in Figure 4a. As shown in Figure 4d, the electric fields are enhanced in the horizontal and vertical directions at the nanoholes. The locations of these enhanced fields are in good agreement with those of the extinction spots in the near-field image shown in Figure 4a. The results indicate that the nearfield transmission image visualizes the near-field distribution on the plasmonic chip. We also found that strongly enhanced fields induced by the lightning rod effect are observed around the nanoholes in the simulation, which are not consistent with the experimental near-field image. This disagreement might be explained by the morphological differences between the actual sample and the simulation model. The enhanced fields around D
DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 5. (a) Unpolarized near-field transmission image observed near 570 nm. Black dotted circles represent the approximate shapes of the nanoholes. The scale bar is 400 nm. (b) Electric field amplitude distribution on the 2D plasmonic chip simulated at 620 nm.
relative to the nanoholes. This spatial feature is entirely different from that observed near 530 nm in Figure 4a, indicating that a different plasmon mode is excited at 570 nm. We compared the observed near-field image with the simulated electric field distribution. Because the experimental resonance peak at 570 nm is attributable to the simulated peak at 620 nm (see Figures 2b and 3c), we obtained the electric field amplitude distribution at 620 nm, as shown in Figure 5b. The simulated field distribution qualitatively reproduces the experimental near-field image in Figure 5a, which again demonstrates that the near-field transmission image reflects the near-field distribution induced by plasmon resonance. From the spatial patterns in Figure 5a,b, it is expected that the near-field distribution near 570 nm can be attributed to plasmons propagating along the diagonal direction on the sample. We also found that the near-field image taken near 770 nm is not consistent with the simulated image (see Figure S1 in the Supporting Information). The reason is not clear, but it may be related to the overlapping of plasmons excited on the top and bottom surfaces of the sample. To obtain further insight into the observed results, we estimated the resonance wavelengths considering the grating coupling conditions. For the sake of simplicity, we considered the plasmon dispersion relation for the metal−dielectric interface given by the following equation1
Figure 6. Dispersion relations of surface plasmon polaritons induced at the Au/air (red curve) and Au/glass (blue curve) interfaces. The horizontal dotted lines indicate the grating coupling conditions.
using eq 2. The horizontal dotted lines in Figure 6 represent the grating coupling conditions for (m, Λ) = (1, 679 nm), (1, 480 nm), and (2, 679 nm). The plasmon resonance wavelengths can be estimated from the intersections between the dispersion curves and horizontal lines, as indicated in Figure 6. As is evident in Figure 6, the first-order diffraction mode in the horizontal and vertical directions (m = 1, Λ = 480 nm) is excited on the top surface of the plasmonic chip at 525 nm, which is in good agreement with the near-field image near 530 nm shown in Figure 4a. We also found from Figure 6 that the second-order diffraction mode in the diagonal direction (m = 2, Λ = 679 nm) is excited on the bottom surface of the chip at 587 nm. This result agrees with the near-field observation (570 nm) in Figure 5a, where plasmons propagating along the diagonal direction are induced. On the other hand, in the nearinfrared region, not only the first-order horizontal bottom mode but also the first-order diagonal top mode is excited. We should note here that the first-order diagonal top mode is dipole-forbidden (see Figure S2 in the Supporting Information) and is feasible to be excited by near-field local illumination. Spatial features in Figure S1a are very similar to those observed in Figure 5a, indicating that the diffraction mode in the diagonal direction contributes to the near-field distribution at 770 nm. We also found that a shoulder attributable to the diagonal mode on the top surface was observed near 700 nm in the near-field extinction spectrum (see also Figure S3 in the Supporting Information). These results strongly support that not only the horizontal mode but also the diagonal mode is excited in the near-infrared region. Spectral and spatial overlapping of the horizontal and diagonal modes may prevent the explicit visualization of the near-field characteristics in the near-infrared region. Further experimental and theoretical investigations are necessary to clarify the nearfield characteristics in this spectral region.
ij ε ε yz ksp = k 0jjj Au m zzz j εAu + εm z (1) k { where ksp and k0 are the wave vectors of the surface plasmon and incident light, respectively; εAu and εm are the dielectric functions of gold and the dielectric medium, respectively. For normal incidence onto the periodical nanohole array, the grating coupling conditions can be expressed as follows51 2π ksp = m (m = 1, 2, 3, . . . ) (2) Λ where m is an integer representing the diffraction order and Λ indicates the pitch of the nanohole array. In this study, Λ is 480 nm in both horizontal and vertical directions, whereas in the diagonal direction, Λ is 679 nm (= 2 × 480 nm ). We calculated the plasmon dispersion relations for the top [εm = 1.0 (air)] and bottom [εm = 2.25 (glass)] interfaces on the gold film using eq 1, as shown in red and blue curves in Figure 6, respectively. Here, the SiO2 layer on the top surface is not included for qualitative estimations of the resonance wavelength. We also calculated the grading coupling conditions 1/2
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CONCLUSIONS In conclusion, we fabricated a 2D periodic nanohole array covered with a thin gold film and studied its near-field characteristics by an aperture-type near-field optical microscopy. The near-field extinction spectra exhibit several plasmon resonance peaks depending on the positions on the chip. The observed spectral features were qualitatively reproduced by the E
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simulated absorption spectrum. In the near-field images, the extinction spots were observed not only inside the nanoholes but also on the outside. From the electromagnetic simulations, we revealed that the positions of the extinction spots in the near-field images are well correlated with the locations for the enhanced electric fields. We also found that the excitation wavelength dependency on the near-field distributions is rationalized with the classical grating coupling conditions for the plasmonic chip. These findings provide new physical insights into the near-field spectral and spatial characteristics of the metallic nanohole arrays and are essential not only for optimizing the performances in the plasmonic nanohole arrays but also for boosting their practical applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b12495.
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Near-field transmission image and electric field distribution simulated at the near-infrared region, transmission spectrum simulated by rigorous coupled wave analysis, and near-field extinction spectrum of the 2D plasmonic chip (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Keisuke Imaeda: 0000-0001-8877-1085 Wataru Minoshima: 0000-0002-4392-7354 Keiko Tawa: 0000-0002-5736-1187 Kohei Imura: 0000-0002-7180-9339 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by JSPS KAKENHI grant nos. JP26107001, JP26107003, JP15K21725, JP16K13939, JP16H04100, JP15H01100, and JP17H05273 in Scientific Research on Innovative Areas “Photosynergetics” and JP16H02092 in Scientific Research (A) from the Japan Society for the Promotion of Science. K.T. and W.M. thank Toyo Gosei for providing the UV-curable resin PAK-02-A.
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REFERENCES
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DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.8b12495 J. Phys. Chem. C XXXX, XXX, XXX−XXX
