Open Access
Add to BibSonomy
Abstract
A bright spot is observable in the center of Bull’s eye plasmonic pattern with a fluorescence microscope due to the plasmonic nanoantenna effect. In this effect, a propagating wave of surface plasmon resonance concentrates in the center. This study focused on the relationship between the center structure of Bull’s eye pattern and the nanoantenna effect in four fabricated Bull’s eye-type plasmonic chips with centers of different sizes (full- or half-pitch diameter) and shapes (convex or concave). The fluorescence intensity of the fluorescent nanoparticles adsorbed to these plasmonic chips was measured with an upright–inverted microscope to evaluate the plasmonic chip enhancement factor composed of the product of the excitation and emission enhancement and individual factors. When the emission enhancement factor was investigated under nonresonance excitation conditions, by the disappearance of a bright spot, excitation enhancement was found to contribute to the plasmonic nanoantenna effect. The concave Bull’s eye structure with a half-pitch diameter demonstrates the highest nanoantenna effect due to the formation of a larger constructive wave in the superposition of the diffraction wave of incident light under resonance conditions. In addition, the electromagnetic field intensity simulated by discrete dipole approximation agrees with the microscopy results. Overall, the results indicate that the plasmonic nanoantenna effect could be controlled depending on the resonance condition and center structure.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
A Bull’s eye pattern is a concentric circular structure. In 2004, Baccarelli et al. proposed a novel microstrip leaky-wave nanoantenna based on the excitation of a series of concentric, radially periodic annular rings of the fundamental TM0 mode in a leaky regime. With sufficient physical and geometrical structural parameters, it is possible to avoid the presence of any other mode in a guided or physical leaky regime [1]. To apply a Bull’s eye pattern to plasmons, in 2005, Ishihara et al. demonstrated terahertz (THz) near-field imaging using the resonantly enhanced transmission of THz wave radiation through a Bull’s eye structure. The spatial resolution for the Bull’s eye structure (aperture diameter = 100 μm) was evaluated in the near-field region, and a resolution of 50 μm (corresponding to λ/4) was achieved. They obtained THz near-field images of the subwavelength metal pattern with a spatial resolution below the diffraction limit [2]. Since then, the center structure has been improved to a pair of unique semicircular Bull’s eye pattern and so on to make more efficient use of plasmons. Ren et al. have proposed a metal–semiconductor–metal Ge photodetector that overcomes the inherent tradeoff between quantum efficiency and speed response using an SP antenna consisting of a split Bull’s eye structure and φ-shaped gap. The photo response was strongly enhanced compared with a diode with a conventional Bull’s eye nanoantenna, and the area required for the SP antenna was significantly scaled down [3]. Kawano et al. have fabricated a THz-focusing device composed of a displaced Bull’s eye, i.e., a “spiral Bull’s eye structure,” to generate the frequency of plasmon resonance and successfully capture the transmission images and spectra of mouse organs. The frequency is adjusted by rotating the device via locally concentrating the THz light [4]. Aouani et al. have combined Bull's eye with a fluorescence microscopy and presented directional control of emissions from fluorescent molecule in an aqueous solution using an Au-coated Bull’s eye nanoantenna. The luminescence’s directionality is primarily controlled by the phase relationship between the luminescence emitted directly from the nanoaperture and that emitted via the surface waves scattered by the gratings. The distance between the nanoaperture at the center of Bull’s eye and the first groove is adjusted, and the interference effect is evaluated by calculating the phase from the center of the propagating waves for each groove from the center of Bull’s eye and determining the electric field intensity. Based on these phase calculations, the superposition of the surface waves at the center of the Bull’s eye can be determined [5].
Applying the Bull’s eye structure to fluorescence microscopy under Köhler illumination is expected to result in the capture of clear images in the center of Bull’s eye. This method has been used to observe cells in the fields of medical science and biology [6,7,8]. In addition to electric field enhancement, spatial resolution, which depends on the wavelength of the light and numerical aperture of the objective lens, is one of the important subjects. The diffraction limit is generally several hundred nanometers in visible light; studies to improve this are underway [9,10].
The authors’ laboratory has applied a plasmonic chip (i.e., a glass substrate with a light wavelength scale periodic structure coated with a thin metal film [11,12]) to fluorescence microscopy and found that fluorescently labeled material can be observed on the chip brighter than on a glass slide. In the field of biodetection, the authors have studied the uses of plasmonic chips in the highly sensitive multicolor imaging of breast cancer cells [13,14] and the detection of exosomes for early disease diagnosis.
The cross-section through the center of the Bull’s eye pattern is a periodic structure with numerous grating vectors. Under a microscope with Köhler illumination, incident light with all azimuthal angles from the objective lens can be utilized for grating–coupled surface plasmon resonance (GC-SPR). Compared with the fluorescence intensity on a glass, the fluorescence intensity observed under epi-fluorescence microscope was 36 and 18 folds on a plasmonic chip with Bull’s eye pattern and a line-and-space pattern [15], respectively. Further, the larger fluorescence enhancement was shown in the center of Bull’s eye pattern as 42 fold. Bull’s eye structure has also been applied to enhanced two-photon excited emission [16]. A particularly large electric field enhancement can be induced at the center of Bull's eye. However, it was not studied which shape and size of a center structure and experimental condition can more effectively contribute to the concentration or focusing effect, i.e., the nanoantenna effect. To improve these fluorescence enhancement factors in the center, further studies were required. Therefore, in this study, four Bull’s eye-type plasmonic chips with centers of different sizes and shapes are fabricated. The fluorescence intensities of the fluorescent nanoparticles adsorbed to the chips’ surfaces are measured via fluorescence microscopy under transmitted light [17]. The results reveal that the plasmonic nanoantenna effect is dependent on the excitation or emission enhancement via controlling the excitation condition with or without excitation enhancement. The fluorescence intensities at the center and edge of each Bull’s eye are evaluated, and the nanoantenna enhancement rates are calculated as the ratio of those intensities. Furthermore, the electromagnetic field is simulated by discrete dipole approximation (DDA) [18], and the superposition of the propagation waves is calculated [5]. The results are compared with those evaluated via microscopy.
2. Principle
Surface plasmon resonance is a phenomenon in which the oscillation of free electrons on a metal surface combines with electromagnetic waves [19]. It is classified as localized surface plasmon resonance for nanostructures, including metal nanorods and nanodiscs [20,21], and propagating surface plasmon resonance for continuous metal surfaces [22,23]. A plasmonic chip produces an electric field enhanced by GC-SPR [24,25], a type of propagating resonance with conditions that can be expressed as follows:
3. Experiments
3.1 Fabrication of a plasmonic chip
The four fabricated chips had either columnar or round well center structures with either half- or full-pitch diameters. Two Bull’s eye pattern quartz molds (specially made, NTT-AT) were prepared with round well center structures and either full- or half-pitch diameters. The diameter of the pattern outline was 20 μm, and the pattern array comprised 2,000 pieces with 5-μm spaces in hexagonal lattices for both molds (Fig.1[a]). These molds were individually overlaid onto the glass slide dropping an ultraviolet (UV)-curable resin (PAK-TRAD03W, Toyo Gosei) and exposed to UV light. Then, two kinds of replica molds were prepared. The center structure was reversed in the UV nanoimprint method, and both replica molds had columnar center structures.
After UV-curable resin (PAK-02-A, Toyo Gosei) was dropped on a cover slip, the Bull’s eye original molds and replica molds were individually overlaid on the cover slip, and four different replicas were fabricated via exposure to UV light. These replicas were coated with four layers of Ti, Ag, Ti, and SiO2 layers by an RF sputtering machine. The film thickness was controlled at 60 ± 5 nm for the Ag layer and 20 nm for the SiO2 layer (Fig.1[b]). The pitch and groove depth were evaluated as 480 and 30 nm, respectively, for the periodic structures by atomic force microscopy (AFM). As expected, the four chips had either columnar or round well center structures with a half- or full-pitch diameter of 240 or 480 nm, respectively. Therefore, the four fabricated chips were referred to as half Λ–round well, half Λ–column, 1Λ–round well, and 1Λ–column (Figs. 1[c]–[f]).
Fig. 1. (a) Microscopic bright field image of Bull’s eye array, (b) cross section of a grating structure, and (c)-(f) AFM images and cross-sectional schematics of the four Bull’s eye-type plasmonic chips with different center sizes and shapes: (c) half Λ–round well, (d) half Λ–column, (e) 1Λ–round well, and (f) 1Λ–column. The scale bar depicted in AFM image corresponds to 1 μm.
3.2 Preparation of interface and sample
In order to make a carboxylate-modified fluorescent nanoparticle adsorb, the surface of a plasmonic chip was modified with 1 wt% (3-aminopropyl)triethoxysilane (Shin-Etsu Chemical). Next, a cover glass was attached to the plasmonic chip with double-sided tape, and Milli-Q water was injected into the gap to measure the background light intensity. Then, 30 μL of 5.1 × 10−4 wt% carboxylate-modified microspheres (dark red, ϕ = 0.04 μm, Thermo Fisher Scientific; absorption: λ = 500-680 nm, emission: λ = 660-750 nm) diluted with phosphate-buffered saline (PBS) was injected. After incubating for five minutes, the nanoparticles adsorbed to the chip surface via electrostatic interaction, and the surface was rinsed with 10 μL of PBS and 20 μL of milliQ water. The maximum absorption and maximum fluorescence wavelengths of the dark red particles were 660 and 680 nm, respectively.
3.3 Microscopy
The fluorescence microscopy images were taken with an upright–inverted microscope (specially made, iX72-BX2$\,\centerdot\,$PH, Olympus) under transmitted light from the inverted side (Fig. 2). The mercury lamp, Cy3 filter unit (Cy3-4040C-000, Opto-Line, λex: 511–551 nm), and Cy5 filter unit (Cy5-4040C-000, Opto-Line, λex: 608–648 nm) equipped to the inverted side were used for the excitation light that illuminated the back side of the chip with a 10× (UPlanFLN10x, Olympus, NA = 0.30) or 20× (UPlanFLN20x, Olympus, NA = 0.50) objective lens. The fluorescence of the dark red particles dispersed on the chip was observed with a Cy5 filter unit (Cy5-4040C-000, Opto-Line, λem: 672–712 nm), 100× objective lens (PlanFLN100x, Olympus, NA = 0.95), and electron multiplying–charge coupled camera (iXon Ultra 888, Andor) installed on the upright side. One pixel in a fluorescence image at 100× objective magnification obtained in this optical system corresponded to 104 nm.
3.4 Evaluation of fluorescence enhancement factor
The Bull’s eye patterns located approximately 100∼200 μm outside from the array edge were selected to evaluate the fluorescence enhancement. The evaluation method of the fluorescence image is explained. As shown in Fig.3, the gray scale (brightness) corresponds to the fluorescence intensity. The central coordinate was determined from a fluorescence image, and the brightest pixel within 3 × 3 pixels around the center (red box depicted in Fig. 3) was evaluated as the fluorescence intensity at Bull’s eye center, Fc. The mean fluorescence intensity for the area composed of 13 × 48 pixels at 6 pixels toward a center from Bull’s eye outer edge (green box depicted in Fig. 3) was evaluated as the value of the pattern edge (Fe). The Fe was measured at all four edges. The mean fluorescence intensity at a flat metal area (Fm) was evaluated 200 µm from the array edge, where the propagating plasmon field could not reach. The background light intensity measured before the fluorescent nanoparticles were added was evaluated as Bc, Be, and Bm for the center, edge, and flat metal areas, respectively. In the plasmonic chip enhancement factor Ep, the values for the center and edges described as Ec and Ee were calculated from (Fc-Bc)/(Fm-Bm) and (Fe-Be)/(Fm-Bm), respectively. The nanoantenna enhancement rate A(x, y) was defined as Ec/Ee, where x and y are the conditions of the center structure of a pattern and the optics in the microscope, respectively.
Fig. 3. Center and edge areas used to evaluate the fluorescence intensity within a Bull’s eye pattern. The scale bar corresponds to 5 μm.
3.5 Electromagnetic field simulation by DDA
Results of microscopic observation were compared to those of the electromagnetic field simulations using the DDA. In the simulation, the Bull's eye structure was modeled as six-fold concentric circles on a silver base plate (8 μm x 8 μm x 20 nm). The number of 6 circles is enough for discussion of a plasmonic nanoantenna effect [3,5]. Pitch and the depth of the circles were 480 nm and 30 nm, respectively. The four different center structures were modeled by adjusting the phase of the periodic structure. Dielectric functions of silver were adopted from the literature [26]. The refractive indices of water and the resin were 1.3 and 1.5, respectively, and they were almost the same as the values previously obtained from experimental results of resonance reflection spectra [17]. The structure was irradiated by a plane wave from the back-side of the base plate. The incident angle was varied within the solid angle of the objective lens. Scattered field at 20 nm above the structure was calculated every 5-degree of the incident angle, and the results were integrated to obtain the simulated electromagnetic field under the Köhler illumination [27]. The ratio of the center electric enhancement to the edge electric enhancement was evaluated from the simulated field. |E|2 and |EM|2 were the electric field intensity 20 nm above surface of grating structure and the mean value of electric field intensity 20 nm above flat metal for 1 μm2 area, respectively.
4. Results and Discussion
As described previously, there are two types of excitation and emission enhancement in a plasmonic chip fluorescence enhancement, that can be understood from a theoretical curve representing the relationship between the resonance angle and the wavelength (Fig. 4). The numerical aperture of an objective lens defines the range of incident or detection angles, and the filter set defines the range of excitation and emission wavelength. ϕ is the azimuth angle that is the angle between the ${{\boldsymbol k}_{\textrm{phx}}}$ and ${{\boldsymbol k}_\textrm{g}}$. If the curve exists, the enhancement is taking effect.
Fig. 4. Theoretical curves representing the relationship between the resonance angle and the wavelength. The solid and dashed lines correspond to the Ag–water and Ag–resin interfaces, respectively, with ϕ = 0° (red), ϕ = 45° (blue), and ϕ = 90° (green). The yellow line indicates θ = 17°, the outermost illumination angle under 10× objective magnification. A green and two red zones show the wavelength range of Cy3 excitation, Cy5 excitation and Cy5 emission, from left to right.
Here, Ex and Em are the excitation enhancement factor and emission enhancement factor, respectively. The plasmonic chip enhancement factor Ep is calculated from Ex × Em, corresponding to the fluorescence intensity of a fluorescent molecule on the plasmonic chip relative to that of a fluorescent molecule on the metal surface that the plasmon resonance field does not propagate. (Ec and Ee are Ep values for the center and edge of Bull’s eye pattern, respectively.) Additionally, the Ep is equivalent to Em under excitation in nonresonance conditions and under observation conditions including resonance in the emission. In this study, the enhanced fluorescence always included the effect of Em. Using the evaluated values of Ep and Em, Ex could be calculated independently.
4.1 Relationship between emission enhancement and the nanoantenna effect
Figures 5(a)–(d) show the fluorescence images of the four types of chips when illuminated with a Cy3 filter at an irradiation angle of 0°–17° with a 10× objective lens (NA = 0.30) without excitation enhancement.
Fig. 5. Fluorescence images of the dark red particles adsorbed on the four types of chips when illuminated with a Cy3 filter under a 10× objective lens: (a) half Λ–round well, (b) half Λ–column, (c) 1Λ–round well, and (d) 1Λ–column. The scale bars correspond to 5 μm.
The Ep was equivalent to the emission enhancement factor Em under these nonresonance excitation conditions (Ex = 1). Based on the fluorescence image analysis, the plasmonic nanoantenna enhancement in Bull’s eye center was not observed in all patterns. The Ep of the half Λ–round well chip was 2.9 ± 0.7, with the same value for the center and the edge. The Ep values at the half-pitch centers were 2.9 ± 0.7 and 3.0 ± 0.7, higher than the values of 2.2 ± 0.3 and 2.6 ± 0.7 for the full-pitch centers (Fig. 6). All nanoantenna enhancement rates A(Cy3-Cy5) were 1.0 (Table 1). As a result, the plasmonic nanoantenna effect based on emission enhancement did not occur for all four structures in this experimental system.
Fig. 6. Ep values evaluated from the fluorescence images shown in Fig. 5 for the four types of Bull’s eye patterns at the center (red) and edge (green), i.e., Ec and Ee.
Table 1. Nanoantenna enhancement rates A(Cy3-Cy5) evaluated in the experimental conditions including only Em, without Ex.
4.2 Single interface: nanoantenna effect on Ag–water interface
The excitation enhancement had two effects under the light transmitted from the bottom side: The transmitted light reached the water interface, and the enhancement field was formed by coupling the light with the plasmons at the Ag–water interface. The plasmon field coupled at the Ag–resin interface on the back of the chip leaked out to the water interface. Figure 7 displays the fluorescence images taken when applying emission enhancement and excitation enhancement with a plasmon field only at the Ag–water interface.
Fig. 7. Fluorescence images of the dark red particles adsorbed on the four types of chips when illuminated with a Cy5 filter at an irradiation angle of 0° to 17° under a 10× objective lens: (a) half Λ–round well, (b) half Λ–column, (c) 1Λ–round well, and (d) 1Λ–column. The scale bars correspond to 5 μm.
The fluorescence image of the half Λ–round well chip showed a clear bright spot in Bull’s eye center (Fig. 7[a]). Ep value is composed of Ex and Em in this experimental condition and the component of Ex and Em was not analyzed separately. The Ep values in the center (Ec) were 10.5 ± 2.3, 6.8 ± 1.5, 6.2 ± 1.0, and 6.6 ± 1.8 for the half Λ–round well, half Λ–column, 1Λ–round well, and 1Λ–column chips, respectively, while the Ep values at the edge (Ee) were 4.7 ± 0.7, 3.8 ± 1.1, 3.6 ± 0.4, and 3.7 ± 0.8, respectively (Fig. 8).
Fig. 8. Ep values evaluated from the fluorescence images shown in Fig.7 for the four types of Bull’s eye patterns at the center (red) and edge (green).
Therefore, the nanoantenna enhancement rate of the half Λ–round well structure A(half well, Cy5 10x-Cy5) was 2.2 ± 0.3, i.e., the highest among the four chips. The rates were 1.9 ± 0.4, 1.8 ± 0.2, and 1.8 ± 0.3 for the half Λ–column, 1Λ–round well, and 1Λ–column chips, respectively (Table 2). A bright spot was observed in the center of the Bull’s eye pattern of all four chips.
Table 2. Nanoantenna enhancement rate A(Cy5 10x-Cy5) evaluated in the experimental conditions including both Em and Ex at the Ag–water interface.
On the other hand, results of DDA electromagnetic field simulation for the illumination angles of 0-17° are shown in Fig. 9. In all structures, bright spots were observed at the center of the pattern. Similar to the microscopy results, the structure with the maximum plasmonic nanoantenna enhancement rate was the half Λ–round well chip. The ratio of the electromagnetic field intensity at a center to that at an edge was 2.22, 1.78, 1.84, and 1.52 for the half Λ–round well, half Λ–column, 1Λ–round well, and 1Λ–column chips, respectively.
Fig. 9. The DDA electromagnetic field simulation with the Ag–water interface at λ = 630 nm: (a) half Λ–round well, (b) half Λ–column, (c) 1Λ–round well, and (d) 1Λ–column. The scale bars correspond to 4 μm.
The phase shift of the waves from the first and the second teeth counting from the center were calculated at a center and the superposition of them was calculated. The results indicated that the half Λ–round well structure showed the largest constructive superposition (see SI Fig. S1). This corresponded with the results of the enhancement at the center measured by microscopy. The simulation results were consistent with the experimental results and the theory of wave superposition, demonstrating that the half Λ–round well chip had the highest nanoantenna enhancement rate.
4.3 Double interfaces: nanoantenna effects at Ag–water and Ag–resin interfaces
Figure 10 shows the fluorescence images of the four plasmonic chips taken under illumination from the backside with a Cy5 filter at an irradiation angle of 0° to 30° under a 20× objective lens (NA = 0.50) to apply excitation enhancement at the Ag–resin and Ag–water interfaces.
Fig. 10. Fluorescence images of the dark red particles adsorbed on the four types of chips when illuminated with a Cy5 filter and a 20× (NA = 0.50) objective lens: (a) half Λ–round well, (b) half Λ–column, (c) 1Λ–round well, and (d) 1Λ–column. The scale bars correspond to 5 μm.
The Ep values evaluated at the center and edge of the half Λ–round well chip were 26.4 ± 7.0 and 11.9 ± 2.4, respectively (Fig. 10[a]). The values for the center of the half Λ–column, 1Λ–round well, and 1Λ–column chips were 23.7 ± 3.9, 15.5 ± 1.9, and 19.2 ± 3.0, respectively, while those for the chips’ edges were 11.7 ± 2.5, 8.7 ± 1.0, and 9.9 ± 0.4, respectively (Fig. 11).
All excitation enhancement factors for the center and edge increased by adding the Ag–resin interface to the Ag–water interface, as expected. The nanoantenna enhancement rate A(half well, Cy5 20x-Cy5) was 2.2 ± 0.3 at the half Λ–round well chip, the highest among the four chips. The rate was 2.1 ± 0.4 at the half Λ–column chip, 1.8 ± 0.1 at the 1Λ–round well chip, and 1.9 ± 0.3 at the 1Λ–column chip (Table 3). A(half well, Cy5 20x-Cy5) was almost the same as A(half column, Cy5 20x-Cy5) under the condition of using resonance fields at two interfaces for excitation field. The round well structure from the top view in the Ag–water interface was flipped to the column structure from the bottom view in the Ag–resin interface. At all interfaces, the nanoantenna enhancement rate of a concave pattern was considered to be superior to that of a convex structure. Therefore, the nanoantenna enhancement rate for a well with a column at bottom side was not a lot different from that for a column with a well at bottom side. In the half Λ–column chip, the nanoantenna enhancement rate in the optical system improved to more than two; however, it was less than two in the optical system upon applying the excitation enhancement at only the Ag–water interface.
Table 3. Nanoantenna enhancement rate A(Cy5 20x-Cy5) evaluated in the experimental conditions including both Em and Ex at both the Ag–water and Ag–resin interfaces.
Figure 12 shows the DDA simulation results with an incidence angle range of 0°–30° to utilize the excitation enhancement at the double interfaces.
Even though not only the parallel component (${{\boldsymbol k}_{\textrm{phx}}}$ // ${{\boldsymbol k}_\textrm{g}}$) but also the available azimuthal (ϕ) components increased in the double interfaces, the ratio was more than two in only the half Λ–round well chip under both the single and double interfaces. The ratio of ${|{E{|^2}/|{E_M}} |^2}$ in the Ag–resin interface was the largest for the half Λ–column structure and the smallest for the half Λ–round well structure. The difference in the center structure indicating the largest${|{E{|^2}/|{E_M}} |^2}$, i.e., the largest nanoantenna effect, was because the round well structure from the top view in the Ag–water interface was flip to that of the column structure from the bottom view in the Ag–resin interface. At all interfaces, the nanoantenna enhancement rate of a concave pattern was superior to that of a convex structure.
Furthermore, the theory of wave superposition was considered to interpret the bright spot in the center of Bull’s eye pattern. The largest constructive waves were achieved in the half Λ–round well chip because the propagating waves of diffraction overlapped to a fundamental wave at the center, i.e., all phases of the diffraction waves were in accordance with a fundamental wave. The superposition of the propagating waves explained the nanoantenna enhancement rate. The bright spot observed in a center of the Bull’s eye was understood by the plasmonic electric field intensity evaluated by the simulation and the constructive wave formed via the superposition of the plasmonic propagating waves.
The plasmonic chip enhancement at edge Ee did not depend on the center structure, but Ec at center depended as shown in Figs. 8 and 11. The larger Ee can be obtained when the resonance angle is small because the electric field intensity depends on the cosine of incident angle [24], and when both excitation and emission enhancement can be utilized, i.e., both angle ranges of illumination and detection include the resonance condition at the wavelength range of excitation and emission, respectively. It means that the relationship between pitch of a periodic structure and microscopic observation condition are important. On the other hand, the nanoantenna enhancement rate can be interpreted as the constructive wave by superposition of a plasmonic waves at center. Therefore, larger Ec at center can be obtained under constructive wave condition depending on the center structure in addition to the condition for providing a larger Ee. Furthermore, the nanoantenna effect and plasmon enhancement depending on the resonance wavelength and angle can be controlled by the making grating pitch tunable post-fabrication using a graphene [28].
Fig. 11. Ep values evaluated from the fluorescence images shown in Fig. 10 for the four types of Bull’s eye patterns at the center (red) and edge (green).
Fig. 12. The electromagnetic field simulation with the Ag–water and Ag–resin interfaces at λ = 630 nm. (a) Half Λ–round well, (b) half Λ–column, (c) 1Λ–round well, and (d) 1Λ–column for the Ag–water interface. (e) Half Λ–round well, (f) half Λ–column, (g) 1Λ–round well, and (h) 1Λ–column for the Ag–resin interface. The scale bars correspond to 4 μm.
5. Conclusions
Concerning the bright spot shown in the center of Bull’s eye pattern of a plasmonic chip under fluorescence microscopy, the relationship between the center structure and the plasmonic nanoantenna effect was investigated. The fluorescence intensity of fluorescent nanoparticles adsorbed to four different Bull’s eye-type plasmonic chips was measured with a microscope under transmitted light, and the nanoantenna effect was evaluated. The bright spot due to the nanoantenna effect was not observed in the experiment when using only emission enhancement, and it was found that excitation enhancement influenced the nanoantenna effect. In the concave structure of a Bull’s eye’s center with a half-pitch diameter, the largest nanoantenna enhancement rate was obtained by the excitation enhancement effects at any Ag–water and Ag–resin interface. The Ep in the center was 26.4 ± 7.0 using the resonance fields at both interfaces. The results of the microscopy experiments agreed with the electromagnetic field intensity simulated by DDA and were interpreted via the theory of wave superposition of propagating waves at the center.
Funding
The Murata Science Foundation.
Acknowledgments
TN, TS, and KT thank Toyo Gosei for providing the UV-curable resins.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. P. Baccarelli, P. Burghignoli, G. Lovat, and S. Paulotto, “A novel printed leaky-wave ‘bull-eye’ antenna with suppressed surface-wave excitation,” Antennas and Propagation Soc. Int. Symp. 4, 1078–1081 Vol.1 (2004). [CrossRef]
2. K. Ishihara, T. Ikari, H. Minamide, J. Shikata, K. Ohashi, H. Yokoyama, and H. Ito, “Terahertz near-field imaging using enhanced transmission through a single sub wavelength aperture,” Jpn. J. Appl. Phys. 44(No. 29), L929–L931 (2005). [CrossRef]
3. F. F. Ren, K. W. Ang, J. Ye, M. Yu, G. Q. Lo, and D. L. Kwong, “Split bulls eye shaped aluminum antenna for plasmon-enhanced nanometer scale germanium photodetector,” Nano Lett. 11(3), 1289–1293 (2011). [CrossRef]
4. X. Deng, L. Li, M. Enomoto, and Y. Kawano, “Continuously frequency-tuneable plasmonic structures for terahertz bio-sensing and spectroscopy,” Sci. Rep. 9(1), 3498–3506 (2019). [CrossRef]
5. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Plasmonic antennas for directional sorting of fluorescence emission,” Nano Lett. 11(6), 2400–2406 (2011). [CrossRef]
6. S. T. Hess, T. P. K. Girirajan, and M. D. Mason, “Ultra-high resolution imaging by fluorescence photoactivation localization microscopy,” Biophys. J. 91(11), 4258–4272 (2006). [CrossRef]
7. B. Huang, M. Bates, and X. Zhuang, “Super-resolution fluorescence microscopy,” Annu. Rev. Biochem. 78(1), 993–1016 (2009). [CrossRef]
8. K.-F. Giebel, C. Bechinger, S. Herminghaus, M. Riedel, P. Leiderer, U. Weiland, and M. Bastmeyer, “Imaging of cell/substrate contacts of living cells with surface plasmon resonance microscopy,”,” Biophys. J. 76(1), 509–516 (1999). [CrossRef]
9. S. Yang, Y. Cao, Q. Shi, X. Wang, T. Chen, J. Wang, and Y.-H. Ye, “Label-free super-resolution imaging of transparent dielectric objects assembled on a silver film by a microsphere-assisted microscope,” J. Phys. Chem. C 123(46), 28353–28358 (2019). [CrossRef]
10. M. Chen, X. H. Pan, Q. Liu, S. X. Huo, S. H. Cao, Y. Y. Zhai, Y. Zhao, and Y. Q. Li, “Variable-angle nanoplasmonic fluorescence microscopy: An axially resolved method for tracking the endocytic pathway,” Anal. Chem. 91(21), 13658–13664 (2019). [CrossRef]
11. K. Tawa, F. Kondo, C. Sasakawa, K. Nagae, A. Nozaki, and T. Kaya, “Sensitive detection of a tumor marker, α-fetoprotein, with a sandwich assay on a plasmonic chip,” Anal. Chem. 87(7), 3871–3876 (2015). [CrossRef]
12. K. Tawa, M. Umetsu, H. Nakazawa, T. Hattori, and I. Kumagai, “Application of 300× enhanced fluorescence on a plasmonic chip modified with a bispecific antibody to a sensitive immunosensor,” Anal. Chem. 5(17), 8628–8632 (2013). [CrossRef]
13. K. Tawa, S. Yamamura, C. Sasakawa, I. Shibata, and M. Kataoka, “Sensitive detection of cell surface membrane proteins in living breast cancer cells using multicolor fluorescence microscopy with a plasmonic chip,” ACS Appl. Mater. Interfaces 8(44), 29893–29898 (2016). [CrossRef]
14. M. Yoshida, H. Chida, F. Kimura, S. Yamamura, and K. Tawa, “Multi-color enhanced fluorescence imaging of a breast cancer cell with a hole-arrayed plasmonic chip,” Micromachines 11(6), 604–616 (2020). [CrossRef]
15. K. Tawa, S. Izumi, C. Sasakawa, C. Hosokawa, and M. Toma, “Enhanced fluorescence microscopy with the Bull’s eye-plasmonic chip,” Opt. Express 25(9), 10622–10631 (2017). [CrossRef]
16. Y. Omura, Y. Matsubayashi, T. Kishimoto, S. N. Kudoh, C. Hosokawa, and K. Tawa, “Two-photon-excited emission of quantum dots with a plasmonic chip,” J. Phys. Chem. C 124(29), 16076–16082 (2020). [CrossRef]
17. H. Chida and K. Tawa, “Microscopic study on excitation and emission enhancement by the plasmon mode on a plasmonic chip,” Sensors 20(22), 6415–6427 (2020). [CrossRef]
18. B. T. Draine and P. J. Flatau, “Discrete-dipole approximation for scattering calculations,” J. Opt. Soc. Am. A 11(4), 1491–1499 (1994). [CrossRef]
19. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef]
20. K. M. Mayer and J. H. Hafner, “Localized surface plasmon resonance sensors,” Chem. Rev. 111(6), 3828–3857 (2011). [CrossRef]
21. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, “Biosensing with plasmonic nanosensors,” Nat. Mater. 7(6), 442–453 (2008). [CrossRef]
22. E. Wijaya, C. Lenaerts, S. Maricot, J. Hastanin, S. Habraken, J. P. Vilcot, R. Boukherroub, and S. Szunerits, “Surface plasmon resonance-based biosensors: From the development of different SPR structures to novel surface functionalization strategies,” Curr. Opin. Solid State Mater. Sci. 15(5), 208–224 (2011). [CrossRef]
23. K. Tawa, H. Hori, K. Kintaka, K. Kiyosue, Y. Tatsu, and J. Nishii, “Optical microscopic observation of fluorescence enhanced by grating-coupled surface plasmon resonance,” Opt. Express 16(13), 9781–9790 (2008). [CrossRef]
24. H. Raether, Surface plasmons on smooth and rough surfaces and on gratings (Springer-Verlag, 1988), pp. 1–117.
25. W. Knoll, “Interfaces and thin films as seen by bound electromagnetic waves,” Annu. Rev. Phys. Chem. 49(1), 569–638 (1998). [CrossRef]
26. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
27. A. Taflove, A. Oskooi, and S. G. Johnson, Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology, (Artech House, 2013).
28. O. Hemmatyar, B. Rahmani, A. Bagheri, and A. Khavasi, “Phase Resonance Tuning and Multi-Band Absorption Via Graphene-Covered Compound Metallic Gratings,” IEEE J. Quantum Electron. 53(5), 1–10 (2017). [CrossRef]
