Nano-Scale Refractive Index Sensors with High Figures of Merit via

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Nano-Scale Refractive Index Sensors with High Figures of Merit via Optical Slot Antennas Baowei Gao, Yilun Wang, Tongzhou Zhang, Yi Xu, Axin He, Lun Dai, and Jiasen Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03406 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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Nano-Scale Refractive Index Sensors with High Figures of Merit via Optical Slot Antennas Baowei Gao†, Yilun Wang†, Tongzhou Zhang†, Yi Xu†, Axin He†, Lun Dai†,‡, and †State

Jiasen Zhang†,‡,*

Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing

100871, China. ‡Collaborative

Innovation Center of Quantum Matter, Beijing 100871, China.

*[email protected]

ABSTRACT

Nano-scale refractive index (RI) sensors based on a single nanorod or nanoantenna typically suffer from a low figure of merit (FOM) due to the large full width at half maximum of the plasmonic dipole resonance. Here, we demonstrate nanosensors with a high FOM and a sensing volume that is much smaller than 3 using slot antennas. Two configurations, one based on a bowtie slot antenna (BSA) and one based on a slot antenna pair (SAP), are proposed. The RI information is obtained from the extinction dip that is due to the interference of surface plasmon polaritons (SPPs), which are launched at different nodes of a thirdorder resonant mode of the BSA or different antennas of the SAP. The high FOM is attributed to the dependence of the extinction spectrum on both the amplitude and the phase of the SPPs. There are

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important applications for these nanosensors, which can measure the local RI beyond the diffraction limit and can be flexibly integrated.

KEYWORDS nano sensing·optical slot antennas·surface plasmon polaritons·slot antenna pairs·refractive index

Surface plasmon polaritons (SPPs), which can exhibit subwavelength lateral mode confinement and a very large electric field enhancement,1 have been extensively used in fundamental physics, chemistry, biology, and materials science in the past several decades.2-6 One important application of SPPs is sensing.7,8 In 1982, Nylander et al. demonstrated that gas can be detected by measuring the surface plasmon resonance (SPR) via the refractive index (RI) changes of an organic layer that can absorb the gas.9 This initiated the study of SPP sensing and in the years that followed, numerous noble metal structures that are based on SPPs were proposed in RI sensing. As fundamental methods of SPP coupling, Kretschmann prisms,10-12 metallic gratings,13-15 and optical fibers14 are commonly used in RI sensing. However, these are all macro-scale structures (~cm2), which cannot be used in micro-environments, and are difficult to integrate. Combined with the microfabrication technologies, nanostructure array sensors reduced the sensing scales. Nevertheless, most previous studies have been based on SPPs propagating on metal films; meaning that the sensing areas cannot be very small due to the large SPP propagation distances. The general sensing area in nanostructure array sensors is as large as 100 𝜇𝑚 × 100 𝜇𝑚.16,17 They have sensitivities of 300-600 nm/RIU16,17 and their figures of merit (FOMs) can reach 3.8/RIU.17 FOMs of more than 100/RIU have been reported from arrays of nanostructures that combine the localized surface plasmon resonances with magnetoplasmons or wood anomaly,18,19 however, their sensing areas are still very large and they are hard to be integrated. The only designs that can reduce the sensing scale


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to the nanometer level are nanoparticle sensors, which have sensitivities of 200-400 nm/RIU and lower FOMs of 3/RIU on average.20-23 Unfortunately, when such sensors are used, the nanoparticles must suffuse the target medium and thus will inevitably pollute the medium. Plasmonic cavities with a sensing area of 600 × 800 nm have also been theoretically proposed for RI sensing;24 however, their results have not been experimentally verified. Thus, the development of label-free and integratable RI sensors at the nano-scale with a high FOM is desirable for many applications. In this article, we propose sub-wavelength RI sensors based on optical slot antennas, which have sensing areas as small as 0.048 μm2 and a high FOM of 28.8/RIU. The proposed sensors are based on either a bowtie slot antenna (BSA) or a slot antenna pair (SAP) that is fabricated in a gold film. When a white light is incident upon the sensor, the launched SPPs that propagate at the interface between the gold film and the specimen interfere, resulting in a narrow extinction dip in the spectrum, and the center wavelength of this dip depends on the RI of the specimen. A sensitivity of 574 nm/RIU has been realized in experiment. The high FOM due to the small full width at half maximum (FWHM) of the extinction dip is attributed to the obvious changes in the phase and amplitude of the launched SPPs near the resonant wavelength of the antennas. The proposed nanosensors can measure the local RI beyond the diffraction limit within a sensing volume as small as 0.0091 μm3 and can be flexibly integrated. Due to these characteristics, the nanosensors have important applications in many fields, especially microfluidics and nanofluidics. RESULTS AND DISCUSSION


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Figure 1. (a) Schematic diagram of the BSA sensor. (b) Calculated electric field amplitude distribution at the resonant wavelength of 803 nm in the xy plane. (c) Normalized amplitudes of the electric field at the centers of the center and side nodes in the xy plane versus the wavelength. (d) Amplitude ratio (𝐸10/𝐸20) and the phase difference (∆𝜑) between the center and side nodes versus the wavelength. (e) Normalized integral SPP intensity (blue line) and extinction parameter (red line) versus the wavelength. (f) Extinction parameters versus the wavelength for specimens with various RIs. Figure 1a presents a schematic diagram of the proposed nanosensor based on a BSA. A bowtie-shaped slot antenna is fabricated in a gold film with a thickness of T that has been deposited on a glass substrate. The antenna has a length of L, a maximum width of W1, and a minimum width (along the short axis) of W2. The antenna is covered by the specimen, which was ethanol-water solution in the experiment, and its RI was varied by adjusting the concentration of ethanol. When x-polarized white light is normally incident from the substrate side, resonant modes of the antenna are excited, and SPPs are launched and propagate


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along the interface between the gold film and the specimen. The resonant modes of the BSA depend on its geometry,25 and only the third-order resonant mode was considered in this experiment. There are three nodes in the third-order resonant mode, which can be regarded as three SPP point sources. The SPPs that are launched by the BSA propagate along the x-axis at the gold/specimen interface and interfere with each other. The electric field that is generated by an SPP point source can be expressed in dipolar form as follows:26

(

E(𝑟,𝜃) = 𝐸𝑠cos2 (𝜃)exp ( ― 𝑟 𝛿)cos 𝜔𝑡 ―

2𝜋𝑟

𝜆𝑆𝑃𝑃

)

𝑟,

(1)

where 𝐸𝑠 is the amplitude of the SPP point source and δ is the propagation length of the SPP. The xy plane is defined to overlap with the top surface of the gold film, and the center of the antenna is located at the origin. When the distance between the antenna and a field point is much larger than L, the intensity of the SPPs on the +x-axis can be approximated by the following equation: 𝑒𝑥𝑝( ―𝑥/𝛿) 𝑒𝑥𝑝 𝑥

𝐼𝑥 = |𝐶10𝐸10

𝑒𝑥𝑝( ―𝑥/𝛿) 𝑒𝑥𝑝 𝑥

[𝑖(𝑘𝑆𝑃𝑃𝑥 + 𝜑1)] + 2𝐶20𝐸20

[𝑖(𝑘𝑆𝑃𝑃𝑥 + 𝜑2)]|2,

(2)

where 𝐸10 and 𝐸20 represent the amplitudes of the electric field at the centers of the center node and the side nodes, respectively, at the gold/specimen interface; 𝜑1 and 𝜑2 are the corresponding initial phases; and 𝐶10 and 𝐶20 are the ratios between the amplitudes of the SPP point sources and 𝐸10 and 𝐸20, respectively, and are approximately equal. For the third-order resonant mode, the phase difference ∆𝜑 = 𝜑1 ― 𝜑2 depends on the wavelength and is equal to  at the resonant wavelength. If we tune the ratio 𝐸10/ 𝐸20 to 2, the intensity should be nearly zero at the resonant wavelength, and thus, a narrow extinction dip in the spectrum with a lowest point of nearly zero should appear around the resonant wavelength. The center wavelength of the extinction dip depends on the RI of the specimen; thus, we can obtain the RI by measuring the spectrum of the SPPs.


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Although the resonant modes of rectangular slot antennas are similar to those of BSAs, a BSA allows the ratio 𝐸10/𝐸20 to be more flexibly tuned by adjusting the widths W1 and W2. Here, we used the finitedifference time-domain (FDTD) method to design the geometry of the BSA. If T, L, W1, and W2 are 200, 600, 80, and 60 nm, respectively, and the light source is an x-polarized plane wave, the BSA has a resonant wavelength of 803 nm with a water specimen. The mode pattern at the resonant wavelength in the xy plane, which exhibits a third-order resonant mode, is shown in Figure 1b. The normalized amplitudes of the electrical field at the centers of the center and side nodes in the xy plane with respect to the incident wavelength are plotted in Figure 1c, in which a resonance peak is observed at 803 nm. The amplitude ratio 𝐸10/𝐸20, which has a value of 1.96 at  = 803 nm, is shown in Figure 1d. Figure 1d also shows the phase difference ∆φ = 𝜑1 ― 𝜑2, which is equal to  at  = 803 nm and obviously changes near the resonant wavelength. Then we calculated the intensity of the SPPs propagating along the +x-axis and the normalized intensity is shown in Figure 1e. A narrow extinction dip appears at the resonant wavelength, which originates from the interference between the SPPs that are launched at the three nodes. For comparison with other sensors, we use an extinction parameter, defined as the reciprocal of the normalized SPP intensity. The calculated extinction parameter is plotted in Figure 1e and exhibits a FWHM of 7.2 nm and a maximum value of 109. The peak in the extinction parameter shows a redshift as the RI of the specimen increases. The result is plotted in Figure 1f and reveals a sensitivity of 450 nm/RIU, which is similar to the sensitivities of many other types of nanosensors.16-18,20-23 Because the FWHM of the extinction parameter is an order of magnitude smaller than those of the dipole resonances in nanorods27 or slot antennas,17,26 the proposed nanosensor should have a high FOM of 62.5/RIU, which is defined as the ratio of the sensitivity to the FWHM. We also calculated the sensitivities and FOMs of the BSA sensor in a RI range from 1.25 to 1.5. The results show that the sensor can be used in such a wide RI range (see supporting information S1).


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To understand the proposed sensor, Lorentz oscillator model can be used to explain the fundamental physical mechanism. A slot antenna with a dipole resonance mode acts as a Lorentz oscillator.28 The thirdorder resonance mode of the BSA is more complex. It is seen that each curve in Figure 1c can be described by a sum of three Lorentz oscillators. The signal of the proposed sensor can be considered as the interference of the SPPs launched by 9 Lorentz oscillators, and the results are similar to that in Figure 1e (see supporting information S2).

Figure 2. (a) SEM image of the fabricated slot antenna and the diffraction grating. Inset: A magnified SEM image of the BSA. (b) Schematic diagram of the experimental setup. (c) CCD image of the BSA.


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To experimentally demonstrate the proposed nanosensor, a BSA was fabricated via focused ion beam milling in a 200-nm-thick gold film that was deposited on a glass substrate with geometric parameters of L = 600 nm, W1 = 80 nm, and W2 = 60 nm, the same as the designed parameters. Figure 2a shows a scanning electron microscope (SEM) image of the sample and the inset shows a magnified view of the BSA. To measure the intensity of the launched SPPs, a grating with a period of 610 nm was also fabricated at a distance of 8 μm from the BSA. The calculated normalized diffraction efficiency of the grating had a FWHM of 163 nm (see supporting information S3), much larger than that of the calculated extinction parameter. The influence of the diffraction efficiency of the grating on the dip wavelength of the spectrum can be ignored. Although the distance between the BSA and the grating is 8 μm, the SPPs launched by the different nodes in the BSA sensor propagate along almost the same path on the gold film, resulting in a very small optical path difference. And such a small path difference does not affect the SPPs interference despite the variation of the RI in the portion of the film required for the SPPs propagation. The experimental setup is illustrated in Figure 2b. An x-polarized white light laser beam was focused by an objective and normally incident upon the BSA from the glass substrate. The glass substrate was fixed in a stainless steel cuvette with an output glass window and two channels for changing the specimen (see supporting information S4). The launched SPPs propagated along the +x-axis and were diffracted by the grating. The BSA and the grating were imaged onto a charge-coupled device (CCD) by another objective. Figure 2c shows an optical image of the BSA and the grating. The white dashed box indicates the position of the grating. The light that directly passed through the BSA was much stronger than the diffracted light from the grating; thus, the image of the BSA is saturated. To measure the spectrum, the diffracted light from the grating was collected by the objective and focused into the fiber of a spectrograph, and the light directly passing through the BSA was blocked using a spatial filter.


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Figure 3. (a) Normalized spectrum of the light diffracted from the grating (black dot) with a pure water specimen. The blue line is a smoothed curve representing the experimental results, and the red line shows the extinction parameter obtained from the blue line. (b) Extinction parameters measured for ethanolwater solution with various concentrations of ethanol. (c) Peak wavelength and FWHM of the extinction parameter versus the RI of the specimen. The red line is a linear fit, and the blue line is a guide for the eyes. (d) Electric field amplitudes at the centers of the nodes versus the z-coordinate. The spectrum of the light that was diffracted by the grating when the specimen was pure water is shown in Figure 3a. The interference of the SPPs resulted in a narrow extinction dip with a center wavelength of 799 nm, and the decreases in intensity on both sides of the spectrum originated from the decrease in the diffraction efficiency of the grating. The center wavelength showed a slight shift of 4 nm relative to the calculated wavelength. The extinction parameter, with a maximum value of 52 and a FWHM of 20 nm, is also plotted in Figure 3a. These parameters deviated from the calculated results due to the imperfect fabrication of the nanostructure and the noise caused by light scattering in the device. We prepared specimens with various RIs by varying the ethanol concentration of the ethanol-water solution. The examined concentrations were 5%, 10%, 15%, 20% and 25% and the corresponding RIs were 1.3325, 1.3365, 1.3402, 1.3436, and 1.3468,29 respectively. Figure 3b plots the extinction parameters for specimens with various RIs, showing that the peak wavelength exhibited a redshift with the increasing RI. The peak wavelength and the FWHM are plotted with respect to the RI of the specimen in Figure 3c. The red line is a linear fit; it indicates a sensitivity of 574 nm/RIU, which is slightly larger than the calculated result. The FWHMs vary from 19.2 nm to 20.5 nm and have an average value of 19.9 nm. The FOM is 28.8/RIU, which is one order of magnitude larger than those of sensors that are based on a single nanorod27 or arrays of slot antennas17. For comparison, we also fabricated a rectangular slot antenna with a length of 190 nm and a width of 60 nm. The measured spectrum exhibits two resonant peaks of dipole


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modes located at 775 nm and 823 nm with a FWHM of 150 nm (see supporting information S5), which is an order of magnitude larger than that of the extinction parameter of the BSA. Though the extinction parameter we adopted has a different physical mechanism from the spectrum of the dipole slot antenna, the FOM calculated from the extinction parameter can present useful information about the sensors. Because of the high subwavelength-scale confinement of the resonant mode of the BSA, only the RI of the specimen in a very small volume near the BSA influences the peak wavelength of the extinction parameter. Therefore, the proposed nanosensor has an ultra-small sensing volume or area. The propagation distance of the SPPs between the antenna and the grating on the gold film does not affect the results. The grating is only used to couple the output signal. Other kinds of couplers or waveguides are alternatives for practical applications. Because the electric field is mainly confined inside the slot sensor that is shown in Figure 1b, we estimate the sensing area to be 𝑆 = 𝐿 × 𝑊1, thus obtaining a value of 0.048 𝜇𝑚2. We calculated the electric field distribution of the centers of the nodes in Figure 1b with respect to the zcoordinate; the results are shown in Figure 3d. The penetration depths of the center and side nodes for z > 0 are 49 and 55 nm, respectively. The average penetration depth, denoted by z, is 52 nm. We define the sensing volume as 𝑉 = 𝑆 × (𝑇 + 𝑧), corresponding to V = 0.012 𝜇𝑚3 for the BSA sensor. By taking advantage of the interference of the SPPs that are launched at a single slot antenna, the proposed nanosensor can simultaneously achieve an ultra-small sensing volume and a high FOM, making it highly suitable for local RI measurements, especially in nanofluidics. In the proposed nanosensor, the interference between the SPPs that are launched at different nodes of a third-order mode is exploited to generate a narrow dip in the SPP intensity spectrum. The obvious change in the phase near the resonant wavelength causes the FWHM of the extinction parameter to be one order of magnitude smaller than that of the dipole resonance in a slot antenna or nanorod. In addition to the third-order mode in a slot antenna, there is another way to realize a similar result. The SPPs that are


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launched at two slot antennas with a dipole mode can also interfere, resulting in a similar dip in the SPP intensity spectrum. Here, we used two adjacent rectangular slot antennas to construct an SAP. An SEM image of the designed SAP is shown in Figure 4a. The two slot antennas have the same width, W, and different lengths, L1 and L2. In the experiment, L1 < L2. The distance between the antennas is d. When a white light source is incident upon the SAP from the substrate, SPPs are launched at the two antennas, propagate along the x-axis and interfere. The same grating shown in Figure 2a was again fabricated to measure the spectrum of the SPPs.

Figure 4. (a) SEM image of the SAP. (b) Normalized calculated electric field amplitudes at the centers of the antennas in the xy plane for independent slot antennas (solid lines) and for the antennas in the SAP with d = 125 nm (dashed lines) versus the wavelength. (c) Coefficients (C1 and C2) for independent antennas of various lengths versus the wavelength. (d) Calculated amplitude ratio of the SPPs launched


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by the two antennas (blue line) and the phase difference between the two antennas (red line) in the SAP with d = 125 nm versus the wavelength. (e) Normalized calculated SPP intensity (blue line) and the extinction parameter (red line) versus the wavelength. The intensity of the SPPs that are propagating along the +x-axis can be approximated as follows for x ≫ d: 𝑒𝑥𝑝( ― (𝑥 ― 𝑑/2)/𝛿) 𝑒𝑥𝑝 (𝑥 ― 𝑑/2)

[𝑖(𝑘𝑆𝑃𝑃(𝑥 ― 𝑑/2) + 𝜑′1)]

𝐼𝑥 = |𝐶1𝐸′1

𝑒𝑥𝑝( ― (𝑥 + 𝑑/2)/𝛿) 𝑒𝑥𝑝 (𝑥 + 𝑑/2)

+ 𝐶2𝐸′2

[𝑖(𝑘𝑆𝑃𝑃(𝑥 + 𝑑/2) + 𝜑′2)]|2,

(3)

where 𝐸′1 and 𝐸′2 represent the amplitudes of the electric field at the centers of the antennas at the gold/specimen interface, 𝜑′1 and 𝜑′2 are the corresponding phases, and 𝐶1 and 𝐶2 are coefficients. To obtain an extinction dip in the spectrum of the launched SPPs, two conditions should be satisfied: 𝐶1𝐸′1 = 𝐶2𝐸′2 and 𝜑′2 ― 𝜑′1 + 𝑘𝑆𝑃𝑃𝑑 = (2𝑛 ― 1)𝜋 (where n is an integer). The parameters of the two antennas were selected to be W=60 nm, L1=190 nm, and L2=300 nm via the FDTD method. The calculated electric field amplitudes at the centers of the two antennas, namely, 𝐸1 and 𝐸2, with respect to the incident wavelength for two independent antennas are plotted in Figure 4b for a water specimen. The 190-nm-long antenna has two resonance peaks at 755 nm and 900 nm and the 300nm-long antenna has only one resonance peak at 860 nm in the calculated wavelength range. When the two antennas are coupled, the electric field amplitudes 𝐸′1 and 𝐸′2 and the initial phases 𝜑′1 and 𝜑′2 depend on the distance d. For d = 125 nm, the calculated amplitudes are plotted in Figure 4b. Using Eq. (1), we determine the coefficients 𝐶1 and 𝐶2 for the independent antennas by calculating the amplitudes of the launched SPPs (see supporting information S6); the results are shown in Figure 4c. We ignored the scattering by the right antenna of the SPPs launched at the left antenna and calculated the amplitude ratio of the SPPs, namely, R = 𝐶1𝐸′1/𝐶2𝐸′2, and the phase difference, namely, ∆φ = 𝜑′2 ― 𝜑′1, for d = 125 nm


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(Figure 4d). According to the results, R is equal to 1 at  = 842 nm and the corresponding phase difference is ∆φ = 118°. Therefore, the distance d should introduce a phase delay of 62° to satisfy the conditions for the extinction dip at n = 1. For the case of a pure water specimen, the calculated intensity of the launched SPPs is shown in Figure 4e, along with the extinction parameter. A center wavelength of 850 nm is obtained, which slightly deviates from the designed value. We attribute this phenomenon to the scattering by the right antenna of the propagating SPPs that were launched at the left antenna, which slightly influences the amplitude ratio and the phase difference. Compared with the values in Figure 1e, the extinction parameter has a much higher peak value of 1420 and a slightly larger FWHM of 8.1 nm. Using two separated slot antennas that are operating in dipole mode, it is easy to modulate the interference of the SPPs launched by the antennas by varying their geometries and separation distance. Thus, a higher extinction parameter can be realized.


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Figure 5. (a) Normalized spectrum of the light diffracted from the grating (black dot) with a pure water specimen. The blue line is a smoothed curve representing the experimental result, and the red line shows the extinction parameter obtained from the blue line. (b) Extinction parameters measured for ethanolwater solutions with various concentrations of the ethanol. (c) Peak wavelength and FWHM of the extinction parameter versus the RI of the specimen. The red line is a linear fit and the blue line is a guide for the eyes.


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The experimental results for the extinction spectrum of the SPPs obtained using the SAP shown in Figure 4a when the specimen was pure water are plotted in Figure 5a. The corresponding extinction parameter has a peak wavelength of 803 nm, a maximum value of 160, and a FWHM of 19.5 nm. The difference in the peak wavelengths between the experimental and the calculated results is ascribed to the slight discrepancy between the design and the imperfectly fabricated device. The extinction parameters are plotted versus the ethanol concentration of the ethanol-water solution in Figure 5b and the peak wavelengths of the extinction parameters and their FWHMs are plotted versus the RI of the specimen in Figure 5c. The results indicate that this nanosensor has a sensitivity of 581 nm/RIU and an average FWHM of 22.5 nm. Hence, the FOM is 25.8/RIU. Considering the strong confinement of the electric field at the antennas (see supporting information S7), we estimated the sensing area of the SAP sensor to be 𝑆 = 𝐿1 × (𝑑 + 𝑊), corresponding to S = 0.055 𝜇𝑚2 in the experiment. The corresponding sensing volume is 𝑉 = 𝐿1 × (𝑑 + 𝑊) × 𝑧 + (𝐿1 + 𝐿2) × 𝑊 × 𝑇, where the calculated average penetration depth, denoted by 𝑧, is equal to 58 nm (see supporting information S7). Thus, for the SAP sensor, V = 0.0091 𝜇𝑚3, which is slightly smaller than that of the BSA sensor.

Figure 6. (a) Calculated electric field amplitudes at the centers of the antennas in the SAP with d = 740 nm. (b) Phase differences between two independent antennas and between the antennas in the SAP with


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d = 740 nm. (c) Normalized calculated intensities of the SPPs in the two SAP sensors with different values of d. When designing the SAP, we set n = 1 to realize a small sensing volume. However, n can also be another integer. If we set n = 2 and the other parameters remain unchanged, then d = 740 nm. The calculated amplitudes of the electric field at the centers of the antennas and the phase differences with and without coupling are shown in Figure 6a and 6b, respectively. Although the distance d is comparable to the wavelength of the SPPs, the coupling between the antennas still has a strong influence on the amplitudes and phases. From Eq. (3) and the simulation results in Figure 4b, 4c and 4d and Figure 6a and 6b, we can estimate the intensity of the launched SPPs; the results are shown in Figure 6c. For d = 125 nm, there is an extinction dip at 847 nm, which is consistent with the result in Figure 4f. For d = 740 nm, the center wavelength of the extinction dip redshifts to 960 nm due to the coupling between the two antennas. In the two proposed types of nanosensors, the extinction conditions depend on both the amplitude ratio and the phase difference of the SPPs that are launched, either at different nodes of a slot antenna with a third-order mode or at two dipole slot antennas. This is why the extinction parameter has a much smaller FWHM than that of the dipole resonance of a single antenna or nanorod, resulting in a higher FOM. In a BSA, the center node and the side nodes have a similar resonant wavelength. In an SAP sensor, antennas of different lengths are used to satisfy the extinction conditions, the center wavelength of the extinction parameter typically deviates from the resonant wavelengths of the antennas. Due to the strong field confinement effect of the antennas, the sensing result depends only on the RI in the near-field of the single antenna or an antenna pair, resulting in an ultra-small sensing volume. CONCLUSIONS We have demonstrated two configurations for nanosensors with high FOMs both theoretically and experimentally. The proposed nanosensors are based on the interference of the SPPs that are launched at


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different nodes of a BSA operating in the third-order resonant mode or at two antennas in an SAP operating in the dipole mode. The extinction of the SPPs depends not only on the amplitudes of the launched SPPs but also on their phases, resulting in the formation of a narrow dip in the extinction spectrum. In our experiments, the FOM reached 28.8, which is one order of magnitude larger than those of previously reported sensors based on the dipole resonance in a single nanorod or slot antenna. Because either a single antenna or an SAP was used, a sensing volume of 0.0091 μm3 was realized, which is much smaller than

3. The high FOM and the small sensing volume beyond the diffraction limit make these nanosensor highly suitable for local RI measurements for label-free sensing, which is an important capability for many applications, especially in micro-nanofluidic chips. METHOD Fabrication of slot antennas. A 200-nm-thick Au film was deposited on a silica cover glass via electron beam evaporation. The slot antennas and the diffraction gratings were fabricated on the Au film via focused ion beam milling. Experimental methods. The cover glass with the Au film and another cover glass were coupled in parallel, separated by a small gap and sealed in a stainless steel module. There were two channels in the sidewalls of the stainless steel module for changing the ethanol-water solution between the two cover glasses. The white light source was a supercontinuum source with a wavelength range from 400 nm to 2000 nm (YSL Photonics SC-5). The spectrum of the launched SPPs was measured using a spectrograph (Zolix Omni-λ300i). Calculation methods. The FDTD method was used to calculate the electromagnetic field distribution. The Yee cell size was set to 2  2  2 nm3, and the perfectly matched layer boundary condition was used. The optical properties of gold were obtained by fitting the data from Johnson and Christy.30 ACKNOWLEDGEMENT


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This work was supported by the National Natural Science Foundation of China under Grant Nos. 11574011, 91850104, 61874003, and 61521004. SUPPORTING INFORMATION AVAILABLE Figure S1 presents the extinction parameters and FOMs of the BSA sensor for varying RIs from 1.25 to 1.5 to examine the RI range that can be detected by the sensor. S2 presents a Lorentz oscillator model to explain the operation principle of the proposed sensors, of which the parameters of the Lorentz oscillators are listed in Table S1 and the interference results of the oscillators are shown in Figure S2. Figure S3 illustrates the calculated normalized diffraction efficiency of the grating used in the experiment to measure the intensity of the launched SPPs. A chirped grating was proposed to increase the FWHM of the diffraction efficiency. The stainless steel cuvette and the fluid exchange system used in the experiment are shown in Figure S4. Figure S5 shows the SEM image of a rectangular slot antenna and the light diffracted from it in the experiment. Figure S6 exhibits the amplitudes of the propagation SPP point sources launched by slot antennas with lengths of 190 and 300 nm, and the electric field amplitude distributions in the SAP are shown in Figure S7. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1.

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10. Roh, S.; Chung, T.; Lee, B. In Overview of Plasmonic Sensors and Their Design Methods, Advanced Sensor Systems and Applications IV, International Society for Optics and Photonics: 2010; p 785303. 11. Nenninger, G. G.; Piliarik, M.; Homola, J., Data Analysis for Optical Sensors Based on Spectroscopy of Surface Plasmons. Meas. Sci. Technol. 2002, 13, 2038. 12. Lahav, A.; Auslender, M.; Abdulhalim, I., Sensitivity Enhancement of Guided-Wave Surfaceplasmon Resonance Sensors. Opt. Lett. 2008, 33, 2539-2541. 13. Bin, W.; Qing-Kang, W., High Sensitivity Transmission-Type SPR Sensor by Using Metallic– Dielectric Mixed Gratings. Chin. Phys. Lett. 2008, 25, 1668.


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14. Lee, B.; Roh, S.; Kim, H.; Jung, J. In Waveguide-Based Surface Plasmon Resonance Sensor Design, Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications III, International Society for Optics and Photonics: 2009; p 74200C. 15. Zeng, B.; Gao, Y.; Bartoli, F. J., Rapid and Highly Sensitive Detection Using Fano Resonances in Ultrathin Plasmonic Nanogratings. Appl. Phys. Lett. 2014, 105, 161106. 16. Teo, S. L.; Lin, V. K.; Marty, R.; Large, N.; Llado, E. A.; Arbouet, A.; Girard, C.; Aizpurua, J.; Tripathy, S.; Mlayah, A., Gold Nanoring Trimers: A Versatile Structure for Infrared Sensing. Opt. Express 2010, 18, 22271-22282. 17. Liu, N.; Weiss, T.; Mesch, M.; Langguth, L.; Eigenthaler, U.; Hirscher, M.; Sonnichsen, C.; Giessen, H., Planar Metamaterial Analogue of Electromagnetically Induced Transparency for Plasmonic Sensing. Nano Lett. 2009, 10, 1103-1107. 18. Maccaferri, N.; Gregorczyk, K. E.; De Oliveira, T. V.; Kataja, M.; Van Dijken, S.; Pirzadeh, Z.; Dmitriev, A.; Åkerman, J.; Knez, M.; Vavassori, P., Ultrasensitive and Label-Free Molecular-Level Detection Enabled by Light Phase Control in Magnetoplasmonic Nanoantennas. Nat. Commun. 2015, 6, 6150. 19. Shen, Y.; Zhou, J.; Liu, T.; Tao, Y.; Jiang, R.; Liu, M.; Xiao, G.; Zhu, J.; Zhou, Z.-K.; Wang, X., Plasmonic Gold Mushroom Arrays with Refractive Index Sensing Figures of Merit Approaching the Theoretical Limit. Nat. Commun. 2013, 4, 2381. 20. Jain, P. K.; El-Sayed, M. A., Noble Metal Nanoparticle Pairs: Effect of Medium for Enhanced Nanosensing. Nano Lett. 2008, 8, 4347-4352.


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This figure exhibits the schematic diagram of the bowtie slot antenna refractive index sensor which has a sensing volume as small as 0.0091 μm3 and a sensitivity of 574 nm/RIU that is shown in the right part of the figure. And it indicates that the sensor can obtain a figure of merit of 28.8/RIU due to the small full width at half maximum of the extinction parameter. 90x33mm (300 x 300 DPI)


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Nano-Scale Refractive Index Sensors with High Figures of Merit via

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