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Abstract
We report a plasmonic refractive index sensor with a wide measurement range based on periodic gold nanocubes coupled with a gold film. The theoretical sensing range is 1.0–1.8. The structure consists of two-dimensional gratings composed of periodic nanocubes that both excite local surface plasmon resonance and stimulate propagating surface plasmon resonance. The strong resonance of the multiple surface plasmons is suitable for use in refractive index sensing and effectively reduces the full width at half maximum of the resonance peak. The sensing performance of each resonant mode in the reflected spectrum is discussed in detail. The highest sensitivity and figure of merit of the proposed sensor are 1002 nm per refractive index units (RIU) and 417 RIU−1, respectively. The proposed sensor will be useful for bio-chemical sensing applications such as measuring changes in the refractive index of gases or liquids.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past ten years, research on surface plasmons has made great progress both theoretically and experimentally. Surface plasmons have attracted much attention from researchers globally because of their wide range of potential applications in photolithography [1–3], photocatalysis [4,5], Absorber [6,7], sensor [8,9], meta materials [10–12], magnetic field enhancement [13,14], Raman enhancement [15,16] and other fields [17–19]. Surface plasmons are waves that propagate along the surface of a conductor [20]. Its essence is the electromagnetic oscillation caused by the interaction between light and free electrons on the interface between a metal and dielectric. It has the optical characteristics of short wavelength, near-field enhancement, and surface electromagnetic localization. Recent optical detection technology, which uses the sensitivity of the electromagnetic field of a surface plasmon to the surrounding media, has become an important means of measuring the interaction of biological molecules qualitatively. Compared with traditional detection techniques, surface plasmon biosensors have the following advantages: no need to purify the substance to be detected, real-time monitoring [21,22] of the dynamic reaction between biological molecules, low background interference, high sensitivity, and no need to label the samples [23].
At present, propagating surface plasmon (PSP) sensors [24,25] are generally more sensitive than local surface plasmon (LSP) sensors [26] because of their wide range of action with the surrounding media environment, and they have been initially commercialized in medical diagnosis and drug screening. However, because of the limitation of its excitation method, the realization of PSP sensing requires expensive and high-precision optical instruments, which is not in line with the aim of developing low-cost and portable surface plasmon sensing. Compared with prism-excited [25] surface plasmon sensors, nano-array [27–29] surface plasmon sensors yield more advantages. For example, the surface plasmon excitation of a nano-array structure is simple and insensitive to incident angle or polarization direction [27], has a free structure design [28] and flexible sensing mechanism, and is more miniaturized and integrated. Surface plasmon sensors made of metal nano-arrays penetrate into different research areas, driving the interdisciplinary development of many topics.
In our previous work, we proposed a composite structure for refractive index sensing with a sensing range of 1.3–1.4 [30]. This composite structure comprises a gold nano-disk array on a gold film with a SiO2 thin film. However, the refractive index sensing characteristics and sensing range under different modes are not discussed in detail. In this work, we use gold nanocubes instead of the gold disks used previously in order to excite stronger LSPs, and use SiO2 cube spacers instead of the SiO2 thin film to increase the range of interaction between the surface plasmon resonances (SPRs) and analyte. Thus, the refractive index sensor with a wide range, high sensitivity, and high figure of merit (FOM) is realized at the same time. The multiple SPR modes in the structure are analyzed in detail using the finite-difference time-domain method, and the reflection spectra under different refractive indexes are simulated. The results show that the structure we designed can use different resonant modes for a wide range refractive index sensing, and the sensor can achieve a high FOM due to the effective reduction of the full width at half maximum (FWHM). We showed the sensitivity and FOM of resonance peaks at different resonance modes, which are two important physical parameters for describing the performance of refractive index sensor. Finally, we propose a similar alternative structure to improve its reusability as a refractive index sensor.
2. Sensor design and analysis method
Figure 1(a) illustrates the three-dimensional geometry structure of the refractive index sensor, which mainly consists of two functional layers for exciting surface plasmons. The upper layer is a two-dimensional square array composed of gold nanocubes, and the lower layer is a gold film. This means that our structure is periodic along the X and Y axes, as shown in Fig. 1(a). Here, we use square SiO2 spacers with the same length and width as the gold nanocubes to separate the two functional layers. The whole composite structure is situated on a glass substrate. In practical structure fabrication, a layer of gold film is first deposited on a glass substrate by magnetron sputtering. Then, a photoresist is coated on the gold film and nanocube structures are fabricated by electron beam lithography. After that, SiO2 and gold are deposited again by magnetron sputtering. Finally, the photoresist is removed by the lift-off process, leaving SiO2 spacers and gold nanocubes. The length and width of the gold nanocubes are 300 nm, and their height is 40 nm. The length and width of the SiO2 spacers are equal to that of the gold nanocubes and their thickness is 24 nm. The thickness of the gold film is 80 nm and the period of gold nanocubes arrays is 1000 nm, as indicated by the red line in Fig. 1(a).
Fig. 1. (a) Three-dimensional geometric structure of the refractive index sensor. From top to bottom, the components consist of a gold nanocube square array, SiO2 spacers, gold film, and glass substrate (the black arrow indicates the direction of propagation of incident light, the blue arrow indicates the polarization direction, and the red line shows the period). (b) Schematic diagram of the calculation unit model, in which the length of the unit is set to the length of the structural period.
In this work, we use the finite-difference time-domain method (FDTD Solutions) to analyze the surface plasmon modes and study the structure sensing characteristics. As shown in Fig. 1(a), the planar polarized light is propagated by following the positive direction of the Z-axis to excite the surface plasmon in the structure, where the polarization direction is along the X-axis and parallel to the edge of the gold nanocube. As mentioned above, the composite structure is periodic along the X and Y axes, so we select a period as the unit of calculation. The calculated unit is shown in Fig. 1(b), where the surface of the gold nanocube and the gold film are covered with the analyte (background refractive index) to be measured, which is considered to be infinitely thick in the calculation. The boundary around the calculated unit is used as the periodic boundary condition and the plane wave is vertically incident. Absorption boundary conditions are used in glass and analyte, which are the upper and lower surfaces of the calculation unit. The length and width of the unit are set to be the period of the sensor structure, which is 1000 nm. In the simulation, the dielectric constant of gold is selected from the experimental data supported by the Drude model [31,32].
3. Theoretical analysis of the structure
In our structure, the gold nanocubes are mainly used as particles for exciting LSPs. The array of gold nanocubes can be regarded as a two-dimensional grating that provides additional momentum for exciting PSPs on the top surface of the gold film. In order to excite the strong coupling between LSPs and PSPs, the SiO2 spacers are used to separate the array of gold nanocubes and gold films. Figure 2(a) shows the reflection spectrum of the structure with a analyte refractive index of 1.3. It is obvious that multiple SPRs of the composite structure can be stimulated. The three main SPRs are mode 1 (949 nm), mode 2 (1310 nm), and mode 3 (1715 nm). As discussed, the arrays provide an additional momentum $\textrm{G} = \frac{2\pi }{d}\sqrt {{n^2} + {m^2}}$ [33], where d is the grating period, and n and m are integers. Modes 1 and 2 in the reflection spectrum correspond to (n, m) = (1, 1) and (1, 0), respectively. According to the momentum equation, the resonance wavelengths of modes 1 and 2 are related to the grating period. In fact, the resonant wavelengths of both the modes increase as the period increases [33]. Mode 3 is the result of LSPs produced by the gold nanocubes, and its resonance wavelength is related to their size.
We also calculated the electric field distribution at resonance wavelengths under three modes. Figures 3(a), 3(b), and 3(c) respectively show the electric field distribution of the three modes in the X-Z plane along the center of the gold nanocube. Figures 3(d), 3(e), and 3(f) show the electric field distribution of the three modes on the upper surface of the gold nanocubes, respectively; Figs. 3(g), 3(h), and 3(i) show the electric field distribution of the three modes on the lower surface of gold nanocubes, respectively; and Figs. 3(j), 3(k), and 3(l) show the electric field distribution of the three modes on the surface of the gold film. For the electric field distributions, Figs. 3(a), 3(d), 3(g), and 3(j) show those of mode 1; Figs. 3(b), 3(e), 3(h), and 3(k) show those of mode 2; and Figs. 3(c), 3(f), 3(i), and 3(l) show those of mode 3.
Fig. 3. Electric field distribution |E|/|E0| at the resonance wavelengths of the three modes. (a), (d), (g), and (j) Electric field distribution at the resonant wavelength of 949 nm (mode 1). (b), (e), (h), and (k) Electric field distribution at the resonant wavelength of 1310 nm (mode 2). (c), (f), (i), and (l) Electric field distribution at resonant wavelength 1715 nm (mode 3). (a), (b), and (c) Electric field distribution along the center of the gold nanocube in the X-Z plane. (d), (e), and (f) Electric field distribution on the top surface of the gold nanocube. (g), (h), and (i) Electric field distribution at the interface between the gold nanocubes and SiO2 spacers. (j), (k), and (l) Electric field distribution at the interface between the SiO2 spacers and gold film.
For modes 1 and 2, the conversion of incident light energy into PSPs by the two-dimensional gratings is the reason for their low reflectivity in the reflection spectrum, which can be clearly observed in Figs. 3(g) and 3(j) for mode 1 and Figs. 3(h) and 3(k) for mode 2. Two different PSP modes can also be observed from the different electric field distributions. Figures 3(d) and 3(e) show that although there are LSPs in these two modes, they are negligible compared with the PSPs on the gold films. In other words, most of the energy of the incident light mainly forms the PSP in modes 1 and 2. An observation of all the electric field distributions of mode 3 reveals that the energy is well localized. For mode 3, the strong LSPs at the eight apex corners of the gold nanocubes are responsible for the low reflectivity in the reflectance spectra, as shown in Figs. 3(f) and 3(i), where the electric field enhancement at the lower four apex corners of the gold nanocube can reach 96 times that of the incident light. This is much higher than that stimulated by the PSPs of modes 1 and 2. Note that these three modes are the result of the PSP and LSP coupling caused by the structural composite. In other words, both PSPs and LSPs exist in these three modes, but the difference between them lies in the levels of contribution of the two types of surface plasmons.
4. Sensing characteristics of the structure
Refractive index sensing is realized by measuring the change in the resonance peak position in the corresponding reflection spectra. Therefore, for measuring different dielectrics, a larger displacement of the resonance peak indicates a better sensing performance and higher sensitivity. As shown in Fig. 4, the reflective spectra of the analyte refractive index from 1.0 to 1.8 is shown in the case of wavelength scanning ranging from 500 nm to 2000 nm. The contour map of the reflection spectra reveals the suitability of our structure as a multiple SPR refractive index sensor. The resonant wavelengths of the three modes are linearly sensitive to the refractive index of analyte, especially in modes 1 and 2. Although mode 1 is no longer suitable for sensing when the refractive index is 1.0 to 1.1, modes 2 and 3 can be used to sense a wide range of refractive index values from 1.0 to 1.8. From the perspective of refractive index sensing, the simulation results of mode 2 are almost perfect because they cover a wide range of refractive index values, exhibit excellent linear sensitivity to the environmental dielectric (discussed in detail below), and have an extremely low reflectivity and narrow FWHM (also shown in Fig. 2).
Fig. 4. Reflective spectra of the composite structures. The analyte refractive index ranges from 1.0 to 1.8 and the color represents the reflectivity distribution.
For a resonance-based refractive index sensor, two important physical parameters describe its sensing performance. The sensitivity is defined as S =Δλ/Δn, where Δλ is the shift of the resonance peak wavelength, Δn is the refractive index change of the dielectric, and the FOM is defined as FOM = S/FWHM [34]. As shown in Fig. 5, to better describe the sensing characteristics of the three resonance peaks, we plot the curves of the resonance wavelength and FOM with respect to the analyte refractive index under different modes.
Fig. 5. Sensitivity curves and FOM under the three resonance modes. (a), (c), and (e) Relationship between the resonance wavelength and refractive index for modes 1, 2, and 3, respectively. (b), (d), and (f) Relationship between the FOM and refractive index for modes 1, 2, and 3, respectively.
Figure 5(a) shows the relationship between the resonant wavelength and refractive index for mode 1, and a good linear relationship between them can be seen. For mode 1, the simulated sensing range is evaluated from 1.1 to 1.8. Because the resonance peak is linearly sensitive to the refractive index, the sensitivity of mode 1 is fixed over a wide range (698 nm per refractive index unit (RIU)). The FOM of mode 1 varies with the analyte refractive index. As shown in Fig. 5(b), the FOM increases nonlinearly with respect to the refractive index up to a value of 1.4, at which point the FOM stabilizes. The maximum FOM of mode 1 is 112 RIU−1. Figure 5(c) shows the relationship between the resonant wavelength and analyte refractive index in mode 2. It can be seen that mode 2 has the same good linear relationship as mode 1, but has stronger resonance intensity, which leads to a lower reflectivity (as shown in Fig. 2). The sensitivity of mode 2 can reach 1002 nm/RIU. As shown in Fig. 5(d), the FOM of mode 2 decreases as the refractive index increases because the FWHM broadens in this case. The reason may be that, as the refractive index increases, the resonance wavelengths of mode 2 and 3 become close, which leads to interaction between the modes. In fact, the FWHM of mode 3 also becomes broader as the refractive index increases, as can be seen from Fig. 4. The maximum FOM of mode 2 is 417 RIU−1 and occurs at a low refractive index. The sensitivity curve and FOM of mode 3 are shown in Figs. 5(e) and 5(f), respectively. It can be seen that the linear relationship between the resonant wavelength and the analyte refractive index is not as good as that in modes 1 and 2. The sensitivity of mode 3 is 530 nm/RIU, obtained by fitting the curve of Fig. 5(e) linearly. Because of the very wide FWHM (shown in Figs. 2 and 4), the FOM of mode 3 is much smaller than that of modes 1 and 2 and is similar to that of other LSP refractive index sensors.
Analysis of the three modes reveals that mode 2 shows the best refractive index sensing characteristics, and can theoretically reach the sensitivity of 1002 nm/RIU and FOM of 417 RIU−1; the sensitivity is higher than that in the previously reported work (737 nm/RIU [34]; 693.88 nm/RIU [35]; 580 nm/RIU [36]; 853 nm/RIU [30]), and the FOM is higher than that obtained in other previous studies [37,38]. This is mainly due to the wide range of interaction between mode 2 and the dielectric environment and the strong resonance intensity. In practical refractive index sensing, three modes can be used simultaneously to improve the accuracy of the sensor. In addition, the polarization direction of the incident light has no effect on the reflection spectrum of the structure. This means that the angle between the incident electric vector and the nanocube has no effect on the sensitivity and FOM.
5. Similar alternative structure
A surface plasmon is a collective charge oscillation existing at the interface between a conductor and dielectric. Therefore, when designing a surface plasmon refractive index sensor, it is necessary to increase as much as possible the area of the contact surface of the dielectric (to be measured) with the metal that generates the surface plasmon, to increase the refractive index sensitivity. The structure of Fig. 1 is designed on the basis of such a principle. However, the uneven surface structure in Fig. 1 makes it difficult to clean the analyte of the dielectric, which reduces the accuracy of the measurement and the reusability of the structure. Therefore, we suggest a similar substitute to the original structure. As shown in Fig. 6, we fill the gap between the gratings of the original structure with SiO2 to make its surface smooth. Note that the height of the SiO2 filling the gap is the same as that of the nanocubes, which means that the upper surfaces of the gold nanocubes are still in contact with the dielectrics, as shown in Fig. 6. Obviously, although we have improved the reusability of the sensor, we have also reduced its sensitivity.
Fig. 6. Three-dimensional structure of the improved sensor. The inset shows the side view of the sensor structure.
The simulated range of refractive index is set to 1.0–1.8, and the wavelength scanning range is set to 500–2000 nm. The reflective spectra of the new structure are shown in Fig. 7. The main SPR modes of the new structure are the same as those of the original structure (shown in Fig. 4). Therefore, we believe that the previous analysis of the resonance modes is still applicable to the new structure. The sensitivities of modes 1 to 3 of the new structure are calculated to be 581, 924, and 134 nm/RIU, respectively. The sensitivities of the three resonance modes are lower than those of the original structure, which is consistent with the prediction. The sensitivity of mode 3 decreases most obviously. This is because mode 3 is caused by LSPs, and the strong LSPs are generated at the lower four apex corners of the gold nanocube, which are covered by SiO2 in the new structure. A comparison of Figs. 4 and 7 also reveals that the sensing range of the new structure will decrease for modes 1 and 2.
Fig. 7. Reflective spectra of the new structure, in which the refractive index ranges from 1.0 to 1.8 and the color represents the reflectivity distribution.
6. Conclusions
We proposed a wide range refractive index sensor based on multiple SPRs excited by the coupling of gold nanocubes and gold film. The main SPR modes in the structure were analyzed in detail through simulation. The results show that, because our structure contains both metal particles and two-dimensional gratings composed of gold nanocubes, it can be used for refractive index sensing using not only LSPs but also PSPs. The refractive index sensing range of the sensor theoretically extends from 1.0 to 1.8. Moreover, the maximum sensitivity and FOM of the sensor can theoretically reach 1002 nm/RIU and 417 RIU−1 respectively. Using the foundation of the original structure, a method to improve the reusability of the structure is proposed. We believe that the proposed refractive index sensor can be applied in future biosensors, detection sensors, and sensor integration.
Funding
National Natural Science Foundation of China (NSFC) (61865008).
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