Open Access
Add to BibSonomy
Abstract
An extremely sensitive multi-order mode refractive index (RI) sensor was fabricated by coupling titanium dioxide nanograss film coated FTO conductive glass with Kretschmann prism. Both calculation and experimental studies were carried out. Theoretical analysis by employing resonant waveguide modes indicated that the maximum sensitivity could be achieved when the mode worked at the weakly-bounded condition. The experimental results showed that for p-polarized and s-polarized light, the sensor exhibited a maximum RI sensitivity of 2938.21 nm/RI unit (RIU) and 1484.39 nm/RIU in the 1st order mode, respectively. Its maximum figure of merit was as high as 77.77. The proposed sensor is promising to be applied in environmental monitoring, immune analysis, nucleic acid test, etc.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Refractive index (RI) sensors have attracted much attention in recent years in the sensing field since the RI measurement is of great significance for biology, chemistry, environment monitor, etc. As fundamental methods of RI sensing, Kretschmann prisms coupling [1], grating coupling [2], multimode interference [3,4], waveguide coupling [5,6], and localized surface plasmon [7] are commonly used to design RI sensors. So far, a number of optical RI sensors have been developed, such as Mach–Zehnder interferometer (MZI) [8,9], fiber Bragg gratings [10,11], long period gratings [12,13], optical fiber Fabry-Perot sensors [14,15], microring resonator sensors [16], surface plasmon resonance (SPR) sensors [17], localized surface plasmon sensors [18,19] and so on. Among these different sensors, the SPR sensor based on the classical Kretschmann configuration of prism coupling is the most important one because of its simply construction and convenient operation. The SPR sensors function on the basis of utilizing a special type of electromagnetic waves, viz., surface plasmon polaritons [20]. They can detect the change of refractive index near the chip surface and is a powerful tool for analysis of the kinetics of biochemical reactions. Compared with Bragg gratings, long period gratings and Fabry-Perot sensors, SPR sensors offer the advantages of high sensitivity and better stability [21]. By comparison with other biomolecular detection technologies such as chromatography methods, chemiluminescence method, fluorescence assay, immunoradiometric assay, SPR sensors have the unique characteristics of low cost, label-free, non-invasive, rapid, direct, real-time and in situ monitoring of biomolecular interaction [22,23]. Moreover, it does not require complex sample pre-treatment, separation process and also no need to consider the turbidity of the solution. Therefore, SPR sensors have been extensively used in various fields including fundamental physics, chemistry, biology, and materials science in the past several decades.
To evaluate the sensing performance of an RI sensor, the RI sensitivity (S) is generally used. The sensitivity with wavelength interrogation is defined as the ratio between the shift of the resonance wavelength and the change of the RI of analyte, i.e., the spectral shifts per RI unit (RIU) [24], as expressed by the following formula [25]:
where λres represents the resonance wavelength and nS stands for the RI of the measured media. It is quite important that the resonance wavelength is highly sensitive to any RI change of surrounding media for an RI sensor. In order to improve the sensitivity of the prism coupling sensor, many methods have been proposed such as adding dielectric layer [26,27], complex noble metal structures [28], semiconductor layer [29] or graphene and air gap [30].In addition to the sensitivity, the sensing accuracy, namely the figure of merit (FOM), is another important parameter in characterizing the performance of the Kretschmann configuration sensor. FOM should be as high as possible for a RI sensor. The FOM depends on the full width at half maximum (FWHM) of the spectral curve and sensitivity. The FOM in wavelength units is defined as the ratio of the RI sensitivity to FWHM as described by the following equation [31]:
In our previous work, we have replaced the conventional gold layer with titanium dioxide (TiO2) nanowire arrays coated FTO in the Kretschmann configuration, and the sensitivity of the proposed sensor is 1.4 times higher than that of the SPR sensor [32]. However, the spectral dip becomes dull at the expense of the sensitivity. This blunted sharpness of the dip deteriorates the sensing accuracy.
In this paper, we improved the sensing accuracy of the sensor by utilizing TiO2 nanograss films and designing the sensing mode work at the weakly-bounded condition. The multi-order waveguide modes in the resonance spectra of the sensor, including 1st, 2nd and 3rd order mode, were presented by combining TiO2 nanograss film with Kretschmann configuration. Our theoretical analysis showed that the sensitivity can be enhanced significantly by making the waveguide mode work at the weakly-bounded condition, under which stronger light fields are concentrated inside the detected material. The responses of the sensor to RI for s-polarized and p-polarized light were performed by measuring the different water-glycerol solutions with refractive indices from 1.33303 to 1.47399. The results showed that for the glycerol concentration lower than 40%, under the circumstances of p polarization the sensor has higher RI sensitivity, whereas for glycerol concentration higher than 60%, the sensor with s polarization situation is more sensitive. The maximum FOM of the sensor was as high as 77.77, which is much higher than that of the sensor with a nanowire array we have reported previously [32]. Moreover, the maximum FOM of the sensor is also much better than that of some nanostructured sensors in former reports [33,34].
2. Construction of the experimental setup
The proposed sensor configuration is presented schematically in Fig. 1. It is mainly composed of a LS-1 type tungsten-halogen lamp (Ocean Optics, USA), a HR4000 type CCD spectrum analyzer (Ocean Optics, USA), a glass prism coupler with central symmetry structure (Beijing Beidong Photoelectric Automation Development Co., Ltd, China, 45°/45°/90°, the refractive index is 1.799 under the wavelength of 633 nm), a FTO conductive glass (Asahi Glass Co., Ltd, Japanese, the refractive index is ∼1.9 under the wavelength of 633 nm), a pair of fiber collimators and a linear polarizer (Beijing Daheng Optoelectronic Technology Co., Ltd, China), multimode silica fibers (Zhejiang Leichou Technology Co., Ltd, China), a silicone rubber test tank (home-made), and a peristaltic pump (Baoding Qili Constant Flow Pump Co., Ltd, China). The tungsten-halogen lamp was jointed with the fiber collimator. The linear polarizer was fixed in front of a collimator. The CCD spectrometer was connected with another fiber collimator.
The proposed Kretschmann-type sensor was constructed using the optical components mentioned above. Firstly, the 45°/45°/90° glass prism was tightly attached to the back of FTO glass chip with TiO2 nanograss film by coupling liquid CH2I2 which has a high refractive index of 1.741. Then the test tank was mounted firmly onto the surface of the chip and the TiO2 nanograss film was exposed to the tank. Broadband light from the tungsten halogen lamp traversed successively a quartz fiber, a collimator lens, and the linear polarizer to transform into a collimated p-polarized beam or s-polarized beam. The collimated beam was diffracted into the prism at a given incident angle with respect to the normal to the input surface of the prism. Incident angle of the light beam was managed by turning the rotating stage. The incident light suffered attenuated total reflectance from the interface between the glass substrate and FTO/TiO2 nanograss film layer. The corresponding dips at specific wavelengths can be observed in the resonance spectra. The reflected light signal was determined by the CCD spectrum analyzer with a time resolution of 1 ms. All experiments were performed at room temperature of 20 ± 0.1°C.
3. Preparation of the TiO2 nanograss film
The TiO2 nanograss film was prepared on a FTO glass substrate by using a hydrothermal method. In brief, the FTO glass substrates were placed within a sealed reactor, containing 20 mL of water, 20 mL of hydrochloric acid (37 wt %), and 0.47 mL of tetrabutyl titanate. A reaction temperature of 150 °C was used. After the reaction finished, the substrate was successively rinsed with ethanol and distilled water, and then dried in air. Scanning electron microscopy (SEM) micrograph of TiO2 nanograss film was taken with Hitachi S4800, Japan.
Figure 2 gives the SEM image of TiO2 nanograss film. As can be seen from the Fig. 2, the TiO2 nanograss film is composed of many nanowires with the widening of diameter from 50 to 300 nm. There exists a large number of gaps among the nanowires, which increase the surface area of the TiO2 nanograss film and are conducive to the detection of biochemical molecule.
4. Results and discussion
The responses of the sensor with TiO2 nanograss film to RI are investigated using a series of aqueous glycerol solutions with different concentrations as the samples. The incident angle θ is a key parameter of the device. We change it in experiment and find that when θ = 15°, the device shows the highest sensing sensitivity. This is because that under such angle, the excited waveguide mode supported in the FTO layer works in the weakly-bounded condition, meaning that the electromagnetic fields penetrate into the glycerol solution greatly which can help increase the sensing sensitivity. To prove this point, we performed a theoretical analysis on the sensing mechanism. We employed the transfer matrix method to calculate the reflection spectra of the sensor. The calculated reflectivity as a function of wavelengths with different concentration ranges of glycerol are plotted in Figs. 3(a) and 3(b), respectively, for p-polarized and s-polarized incidence. The RIs of the solution are 1.33303 (0% glycerol), 1.34481 (10% glycerol), 1.35749 (20% glycerol), 1.37070 (30% glycerol), 1.38413 (40% glycerol), 1.39809 (50% glycerol), 1.41299 (60% glycerol), and 1.42789 (70% glycerol), respectively, which are extracted from experimental measurement. We find that in the wavelength range of interest, there are two resonances, and the resonant dips shift to red with the increase in the concentration range of glycerol for both modes. As will see later, they are 1st order mode and 2nd order mode, and the sensitivity of the former mode is higher than that of the latter one, since the 1st order mode exhibits a larger bandwidth. To check the origin of the dips, we show in Fig. 3(c) the simulated out-of-plane magnetic field distributions as a function of wavelengths with a p-polarized incidence and 0% concentration. We see that the two dips are associated with waveguide modes bounded in the FTO layer. A great proportion of the power flow is propagating inside the glycerol solution, indicating that the waveguide mode is in fact weakly-bounded, and such condition is obviously good for the sensing sensitivity.
Fig. 3. Calculated reflectance spectra with different concentration ranges of glycerol for (a) p- and (b) s-polarized incidence. (c) Simulated out-of-plane magnetic field distributions as a function of wavelength for p-polarized incidence with 0% concentration. (d) Calculated wavelength shift as a function of the transverse wave vector of the incidence.
The weakly-bounded condition indicates that the incident transverse wave vector k// (which is also the propagation constant of the excited waveguide mode) is slightly higher than that in the detected material, and such condition is actually highly related to the incident angle θ. To verify this, we study the dependence of the sensing performance on the transverse wave vector k///k0 (which is related to θ where k0 is the wave vector in vacuum). Figure 3(d) plots the calculated wavelength shift, defined as the wavelength difference between the case with 0% glycerol and that with 70% glycerol, as a function of k///k0. We find that the maximum sensitivity can be achieved when the transverse wave vector is close to that of the 70% glycerol, which is exactly the weakly-bounded condition. Decreasing k///k0 further will push the system into the non-detectable region, where the detection for glycerol in the full range (0%∼70%) is not possible as we aim for a large detection range. Increasing k///k0 will lower the sensitivity, and exceeding that of glass will push the system into the non-coupled region, where light cannot be coupled into the device. Therefore, the weakly-bounded condition that enables strong evanescent fields localized in glycerol is the key to the high sensitivity of the device in Figs. 3(a) and 3(b), and such condition corresponds to a critical incident angle θ = 15° in experiment.
We performed experiments based on the above theoretical analysis. The measured response of the sensor to RI is shown in Fig. 4 for p-polarized incidence. Figure 4(a) shows the reflectance spectra of the sensor measured with eight samples exhibiting different concentration ranges of glycerol. Figure 4(b) provides the reflectance spectra measured with three samples whose refractive indices are 1.4429 (80% glycerol), 1.45839 (90% glycerol), and 1.47399 (100% glycerol), respectively. From Fig. 4(a), it can be clearly seen that the resonance peak moved to longer wavelengths with increasing the RI of glycerol solution. When the glycerol concentration of aqueous solutions gradually increased from 0% to 70%, the resonance wavelength was gradually shifted from 636.97 nm to 777.78 nm, corresponding to a wavelength shift of 140.81 nm. In this case, the RI sensitivity of the sensor is 1484.39 nm/RIU. Moreover, we can obviously see that the 2nd order mode occurs in the resonance spectra from 30% glycerol solution.
Fig. 4. Measured reflectance spectra of the sensor for p-polarized incidence: (a) in aqueous glycerol solutions from 0 to 70%; (b) in aqueous glycerol solutions from 80% to 100%.
However, it can be observed from Fig. 4(b) that for the glycerol solutions with concentration of more than 80%, the intensity of the resonance peak in the spectra decreases. This is due to the fact that the RI of the solution exceeds that of the transverse wave vector of the incidence. Meanwhile, we can also see that the resonance peak for the 2nd order mode continues to move to longer wavelengths, and the resonance peak of the 3rd order mode begins to appear in the resonance spectra.
Figures 5(a) and 5(b) present the response spectrum of the sensor to glycerol solutions with different concentrations for s polarization. As can be seen from Fig. 5(a) that for glycerol concentration from 0% to 70%, there are also two modes in the resonance spectrum. In such situation, with increasing the glycerol concentrations, the resonance wavelengths of both 1st and 2nd order mode also gradually move to longer wavelengths. In the range of glycerol concentration of 0% to 40%, there is a good linear relationship between shifted resonance wavelength and glycerol concentration in aqueous solution for the 1st order mode and the 2nd order mode of s polarization, respectively. In this range, the RI sensitivities of the sensor are 441.29 nm/RIU for the 1st order mode and 335.23 nm/RIU for the 2nd order mode. More surprisingly, for more than 50% glycerol concentration, the shift of resonance wavelength takes on an exponential function change with the increase of the glycerol concentration, which is significantly different from that of p polarization. Of particular note is that within the glycerol concentration range of 60% to 70%, the average RI sensitivities reach up to 2938.21 nm/RIU for the 1st order mode and 1742.28 nm/RIU for the 2nd order mode, respectively, evidencing that the sensor has a better sensing capability to RI for 1st order mode.
Fig. 5. Measured reflectance spectra of the sensor for s-polarized incidence: (a) in aqueous glycerol solutions from 0 to 70%; (b) in aqueous glycerol solutions from 80% to 100%.
Nevertheless, similar to the situation of p polarization, the reflected light intensity of the resonance peak becomes smaller as the glycerol concentration exceeds 80%, as shown in Fig. 5(b). This is probably because that the resonance peak of the 1st order mode shifts to larger wavelength that is out of the measuring scope of the sensor. In the meantime, the resonance peak for the 2nd order mode moves to longer wavelength. Compared with glycerol solutions below 70%, the resonance intensity of the 2nd order mode for glycerol concentration of more than 80% is obviously increased. Additionally, for glycerol concentration of more than 80%, a weak resonance peak of 3rd order mode appears in the resonance spectrum.
Figure 6 summarizes the above experimental results, showing the magnitude difference of the resonance wavelength shift with increasing glycerol concentration in aqueous solution for both p polarization and s polarization. It is clear that between 0% and 70% glycerol concentration, a wavelength shift of 140.81 nm occurs in the case of p polarization. While in the case of s polarization, the wavelength shift is 65.62 nm for the 2nd order mode and 84.71 nm for the 1st order mode. Therefore, the sensor with p polarization is more sensitive as refractive indices are less than 1.38413, while in the case of s polarization it can give a higher RI sensitivity for refractive indices more than 1.41299. All these experimental results are in accordance with the numerical results shown before, demonstrating the role of the weakly-bounded waveguide mode for realizing a high sensing performance.
Fig. 6. The relationship between shifted resonance wavelength and glycerol concentration in aqueous glycerol solutions from 0 to 70%.
It is worth noting that we have substituted TiO2 nanograss film coated FTO layer for the classic noble metal gold layer because fluorine-doped tin oxide has low dissipation performance and low cost [35]. The FTO layer coated onto the prism surface is used as a sensitive element and plasmon-carrying layer, and TiO2 nanograss film is as the dielectric layer. The refractive index sensitivity of the sensor is enhanced due to the effect of the FTO layer and TiO2 nanograss film at the weakly-bounded waveguide mode. Furthermore, FTO is more easily integrated into optoelectronic devices than Au and Ag.
On the other hand, to further evaluate the sensing performance of the proposed sensor with nanograss film, the FOM of the sensor was calculated using Eq. (2). The calculated results are given in Table 1.
Table 1. FOM of the multi-order mode sensor with TiO2 nanograss film in glycerol solution
It can be seen from Table 1 that the maximum FOM value of the sensor is up to 77.77, which is significantly higher than that of the nanostructure-based and nanoparticle-based sensors reported in other literatures, for instance, the antenna structures in a thin gold film (FOM of 3.8) [33], the ordered arrays of nanoholes (FOM of 23) [34], the periodic nanowell structure (FOM of 14.5) [36], gold nanoparticles of different shapes and sizes (FOM = 1.7 - 4.5) [37], planar metamaterial with ring resonator and pair resonator (FOM of 34.33) [38], the nonconcentric ring/disk cavity metamaterial with Fano-type resonances (FOM of 8.34) [39], the three-dimensional (3D) plasmonic nanograter nanostructures with unusual Fano resonances (FOM of 12.5 and 35) [40], nanoparticle clusters (FOM of 10.6) [41], nanoshell clusters (FOM of 22.25) [42], and nano-cross-structures (FOM of 4.6) [43].
Table 2 exhibits the FOM of the sensor based on a TiO2 nanowire array in NaCl solution in the literature 32. From Table 2 we can see that the maximum FOM of the sensor with nanograss film is also larger than that of the sensor with nanowire array in our previous work [32]. This is mainly because that the FWHM of its spectral curve is smaller in comparison with the sensor with nanowire arrays. The TiO2 nanograss film plays an important role in boosting the FOM. The higher FOM of the proposed sensor should mainly be attributed to the growth of sparse disordered nanowires on the surface of FTO films, which leads to the change of the sensing mode and the FWHM of the spectral curve of the sensor.
Table 2. FOM of the sensor based on a TiO2 nanowire array in NaCl solution in Ref. 32
5. Conclusions
In summary, we have fabricated a novel sensor by coupling TiO2 nanograss coated FTO conductive glass with a Kretschmann prism to modulate the optical field transmission mode. The proposed sensor exhibits multi-order modes for RI sensing in the two cases of p polarization and s polarization. In the case of s polarization, the sensor shows extremely high RI sensitivities of 2938.21 nm/RIU for the 1st order mode and 1742.28 nm/RIU for the 2nd order mode over the glycerol concentration range from 60% to 70%. In the case of p polarization, the sensor has an average sensitivity of 1484.39 nm/RIU in the glycerol concentration range from 0% to 70% for the 1st order mode, and the resonance wavelength has a good linear relationship with glycerol concentration. The sensor reaches to a FOM of 77.77, which is larger than that of our former proposed sensors with nanowire arrays. Moreover, the proposed sensor has an advantage of simplicity and low cost. It is promising for sensing applications in many fields such as food safety, drug screening, clinical detection, biological detection, environmental monitoring, and so on.
Funding
Natural Science Foundation of Shandong Province (ZR2019MC069); Major Scientific and Technological Innovation Project of Shandong Province (2018CXGC0608); National Natural Science Foundation of China (61535010, 62027825); National Key Research and Development Program of China (2020YFC2004600).
Disclosures
The authors declare no conflicts of interest.
References
1. X. X. Wang, X. L. Bai, Z. Y. Pang, H. Yang, and Y. P. Qi, “Investigation of surface plasmons in Kretschmann structure loaded with a silver nano-cube,” Results Phys. 12, 1866–1870 (2019). [CrossRef]
2. Z. Li, L. L. Liu, B. Z. Xu, P. P. Ning, C. Chen, J. Xu, X. L. Chen, C. Q. Gu, and Q. Qing, “High-Contrast Gratings based Spoof Surface Plasmons,” Sci. Rep. 6(1), 21199 (2016). [CrossRef]
3. H. Z. Dong, L. H. Chen, J. J. Zhou, J. H. Yu, H. Y. Guan, W. T. Qiu, J. L. Dong, H. H. Lu, J. Y. Tang, W. G. Zhu, Z. G. Cai, Y. Xiao, J. Zhang, and Z. Chen, “Coreless side-polished fiber: a novel fiber structure for multimode interference and highly sensitive refractive index sensors,” Opt. Express 25(5), 5352–5365 (2017). [CrossRef]
4. H. D. Zheng, B. C. Huang, Y. H. Li, R. J. Zhang, X. H. Gu, Z. B. Li, H. Y. Lin, W. G. Zhu, J. Y. Tang, H. Y. Guan, H. H. Lu, Y. C. Zhong, J. B. Fang, Y. H. Luo, J. Zhang, J. H. Yu, F. K. Tittel, and Z. Chen, “Residual thickness enhanced core-removed D-shaped single-mode fiber and its application for VOC evaporation monitoring,” Opt. Express 28(10), 15641–15651 (2020). [CrossRef]
5. J. Y. Tang, J. B. Fang, Y. L. Liang, B. Zhang, Y. H. Luo, X. Y. Liu, Z. B. Li, X. J. Cai, J. Q. Xian, H. Lin, W. G. Zhu, H. Y. Guan, H. H. Lu, J. Zhang, J. H. Yu, and Z. Chen, “All-fiber-optic VOC gas sensor based on side-polished fiber wavelength selectively coupled with cholesteric liquid crystal film,” Sens. Actuators, B 273, 1816–1826 (2018). [CrossRef]
6. Y. M. Huang, W. G. Zhu, Z. B. Li, G. L. Chen, L. H. Chen, J. J. Zhou, H. Lin, J. W. Guan, W. X. Fang, X. Liu, H. Z. Dong, J. Y. Tang, H. Y. Guan, H. H. Lu, Y. Xiao, J. Zhang, H. C. Wang, Z. Chen, and J. H. Yu, “High-performance fibre-optic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film,” Sens. Actuators, B 255, 57–69 (2018). [CrossRef]
7. V. V. Kulish and P. M. Tomchuk, “Optical properties of metal nanotubes and metal nanoshells,” Surf. Sci. 602(5), 1045–1052 (2008). [CrossRef]
8. J. Yang, M. Yang, C. Y. Guan, J. H. Shi, Z. Zhu, P. Li, P. F. Wang, J. Yang, and L. B. Yuan, “In-fiber Mach-Zehnder interferometer with piecewise interference spectrum based on hole-assisted dual-core fiber for refractive index sensing,” Opt. Express 26(15), 19091–19099 (2018). [CrossRef]
9. Y. F. Zhang, C. P. Lin, C. R. Liao, K. M. Yang, Z. Y. Li, and Y. P. Wang, “Femtosecond laser-inscribed fiber interface Mach-Zehnder interferometer for temperature-insensitive refractive index measurement,” Opt. Lett. 43(18), 4421–4424 (2018). [CrossRef]
10. Y. Tian, B. Xu, Y. Chen, C. Duan, T. Tan, Q. Chai, J. J. C. Marti, J. Z. Zhang, J. Yang, and L. B. Yuan, “Liquid Surface Tension and Refractive Index Sensor Based on a Side-Hole Fiber Bragg Grating,” IEEE Photonics Technol. Lett. 31(12), 947–950 (2019). [CrossRef]
11. G. M. Noah, Y. Bao, and T. K. Gaylord, “Cross-sectional refractive-index variations in fiber Bragg gratings measured by quantitative phase imaging,” Opt. Lett. 45(1), 53–56 (2020). [CrossRef]
12. Y. Cho, F. Ahmed, H. E. Joe, H. Yun, B. K. Min, and M. B. G. Jun, “Fabrication of a screw-shaped long-period fiber grating for refractive index sensing,” IEEE Photonics Technol. Lett. 29(24), 2242–2245 (2017). [CrossRef]
13. Q. S. Li, X. L. Zhang, J. G. Shi, D. Xiang, L. Zheng, Y. Yang, J. H. Yang, D. Feng, and W. F. Dong, “An Ultrasensitive Long-Period Fiber Grating-Based Refractive Index Sensor with Long Wavelengths,” Sensors 16(12), 2205 (2016). [CrossRef]
14. Y. Gong, T. Zhao, Y. J. Rao, Y. Wu, and Y. Guo, “A ray-transfer-matrix model for hybrid fiber Fabry-Perot sensor based on graded-index multimode fiber,” Opt. Express 18(15), 15844–15852 (2010). [CrossRef]
15. M. S. Jiang, Q. S. Li, J. N. Wang, W. G. Yao, Z. W. Jin, Q. M. Sui, J. G. Shi, F. Y. Zhang, L. Jia, and W. F. Dong, “Optical Response of Fiber-Optic Fabry-Perot Refractive-Index Tip Sensor Coated With Polyelectrolyte Multilayer Ultra-Thin Films,” J. Lightwave Technol. 31(14), 2321–2326 (2013). [CrossRef]
16. Y. Q. Chen, F. Yu, C. Yang, J. Y. Song, L. H. Tang, M. Y. Li, and J. J. He, “Label-free biosensing using cascaded double-microring resonators integrated with microfluidic channels,” Opt. Commun. 344, 129–133 (2015). [CrossRef]
17. Y. S. Zhan, Y. L. Li, Z. Q. Wu, S. Hu, Z. B. Li, X. Y. Liu, J. H. Yu, Y. M. Huang, G. Y. Jing, H. H. Lu, H. Y. Guan, W. T. Qiu, J. L. Dong, W. G. Zhu, J. Y. Tang, Y. H. Lu, J. Zhang, and Z. Chen, “Surface plasmon resonance-based microfiber sensor with enhanced sensitivity by gold nanowires,” Opt. Mater. Express 8(12), 3927–3940 (2018). [CrossRef]
18. J. S. Chen, P. F. Chen, H. T. H. Lin, and N. T. Huang, “A Localized surface plasmon resonance (LSPR) sensor integrated automated microfluidic system for multiplex inflammatory biomarker detection,” Analyst 145(23), 7654–7661 (2020). [CrossRef]
19. S. Nishiuma, Y. Handa, T. Imamura, M. Ogino, T. Yamada, K. Furusawa, and R. Kuroda, “Localized surface plasmon resonant metal nanostructures as refractive index sensors,” Jpn. J. Appl. Phys. 47(3), 1828–1832 (2008). [CrossRef]
20. M. A. Najeeb, Z. Ahmad, R. A. Shakoor, A. M. A. Mohamed, and R. Kahraman, “A novel classification of prostate specific antigen (PSA) biosensors based on transducing elements,” Talanta 168, 52–61 (2017). [CrossRef]
21. M. Wang, H. Li, T. Xu, H. Zheng, M. Yu, G. Li, J. Xu, and J. Wu, “Probing bianisotropic biomolecules via a surface plasmon resonance sensor,” Opt. Express 26(22), 28277–28287 (2018). [CrossRef]
22. Y. Yanase, T. Hiragun, T. Yanase, T. Kawaguchi, K. Ishii, and M. Hide, “Application of SPR Imaging Sensor for Detection of Individual Living Cell Reactions and Clinical Diagnosis of Type I Allergy,” Allergol. Int. 62(2), 163–169 (2013). [CrossRef]
23. C. M. Miyazaki, D. E. Camilo, F. M. Shimizu, and M. Ferreira, “Improved antibody loading on self-assembled graphene oxide films for using in surface plasmon resonance immunosensors,” Appl. Surf. Sci. 490, 502–509 (2019). [CrossRef]
24. Q. Q. Meng, X. Zhao, S. J. Chen, C. Y. Lin, Y. C. Ding, and Z. Y. Chen, “Performance analysis of surface plasmon resonance sensor with high-order absentee layer,” Chin. Phys. B 26(12), 124213 (2017). [CrossRef]
25. J. K. Nayak, P. K. Maharana, and R. Jha, “Dielectric over-layer assisted graphene, its oxide and MoS2-based fibre optic sensor with high field enhancement,” J. Phys. D: Appl. Phys. 50(40), 405112 (2017). [CrossRef]
26. A. Lahav, A. Shalabaney, and I. Abdulhalim, “Surface plasmon sensor with enhanced sensitivity using top nano dielectric layer,” J. Nanophotonics 3(1), 031501 (2009). [CrossRef]
27. A. Lahav, M. Auslender, and I. Abdulhalim, “Sensitivity enhancement of guided-wave surface-plasmon resonance sensors,” Opt. Lett. 33(21), 2539–2541 (2008). [CrossRef]
28. A. K. Mishra and S. K. Mishra, “Gas sensing in Kretschmann configuration utilizing bi-metallic layer of Rhodium-Silver in visible region,” Sens. Actuators, B 237, 969–973 (2016). [CrossRef]
29. S. Ghosh and M. Ray, “Performance analysis of semiconductor based surface plasmon resonance structures,” Sens. Actuators, B 205, 298–304 (2014). [CrossRef]
30. A. Verma, A. Prakash, and R. Tripathi, “Sensitivity enhancement of surface plasmon resonance biosensor using graphene and air gap,” Opt. Commun. 357, 106–112 (2015). [CrossRef]
31. M. A. Otte, B. Sepulveda, W. H. Ni, J. P. Juste, L. M. Liz-Marzan, and L. M. Lechuga, “Identification of the Optimal Spectral Region for Plasmonic and Nanoplasmonic Sensing,” ACS Nano 4(1), 349–357 (2010). [CrossRef]
32. Q. S. Li, D. Xiang, Z. M. Chang, J. G. Shi, Y. H. Ma, L. Cai, D. Feng, and W. F. Dong, “Highly sensitive refractive index sensor based on a TiO2 nanowire array,” Appl. Opt. 56(7), 1930–1934 (2017). [CrossRef]
33. N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sonnichsen, and H. Giessen, “Planar Metamaterial Analogue of Electromagnetically Induced Transparency for Plasmonic Sensing,” Nano Lett. 10(4), 1103–1107 (2010). [CrossRef]
34. J. Henzie, M. H. Lee, and T. W. Odom, “Multiscale patterning of plasmonic metamaterials,” Nat. Nanotechnol. 2(9), 549–554 (2007). [CrossRef]
35. L. Dominici, F. Michelotti, T. M. Brown, A. Reale, and A. Di Carlo, “Plasmon polaritons in the near infrared on fluorine doped tin oxide films,” Opt. Express 17(12), 10155–10167 (2009). [CrossRef]
36. E. M. Hicks, X. Y. Zhang, S. L. Zou, O. Lyandres, K. G. Spears, G. C. Schatz, and R. P. Van Duyne, “Plasmonic properties of film over nanowell surfaces fabricated by nanosphere lithography,” J. Phys. Chem. B 109(47), 22351–22358 (2005). [CrossRef]
37. H. J. Chen, X. S. Kou, Z. Yang, W. H. Ni, and J. F. Wang, “Shape- and size-dependent refractive index sensitivity of gold nanoparticles,” Langmuir 24(10), 5233–5237 (2008). [CrossRef]
38. R. Li, X. K. Kong, S. B. Liu, Z. M. Liu, and Y. M. Li, “Planar metamaterial analogue of electromagnetically induced transparency for a miniature refractive index sensor,” Phys. Lett. A 383(32), 125947 (2019). [CrossRef]
39. F. Hao, Y. Sonnefraud, P. Van Dorpe, S. A. Maier, N. J. Halas, and P. Nordlander, “Symmetry Breaking in Plasmonic Nanocavities: Subradiant LSPR Sensing and a Tunable Fano Resonance,” Nano Lett. 8(11), 3983–3988 (2008). [CrossRef]
40. A. J. Cui, Z. Liu, J. F. Li, T. H. H. Shen, X. X. Xia, Z. Y. Li, Z. J. Gong, H. Q. Li, B. L. Wang, J. J. Li, H. F. Yang, W. X. Li, and C. Z. Gu, “Directly patterned substrate-free plasmonic “nanograter'‘ structures with unusual Fano resonances,” Light: Sci. Appl. 4(7), e308 (2015). [CrossRef]
41. N. A. Mirin, K. Bao, and P. Nordlander, “Fano Resonances in Plasmonic Nanoparticle Aggregates,” J. Phys. Chem. A 113(16), 4028–4034 (2009). [CrossRef]
42. S. Golmohammadi, A. Ahmadivand, and N. Pala, “Fano Resonances in Nanoshell Clusters Deposited on a Multilayer Substrate of beta-SiC/SiO2/Si to Design High-Quality Plasmonic Sensors,” J. Lightwave Technol. 33(13), 2817–2823 (2015). [CrossRef]
43. N. Verellen, P. Van Dorpe, C. J. Huang, K. Lodewijks, G. A. E. Vandenbosch, L. Lagae, and V. V. Moshchalkov, “Plasmon Line Shaping Using Nanocrosses for High Sensitivity Localized Surface Plasmon Resonance Sensing,” Nano Lett. 11(2), 391–397 (2011). [CrossRef]
