A novel refractive index (RI) sensor based on microfiber (MF) coated with gold nanowires has been proposed and theoretically investigated. Compared with gold film, the gold nanowires can significantly improve the performance of the MF sensor. The influence of the diameters of gold nanowire and microfiber on the sensing properties are investigated. For analyte RI ns = 1.33, the maximum sensitivity of 5200nm/RIU (DMF = 3μm, Dwire = 120nm) and the maximum figure of merit (FOM) of 150.38RIU−1 (DMF = 9μm, Dwire = 30nm) can be achieved. Both the sensitivity and the FOM will increase when the RI increases from 1.33 to 1.40. For ns = 1.40, an extremely high RI sensitivity of 12314nm/RIU (DMF = 10μm, Dwire = 50nm) can be obtained.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Surface plasmon resonance (SPR) is an optical phenomenon that occurs from free electron oscillation between metallic surface and dielectric layer under a p-polarized light radiation when the phase matching condition is satisfied. SPR sensor has been shown to play an important role in monitoring and prevention of disease, food safety, environmental pollution, biological, military, and medical treatment. In the past decades, different kinds of SPR sensors based on prism [1,2], slot waveguide , and V-groove waveguide  have been fabricated. However, the conventional SPR sensors suffer from many defects such as expensive, narrow range of applications and hard fabrication.
In 1993, optical fiber based on SPR has been first proposed by Jorgenson where a section of the fiber cladding was removed and a thin gold layer was deposited onto the fiber core to excite SPR . This sensor possesses simple preparation, special optical properties, and all fiber miniaturization. Since then, many different structures for optical fiber SPR sensors have been reported, such as microfiber structure , tapered structure , side-polished structure , fiber Bragg grating , long-period fiber grating [10,11], and Photonic Crystal Fiber . Among these structures, photonic crystal fiber (PCF)-based SPR sensors have been extensively studied due to their unusual and appealing optical characteristic such as compact structure, fast response, and efficient light-controlling capabilities . Until now, PCF SPR sensors with microfluidic slot-based structures, external metal-coated, long-period fiber Bragg grating, and internal metal-coated, and D-shaped structures have been reported . Recently, Rifat et al. designed a novel PCF SPR sensor by introducing a large cavity into the first ring of the PCFs . A high wavelength sensitivity of 11000nm/RIU and a 10−6 scale sensor resolution in the sensing range of 1.33–1.42 are predicted .
In addition, microfiber tapered to micrometer range shows advantages owing to its low propagation loss, low price cost as well as easy to batch production. And microfiber with small size, large surface volume ratio, and strong evanescent field shows high sensitivity, fast responding and low detective limitation to the outside environment. Therefore, microfiber achieves the vital application in optical sensing. For instance, Chiam et al. employed an optical microfiber sensor coated with conducting polymer to detect alcohol . Due to the strong evanescent field of microfiber, the sensor has higher sensitivity compared with the traditional alcohol detection sensor . Li et al. designed a refractive index sensor based on optical micro/nanofibers coated by Au nanoparticles . By exploiting the evanescent field of microfiber, the sensor can obtain a detection limit of 1 pg/mL .
For most SPR fiber sensors, the sensitive layer on the surface of fiber is metal film. Tabassum et al. studied the effects of different oxides on the surface of the bimetallic (Al/Cu) layer on the performance of the SPR sensor . Although they found that the sensitivity and figure of merit (FOM) of the sensor can be improved, the structure is complex. Wieduwilt et al. focused on the optical fiber micro-taper with circular symmetric gold coating for sensor . The use of circularly symmetric gold film allows the plasmon resonance in the fiber taper not to depend on the polarization of the incident light. The disadvantage is that the original ratio of the core/cladding radius remains constant during tapering. Recently, microfiber sensor with nanoscale metal nanowires has been proposed and attracted much attention . Comparing to metal film, nanoscale metal nanowires possess strong absorbing ability around visible and near-infrared light as they are small enough to confine their electrons and produce quantum effects . Luan et al. studied SPR temperature Sensor based on photonic crystal fibers randomly filled with silver nanowires . The filled nanowires can support resonance peaks which are extremely sensitive to temperature. Santos et al. proposed a gold wire partially incrusted on the surface of a D-type fiber . Their numerical results show that the use of the gold wire provides a higher sensitivity, compared with a conventional D-type fiber coated with a gold film .
To fully take the advantages of microfiber and nanowires, we design a gold nanowires coated-microfiber (GNC-MF) sensor. A systematically comparison between the GNC-MF sensor and the conventional gold film coated-microfiber (GFC-MF) sensor has been made. It is found that both the refractive index (RI) sensitivity and figure of merit FOM can be significantly improved by the gold nanowires owing to the localized SPR.
2. Simulation model and theoretical simulation
The three-dimensional schematic of the structure of the GNC-MF SPR sensor is illustrated in Fig. 1(a), with its cross-section in Fig. 1(b). As shown in Fig. 1(a), gold nanowires are arranged tightly around the surface of MF, where these nanowires are parallel to MF.
Owing to the symmetry of the cross section of proposed sensor, the simulation region can be reduced to a quarter of the cross section (see Fig. 1(c)) to improve the efficiency of calculation. In order to ensure the structural symmetry, the number of nanowire must be multiple of 4. According to geometric relationship shown in Fig. 1(e), the central angle in right-angled triangle is calculated as follow:22], as shown in Fig. 1(c). By doing so, only TE mode can be excited although both TE and TM modes can be supported by our model. To ensure the sufficient accuracy, free-triangle meshing is used and the minimum element size is set to be 1nm. For the microfiber, the refractive index of fused silica is 1.45. And the dielectric constant of the gold nanowire is described by the Drude model :22], and λ is wavelength of light in vacuum.
To give a physical insight into the sensing mechanism for the proposed sensor, the dispersion relations of MF-guided mode and plasmon mode are calculated by FEM. The calculation results for an analyte RI of 1.33 are shown in Fig. 2 with the black and red lines denoting the MF-guided and plasmon modes, respectively. As shown by inset of Fig. 2, the optical field of MF-guided mode predominantly concentrated in the fiber (see inset (a)), while that of the plasmon mode mainly distributes in the gold nanowire (see inset (b)). When the wave vector of surface plasmon mode is equal to that of the guided mode of MF, i.e., phase matching condition, the sensor will generate plasma resonance. And two dispersion relation curves intersect at a resonant wavelength of 591nm, where much energy of the MF-guided mode will couple into the plamson mode. The energy coupling is evidently seen in the imaginary part of effective refractive index of the core mode, which reaches its maximum at 591nm, as shown in Fig. 2(c). With the RI variation, the intersection position of the two dispersion curves will change accordingly and thus leads to the shift of resonant wavelength. Whereby, the shift of resonant wavelength is used to detect the analyte RI.
3. Results and discussion
Transmission loss of the SPR mode is determined by the imaginary part of the effective refractive index (Im(neff)) of the core mode. Therefore, the loss is :
When the analyte RI varies, the wavelength of the loss peak will shift, due to the change of the phase matching condition between the MF-guided and plasmon modes. Therefore, the analyte RI can be detected by tracing the wavelength shift of the loss peak. One of important parameters to assess the performance of the sensor is sensitivity, which is defined by:
Although high sensitivity is desirable for a SPR sensor, larger full width at half maximum (FWMH) of the loss peak will leads to larger uncertainty in tracing the loss peak and thus a lower resolution. To access comprehensively the performance of the SPR sensor, another important parameter called FOM is necessary for the assessment of the SPR sensor. The definition of FOM is given by:
In the following, we compared two different types of microfiber based SPR sensors: GNC-MF and GFC-MF. For comparison, the diameter of gold nanowire in GNC-MF (Dwire) is equal to the thickness of the gold film (Tfilm) of GFC-MF, which ranges from 10nm to 120nm. In the simulations, the diameters of the microfiber are 3.0μm~10.0μm.
3.1 Comparison of electric light fields on the gold surfaces of the GNC-MF and GFC-MF sensors
The light field distributions of GNC-MF and GFC-MF at resonance are shown respectively in Fig. 3(a) and (b), where the diameter of MF is 6μm and Dwire = Tfilm = 60 nm. Comparing Fig. 3(a) and Fig. 3(b), one finds that the electric field strength on the surface of gold nanowires is higher than that of gold film. To get a more intuitive view of the local plasma enhancement, we plot the normalized field strengths along the y-axis. As shown by Fig. 3(c), the normalized field strength at the surface of gold nanowires is about 1.43 times higher than that of the gold film, which indicates that the more energy of the core is coupled into the surface plasmon of gold nanowires. This is evident from the energy loss of MF-guided mode. The transmission loss spectra of the GFC-MF (black) and the GNC-MF for analyte RI ns = 1.33 and 1.34 are shown in Fig. 3(d), where the simulation results (marks) are interpolated by B-Spline method (solid curves) in order to obtain the peaks and FWHM of the transmission loss. As shown in Fig. 3(d), when ns = 1.33, the maximum propagation loss of GNC-MF is 27000 dB/m at the resonance peak, which is higher than the propagation loss of 11000 dB/m for GFC-MF. And the GNC-MF has a narrower loss peak comparing to the GFC-MF . With the refractive index increases to 1.34, the resonance wavelength shifts towards the longer wavelength which results in the redshift. A higher transmission loss of 33000 dB/m appears for GNC-MF, which is about 2.5 times higher than that of the GFC-MF (13000 dB/m) under the same refractive index. The data of transmission loss shows that the increase in loss of GNC-MF at ns = 1.33 and 1.34 is great than that of GFC-MF, which indicates that the GNC-MF is more sensitive to surrounding analyte. By calculating the sensitivity of the sensor, the sensitivity of GNC-MF (3000 nm/RIU) is higher than that of GFC-MF (2900 nm/RIU). Therefore, gold nanowire structure can improve sensor performance compared to gold film structure.
3.2.1 Influence of the microfiber diameter on the sensitivities and FOM for the GNC-MF and the GFC-MF sensors
The diameter of the MF is critical the sensing performance since it determines the strength of the evanescent wave and thus influences the coupling efficiency between the MF-guided and the plasmon modes. Figure 4(a) and (b) show the transmission loss spectra of GNC-MF and the GFC-MF with the diameter being DMF = 4.0, 5.0, 6.0 and 7.0μm, respectively. Figure. 4(c) and (d) plot the dependences of loss peaks, sensitivity, and FOM changing with the MF diameter, respectively. With the decrease in DMF, the loss peak will increase and the resonant wavelength of the loss peak will red-shift. This is because with the increase of the MF diameter, the real part of the effective index of the MF-guided mode decreases, causing the intersection point of the dispersion curves (see Fig. 2) between the MF-guided and plasmonic modes move to the long wavelength . Meanwhile, the increase of the MF diameter will result in weaker plasmonic mode, thus the decrease of loss peak. As shown by Fig. 4(c), the loss peaks of GNC-MF sensors are always higher than those of GFC-MF sensors, resulting from the enhancement of the SPR by the Au nanowires.
Figure 4(d) shows the dependences of the sensitivities and FOM of the SPR sensors on the MF diameter when the analyte refractive index is 1.33. When the diameter of the MF increases, the sensitivity and FOM of two SPR sensors will decrease. Both the sensitivities and FOM of the GNC-MF sensors are always higher than those of GFC-MF sensors.
3.2.2 Influence of gold layer thickness on the sensitivities and FOM of the GNC-MF and GFC-MF sensors
In the following, we investigate the influence of the diameter of gold nanowire on the GNC-MF sensor, and make a comparison to the GFC-MF sensor. When the gold nanowire diameter increases, the resonant wavelength of GNC-MF sensor will red-shift and the transmission loss at the peak will decrease (see Fig. 5(a)). The red-shift phenomenon of the resonant wavelength is caused by variance of the real part of the effective index of the plasmon mode . For gold film structure, the resonant wavelength will red-shift with the film thickness increasing from 50 nm to 70 nm. When the film thickness increases further to 80 nm, the resonant wavelength keeps unchanged . As shown by Fig. 5(c) and (d), GNC-MF sensors have higher loss peak, larger sensitivity and FOM compared to the GFC-MF sensors.
3.2.3 Comparison of the GNC-MF and GFC-MF sensors on the sensitivity and FOM
In the following, we will compare the sensitivity and FOM between the GNC-MF and GFC-MF sensors systematically. Figure 6 shows the dependences of sensitivity on the diameters of gold nanowire (thickness of Au film) for different MF diameters. For each MF diameter, the GNC-MF sensor with gold nanowire shows advantage when the diameters of gold nanowire larger than ~50 nm. For each MF diameter, we can obtain a maximum sensitivity of two different structures. The specific results are shown in Table 1, from which one finds that the maximum sensitivities of the GNC-MF sensor are always higher than those of GFC-MF sensor with the MF diameter ranging from 3μm to 10μm. And the highest sensitivity of the GNC-MF sensor is 5200nm/RIU (DMF = 3μm, Dwire = 120nm), higher than the PCF SPR sensor of 2000 nm/RIU .
In order to accurately measure the enhancement effect on the sensitivity of the SPR sensor, we introduce a sensitivity enhancement factor:Table 1 one can find that, the gold nanowire can enhance the maximum sensitivity of the SPR sensor by 13.5%~36.8%.
Figure 7 shows the FOM of two types of sensor under analyte with RI of 1.33, where the solid curves from the B-spline interpolation deviate from the marks owing to the sparseness of the simulation points. For each MF diameter, the FOM of GNC-MF sensor will be larger than that of GFC-MF, when the thickness of the gold layer is larger than a certain value. Similarly, a FOM enhancement factor is introduced as:Table 2 that the maximum FOM of the GNC-MF sensor are always higher than those of GFC-MF sensor. The maximum enhancement factor is 18.5%.
3.3 Influence of the refractive index on the sensitivities and FOM of the GNC-MF and GFC-MF sensors
Figure 8 shows the transmission loss spectra of the GNC-MF and the GFC-MF sensors under different analytes with RI increasing from 1.33 to 1.41. It is found that the loss peak shifts toward long wavelength and becomes higher as the analyte RI increases. The higher loss results from that the evanescent wave of the MF will become stronger for a larger RI, which leads to the stronger coupling between the MF-guided mode and the surface plasmon mode. Comparing with the gold film structure, gold nanowire structure has stronger loss, which indicates that more electric field has been coupled into the gold nanowire.
Based on the transmission loss spectra, the sensitivity and FOM for different analyte RIs can be calculated according respectively to Eq. (5) and (6), and the numerical results are shown in Fig. 9(a) and (b). The sensitivities increase with analytes RI. The optical field distribution of the SPR mode of the GNC-MF and the GFC-MF sensors for different analytes RIs are shown in Fig. 9(c) and (d), respectively. It can be seen that the guided mode inside the MF becomes weaker as the analyte RI increase. And more energy inside the MF is transferred to the plasmon mode in gold. The maximum sensitivity of the GNC-MF sensor is up to 12314 nm/RIU, which is almost two times higher than that of SPR sensor based on four-hole grapefruit microstructure fiber (7400nm/RIU) . For the GFC-MF sensor, the sensitivities are lower than that of the gold nanowire structure in the RI ranging from 1.33 to1.40. As show in Fig. 8(b), the FOM of two structures will increase with the RI and the maximum FOM of 209 RIU−1 for the GNC-MF sensor is achieved at ns = 1.40, which is about 1.4 times higher than that of the GFC-MF sensor (149 nm/RIU).
For fabricating the proposed microfiber in this paper is using the traditional way which a single mode fiber (the core diameter of 8μm and a cladding diameter of 125μm from Corning Inc.) by heating with a flame, then the fiber is elongated at a drawing speed of 0.1 mm/s . Covering a uniform gold layer on the microfiber requires a special coating technique. Fortunately, a DC magnetron sputtering technique has been demonstrated, which can realize circular symmetric gold coating on optical microfiber effectively . Compared to gold film deposition, gold nanowires coated microfiber is more challenge. A self-assembly technique could be used to coat single-layer gold nanowires over microfiber by combining the dip-coating and self-assembly evaporation techniques , because the optical fiber in present work has a typical diameter of several micrometers, which is much larger than the diameter of the gold nanowires, allowing the curvature of the substrates having little impact on the self-assembly. In addition, the interaction between nanowires for self-assembly, including the van der Waals and electric double layer interactions, plays the weaker role in the alignment induced by the drying induced internal flow . Therefore, the single-layer gold nanowires could be aligned uniformly on microfiber by properly controlling the temperature, humidity and withdraw speed during the fabrication process . The self-assembled gold nanowires may exist interspaces between them, so that analyte can infiltrate into the gaps between the microfiber and the gold nanowires by the capillary effect since the sizes of these gaps are of several nanometers .
We have shown that the GNC-MF sensor with nanowire structure performs better in the analyte RI sensing, after a systematically comparison between the GNC-MF and GFC-MF sensors. The RI sensitivity and FOM depend strongly on the sizes of microfiber and gold nanowire. For the case of analyte RI ns = 1.33, the maximum sensitivity and FOM are respectively 5200nm/RIU (DMF = 3μm, Dwire = 120nm) and 150.38RIU−1 (DMF = 9μm, Dwire = 30nm), which increases respectively by 36.8% and 9.2% compared with GFC-MF sensor. For ns = 1.40, an extremely high RI sensitivity of 12314nm/RIU and high FOM 209 RIU−1 can be obtained for GNC-MF sensor. The FOM of GNC-MF sensor is about 1.4 times higher than the GFC-MF sensor (149nm/RIU). Therefore, our GNC-MF sensor has potential applications in biological and biochemical analyte detections.
NSFC (No.61675092, 61705086, 61275046, No.61361166006, 61475066, No.61405075, 61401176, 61505069, No.61575084, 11604050); NSFGP (2017A030313375, 2015A030306046, 2015A030313320, 2016A030311019, 2016A030313079, 2016A030310098); STPGP (2015A020213006, 2015B010125007, 2016B010111003, 2016A010101017, 2016TQ03X962, 2014B010120002,2014B090905001); PGHE (YQ2015018); STPG (201707010396, 201605030002, 201607010134, 201704030105, 201604040005); RTHOCICZ(55560307).
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