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Abstract
In order to broaden the sensing bandwidth of surface plasmon resonance (SPR) sensors, we propose and demonstrate a dual-channel SPR fiber optic sensor with wide bandwidth. The sensor is fabricated using no-core fiber (NCF), in which the film consists of a silver film and a ZnO film. The sensing characteristics are investigated by simulation and experiment. The resonance wavelength range of the SPR sensor can be significantly tuned by varying the thickness of the ZnO film. In the experiments, a dual-channel SPR sensor that can be used for simultaneous detection of temperature and refractive index was realized by cascading ZnO/Ag film with Ag film. The experimental results show that the two sensing channels are independent without crosstalk. The sensitivity of this sensor is 3512 nm/RIU in the range of 1.333 ∼ 1.385 and 4.6 nm/°C in the range of 0 ∼ 60 °C, which is better than most of the current dual-channel SPR sensors. In addition, the experimental results show that this sensor has good stability in use. The sensor proposed in this work has the advantages of a wide operating wavelength range, simple and compact structure, and high sensitivity. It has a broad application prospect in the simultaneous measurement of refractive index and temperature of liquids.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface plasmon resonance (SPR) sensors have the capability of fast detection, label-free analysis, and real-time detection, and it has attracted much attention for its promising applications in the field of optical sensing [1–3]. Among many SPR sensors, those based on fiber optic substrates have additional advantages, such as simple structure, miniaturization, easy integration, and the potential to form sensing networks [4–6]. SPR fiber optic sensors operate under total internal reflection conditions. When the incident light is transmitted in an optical fiber, part of the light is sent back to the core through reflection, while the rest is transmitted as evanescent wave near the metal film. When the metal film is thin enough, the evanescent wave will excite the free electrons on the metal surface to oscillate, thus forming a surface plasma wave. Therefore, when the momentum of the incident light is equal to that of the surface plasma wave, resonance will occur at a specific wavelength, and the spectrum will show a sharp decrease. This specific wavelength is called the resonance wavelength, which is very sensitive to changes in external refractive index (RI) and is widely used in RI sensing [7]. However, the magnitude of the RI can be perturbed by temperature, resulting in inaccurate data obtained. Currently, the mainstream solution is to measure both the RI and the temperature of the object to be measured at the same time to minimize the negative effect caused by temperature in the SPR signal.
SPR fiber optic sensors provide an effective solution to the temperature crosstalk problem. The solution integrates multiple sensing probes into a single optical path in a cascade manner and uses wavelength multiplexing to demodulate the SPR optical signal in different wavelength ranges. Therefore, multiple resonance wavelengths appear in the transmission spectrum, and environmental changes can be determined by monitoring the location of the resonance wavelengths [4–9]. However, the resonance wavelengths of conventional SPR fiber optic sensors are excited only in the visible wavelength band. For multi-channel sensors, multiple SPR valleys can only move within a limited range, which significantly limits the sensor's performance. To solve this problem, various methods have been proposed to broaden the wavelength range of SPR sensors so that the SPR valleys can move over a wide range. These effective methods can be broadly classified into two main categories: fiber structure optimization and thin film material modification. There are many methods for fiber structure optimization, including polishing [10–12], tapping [13], and using photonic crystal fibers (PCFs) with multiple air-hole structures [14–16]. These optimization methods can modulate the evanescent field, improve the SPR spectral range, and modulate the resonance wavelength. The essence of these methods is to change the angle of incident light in the optical fiber; however, the mechanical strength of the optical fiber is inevitably damaged during the optimization of the structure, and the implementation of complex post-processing techniques remains difficult. Therefore, in addition to structural optimization, using two-dimensional (2D) materials to regulate the resonance wavelength range is also a proven method. This method not only does not damage the mechanical strength of the fiber but also serves to prevent metal oxidation. Recent studies have shown that 2D materials such as graphene and phosphene can improve the sensitivity and bandwidth of SPR sensors thanks to their excellent optoelectronic properties, high carrier mobility, and tunable direct bandgap [17,18]. In addition, it has also been shown that the high carrier mobility and optical absorption of transition metal disulfides allow strong coupling of the evanescent field at the interface, which changes the resonance conditions and allows the resonance wavelength range to be tuned [19]. 2D materials with these properties also include metal oxides, of which ZnO is a strong candidate. ZnO is a strong candidate because it has a broadband direct bandgap compared to the above materials, with a bandwidth of about 3.3 eV at room temperature, and it is simple to prepare, low-cost, and easy to obtain [20]. In addition, ZnO benefits from its good chemical stability and does not need to be coated with other protective layers. In summary, ZnO is used in this work to modulate the resonance dip, thus achieving the purpose of broadening the sensing bandwidth and improving the performance of the sensor.
In this work, we investigated the role of ZnO in modulating the resonance wavelength of SPR sensors and proposed a broadband dual-channel fiber optic sensor. This sensor can be used to detect temperature and RI. The effect of ZnO thin films on the transmission spectrum of the SPR sensor was theoretically investigated to verify the feasibility of ZnO in modulating the resonance wavelength. The effect of different thicknesses of ZnO films on the sensing performance was experimentally investigated, and the optimal film thickness was determined. The results show that ZnO can broaden the sensing bandwidth by 44 nm. Finally, the proposed dual-channel sensor was tested for performance and stability. The test results show that the maximum sensitivity can reach 3512 nm/RIU and 4.6 nm/°C, respectively. The corresponding RI range is 1.333-1.385, and the temperature range is 0-60 °C. In addition, the test result show that the proposed sensor has good stability and repeatability. The novelty of this work lies in the broadening of the resonance wavelength range of the SPR sensor using ZnO film, and this simple tuning of the bandwidth provides new insights into the design of multichannel sensors.
2. Structure, preparation and principle of sensor
The structure of the proposed sensor is shown in Fig. 1(a), and the main part of the sensor is a no-core fiber (NCF), whose actual image is shown in Fig. 1(b). The diameter of the NCF is 125 um, which is simple in structure, low cost, and easy to obtain [21]. On the outer surface of the NCF, there is a layer of silver film used to excite the plasma wave, where the silver film thickness is 40 nm. The silver film is divided into two parts; one part is only coated with the silver for the RI detection, and the other has a ZnO film and a PDMS layer deposited on top of the silver film for temperature detection. The ZnO adjusts the resonance wavelength range, and its thickness is 70 nm. The PDMS layer is used to sense temperature changes and is a structurally stable solid temperature-sensitive material. At both ends of the sensor is a multimode fiber (MMF) for transmitting the SPR optical signals, which has a structure shown in Fig. 1(c), with a core diameter of 62.5 um and an optical fiber diameter of 125 um. This is a standard MMF with the same size and NCF, and it can be fusion-spliced without loss. The above is a detailed description of the proposed sensor structure and the role played by each material.
The preparation process of the sensor is described next. For SPR sensors, thin film deposition is a critical step [21]. During the experiment, magnetron sputtering was used to deposit silver and ZnO film. The whole process of coating should be carried out under high vacuum; therefore, after fixing the optical fiber, it is necessary to carry out the evacuation process first when the vacuum level reaches 6.6 × 10−4 Pa before flushing argon gas into the vacuum cavity shown in Fig. 2(a). Finally, the coating operation is carried out, and the flow chart is shown in Fig. 2(b). It should be noted that the magnetron sputtering machine is unable to control the thickness accurately, and the optimal sensing performance needs to be determined by controlling the deposition time. Finally, a scanning electron microscope (SEM) was utilized to observe the film thickness.
Figure 3(a) shows the dual-channel sensor preparation process. After depositing the silver film on the outer of two NCFs, a ZnO film was deposited on one of them. Subsequently, the two prepared sensor were cascaded and connected to the optical path of the multimode fiber. Finally, the outer surface of the ZnO is covered with a layer of PDMS, which consists of a mixture of silicone oil and curing agent at ambient temperature (25 °C) [16]. However, the RI of PDMS can be affected by volatile organic compounds (VOCs), which will cause measurement errors. Therefore, direct contact between PDMS and VOCs needs to be avoided. The above steps resulted in the proposed RI-temperature sensor. In addition, we used an optical microscope to photograph the outer surface of each part of the sensor, where Fig. 3(b) shows the outer surface of the NCF, which is transparent. Figure 3(c) shows the outer of NCF after silver coating, which has a metallic luster. Figure 3(d) shows the outer surface of Ag-based NCF after ZnO coating, which is grayish-white.
Fig. 3. (a) Sensor preparation flow. (b) NCF side view. (c) Ag-based NCF side view. (d) ZnO/Ag-based NCF side view.
The working principle of the sensor is presented next. Throughout, the finite element method (FEM) was used to analyze transmission spectrum of this sensor. The transmission spectra, whose horizontal coordinate is the wavelength and vertical coordinate is the optical loss, are calculated as [16]:
Δλ is the distance shifted by the dip and Δn is the change of the. In addition, the results also show that the ZnO film increases the optical loss, resulting in the enhancement of the resonance intensity. During the simulation, the dielectric constant of gold film and silver film can be expressed by Lorentz-Drude model, and its expression is as follows [15]:
In summary, the ZnO film aligns with our expectation that it can tune the resonance wavelength of the SPR sensor. This is thanks to its excellent optical properties. It has a bandgap of 3.3 eV, which is a good enhancement of the evanescent field and thus can enhance the resonance intensity and increase the optical loss. In addition, ZnO is a high RI oxide from the RI point of view. As shown in Fig. 5, it exhibits the variation of the effective RI of ZnO with wavelength [22]. As can be seen from the figure, the effective RI of ZnO is larger in the real part, so when the ZnO film is covered on top of the silver film, it is equivalent to boosting the RI of the object to be measured and therefore red-shifting the SPR valley.
Subsequently, the transmission spectrum of the SPR sensor was calculated. At this time, the external RI was set to 1.36, the temperature was set to 60 °C, and the simulation results are shown in Fig. 6. The results show that the transmission spectrum shows two resonance peaks at 589 nm and 803 nm, the principle of which can be explained by the mode field diagram in Fig. 6. The inset (a) shows the resonance mode of the RI channel, where the SPR effect is excited at the point when the effective RI of the surface plasmon polariton (SPP) mode excited by the plasma wave is equal to that of the fiber core mode. At this point, the energy in the fiber core is coupled to be transmitted near the silver film, resulting in a weakening of the output light intensity and a loss peak in the transmission spectrum. The inset (b) shows the resonance mode of the fiber core for the temperature sensing channel, and it can be seen from the electric field diagram that there is energy transmitted near the silver film. In addition, at non-resonance wavelengths, the energy is transmitted stably and independently in fiber core. In summary, we can determine the external environmental conditions by tracking the resonance wavelength positions of the two channels. The above is the working principle of the proposed sensor.
Next, the sensing performance of the proposed sensor was calculated, and the results are shown in Fig. 7, where (a) shows the transmission spectra of the sensor at different RIs (1.33-1.38) when the temperature was set to 60 °C. The simulation results show that the SPR valley in the short-wavelength band shifts with the change of the RI, while the one for the detected temperature does not, and the sensitivity was calculated to be 3900 nm/RIU. Figure 7(b) shows the transmission spectra of the sensor at different temperatures (20-60 °C) when the RI is set to 1.37. The simulation results show that the SPR valley in the long wavelength band moves with the change of temperature while the SPR valley for the detection of RI does not move. The temperature sensitivity was calculated to be 5.0 nm/°C. In summary, the proposed sensor can be used for temperature-dual parameter measurements, and there is no interference between the two channels.
3. Results and discussion
Next, we carried out experimental tests on the proposed sensor by connecting the prepared sensor to the optical path shown in Fig. 8, in which the light source is a halogen lamp, which is capable of emitting broadband optical signals from 200 to 2000 nm, the optical signal is transmitted into the sensor by a multimode patch cable. Then, the optical signal is output from the multimode patch cable. The spectrometer (USB 4000) is used to receive and demodulate the optical signal, and finally, the computer presents the transmitted Spectrum. The whole process of the experiment was done in a temperature-controlled box. Using this optical path, we implemented the following five experiments.
For SPR sensors, there exists an optimal thickness of the meta. Therefore, we first investigated the effect of the silver coating time on the transmission spectrum and determined the optimal coating time. As shown in Fig. 9, when the external RI is 1.333, and the transmission spectra of the sensor are at different silver coating times. The experimental results show that when the silver coating time is 195s, the SPR valley shows a sharp and narrow shape, which is suitable for sensing. If the silver coating time is too short, the condition of excitation resonance is not satisfied, and no obvious SPR valley appears in the transmission spectrum. If the coating time is too long, it will cause the silver film to be too thick, which will hinder the interaction between the evanescent wave and the equipartition wave, thus weakening the resonance strength. In this case, the SPR valley shows a wide and shallow shape, which is unsuitable for sensing, so the silver coating time is determined to be 195s.
Next, we determined the optimal NCF length, as shown in Fig. 10. It shows the transmission spectra of the sensor at different (1, 2, and 3 cm) NCF lengths when the RI is 1.333 and 1.405. The experimental results show that the NCF length does not affect the position of the resonance wavelength. According to Eq. (2), the sensitivity does not change with the NCF length. However, the length of NCF will change the shape of the SPR valley because the length of NCF determines the coating area of the silver film, and a larger area of the silver film will enhance the SPR effect. As a result, the SPR valley will become deeper and broader but too deep and wide. The SPR resonance valley is not favorable for detecting dual-channel sensors. Therefore, we chose the length of NCF as 2 cm. The length of NCF after cascade is 4 cm.
After determining the optimal silver coating time and NCF length, we tested RI sensing performance of the Ag-based NCF sensor. The test results are shown in Fig. 11, where (a) is the sensor's response to different RIs and (b) is the linear fitting relationship between the resonance wavelength and RI. The results show that the Ag-based RI sensor has a sensitivity of 3649 nm/RIU and a fitting coefficient 0.97351. In addition, it has a resonance wavelength range of 323 nm. In the subsequent study, we are committed to extending this value.
Fig. 11. (a) RI sensing performance of Ag-based NCF sensor. (b) Fitted relationship between RI and resonance wavelength.
Next, the effect of ZnO coating time on the resonance wavelength of Ag-based RI sensors was explored. The experimental results are shown in Fig. 12, (a) (b) (c) demonstrating the response of the sensor to different RIs with different coating times (3 min, 4 min, 5 min). From the experimental results, we can draw three patterns: firstly, the ZnO film can significantly broaden the resonance wavelength range of the Ag-based SPR NCF sensor, and the resonance wavelength range reaches up to 367 nm with the increase of the coating time. The bandwidth was expanded by 44 nm compared to the experimental results in Fig. 11(a). Secondly, the deposition of the ZnO film can enhance the sensor's sensitivity. In the RI range of 1.40-1.41, the maximal sensitivities of the sensor under the three coating times reach 7120 nm/RIU, 8223 nm/RIU, and 9560 nm/RIU, respectively. Finally, the deposition of ZnO film reduces the quality of SPR valleys, making them broader and deeper, which is unfavorable for spectrometer recognition. Therefore, to measure the full width at half maximum (FWHM) of the resonance wavelength range and the SPR valley, we took a compromise solution and determined the optimal time for coating the ZnO film as 4 min.
Fig. 12. Performance of Ag/ZnO-based NCF sensors with different ZnO coating times. (a) 3 min; (b) 4 min; (c)5 min.
In addition, to make the sensors have better reproducibility, we used cold-field SEM to measure the thicknesses of the silver and ZnO films, respectively. The test results are shown in Fig. 13, where (a) shows the morphology of the silver film under SEM, and the thickness of the silver film was measured to be about 40 nm; (b) shows the morphology of Ag/ZnO under SEM, and it is difficult to accurately measure the thickness of the film because ZnO is not conductive. In order to solve this problem, the sensor was sprayed with gold during the measurement process, and it can be seen that the outer surface of the sensor was covered with a layer of gold nanoparticles. The thickness of the ZnO film was detected to be about 70 nm.
After all the preparation parameters of the sensor were finalized, we subjected the sensor to cascade testing. The test result is shown in Fig. 14. The transmission spectrum of the cascade sensor is the black solid line, and two dips appeared in the spectrum when the external RI was 1.333 and the temperature was 10 °C. The blue dashed line is the spectrum of the RI sensor (Ag/NCF) when the outside RI is set to 1.333. The red line represents the spectrum of the temperature sensor (ZnO/Ag/NCF), when the outside temperature is set to 10 °C. The experimental results show that the resonance wavelength of the dual-channel sensor is consistent with that of the single-channel sensor. The cascade operation does not change the performance of the sensing and affects the accuracy of the sensor. Next, the sensors were tested for temperature and RI detection performance.
Fig. 14. Transmission spectra of the sensor at T = 0 °C RI = 1.333. Ag-based NCF (blue dashed line); Ag/ZnO/PDMS-based NCF (red dashed line); cascaded Ag and Ag/ZnO/PDMS-based NCF (black solid line).
First, the RI detection performance of the sensor was investigated. The sensor was immersed in this way into solutions with different RIs, and the transmission spectra were recorded each time the fixed external temperature was 0 °C. The experimental result is shown in Fig. 15(a), which exhibit the same regularity as the simulation results. The resonance valley in the long wavelength band does not move, and the resonance valley in the short wavelength band changes with the change of RI. The relationship between RI and resonance wavelength is shown in Fig. 15(b), and the linear sensitivity of the sensor is 2424 nm/RIU. The maximum RI sensitivity is 3512 nm/RIU, calculated by Eq. (3). Subsequently, the temperature detection performance of the sensor was investigated. The sensor was immersed in water with an RI of 1.333, and the temperature controller was adjusted so that the temperature was varied in the range of 0-60 °C. The transmission spectra were recorded every 10 °C. The experimental result is shown in Fig. 15(c), consistent with the simulation results; the resonance dip in short wavelength band does not move, and the resonance valley in the long wavelength band is red-shifted with the temperature increase. The fitted relationship between temperature and resonance wavelength is shown in Fig. 15(d), and the linear sensitivity of the sensor is 3.28 nm. The maximum temperature sensitivity is calculated by Eq. (2) to be 4.6 nm/°C. Above are the performance metrics of the sensor, which we compared with the recently published two-parameter sensors, and it can be seen from Table 1 that the proposed sensor outperforms the other dual-parameter sensors.
Fig. 15. (a) Test results of RI sensing performance of the sensor. (b) Relationship between RI and resonance wavelength fitted. (c) Test results of temperature sensing performance of the sensor. (d) Relationship fitted between temperature and resonance wavelength.
Table 1. Performance comparison of the proposed dual-channel sensor with other sensors
Finally, we tested the stability of the proposed sensor. Stability is the ability of a sensor to keep its performance unchanged after some time. During the experiment, the temperature and RI detection sensor’s performance was tested every 1 hour for five consecutive times. The experimental results are shown in Fig. 16, where (a) is the stability of the sensor for RI detection and (b) is the stability of the sensor for temperature detection. We recorded the maximum and minimum values of the resonance wavelength at the same RI, and the difference between the two was taken as the maximum error. The maximum error was calculated to be 2.147 nm, which proves that the sensor has good stability.
4. Conclusion
In this paper, we propose a method to broaden the resonance wavelength range of SPR sensors. A dual-channel sensor that can be used to detect temperature and RI is designed. The sensor employs a ZnO thin film to broaden the sensing bandwidth so that the sensitivity and detection range of the sensor are enhanced. The sensor was prepared using a cascade structure, with the structure of deposited Ag film for RI detection and the structure of deposited Ag/ZnO/PDMS for temperature detection. During the preparation process, we analyzed the effects of coating time and NCF length on the sensing performance. The experimental results show that the NCF length only changes the FWHM of the SPR valley. At the same time, the film thickness of ZnO can significantly affect the resonance wavelength range of the sensor. The thicker the ZnO film, the wider the resonance wavelength range. After determining the optimal parameters, the detection performance of the sensor was tested. The experimental results show that the maximum sensitivity of the sensor is 3512 nm/RIU and 4.6 nm/°C, respectively, and the sensor has excellent stability, with an experimental error of only 2.147 nm within five hours. This co-work demonstrates that the ZnO membrane can significantly broaden the sensor bandwidth, which provides a new direction for the application and design of high-performance fiber optic sensors.
Funding
National Natural Science Foundation of China (12074331, 42174162).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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