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Abstract
As the sensitivity of the optical temperature sensor increases, one of the most important noise sources may be from the fluctuation of the surrounding gas environment (refractive index change). In this work, we have designed and fabricated an optical temperature sensor with a device size of 15 μm2. The sensor is constructed by a titanium dioxide grating on top of a double-metal surface plasmon resonance (SPR) structure. Our design can provide minimal gas environment dependence without compromising the performance in terms of temperature sensitivity. In addition, the design also facilitates a generous dimensional tolerance in the device fabrication. Based on the design, a proof-of-concept device has been fabricated and characterized. The obtained sensitivity of the fabricated sensor reaches 135 pm/℃. Meanwhile, the measured resonance wavelength shift is ∼0.004 pm in different gases (air, CH4, and CO2). The presented temperature sensor should be convenient and valuable for high-accuracy temperature measurements and integrated opto-electronic sensing chips.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Accurate measurement of the temperature plays a very important role in various applications, including medical, agriculture, industrial, aerospace, and automotive [1–3]. Traditionally, thermocouple, thermistor, and resistance temperature sensors have been widely used [4,5]. However, these types of sensors are not well suited to the environment of electromagnetic disturbances and inflammable atmospheres [6]. In contrast, optical temperature sensors are considered to be a promising candidate in the harsh environment due to their immunity to electromagnetic interference, passive operation, and real-time monitoring [7,8].
Among optical temperature sensors, the surface plasmon resonance (SPR) technique has attracted growing interest due to its extremely high sensitivity [9]. The SPR is the resonant oscillation of conduction electrons at the interface between a metal film and a dielectric medium [10]. A tiny temperature change of the sensing environment may cause an obvious variation of the resonant spectra. Nevertheless, SPR cannot be excited by a light directly incident from the air onto the metal-dielectric interface, because energy and momentum conservation cannot be satisfied simultaneously [11]. This issue can be solved by using the so-called Kretschmann configuration [12]. By using the prism-based Kretschmann configuration, the temperature dependence resonance wavelength / angular shift has been analyzed and high sensitivity has been obtained [13–16].
To reduce the device footprint, optical waveguides and metal-gratings have been proposed to replace the prism. In the waveguide scheme, a temperature-dependent wavelength shift (TDWS) of 120 pm/℃ has been realized with a 900 nm long waveguide array [17]. For the metal-gratings, a temperature-dependent angular shift of 0.02 deg/℃ has been realized [18]. These waveguide array and metal-grating SPR sensors are not only sensitive to the temperature change but also quite sensitive to the refractive index change of the surrounding media. This feature provides the possibility of simultaneous measurement of the refractive index and temperature [17–19]. However, in some applications, the SPR sensors are required to be only sensitive to temperature changes. For example, gas sector and coal mine are desirable to have temperature sensors with a high-accuracy but immunity to environmental gas fluctuation [20,21]. If SPR sensor is temperature-dependent but gas fluctuation independence can be realized, it should be convenient and valuable for special situation sensing.
In this work, we demonstrate an optical temperature sensor based on a double-metal SPR structure. The double-metal SPR can not only induce a high quality (Q)-factor, but also can confine 92% of the light in the thermal-optic polymer layer while negligible light can skip the sensor. As a result, the designed sensor is able to offer high-temperature sensitivity and gas environmental independence. The achieved design also exhibits sufficient fabrication tolerance of ±40 nm. In the fabricated device, the measured temperature dependence wavelength shift reaches 135 pm/°C, while achieving a resonance shift of ∼0.004 pm in air, CH4, and CO2 gas.
2. Double-metal SPR sensor design
The designed double-metal SPR temperature sensor is shown in Fig. 1(a). The device relies on a polymer (polycarbonate) / silver (Ag) / silicon (Si) substrate cladding with a thin gold (Au) film and a TiO2 grating. The thickness of each layer is given as: Au 50 nm, polymer 2 μm, and Ag 200 nm. The period, width, and depth of the TiO2 grating are set as Λ, W, and D for the performance optimization.
Fig. 1. (a) Schematic diagram of the SPR sensor, and (b) reflected spectrum (blue curve) of our designed sensor when D = 200 nm, W=555 nm, and Λ=925 nm. For comparison, a traditional single-metal-SPR structure is also simulated (red curve). Our design has a higher Q-factor to realize a higher temperature sensitivity.
Figure 1(b) depicts the simulated reflected spectrum when D=200 nm, W=555 nm, and Λ=925 nm. In the simulation, a TM-polarized light is vertically incident on the TiO2 grating. The refractive index of TiO2, polymer, and Si is set as 2.25, 1.6, and 3.47, respectively. The refractive index of Au and Ag is 0.59-10.92i and 0.15-11.57i. It can be observed that two SPR modes are appearing at 1521 and 1546 nm, respectively. Both resonances have almost the same extinction ratio (ER) of 9 dB. The ER is defined as the lowest reflection of the resonance in this work. The Q-factor was also calculated and found to be 360 for both resonances using the equation Q-factor=λres/FWHM, where λres is the resonance wavelength, FWHM is the full width at half maximum of the resonance. For comparison, the reflected spectrum of a traditional single-metal-SPR structure is also simulated (similar structure as depicted in Fig. 1(a), but without bottom Ag). There is one resonance mode with a Q-factor of 189 and ER of 7 dB. To enhance the sensitivity, SPR with a high Q factor is preferred [22]. The Q-factor of our design is larger than those of the traditional case, so that a high-temperature sensitivity can be expected.
Figure 2 shows the optical distributions of the two resonance modes simulated by the FDTD method. We can see that most of the light is confined in the polymer layer due to the surface plasmon resonance. Since the polymer used in this work (polycarbonate) has a high thermal-optic coefficient of 0.9×10−4/°C [23,24], a little temperature fluctuation will lead to a change in the refractive index of the polymer and hence a resonance wavelength shift of the SPR. The surrounding environment can be seen as a part of device when the sensor works. According to the simulations, the optical confinement factor is 92% in the polymer, 5% in the TiO2, and 3% in the surrounding environment. Due to only 3% light in the surrounding environment, the designed SPR sensor is almost independent of the surrounding gas type or concentration variation.
Fig. 2. Optical distribution of the two resonances: most light is confined in the polymer layer and little light in the surrounding environment, contributing to high sensitivity and environmental independence. The surrounding environment is air in this simulation.
For a nano-photonic device, a generous dimensional tolerance gives us more flexibility in the device fabrication. In order to investigate the tolerance, the influence of the TiO2 grating depth D and width W on resonance properties has been calculated. In the calculation, the TiO2 grating period Λ is fixed as 925 nm to enable the two resonance wavelengths to be around 1550 nm. Figure 3(a) shows the resonance wavelength versus the TiO2 depth D when width W=555 nm. We can see that the resonance wavelength keeps constant as the D ranging from 160 to 220 nm. Similarly, we also find that the change of W as 555 ± 40 nm has little influence on the resonance wavelength when D=200 nm, as shown in Fig. 3(b). In the fabrication process, the resolution of the electron-beam lithography is usually ± 10 nm [25]. As a result, the insensitivity of the resonance wavelength to D and W of the TiO2 grating predicts the excellent robustness of the device against fabrication imperfection.
Fig. 3. Simulated reflection spectra: (a) grating depths (D) varying from 160 to 220 nm and (b) widths (W) from 515 to 595 nm. The insensitivity of the resonance wavelength to D and W predicts the excellent robustness of the device against fabrication imperfection.
Figure 4(a) shows the simulated reflected spectra as a function of the temperature, where both resonances undergo a blue shift with the increase of temperature. There is a blue shift because the refractive index of the polymer decreases with the increase of temperature. Based on the spectra in Fig. 4(a), the estimated temperature-dependent wavelength shift Δλi/ΔT (TDWS) is 140 pm/℃. Subsequently, the stability of the designed temperature sensor is examined via changing the surrounding environment. Figure 4(b) shows the simulated reflected spectra in different surrounding environments with a refractive index difference Δn=0.1. From the results, we can obtain that the resonance wavelength shift is less than 1 pm when the refractive index has a change of 0.1.
Fig. 4. (a) Simulated reflected spectra as a function of the temperature, and (b) double resonances shift with different refractive indices of the surrounding environment.
3. Double-metal SPR sensor fabrication and measurement
The fabrication process of the SPR sensor is illustrated in Fig. 5(a): (1) A 200 nm thick Ag was deposited onto the silicon substrate by the thermal evaporation; (2) Polymer was spin-coated and baked at 120℃ for 48 hours to form a 2 μm thick film. (3) A 50 nm thick Au was deposited by the thermal evaporation; (4) A 200 nm thick TiO2 film was RF sputtered onto the Au layer with a sputtering speed of 5 nm/min; (5) TiO2 grating was patterned by electron beam lithography; (6) TiO2 was etched (SAMCO, RIE-400iPB) to form the grating by CHF3 and O2 gases. The scanning electron micrograph (SEM) image of the SPR sensor is exhibited in Fig. 5(b).
We carried out a proof-of-concept experiment to demonstrate the ability of the proposed sensor. The schematic diagram of the optical arrangement is shown in Fig. 6. Light from a supercontinuum source (SC5, Yangtze Soton Laser) was collimated by an iris and its polarization was controlled by a polarizer. After passing a beam splitter, the light was focused by a lens onto the SPR sensor with a spot size of ∼15 μm2. The reflected light from the sample was refracted by the beam splitter and then collected by a multimode fiber connected to the optical spectrum analyzer (OSA) (AQ6370D, Yokogawa) to obtain the reflected spectra. During the measurement, the sample was placed on the heating stage in order to test the TDWS between 30 and 70 °C. The sample and the stage were placed in a chamber to control the gas environment surrounding the SPR sensor.
4. Experimental results
Figure 7(a) shows the reflected spectra of the SPR sensor measured at 30, 50, and 70 ℃. The spectra have been fitted using the Lorentz function according to the measured points. In the spectra, two resonances are observed with an extinction ratio of ∼ 10 and 8 dB and a Q-factor of 304 and 291. From 30 to 70 ℃, the resonance peaks have little change in the Q values and extinctions. In Fig. 7(b), the linear fitting of resonance wavelengths to the various temperatures was obtained, indicating that the TDWS of the SPR sensor was 130 and 135 pm/°C for the 1521.8 and 1543.3 nm resonances. The experimental TDWS are consistent with the theoretical expectation that the designed SPR sensor has a relatively high-temperature sensitivity.
Fig. 7. (a) Experimental spectra at various temperatures, (b) double-resonance wavelengths as a function of temperature.
The stability of the device was characterized by placing the device in different surrounding gas conditions. The measured reflected spectra in air, CH4, and CO2 gas environments are shown in Fig. 8. The resonance wavelength shift in different gases was measured to be ∼0.004 pm, corresponding to ∼1.5 pm with a surrounding refractive index change of 0.1.
5. Conclusion
We have investigated a double-metal SPR temperature sensor. High sensitivity is achieved by generating a high Q-factor and confining most of the light in the thermal-optic polymer. Minimal gas environment dependence comes from little light leaking out of the device. The measured TWDS is 135 pm/ °C from 30 to 70 °C and the resonance shift is ∼0.004 pm in different gases (air, CH4, and CO2), showing good agreement with the simulated results. Combining with its compact footprint of 15 μm2 and generous fabrication tolerance, the presented sensor may be attractive for integrated sensing chips in some special situations, such as gas sector, coal mine, and so on.
Appendix
In Table 1, we compare the performance of our double-metal-SPR temperature sensor with prior works, all of which have recently demonstrated high-sensitive properties. From Table 1, it can be seen that the double-metal-SPR temperature sensor has the potential of being applied in a temperature-dependent but gas fluctuation independent environment.
Table 1. Comparison with the prior works
Funding
National Natural Science Foundation of China (62075184).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
References
1. S. Jiang, Y. Peng, B. Ning, J. Bai, Y. Liu, N. Zhang, and Z. Gao, “Surface plasmon resonance sensor based on molecularly imprinted polymer film for detection of histamine,” Sens. Actuators, B 221, 15–21 (2015). [CrossRef]
2. M. Vala, K. Chadt, M. Piliarik, and J. Homola, “High-performance compact SPR sensor for multi-analyte sensing,” Sens. Actuators, B 148(2), 544–549 (2010). [CrossRef]
3. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef]
4. J. D. Nash, D. R. Caldwell, M. J. Zelman, and J. N. Moum, “A Thermocouple Probe for High-Speed Temperature Measurement in the Ocean,” J. Atmos. Ocean. Technol. 16(11), 1474–1482 (1999). [CrossRef]
5. A. A. Khan, “An Improved Linear Temperature/Voltage Converter Using Thermistor in Logarithmic Network,” IEEE Trans. Instrum. Meas. IM-34(4), 635–638 (1985). [CrossRef]
6. A. Tapetado, P. J. Pinzon, J. Zubia, and C. Vazquez, “Polymer Optical Fiber Temperature Sensor With Dual-Wavelength Compensation of Power Fluctuations,” J. Lightwave Technol. 33(13), 2716–2723 (2015). [CrossRef]
7. H. Wang, P. Xiang, and L. Jiang, “Optical Fiber Sensor Based In-Field Structural Performance Monitoring of Multilayered Asphalt Pavement,” J. Lightwave Technol. 36(17), 3624–3632 (2018). [CrossRef]
8. I. Hernández-Romano, M. A. Cruz-Garcia, C. Moreno-Hernández, D. Monzón-Hernández, E. O. López-Figueroa, O. E. Paredes-Gallardo, M. Torres-Cisneros, and J. Villatoro, “Optical fiber temperature sensor based on a microcavity with polymer overlay,” Opt. Express 24(5), 5654–5661 (2016). [CrossRef]
9. Z. Yang, J. Xia, S. Li, R. Qi, G. Zuo, and W. Li, “Ultrawide temperature range operation of SPR sensor utilizing a depressed double cladding fiber coated with Au-Polydimethylsiloxane,” Opt. Express 28(1), 258–269 (2020). [CrossRef]
10. M. Piliarik and J. Homola, “Surface plasmon resonance (SPR) sensors: approaching their limits?” Opt. Express 17(19), 16505–16517 (2009). [CrossRef]
11. A. Otto, “Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection,” Z. Phys. A: Hadrons Nucl. 216(4), 398–410 (1968). [CrossRef]
12. E. Kretschmann and H. Raether, “Notizen: Radiative Decay of Non Radiative Surface Plasmons Excited by Light,” Z. Naturforsch. A 23(12), 2135–2136 (1968). [CrossRef]
13. S. K. Ozdemir and G. Turhan-Sayan, “Temperature effects on surface plasmon resonance: design considerations for an optical temperature sensor,” J. Lightwave Technol. 21(3), 805–814 (2003). [CrossRef]
14. L. J. Davis III and M. Deutsch, “Surface plasmon based thermo-optic and temperature sensor for microfluidic thermometry,” Rev. Sci. Instrum. 81(11), 114905 (2010). [CrossRef]
15. W. Luo, R. Wang, H. Li, J. Kou, X. Zeng, H. Huang, X. Hu, and W. Huang, “Simultaneous measurement of refractive index and temperature for prism-based surface plasmon resonance sensors,” Opt. Express 27(2), 576–589 (2019). [CrossRef]
16. W. Luo, S. Chen, L. Chen, H. Li, P. Miao, H. Gao, Z. Hu, and M. Li, “Dual-angle technique for simultaneous measurement of refractive index and temperature based on a surface plasmon resonance sensor,” Opt. Express 25(11), 12733–12742 (2017). [CrossRef]
17. Y. Kong, P. Qiu, Q. Wei, W. Quan, S. Wang, and W. Qian, “Refractive index and temperature nanosensor with plasmonic waveguide system,” Opt. Commun. 371, 132–137 (2016). [CrossRef]
18. S. Nazem, M. Malekmohammad, and M. Soltanolkotabi, “Theoretical and experimental study of a surface plasmon sensor based on Ag-MgF2 grating coupler,” Appl. Phys. B 126(5), 96 (2020). [CrossRef]
19. F. Xiao, D. Michel, G. Li, A. Xu, and K. Alameh, “Simultaneous Measurement of Refractive Index and Temperature Based on Surface Plasmon Resonance Sensors,” J. Lightwave Technol. 32(21), 4169–4173 (2014). [CrossRef]
20. Stephen J. Mihailov, “Fiber Bragg Grating Sensors for Harsh Environments,” Sensors 12(2), 1898–1918 (2012). [CrossRef]
21. J. Wang, C. Hu, Z. Zhang, H. Gong, Y. Jin, and C. Shen, “Distributed optical fiber temperature sensor applied in underground coal gasification system,” Proc. SPIE 7990, 79900N (2011). [CrossRef]
22. Z. Li, T. Chen, Z. Zhang, Y. Zhou, D. Li, and Z. Xie, “Highly sensitive surface plasmon resonance sensor utilizing a long period grating with photosensitive cladding,” Appl. Opt. 55(6), 1470–1480 (2016). [CrossRef]
23. J. Teng, P. Dumon, W. Bogaerts, H. Zhang, X. Jian, X. Han, M. Zhao, G. Morthier, and R. Baets, “Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides,” Opt. Express 17(17), 14627–14633 (2009). [CrossRef]
24. Z. Zhang, P. Zhao, P. Lin, and F. Sun, “Thermo-optic coefficients of polymers for optical waveguide applications,” Polymer 47(14), 4893–4896 (2006). [CrossRef]
25. C. Vieu, F. Carcenac, A. Pépin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois, “Electron beam lithography: resolution limits and applications,” Appl. Surf. Sci. 164(1-4), 111–117 (2000). [CrossRef]
26. X. Li, C. Wang, Z. Wang, Z. Fu, F. Sun, and H. Tian, “Anti-External Interference Sensor Based on Cascaded Photonic Crystal Nanobeam Cavities for Simultaneous Detection of Refractive Index and Temperature,” J. Lightwave Technol. 37(10), 2209–2216 (2019). [CrossRef]
27. F. Yu, P. Xue, X. Zhao, and J. Zheng, “Simultaneous Measurement of Refractive Index and Temperature Based on a Peanut-Shape Structure In-Line Fiber Mach–Zehnder Interferometer,” IEEE Sens. J. 19(3), 950–955 (2019). [CrossRef]
28. J. Hu, T. Lang, and G. H. Shi, “Simultaneous measurement of refractive index and temperature based on all-dielectric metasurface,” Opt. Express 25(13), 15241–15251 (2017). [CrossRef]
29. Z. Ding, P. Liu, J. Chen, D. Dai, and Y. Shi, “On-chip simultaneous sensing of humidity and temperature with a dual-polarization silicon microring resonator,” Opt. Express 27(20), 28649–28659 (2019). [CrossRef]
