Open Access
Add to BibSonomyAbstract
In this paper, on-chip spherical-cap-shaped microresonators made from self-assembled UV-curable adhesive are proposed and demonstrated for high sensitivity temperature sensing. We observe the whispering-gallery mode resonances in these spherical-cap-shaped microresonators, and investigate the wavelength shifts of the resonance peaks as a function of both input power and temperature. The resulting devices with various diameters offer a high sensitivity of 0.14-0.22 nm/°C for temperature sensing.
© 2016 Optical Society of America
1. Introduction
Over the years, optical whispering-gallery mode (WGM) microresonators have attracted more and more attention in sensing research. In WGM microresonators, the light is well-trapped and confined inside the cavities because of the total internal reflection of the light between materials with high and low refractive indices. These optical cavities can have very high optical quality (Q) factors and very narrow spectral linewidths. The resulting narrow spectral linewidth resonances are very sensitive to the parameters change of the WGM microresonator itself or the surrounding environment, thus make the WGM microresonators ideal candidates as a power tool for a wide range of sensing applications including chemical, biological, optical, pressure and temperature sensing [1–6].
WGM temperature sensing has been mainly focused on spherical, annular, and toroidal shaped resonators based on the conception of temperature-induced WGM resonance wavelength shifts. WGM resonators with the properties of facile processing, low cost, high temperature sensitivity and integratability are highly demanded in practical thermal sensing applications. Inorganic (such as silicon, silica) and organic (such as polymers) materials have demonstrated their capabilities for thermal sensing [7–13]. Generally, fabrication of inorganic microresonators needs sophisticated process or costly apparatus. For example, lithographic and etching steps are required to define the shapes of the silicon-based microresonators [7]. Moreover, the imperfection of etching step introduces optical losses at the boundary, leading to low Q factors. Meanwhile, formation of silica-based microresonators calls for some special heating requirement, such as carbon dioxide (CO2) laser, electric arc or smelting furnace [1,8–10]. In comparison, organic-based microresonators have received increased attention because they possess simpler fabrication process, high quality factor and are cost-effective [11–13]. Organic-based microresonators can be directly formed by surface tension without other expensive facility. Also, due to surface tension, these microresonators have regular shape and smooth surface, thus resulting in high Q factors. In [11] and [13], spherical microresonators made from Polydimethylsiloxane (PDMS) and UV-curable adhesive (NOA61, produced by Norland Inc [14]) have been fabricated on the tip of half-tapered fibers, and they can be used as thermal sensors. However, these fiber-supported spherical microresonators are difficult to be integrated on chips. The discovered hydrophobic peculiarity in some materials paves a promising way to obtain an on-chip WGM microresonator with high quality factor [15,16]. Recently, Ta and associates reported polymer (Rhodamine B doped epoxy resin) hemispherical microresonators on top of a hydrophobic distributed Bragg reflector surface. Optically pumped lasing phenomenon from these hemispherical microresonators was observed, and application of these hemisphere lasers as gas sensors was explored as well [17,18].
Here we propose and demonstrate an on-chip, self-assembled spherical-cap-shaped microresonators for temperature sensing. The spherical-cap-shaped microresonators are made from NOA61, NOA61 is chosen for its high hardness (after curing), extremely broad transparency (low attenuation loss) and good chemical stability [12–14]. Flexible spherical-cap-shaped microresonators are achieved. The optical properties, power and temperature-dependent behaviors are characterized. By linear fitting the variation of resonance with temperature, these UV-curable adhesive spherical-cap-shaped microresonators can thermally tune the resonant wavelength with a range of 0.14-0.22 nm/°C.
2. Spherical-cap-shaped microresonators fabrication
Figure 1 schematically illustrates the fabrication and self-assembly procedure of spherical-cap-shaped microresonators. Device processing starts with the evaporation of Ag metal layer on polished silicon (Si) substrate by magnetron sputtering. The reflectance of Ag mirror exceeds 95% at the wavelength from 1500 to 1600 nm. A layer of Poly(vinyl alcohol) (PVA) (mixed with H2O, the weight ratio of PVA/H2O is 2 g: 30 g) is then deposited on the Ag metal layer via spin coating [Fig. 1(a)], followed by baking at 60 °C for 30 min in an oven. Subsequently, a half-tapered fiber is dipped inside the NOA61 solution and a small amount of solution is spread on the half-taped fiber. Then the fiber with the NOA61 solution is carefully positioned close to the PVA surface and then moved away after gentle contact. As shown in Figs. 1(b) and 1(d), NOA61 liquid firstly forms a long cylinder and then break into several bottle-like pieces. Because of the surface tension, as shown in Fig. 1(e), these bottle-like pieces contract and tend to minimize their surface areas. After 4 minutes, these pieces eventually form spherical-cap-shaped structures [Figs. 1(c) and 1(f)]. Then, these self-assembled spherical-cap-shaped structures are exposed under a UV lamp with light intensity of 90 mW/cm2 to solidify and be stable to test the coupling with tapered-fiber. The spherical-cap shapes of these self-assembled structures remain unchanged after the curing process, due to the low shrinkage attribute of NOA61 [14]. Figure 1(g) shows the front view of the spherical-cap-shaped structure on PVA surface, the contact angle is ~24°.
Fig. 1 (a)-(c) Schematically diagram illustrates the fabrication process of the UV-curable adhesive spherical-cap-shaped microresonators. (d)-(f) Self-assembling process of spherical-cap-shaped microresonators on PVA surface as a function of time. (g) A front view of the spherical-cap-shaped structure on PVA surface.
Characterization of the spherical-cap-shaped microresonators is carried out using the setup schematically presented in Fig. 2. The WGM resonances in the spherical-cap-shaped microresonators are investigated using an evanescent coupling technique [12,13]. The light source (QPhotonics QSDM-1550-1) is a broadband continuous wave centered at 1550 nm with a spectral width of 56.28 nm. A tapered silica fiber with waist diameter ~2 µm couples light into the spherical-cap-shaped microresonators, excites the WGMs, and couples light out of the spherical-cap-shaped microresonators. The transmitted light output power through the tapered fiber is then detected and recorded by an optical spectrum analyzer (OSA, the resolution is 0.01 nm). In order to study the temperature dependent WGM resonance of the spherical-cap-shaped microresonators, a thermoelectric controller (TEC) is employed to heat or cool the whole microresonator. After every change of temperature, a thermocouple attached to the top of the Ag-PVA layer is used to detect the temperature when the microresonator reaches thermal equilibrium.
3. Results and discussion
Figure 3 shows a typical transmission spectrum of a spherical-cap-shaped microresonator with the diameter (D) of 262 µm. Here the value D is denoted as the diameter of the base of the microresonator. As shown in Fig. 3, the measured free spectral range (FSR) is about 1.91 nm, which is in good agreement with the theoretically calculated FSR~λ/(n0πD)(~1.9 nm), whereas λ~1550 nm is the resonant wavelength, n0~1.542 is refractive index of NOA61 at 1550 nm. It indicates that the coupled light is well confined inside the spherical-cap-shaped microresonator thanks to the high reflectivity Ag layer and high refractive index of NOA61, and finally oscillates in the form of WGM by internal reflection between NOA61 and air [18]. The Q-factor of the spherical-cap-shaped microresonator can be estimated by the resonance linewidth of the WGM spectrum, expressed as Q = λ/δλ, where λ is the central resonance wavelength and δλ is the line-width of the peak. By using the Lorentzian fit of the experimental data, a linewidth of 77 pm at a central resonance wavelength of 1549.13 nm is found, corresponding to a loaded Q factor of 2 × 104. In our previous work [13], the absorption limited Q-factor of NOA 61 is estimated to be 6.4 × 105. Taking into account of the diffraction loss due to the curvature of the boundary, the scattering loss due to the surface roughness, the loss resulting from the external couple and the possible light leakage to the substrate, the real Q-factor of the spherical-cap-shaped microresonators should be lower than this value, which shows a reasonable agreement with the experiment result.
Fig. 3 A typical transmission spectrum of a spherical-cap-shaped microresonator with the diameter of 262 µm. The resonance curves marked with the same color represent the modes of the same order.
Next, we study the WGM resonances as a function of input light power, tuned by input current of the light source from 150 to 500 mA with a single step increment of 50 mA. The experiment proceeds at 25 °C controlled by TEC. Figure 4(a) shows the resonant wavelength shifts of two main peaks versus input pump current of the spherical-cap-shaped microresonator with the diameter of 262 μm. The inset shows the transmission spectrum as a function of the input pump current. It is shown that the resonance peaks are stable with increasing (red line) and decreasing (purple line) of the input current. This is due to the steady temperature provided by TEC and the good dissipation of heat benefit from high thermal conductivity silicon (~148 W/m·K [19],) and Ag mirror (~429 W/m·K [19],). Therefore, we can obtain stable and reliable WGM resonances with the spherical-cap-shaped microresonator at a controlled temperature.
Fig. 4 (a) Power and (b) temperature dependent WGM resonances in a spherical-cap-shaped microresonator with the diameter of 262 µm. (c) Schematic diagram of the interaction between cured-NOA61 and PVA. (d) Calculated results of resonant wavelength shift vs effective thermal expansion coefficient.
Figure 4(b) shows the temperature dependent WGM resonances of the spherical-cap-shaped microresonator with the diameter of 262 μm. The input pump current of the light source is 200 mA. As depicted in Fig. 4(b) (blue hollow circles), a resonance peak experiences a gradual blue-shift of 1.61 nm when temperature increases from 26.8 to 36.6 °C. To verify its repeatability, we also study the WGM resonance when temperature decreases in the same range, and a red-shift of 1.67 nm of the WGM resonance is observed, as presented in Fig. 4(b) (red squares). Then, we theoretically investigate the thermal effect of this spherical-cap-shaped microresonator. Both thermal refraction effect and thermal expansion effect lead to the resonant wavelength shift. The resonant wavelength shift ∆λ versus temperature change can be given by the following equation:
where λ (~1550 nm) is the cavity resonance wavelength, is the NOA61 thermal refraction coefficient and = −2.46 × 10−4 /°C [12,20]. is the thermal expansion coefficient. After UV-curing process, the adhesive force integrate NOA61 and PVA into a composite system, as depicted in Fig. 4(c). For this composite system, however, is an effective value of both the material NOA61 and PVA, which can be estimated by the following equation [21–23]:where represents the Young’s modulus, and represents the Poisson ratio, the subscripts 1 and 2 represent the PVA and NOA61, respectively, where = 7 × 10−5 /°C [24] and = 2.2 × 10−4 /°C [12,20]. Where is the volume fraction of the PVA within the NOA61, which can be approximately as:where represents the volume of PVA layer, represents the volume of NOA61 spherical-cap-shaped microresonator, is the thickness of PVA layer, is the thickness of NOA61 spherical-cap-shaped microresonator and a function of the contact angle and the diameter, which can be expressed as: , then Eq. (3) can be rewritten as:As shown in Eqs. (2) and (4), increases as the diameter increases. According to Eq. (1), it can be concluded that the resonance wavelength shift is smaller for bigger microresonators. Considering two limiting cases = 0 and, the effective thermal expansion coefficient is found to be: and [21]. Based on this consideration, we calculate as a function ofwith = 10 °C, as shown in Fig. 4(d). The corresponding is 1.39 × 10−4 /°C according to the experimental value, indicating a good agreement with the theoretical result.The above WGM resonant wavelength shift characteristics can be further observed in different size spherical-cap-shaped microresonators. Figures 5(a) and 5(b) show the ∆λ versus ∆T of the spherical-cap-shaped microresonators with the diameter of 108, 199 and 383 μm [Experimental results of the spherical-cap-shaped microresonator with the diameter of 262 μm is also plotted in the Figs. 5(a) and 5(b)]. Figures 5(c) and 5(d) depict the transmission spectrum of the spherical-cap-shaped microresonator with the diameter of 108 μm as temperature increases and decreases, respectively. Excellent linear dependences of the resonant wavelength shift against the temperature change are observed. In addition, the result shows that the spherical-cap-shaped microresonator with smaller diameter has larger resonance wavelength shift, which agrees well with the above theoretical analysis. The sensitivities of the spherical-cap-shaped microresonators with the diameter of 383, 262, 199 and 108 μm by linear fitting are 0.14, 0.17, 0.19, 0.22 nm/°C, respectively. The corresponded of these spherical-cap-shaped microresonators are shown in Fig. 4(d).
Fig. 5 Resonant wavelength shift vs temperature change in spherical-cap-shaped microresonators with different diameters. (a) Temperature increase, (b) Temperature decrease. The solid lines are the linear fit. Transmission spectrum of a spherical-cap-shaped microresonator with the diameter of 108 µm. (c) Temperature increase, (d) Temperature decrease.
The nature of large thermal refraction coefficient of NOA61 and the reduced effects of thermal expansion result from the adhesive force make the on-chip spherical-cap-shaped microresonators much more sensitive than the sensors based on the silicon material. The temperature sensitivity is 0.11 nm/°C for the silicon resonator temperature sensor [7], which is only half of the sensitivity of the spherical-cap-shaped microresonator with the diameter of 108 μm. Moreover, compared to the conventional spherical temperature sensor [11,13], it is easy to be integrated on chips. By integrated with other photonic devices, the spherical-cap-shaped microresonators can be served as a possible tool to detect the substrate temperature and applied as a wavelength-selective switch. Further efforts will be focused on packaging the spherical-cap-shaped microcavities-taper coupling system. Some promising methods such as gluing the taper to the cavity by using the low refractive index (RI) UV glue [8] or integrating with optics waveguide [25] will be adopted for practical sensing application. In addition, considering the resolution of OSA (0.01 nm), the minimal detectable temperature change is ~0.1 °C. The practical sensitivity of these spherical-cap-shaped microcavities will also be further explored by using a narrowband tunable laser or a wavelength meter.
4. Conclusions
In summary, we report an on-chip temperature sensor based on NOA61 spherical-cap-shaped microresonators which can be fabricated using an ease and low cost fabrication process. Varied sized microresonators with high temperature sensitivity are obtained. The features of high temperature sensitivity and on-chip make it strikingly attractive and a promising candidate for practical thermal sensing applications.
Funding
National Natural Science Foundation of China (grant nos. 61106044 and 61274052); Major Scientific and Technological Special Project of Guangdong province (No. 2014B010119004); Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2015-T2-1-066), and Nanyang Technological University (Startup grant: Lei Wei).
References and links
1. J. Ward and O. Benson, “WGM microresonators: sensing, lasing and fundamental optics with microspheres,” Laser Photonics Rev. 5(4), 553–570 (2011). [CrossRef]
2. H. K. Hunt and A. M. Armani, “Label-free biological and chemical sensors,” Nanoscale 2(9), 1544–1559 (2010). [CrossRef] [PubMed]
3. K. H. Kim, G. Bahl, W. Lee, J. Liu, M. Tomes, X. D. Fan, and T. Carmon, “Cavity optomechanics on a microfluidic resonator with water and viscous liquids,” Light Sci. Appl. 2(11), e110 (2013). [CrossRef]
4. G. C. Righini, Y. Dumeige, P. Féron, M. Ferrari, G. Nunzi Conti, D. Ristic, and S. Soria, “Whispering gallery mode microresonators: fundamentals and applications,” Riv. Nuovo Cim. 34(7), 435–488 (2011).
5. B. B. Li, Q. Y. Wang, Y. F. Xiao, X. F. Jiang, Y. Li, L. X. Xiao, and Q. H. Gong, “On chip, high-sensitivity thermal sensor based on high-Q polydimethylsiloxane-coated microresonator,” Appl. Phys. Lett. 96(25), 251109 (2010). [CrossRef]
6. B. B. Li, W. R. Clements, X. C. Yu, K. Shi, Q. Gong, and Y. F. Xiao, “Single nanoparticle detection using split-mode microcavity Raman lasers,” Proc. Natl. Acad. Sci. U.S.A. 111(41), 14657–14662 (2014). [CrossRef] [PubMed]
7. M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett. 89(7), 071110 (2006). [CrossRef]
8. Y. Z. Yan, C. L. Zou, S. B. Yan, F. W. Sun, Z. Ji, J. Liu, Y. G. Zhang, L. Wang, C. Y. Xue, W. D. Zhang, Z. F. Han, and J. J. Xiong, “Packaged silica microsphere-taper coupling system for robust thermal sensing application,” Opt. Express 19(7), 5753–5759 (2011). [CrossRef] [PubMed]
9. Q. L. Ma, T. Rossmann, and Z. X. Guo, “Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement,” Meas. Sci. Technol. 21(2), 025310 (2010). [CrossRef]
10. L. He, Y. F. Xiao, J. Zhu, S. K. Ozdemir, and L. Yang, “Oscillatory thermal dynamics in high-Q PDMS-coated silica toroidal microresonators,” Opt. Express 17(12), 9571–9581 (2009). [CrossRef] [PubMed]
11. C. H. Dong, L. He, Y. F. Xiao, V. R. Gaddam, S. K. Ozdemir, Z. F. Han, G. C. Guo, and L. Yang, “Fabrication of high-Q polydimethylsiloxane optical microspheres for thermal sensing,” Appl. Phys. Lett. 94(23), 231119 (2009). [CrossRef]
12. G. Gu, C. Guo, Z. Cai, H. Xu, L. Chen, H. Fu, K. Che, M. Hong, S. Sun, and F. Li, “Fabrication of ultraviolet-curable adhesive bottle-like microresonators by wetting and photocuring,” Appl. Opt. 53(32), 7819–7824 (2014). [CrossRef] [PubMed]
13. G. Q. Gu, L. J. Chen, H. Y. Fu, K. J. Che, Z. P. Cai, and H. Y. Xu, “UV-curable adhesive microsphere whispering gallery mode resonators,” Chin. Opt. Lett. 11(10), 101401 (2013). [CrossRef]
14. Norland Products Inc, https://www.norlandprod.com/adhesives/NOA%2061.html.
15. A. Kiraz, M. A. Dundar, A. L. Demirel, S. Doganay, A. Kurt, A. Sennaroglu, and M. Y. Yuce, “Single glycerol/water microdroplets standing on a superhydrophobic surface: Optical microcavities promising original applications,” J. Nanophotonics 1(1), 011655 (2007). [CrossRef]
16. H. B. Lin, A. L. Huston, B. L. Justus, and A. J. Campillo, “Some characteristics of a droplet whispering-gallery-mode laser,” Opt. Lett. 11(10), 614–616 (1986). [CrossRef] [PubMed]
17. V. D. Ta, R. Chen, D. M. Nguyen, and H. D. Sun, “Application of self-assembled hemispherical microlasers as gas sensors,” Appl. Phys. Lett. 102(3), 031107 (2013). [CrossRef]
18. R. Chen and H. D. Sun, “Single mode lasing from hybrid hemispherical microresonators,” Sci. Rep. 2, 244 (2012). [CrossRef] [PubMed]
19. S. U. S. Choi and J. A. Eastman, “Enhancing thermal conductivity of fluids with nanoparticles,” ASME FED 231, 99–105 (1995).
20. Thorlabs Inc.,http://www.thorlabschina.cn/NewGroupPage9.cfm?ObjectGroup_ID=196.
21. M. Frenkel and Z. X. Guo, “On-chip, dynamic, and cryogenic temperature monitoring via PDMS micro-bead coatings,” Polym. Phys. 54(12), 1118–1124 (2016). [CrossRef]
22. M. Frenkel, M. Avellan, and Z. X. Guo, “Whispering-gallery mode composite sensors for on-chip dynamic temperature monitoring,” Meas. Sci. Technol. 24(7), 075103(2013). [CrossRef]
23. R. R. Tummala and A. L. Friedberg, “Thermal expansion of composite materials,” J. Appl. Phys. 41(13), 5104–5107 (1970). [CrossRef]
24. A. Noorjahan, On the Atomistic Simulation Approach Towards the Estimation of the Polymer/Solvent Mutual Diffusion Coefficient (University of Alberta., 2014)
25. M. K. Chin and S. T. Ho, “Design and modeling of waveguide-coupled single-mode microring resonators,” J. Lightwave Technol. 16(8), 1433–1446 (1998). [CrossRef]
