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Studying polymer thin films with hybrid optical microcavities Hong Seok Choi,1 Shehzad Ismail,2 and Andrea M. Armani1,2,* 1
2
Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, USA
Ming Hsieh Department of Electrical Engineering–Electrophysics, University of Southern California, Los Angeles, California 90089, USA *Corresponding author:
[email protected] Received April 7, 2011; revised May 3, 2011; accepted May 7, 2011; posted May 10, 2011 (Doc. ID 145527); published June 1, 2011
Organic and inorganic polymeric thin films have numerous applications, including solar cells, biodetection, and nanocomposites. Improving our understanding of the fundamental material behavior is critical to designing polymers with ideal behavior and increased lifetime. However, there are limited nondestructive characterization methods that are able to perform these high-resolution measurements. In this Letter, we demonstrate a method that is able to detect temperature-induced changes in the refractive index of polystyrene polymer thin films as small as 10−7 . This approach is based on optical microcavity resonators. The experimental results agree well with the theoretical simulations. © 2011 Optical Society of America OCIS codes: 130.3120, 160.5470, 310.6860.
As the applications for polymer and nanocomposite coatings are increasing, there is additional pressure to more fully understand how changes in temperature affect the mechanical and optical properties of polymer thin films at the nanoscale level [1,2]. Currently, researchers employ a suite of methods, including ellipsometry, nanoindentation, calorimetry, and thin-film spectrophotometry [1,3,4]. However, no single method is able to resolve both the mechanical and optical changes in the thin films in real time (submicrosecond accuracy) with high resolution. For example, indentation methods, like nanoindentation or atomic force microscopy, which have the highest spatial resolution, have limited abilities to determine a sample’s optical properties, and they typically degrade a sample, which limits the ability to perform a measurement iteratively [5]. Optical sensing devices based on evanescent field whispering gallery mode resonant cavities have demonstrated the ability to detect subtle changes in the optical properties of the device. Specifically, the circulating optical field interacts with the microcavity, and any changes in the material are detected as a change in the resonant wavelength of the device, which is determined by the refractive index and the geometry of the resonator, among other properties. Variations in temperature induce changes in the refractive index of the material (thermo-optic behavior) and changes in the material size (expansion). The governing equation for these effects is [6] Δλ=ΔT ¼ λ0 ðεeff þ ðdneff =dTÞ=neff Þ;
plasmon devices and temperature stabilization, this sensitivity can be improved to over 10−7 refractive index units [8]. This sensitivity improvement is the result of several advancements, one of which is longer photon lifetime or higher-quality factor (Q) of the device. If an ultrahigh Q microcavity (Q > 108 ) is used, the sensitivity will be improved further. If the microcavity is coated with a polymer film, creating a hybrid cavity [Fig. 1(a)], it can be used to measure the optical properties and the mechanical behavior of the film. Because the optical and mechanical effects in polymers have very different response times, it is possible to deconvolve the two behaviors. Hybrid systems are slightly more complex to analytically describe because the εeff , dneff =dT, and neff must incorporate the polymer and the device material properties according to the degree of interaction with the circulating optical field, which is determined by the device geometry and material properties [9]. Therefore, by optimizing device design, it is possible to maximize the overlap of the optical field with the polymer, enhancing the device’s ability to study the polymer film. The majority of resonant cavity sensor research to date has focused on either biosensor development or singlematerial systems, such as silica and silicon, with limited efforts in developing multimaterial or hybrid devicebased sensors [6,7,10,11]. Therefore, there has been
ð1Þ
where λ0 , εeff , dneff =dT, and neff are cold cavity resonant wavelength, the effective expansion, effective thermal optic coefficients, and effective refractive indices of the optical microcavities, respectively. For example, using silicon resonant cavities, researchers have demonstrated the sensitivity of 83 pm=°C, which corresponds to a change in the refractive index of the device 1:37 × 10−4 [7]. Alternatively, using surface 0146-9592/11/112152-03$15.00/0
Fig. 1. (Color online) Hybrid optical microtoroid resonant cavity. (a) Pov-Ray rendering of the hybrid device. The whispering gallery mode is confined within the polymer-coated device. Part of the polymer film is cut away in the rendering for clarity. (b) Optical micrograph of a hybrid device. © 2011 Optical Society of America
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minimal research in the area of applying microcavities to characterize polymer thin films. In this Letter, a hybrid device is developed with the express design of exploring the temperature-dependent refractive index of a polymer film. All experiments are performed at temperatures that are significantly away from the glass transition temperature of the polymer to ensure that the polymer does not mechanically deform. The hybrid system consists of a silica resonant cavity conformally coated with polystyrene (PS). PS is chosen as the coating polymer because it has higher refractive index than silica, allowing a large overlap of the optical field with the polymer film, it does not readily absorb water ensuring minimal artifacts, and it has very low optical loss. The hybrid microtoroid silica resonators are fabricated using a previously described method [Figs. 1(a) and 1(b)] [12,13]. In this Letter, 200 k molecular weight PS (SigmaAldrich) was used. For resonator characterization, a narrow linewidth (300 kHz) CW tunable laser centered at 980 nm is coupled to the hybrid device using a single-mode low-loss tapered optical fiber (Newport, F-SC). The input power is controlled by an attenuator, which is placed in-line between the laser and the hybrid cavity. The scan speed and range are optimized to ensure that these parameters do not distort the measurements. To determine the loaded quality factor (Q ¼ λ=δλ, λ ¼ wavelength, δλ ¼ linewidth), the resonance linewidth is recorded in the undercoupled regime. Subsequently, the intrinsic Q is calculated using a mode-coupling model [14]. To determine the resonant shift data as a function of temperature, a heater and thermocouple sensor are integrated directly into the resonator sample holder. The heater (Omega CSH-102100=120 V) is mounted directly under the optical devices. Silver conductive epoxy (MG Chemicals) is used between the heater and the stage to ensure effective heat transfer from the heater to the sample holder, to minimize heat lost to the environment, and to hold the heater in place. A surface-mount fastresponse thermocouple sensor (Omega SA1XL) is attached immediately adjacent to the optical device to accurately read the temperature in real time. The response time for this thermocouple sensor is less than 0:15 s. Both the heater and the sensor are connected to a benchtop controller (Omega CSC32 series). The resonant shift results are automatically recorded by a custom LabView program. Initial control experiments are performed to ensure the reliability and stability of this stage and the testing software. From these experiments, it is determined that there is negligible variation in resonance wavelength over the time frame of a typical experiment (less than 0:3 pm over 4 min in air). Initial experiments studied the response of the hybrid device with that of a single material (silica) device. Specifically, two different experiments were performed: (1) increasing the temperature by the same amount (N > 5 times) and (2) increasing the temperature by a different amount. These results are shown in Figs. 2(a) and 2(b). By performing both experiments, we are able to determine the linearity of the sensor response and the stability of the sensor after the temperature change. The slight decline after each step for both
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Fig. 2. (Color online) Comparison of sensor response between silica and hybrid devices. (a) The ΔT is the same for each increment. (b) The ΔT is increasing for each increment. Because of the polymer layer, the hybrid device has a significantly larger response. Note that the y axis is the absolute value of the resonant wavelength shift or the magnitude, for easy comparison between the two devices.
the silica and the hybrid device is the result of the heater overshooting the set temperature and then correcting. Figure 3 shows all of these results compiled into a single graph. As can be easily observed, the monolithic silica microtoroids exhibit a redshift; however, the hybrid microtoroids show blueshift. A simple linear fit to the data provides a significant amount of insight into the dominant detection mechanism. The slope of the silica data is usually around 11 pm=°C, which agrees well with theoretical value as can be seen in Fig. 3. This indicates that the primary cause for the resonant frequency change in the silica device over this temperature range is the change in refractive index of the device. In the hybrid device, the slope is −36 pm=°C. If the device does not experience an expansion, then this value represents the effective sensitivity of the device, which relates to effective dn=dT. The change from a positive to a negative value is the result of the negative thermooptic coefficient of PS. Using the geometry of the device, the dn=dT values of PS and silica from the literature [15,16], and the testing wavelength, the effective sensitivity was calculated to be between −28 pm=°C (PS ¼ 250 nm) and −40 pm=°C (PS ¼ 300 nm). This value is extremely similar to the experimental results, indicating that, over this temperature range, the polymer film does not deform. At higher temperatures, based on the expansion coefficient of PS and the completion between
Fig. 3. (Color online) The results from experiments like those shown in Fig. 2 were compiled to show the shift versus temperature for both the hybrid and silica devices. Additionally, the theory based on Eq. (1) is included. The shift for the silica device is significantly smaller than the shift for the hybrid device, and the theory and experiment are in good agreement.
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Fig. 4. (Color online) Reproducibility of results. (a) The measurement was performed iteratively. (b) The data in part (a) are replotted to emphasize the hysteretic behavior due to the heating element.
the two effects (thermo-optic inducing a blueshift and expansion inducing a redshift), we would expect to see a shift of approximately −4 pm=°C, which is easily resolvable. Based on the previous results, we can determine the threshold sensitivity of the device to changes in the refractive index of PS. Specifically, based on the measured noise threshold (0:3 pm), the theoretical threshold refractive index sensitivity is 4:5 × 10−7 . Finally, experiments are performed to determine if the hybrid sensor is able to monitor the behavior of PS over several heat and cool cycles, or if it experiences degradation. To perform this experiment, we first heat the sensor, and then let it cool to the initial temperature. As can be seen in Figs. 4(a) and 4(b), the signal produced by the hybrid device in response to the temperature increase and decrease is fairly consistent over all eight cycles. This result verifies that the signal produced by the hybrid device is stable, and the basic device structure does not rapidly degrade. Both of these are key features of a thin-film measurement device. It is important to comment on the hysteretic behavior that is clearly evident in Fig. 4(b). For clarity, the heating–cooling cycle is counterclockwise. The time scale is far too slow for the hysteresis to be related to the hybrid device, which has a millisecond response time, including the limitations in the current detection system (function generator, laser, etc.). Therefore, the rate of heating is determined by the heating element, while the rate of cooling is determined by heat transfer from the sample to the environment and to the substrate. In conclusion, we have experimentally and theoretically verified the ability of optical microcavities to nondestructively detect the temperature-dependent optical properties of polymer thin-film coatings. Specifically, we measured the dn=dT of silica and hybrid devices using an optical cavity and concluded that the sensing signal was solely generated by the change in refractive index, with no contribution from material expansion. By performing finite element method simulations, we
calculated the interaction of the optical field with the polymer film and developed an accurate model of the system that had good agreement with the experimental results. As a result of minimal device degradation over the course of the iterative experiments, the response is extremely reproducible, and, in the temperature range studied, the sensor response is very linear. Because this sensor system accurately monitors the effect of temperature on the optical properties of the polymer material with significantly improved sensitivity over currently available methods, it will find numerous uses in studying how temperature changes affect and degrade the optical properties of polymer thin films, which have applications ranging from solar cells to nanocomposite coatings [17,18]. The authors thank Matthew Reddick for the development of a data acquisition program in LabView. This work was supported by the Office of Naval Research (ONR) Young Investigator Program (grant N00014-09-10898) and Battelle Memorial Institute. Additional information is available at http://armani.usc.edu. References 1. Y. Huang and D. R. Paul, Macromolecules 39, 1554 (2006). 2. J. M. Kropka, V. Pryamitsyn, and V. Ganesan, Phys. Rev. Lett. 101, 075702 (2008). 3. M. K. Mundra, C. J. Ellison, R. E. Behling, and J. M. Torkelson, Polymer 47, 7747 (2006). 4. R. D. Priestley, C. J. Ellison, L. J. Broadbelt, and J. M. Torkelson, Science 309, 456 (2005). 5. Z. Burghard, L. Zini, V. Srot, P. Bellina, P. A. van Aken, and J. Bill, Nano Lett. 9, 4103 (2009). 6. T. Carmon, L. Yang, and K. J. Vahala, Opt. Express 12, 4742 (2004). 7. G. D. Kim, H. S. Lee, C. H. Park, S. S. Lee, B. T. Lim, H. K. Bae, and W. G. Lee, Opt. Express 18, 22215 (2010). 8. L. J. Davis and M. Deutsch, Rev. Sci. Instrum. 81, 114905 (2010). 9. H.-S. Choi and A. M. Armani, Appl. Phys. Lett. 97, 223306 (2010). 10. H. K. Hunt and A. M. Armani, Nanoscale 2, 1544 (2010). 11. M. Han and A. Wang, Opt. Lett. 32, 1800 (2007). 12. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, Nature 421, 925 (2003). 13. H.-S. Choi, X. Zhang, and A. M. Armani, Opt. Lett. 35, 459 (2010). 14. A. Yariv, Electron. Lett. 36, 321 (2000). 15. M. J. Weber, Handbook of Optical Materials (CRC Press, 2003). 16. L. He, Y. F. Xiao, C. Dong, J. Zhu, V. Gaddam, and L. Yang, Appl. Phys. Lett. 93, 201102 (2008). 17. R. Vendamme, S. Y. Onoue, A. Nakao, and T. Kunitake, Nat. Mater. 5, 494 (2006). 18. T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner, and U. Scherf, Nat. Mater. 2, 408 (2003).
