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Suspended ultra-small disk resonator on silicon for optical sensing Xiaokun Wang, Xiaowei Guan, Qiangsheng Huang, Jiajiu Zheng, Yaocheng Shi, and Daoxin Dai* Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China *Corresponding author: [email protected] Received September 13, 2013; revised November 1, 2013; accepted November 13, 2013; posted November 18, 2013 (Doc. ID 197614); published December 11, 2013 An ultra-small disk resonator consisting of a suspended silicon disk with a submicron bending radius sitting on an SiO2 pedestal is demonstrated experimentally. An asymmetrical suspended rib waveguide is integrated as the access waveguide for the suspended submicron disk resonator, which is used to realize an ultra-small optical sensor with an improved sensitivity due to the enhanced evanescent field interaction with the analyte. The present optical sensor also has a large measurement range because of the ultra-large free-spectral range of the submicron-disk resonator. As an example, a suspended submicron disk sensor with a bending radius of 0.8 μm is designed, fabricated, and characterized. The concentration of NaCl aqueous solution and organic liquids is measured with the suspended submicron-disk sensor, and the measured sensitivity is about 130 nm∕RIU, which agrees well with the simulation value. © 2013 Optical Society of America OCIS codes: (130.2790) Guided waves; (130.3120) Integrated optics devices; (230.7370) Waveguides. http://dx.doi.org/10.1364/OL.38.005405
Silicon-photonic-integrated devices have been extensively investigated for many applications (optical interconnect [1], optical sensing [2,3], etc.) because of CMOS compatibility and ultra-compactness. Among various silicon-photonic-integrated devices, an optical microcavity is well known as a versatile element to realize various functionality components, including optical modulators [4] and optical filters [5] as well as optical sensors [6,7]. The optical sensors based on optical microcavities have attracted lots of attention since there is a field enhancement in the microcavity when operating at the resonance wavelength [2,3]. It is well known that small size is universally desirable for most applications of microcavities [8]. In [8], a silicon microring resonator with 1.5 μm radius was demonstrated for the application of optical modulators. Similarly, for optical-sensing applications, there are also some benefits with small microcavities. First, a smaller microcavity sensor has a larger free-spectral range (FSR) so that a larger measurement range is available. Second, the sample volume to be measured is less when using a smaller microcavity sensor. Third, one can integrate more sensor elements in a single chip to have a sensor array with a higher integration density. A photonic crystal microcavity could be used for optical sensors with a small footprint and small mode volume [9]. A drawback is that a photonic crystal cavity usually has high excess loss, e.g., ∼17 dB as shown in [10]. A simple way to have a low-loss optical microcavity is using a conventional optical-cavity waveguide with improved light confinement. For the case with a siliconon-insulator (SOI) platform, which is the most popular for silicon photonics, one can make a suspended silicon optical cavity by replacing the buried oxide (BOX) with another lower-index material (e.g., air). A disk resonator is considered here since it is one of the best options to support a suspended structure among various optical cavities. Furthermore, a microdisk resonator is also helpful to have low loss and a high Q value [11,12]. 0146-9592/13/245405-04$15.00/0
A high-Q-suspended microdisk resonator without any access waveguide integrated has been demonstrated previously [13,14]. In that case, one usually has to use a tapered microfiber to couple light into the resonator, which is not reliable and convenient. It is also not ideal to be extended to form an array by cascading several resonators. A suspended GaAs microdisk resonator with an access waveguide has been demonstrated previously [15]. In this Letter, we demonstrate a suspended silicon ultrasmall disk resonator with an access waveguide. The disk resonator has a submicron bending radius (R < 1 μm) to realize a submicron optical sensor with a large measurement range for measuring the concentrations of NaCl solutions and different organic liquids. Figure 1 shows the schematic configuration of the present suspended submicron-disk resonator for optical sensing. It can be seen that there is a suspended submicron disk sitting on a SiO2 pedestal. An asymmetrical suspended rib waveguide is placed at the side as the access waveguide so that the measurement is convenient and robust. It is also easy to achieve a sensor array by cascading several resonators. When using the suspended disk resonator as an optical sensor to measure
Fig. 1. Present suspended silicon submicron-disk resonator with a suspended access waveguide. © 2013 Optical Society of America
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by choosing a smaller bending radius. Therefore we choose R 0.8 μm in the following design. The intrinsic Q-value of a submicron-disk resonator is calculated by [16]
the refractive index of gas or liquid, the sensitivity is improved intrinsically due to the enhanced evanescent field interaction with the analyte in comparison with the regular unsuspended disk resonator with BOX beneath. In addition, the suspended structure is helpful to reduce the bending loss so that one can realize a submicron-disk resonator, which has an ultra-large FSR and thus a large measurement range for optical sensing. Regarding that the bending loss becomes dominant for a submicron-disk resonator [8], we calculate the complex effective refractive index neff and the field profiles of the whispering-gallery mode of a submicron-disk resonator by using a finite-element method. The bending loss for a 90° bending is then given by L 20 lgexpnim k0 Rπ∕2, where k0 is the wavenumber (k0 2π∕λ), R is the bending radius, nim is the imaginary part for the complex effective index neff . Figure 2(a) shows the calculated bending loss L of the fundamental modes (TE0 and TM0 ) as well as the first higher-order modes (TE1 and TM1 ) of a suspended microdisk resonator as the radius R varies. In order to give a comparison, the result for an unsuspended microdisk resonator with BOX is also given, as shown in Fig. 2(b). For the present calculation, the cladding is air (nc 1) or deionized (DI) water (n 1.316). The SOI wafer has a 340 nm thick top silicon layer and a 1 μm thick SiO2 insulator layer (which is thick enough to guarantee a low substrate leaky loss). The refractive indices of silicon and SiO2 are 3.455 and 1.445, respectively. And the operation wavelength is 1550 nm. Figure 2 shows that the suspended microdisk resonator has lower loss than the unsuspended one, which is due to the improved refractive index contrast. The comparison between Figs. 2(a) and 2(b) also shows that the bending loss for TE mode is lower than that for TM mode, which is due to the higher confinement ability for TE mode. As is known, a microdisk resonator usually supports the higher-order mode resonance, which is undesired for many applications, including optical sensing considered here. Fortunately, the higher-order modes (TE1 and TM1 ) have a large loss for the present submicron-disk resonator with a bending radius of less than 0.8 μm, as shown in Figs. 2(a) and 2(b). Therefore, the resonance for the higher-order modes is depressed significantly, which will be verified from the measured spectral responses below. For the disk with a larger radius, the Q value of the diskresonator improves, while multimode effects become severer because the higher-order modes are supported well with a low bending loss, as shown in Fig. 2(a). Furthermore, regarding that the FSR is inversely proportional to the bending radius, one can have a larger FSR
where λc is the resonant wavelength, and nc is the cladding index. One sees that the sensitivity S can be improved by enhancing either the device sensitivity S d or the waveguide sensitivity S w . For disk resonators, one has S d ∂λc ∕∂neff λc ∕ng . Note that the group index ng usually changes slightly as the waveguide dimension varies. For the present SOI nanowire, the group index ng varies slightly from 4.58 to 4.44 when the waveguide width increases from 300 to 500 nm. As a consequence, the device sensitivity S d does not change notably by optimizing the waveguide dimension. In this Letter, we focus on improvement of the waveguide sensitivity S w by introducing a suspended waveguide, which has enhanced evanescent field interaction with analyte. This can be seen from Fig. 3, which shows the calculated field profile for the TM0 mode of the suspended and unsuspended submicron-disks covered by DI water when R 0.8 μm. For the suspended disk, the mode confinement factor in silicon is about 66%. The calculated waveguide sensitivities for the suspended and unsuspended submicron-disk resonator are S w 0.329 and 0.203, respectively. Their device sensitivities are S d 457 and 462 nm∕RIU. Their sensitivities are S 150 and
Fig. 2. Calculated bending losses of the TE0 , TM0 , TE1 , and TM1 modes in a submicron-disk resonator when the cladding is air (thick curves) or DI water (thin curves). (a) Suspended. (b) Unsuspended.
Fig. 3. Profile for the TM0 mode in a submicron disk when R 0.8 μm. (a) Unsuspended. (b) Suspended.
Qi 2πng ∕αλ0 ng ∕2nim ;
(1)
where ng is the group index. It is expected that the suspended submicron-disk resonator has a higher Q-value than the unsuspended one because of the lower bending loss [Fig. 2(a)]. For a suspended submicron-disk resonator with R 0.8 μm, the calculated intrinsic Q-values are about 12,000 and 1955, respectively, for TE and TM polarizations. When the suspended submicron-disk resonator is covered by, e.g., DI water (n 1.316), the Q-values for the TE and TM modes degrade to 680 and 300, respectively. The Q-value degradation is due to the increased bending loss as shown in Fig. 2(a). When using the submicron-disk resonator for optical sensing, the sensitivity is determined by the device sensitivity S d ∂λc ∕∂neff and the waveguide sensitivity S w ∂neff ∕∂nc [17], i.e., S
∂λc ∂λc ∂neff ≡ Sd Sw ; ∂nc ∂neff ∂nc
(2)
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94 nm∕RIU, respectively. It can be seen that the sensitivity is improved by 60%. The fabrication was started with a SOI wafer with a 340 nm thick top silicon layer and a 1 μm thick buried SiO2 layer. A double-etching process was carried out to form the asymmetrical access optical waveguide and the submicron disk. In our case, the patterns were formed with positive E-beam lithography-resist ZEP520A by using a Raith150-II machine, and a dry etching process is implemented by using an STS inductively coupled plasma system. A third etching process was carried out to fabricate grating couplers for efficient fiberchip coupling [18]. Here the grating coupler working for TM polarization is designed and fabricated with a grating period of 810 nm, a duty cycle of 0.5, and an etching depth of 170 nm. And the coupling efficiency is estimated to be 28% at the central wavelength. A 300 nm thick SiO2 layer is then deposited on the top to protect the silicon layer. A thin chromium layer (∼50 nm) was deposited and patterned as the hard mask in the following SiO2 wet-etching process. Finally, the SiO2 layer is corroded with the buffered SiO2 etchant (a mixture of NH4 F, CH3 COOH, DI water, and ethylene glycol), and the wet-etching depth is about 400 nm. Figure 4(a) shows the scanning electron microscopy (SEM) picture of the fabricated submicron-disk resonator whose bending radius is about R 0.8 μm. From this figure, it can be seen clearly that the silicon submicrondisk is sitting on a SiO2 pedestal. For the asymmetrical access waveguide coupling to the submicron disk, the slab height hslab is about 100 nm, and the rib width wrib is about 305 nm. The coupling gap is about 65 nm, as shown in Fig. 4(b). We note that there is a polarization rotation effect in the asymmetric silicon access waveguide due to the mode hybridization. In order to minimize the influence of the polarization rotation, the coupling region is designed to satisfy the critical coupling condition for the TM0 mode only while the critical coupling condition does not satisfy for the TE0 mode. In this way, only the resonance for the TM0 mode of the disk resonator becomes significant, as observed in the measured spectral responses shown below. And the grating coupler at the output end also plays a role of polarizer so that the TE0 mode is filtered out. In order to characterize the fabricated submicron-disk resonator, we use a tunable laser (Agilent 81600B) as the light source. The polarization of the light output from the tunable laser is adjusted by the polarization controller. A vertical coupling system with grating couplers was used to improve the fiber-chip coupling efficiency, and the response of the device at the output port is detected
Fig. 4. SEM pictures of the suspended submicron-disk (R 0.8 um). (a) Tilt view. (b) Top view.
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by the power meter (Aglient 81635). The spectral response of the fabricated submicron-disk resonator is measured when it is covered by the NaCl solution with different concentrations, i.e., c 0–25%. The corresponding refractive index of NaCl solution changes from 1.316 to 1.361. Before switching to measure the new salt solution with a different concentration, DI water was used to clean the sensor chip, which helps to remove the residual salt solution inside the sample reservoir [7]. Figure 5 shows the measured spectral response of the fabricated submicron-disk resonator when covered by the NaCl solution with different concentrations. The overall insertion loss is about 22 dB, which is mainly from the coupling loss of the grating coupler and the mode conversion loss. These spectral responses are normalized by the transmission of the straight waveguide. The wavelength range shown here is limited by the bandwidth of the grating coupler, while the theoretical FSR of the submicron disk is estimated to be around 130 nm to enable a broad measurement range. Figure 5 shows a major resonant wavelength λMRR0 with a high extinction ratio of >20 dB in the range of 1520–1535 nm. The loaded Q-value is about 100, which is comparable with the theoretical value of 112 obtained from a 3D-FDTD simulation. The extinction ratio and Q-value decrease with the increase of the NaCl solution concentration, which should be due to increased radiation loss and over-coupling of the disk-resonator caused by the increment of ambient refractive index. The inset of Fig. 5 shows the resonance wavelength λMRR0 shifts as the concentration varies. It can be seen that the sensitivity of the suspended submicron-disk resonator is about 130 nm∕RIU, which is close to the theoretical value of 150 nm∕RIU. As a comparison, the ring resonator-based optical sensor demonstrated in [19] has a sensitivity of 70 nm∕RIU. In [20], the detection limit (DL) is given as DL r∕S, where r is the sensor resolution (the minimal detectable change of the resonant wavelength). According to [19], a wavelength shift of 1∕15 of the peak width Δλpeak is usually measurable. In our case, one has Δλpeak 1.5 nm and S 130 nm∕RIU. Thus the estimated detection limit for the refractive index is about 8 × 10−4 RIU
Fig. 5. Measured transmission spectrums of the suspended submicron-disk resonator covered by NaCl solution with different concentrations (0%–25%). Inset: the resonant wavelength shifts as the concentration varies.
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organic liquids. It has been shown that the present suspended submicron-disk optical sensor has an improved sensitivity (∼130 nm∕RIU) due to the enhanced evanescent field. This project was supported by a 973 project (2011CB503700), NSFC project (61077040), Zhejiang provincial NSF (LY12A04010), and the Fundamental Research Funds for the Central Universities.
Fig. 6. Measured transmission spectrums of the suspended submicron-disk sensor covered with different organic liquids. Inset: the resonant wavelength shifts as the refractive index of organic liquid varies.
(corresponding to the concentration change of 0.4%). An improved DL can be achieved when using the present suspended submicron-disk resonator for gas sensing because of the improved Q-value. The present submicron-disk sensor also is used to measure the refractive indices of different organic liquids, including DI water (n 1.316) [7], methanol (n 1.326) [21], ethanol (n 1.354) [7], and isopropyl alcohol (IPA, n 1.364) [22]. The measured transmission responses are shown in Fig. 6. It can be seen that the Q-value and the extinction ratio are similar to those shown in Fig. 5. The inset of Fig. 6 shows the resonance wavelength λMRR0 shifts as the refractive index varies. It can be seen that the sensitivity of the suspended submicron-disk resonator is about 130 nm∕RIU, which is consistent with the measurement result for a salt solution. In summary, we have demonstrated an ultra-small optical sensor by using a suspended-silicon submicron-disk resonator sitting on an SiO2 pedestal. An asymmetrical suspended rib waveguide is introduced to work as the access waveguide of the suspended submicron-disk resonator, so that it is convenient to couple light into/out from the resonator. We have designed and fabricated a suspended submicron-disk sensor with R 0.8 μm, which has a very high extinction ratio (>30 dB) and a moderate Q-value (∼102 ). It also has an ultra-large FSR so that a large measurement range is available. The present submicron-disk sensor has been used to measure the concentration of NaCl aqueous solutions and some
References 1. R. Soref, IEEE J. Sel. Top. Quantum Electron. 12, 1678 (2006). 2. D. X. Xu, A. Densmore, A. Delâge, P. Waldron, R. McKinnon, S. Janz, J. Lapointe, G. Lopinski, T. Mischki, E. Post, P. Cheben, and J. H. Schmid, Opt. Express 16, 15137 (2008). 3. Z. Xia, Y. Chen, and Z. Zhou, IEEE J. Quantum Electron. 44, 100 (2008). 4. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Nature 435, 325 (2005). 5. D.-X. Xu, A. Densmore, P. Waldron, J. Lapointe, E. Post, A. Delâge, S. Janz, P. Cheben, J. H. Schmid, and B. Lamontagne, Opt. Express 15, 3149 (2007). 6. T. Lei and A. W. Poon, Opt. Express 19, 22227 (2011). 7. J. Wang and D. Dai, Opt. Lett. 35, 4229 (2010). 8. Q. Xu, D. Fattal, and R. G. Beausoleil, Opt. Express 16, 4309 (2008). 9. K. Yao and Y. Shi, Opt. Express 20, 27039 (2012). 10. S. H. Mirsadeghi, E. Schelew, and J. F. Young, Appl. Phys. Lett. 102, 131115 (2013). 11. H. Lee, T. Chen, L. Jiang, K. Y. Yang, S. Jeon, O. Painter, and K. J. Vahala, Nat. Photonics 6, 369 (2012). 12. M. Soltani, Q. Li, S. Yegnanarayanan, and A. Adibi, Opt. Express 18, 19541 (2010). 13. J. Shainline, S. Elston, Z. Liu, G. Fernandes, R. Zia, and J. Xu, Opt. Express 17, 23323 (2009). 14. M. Borselli, T. J. Johnson, and O. Painter, Appl. Phys. Lett. 88, 131114 (2006). 15. S. Koseki, B. Zhang, K. De Greve, and Y. Yamamoto, Appl. Phys. Lett. 94, 051110 (2009). 16. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, J. Lightwave Technol. 20, 1968 (2002). 17. C.-Y. Chao and L. Guo, J. Lightwave Technol. 24, 1395 (2006). 18. Z. Wang, Y. Tang, L. Wosinski, and S. He, IEEE Photon. Technol. Lett. 22, 1568 (2010). 19. K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, Opt. Express 15, 7610 (2007). 20. I. M. White and X. Fan, Opt. Express 16, 1020 (2008). 21. W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, Appl. Phys. Lett. 86, 151122 (2005). 22. C. Kim and C. Su, Meas. Sci. Technol. 15, 1683 (2004).
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Suspended ultra-small disk resonator on silicon for optical sensing Xiaokun Wang, Xiaowei Guan, Qiangsheng Huang, Jiajiu Zheng, Yaocheng Shi, and Daoxin Dai* Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China *Corresponding author: [email protected] Received September 13, 2013; revised November 1, 2013; accepted November 13, 2013; posted November 18, 2013 (Doc. ID 197614); published December 11, 2013 An ultra-small disk resonator consisting of a suspended silicon disk with a submicron bending radius sitting on an SiO2 pedestal is demonstrated experimentally. An asymmetrical suspended rib waveguide is integrated as the access waveguide for the suspended submicron disk resonator, which is used to realize an ultra-small optical sensor with an improved sensitivity due to the enhanced evanescent field interaction with the analyte. The present optical sensor also has a large measurement range because of the ultra-large free-spectral range of the submicron-disk resonator. As an example, a suspended submicron disk sensor with a bending radius of 0.8 μm is designed, fabricated, and characterized. The concentration of NaCl aqueous solution and organic liquids is measured with the suspended submicron-disk sensor, and the measured sensitivity is about 130 nm∕RIU, which agrees well with the simulation value. © 2013 Optical Society of America OCIS codes: (130.2790) Guided waves; (130.3120) Integrated optics devices; (230.7370) Waveguides. http://dx.doi.org/10.1364/OL.38.005405
Silicon-photonic-integrated devices have been extensively investigated for many applications (optical interconnect [1], optical sensing [2,3], etc.) because of CMOS compatibility and ultra-compactness. Among various silicon-photonic-integrated devices, an optical microcavity is well known as a versatile element to realize various functionality components, including optical modulators [4] and optical filters [5] as well as optical sensors [6,7]. The optical sensors based on optical microcavities have attracted lots of attention since there is a field enhancement in the microcavity when operating at the resonance wavelength [2,3]. It is well known that small size is universally desirable for most applications of microcavities [8]. In [8], a silicon microring resonator with 1.5 μm radius was demonstrated for the application of optical modulators. Similarly, for optical-sensing applications, there are also some benefits with small microcavities. First, a smaller microcavity sensor has a larger free-spectral range (FSR) so that a larger measurement range is available. Second, the sample volume to be measured is less when using a smaller microcavity sensor. Third, one can integrate more sensor elements in a single chip to have a sensor array with a higher integration density. A photonic crystal microcavity could be used for optical sensors with a small footprint and small mode volume [9]. A drawback is that a photonic crystal cavity usually has high excess loss, e.g., ∼17 dB as shown in [10]. A simple way to have a low-loss optical microcavity is using a conventional optical-cavity waveguide with improved light confinement. For the case with a siliconon-insulator (SOI) platform, which is the most popular for silicon photonics, one can make a suspended silicon optical cavity by replacing the buried oxide (BOX) with another lower-index material (e.g., air). A disk resonator is considered here since it is one of the best options to support a suspended structure among various optical cavities. Furthermore, a microdisk resonator is also helpful to have low loss and a high Q value [11,12]. 0146-9592/13/245405-04$15.00/0
A high-Q-suspended microdisk resonator without any access waveguide integrated has been demonstrated previously [13,14]. In that case, one usually has to use a tapered microfiber to couple light into the resonator, which is not reliable and convenient. It is also not ideal to be extended to form an array by cascading several resonators. A suspended GaAs microdisk resonator with an access waveguide has been demonstrated previously [15]. In this Letter, we demonstrate a suspended silicon ultrasmall disk resonator with an access waveguide. The disk resonator has a submicron bending radius (R < 1 μm) to realize a submicron optical sensor with a large measurement range for measuring the concentrations of NaCl solutions and different organic liquids. Figure 1 shows the schematic configuration of the present suspended submicron-disk resonator for optical sensing. It can be seen that there is a suspended submicron disk sitting on a SiO2 pedestal. An asymmetrical suspended rib waveguide is placed at the side as the access waveguide so that the measurement is convenient and robust. It is also easy to achieve a sensor array by cascading several resonators. When using the suspended disk resonator as an optical sensor to measure
Fig. 1. Present suspended silicon submicron-disk resonator with a suspended access waveguide. © 2013 Optical Society of America
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by choosing a smaller bending radius. Therefore we choose R 0.8 μm in the following design. The intrinsic Q-value of a submicron-disk resonator is calculated by [16]
the refractive index of gas or liquid, the sensitivity is improved intrinsically due to the enhanced evanescent field interaction with the analyte in comparison with the regular unsuspended disk resonator with BOX beneath. In addition, the suspended structure is helpful to reduce the bending loss so that one can realize a submicron-disk resonator, which has an ultra-large FSR and thus a large measurement range for optical sensing. Regarding that the bending loss becomes dominant for a submicron-disk resonator [8], we calculate the complex effective refractive index neff and the field profiles of the whispering-gallery mode of a submicron-disk resonator by using a finite-element method. The bending loss for a 90° bending is then given by L 20 lgexpnim k0 Rπ∕2, where k0 is the wavenumber (k0 2π∕λ), R is the bending radius, nim is the imaginary part for the complex effective index neff . Figure 2(a) shows the calculated bending loss L of the fundamental modes (TE0 and TM0 ) as well as the first higher-order modes (TE1 and TM1 ) of a suspended microdisk resonator as the radius R varies. In order to give a comparison, the result for an unsuspended microdisk resonator with BOX is also given, as shown in Fig. 2(b). For the present calculation, the cladding is air (nc 1) or deionized (DI) water (n 1.316). The SOI wafer has a 340 nm thick top silicon layer and a 1 μm thick SiO2 insulator layer (which is thick enough to guarantee a low substrate leaky loss). The refractive indices of silicon and SiO2 are 3.455 and 1.445, respectively. And the operation wavelength is 1550 nm. Figure 2 shows that the suspended microdisk resonator has lower loss than the unsuspended one, which is due to the improved refractive index contrast. The comparison between Figs. 2(a) and 2(b) also shows that the bending loss for TE mode is lower than that for TM mode, which is due to the higher confinement ability for TE mode. As is known, a microdisk resonator usually supports the higher-order mode resonance, which is undesired for many applications, including optical sensing considered here. Fortunately, the higher-order modes (TE1 and TM1 ) have a large loss for the present submicron-disk resonator with a bending radius of less than 0.8 μm, as shown in Figs. 2(a) and 2(b). Therefore, the resonance for the higher-order modes is depressed significantly, which will be verified from the measured spectral responses below. For the disk with a larger radius, the Q value of the diskresonator improves, while multimode effects become severer because the higher-order modes are supported well with a low bending loss, as shown in Fig. 2(a). Furthermore, regarding that the FSR is inversely proportional to the bending radius, one can have a larger FSR
where λc is the resonant wavelength, and nc is the cladding index. One sees that the sensitivity S can be improved by enhancing either the device sensitivity S d or the waveguide sensitivity S w . For disk resonators, one has S d ∂λc ∕∂neff λc ∕ng . Note that the group index ng usually changes slightly as the waveguide dimension varies. For the present SOI nanowire, the group index ng varies slightly from 4.58 to 4.44 when the waveguide width increases from 300 to 500 nm. As a consequence, the device sensitivity S d does not change notably by optimizing the waveguide dimension. In this Letter, we focus on improvement of the waveguide sensitivity S w by introducing a suspended waveguide, which has enhanced evanescent field interaction with analyte. This can be seen from Fig. 3, which shows the calculated field profile for the TM0 mode of the suspended and unsuspended submicron-disks covered by DI water when R 0.8 μm. For the suspended disk, the mode confinement factor in silicon is about 66%. The calculated waveguide sensitivities for the suspended and unsuspended submicron-disk resonator are S w 0.329 and 0.203, respectively. Their device sensitivities are S d 457 and 462 nm∕RIU. Their sensitivities are S 150 and
Fig. 2. Calculated bending losses of the TE0 , TM0 , TE1 , and TM1 modes in a submicron-disk resonator when the cladding is air (thick curves) or DI water (thin curves). (a) Suspended. (b) Unsuspended.
Fig. 3. Profile for the TM0 mode in a submicron disk when R 0.8 μm. (a) Unsuspended. (b) Suspended.
Qi 2πng ∕αλ0 ng ∕2nim ;
(1)
where ng is the group index. It is expected that the suspended submicron-disk resonator has a higher Q-value than the unsuspended one because of the lower bending loss [Fig. 2(a)]. For a suspended submicron-disk resonator with R 0.8 μm, the calculated intrinsic Q-values are about 12,000 and 1955, respectively, for TE and TM polarizations. When the suspended submicron-disk resonator is covered by, e.g., DI water (n 1.316), the Q-values for the TE and TM modes degrade to 680 and 300, respectively. The Q-value degradation is due to the increased bending loss as shown in Fig. 2(a). When using the submicron-disk resonator for optical sensing, the sensitivity is determined by the device sensitivity S d ∂λc ∕∂neff and the waveguide sensitivity S w ∂neff ∕∂nc [17], i.e., S
∂λc ∂λc ∂neff ≡ Sd Sw ; ∂nc ∂neff ∂nc
(2)
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94 nm∕RIU, respectively. It can be seen that the sensitivity is improved by 60%. The fabrication was started with a SOI wafer with a 340 nm thick top silicon layer and a 1 μm thick buried SiO2 layer. A double-etching process was carried out to form the asymmetrical access optical waveguide and the submicron disk. In our case, the patterns were formed with positive E-beam lithography-resist ZEP520A by using a Raith150-II machine, and a dry etching process is implemented by using an STS inductively coupled plasma system. A third etching process was carried out to fabricate grating couplers for efficient fiberchip coupling [18]. Here the grating coupler working for TM polarization is designed and fabricated with a grating period of 810 nm, a duty cycle of 0.5, and an etching depth of 170 nm. And the coupling efficiency is estimated to be 28% at the central wavelength. A 300 nm thick SiO2 layer is then deposited on the top to protect the silicon layer. A thin chromium layer (∼50 nm) was deposited and patterned as the hard mask in the following SiO2 wet-etching process. Finally, the SiO2 layer is corroded with the buffered SiO2 etchant (a mixture of NH4 F, CH3 COOH, DI water, and ethylene glycol), and the wet-etching depth is about 400 nm. Figure 4(a) shows the scanning electron microscopy (SEM) picture of the fabricated submicron-disk resonator whose bending radius is about R 0.8 μm. From this figure, it can be seen clearly that the silicon submicrondisk is sitting on a SiO2 pedestal. For the asymmetrical access waveguide coupling to the submicron disk, the slab height hslab is about 100 nm, and the rib width wrib is about 305 nm. The coupling gap is about 65 nm, as shown in Fig. 4(b). We note that there is a polarization rotation effect in the asymmetric silicon access waveguide due to the mode hybridization. In order to minimize the influence of the polarization rotation, the coupling region is designed to satisfy the critical coupling condition for the TM0 mode only while the critical coupling condition does not satisfy for the TE0 mode. In this way, only the resonance for the TM0 mode of the disk resonator becomes significant, as observed in the measured spectral responses shown below. And the grating coupler at the output end also plays a role of polarizer so that the TE0 mode is filtered out. In order to characterize the fabricated submicron-disk resonator, we use a tunable laser (Agilent 81600B) as the light source. The polarization of the light output from the tunable laser is adjusted by the polarization controller. A vertical coupling system with grating couplers was used to improve the fiber-chip coupling efficiency, and the response of the device at the output port is detected
Fig. 4. SEM pictures of the suspended submicron-disk (R 0.8 um). (a) Tilt view. (b) Top view.
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by the power meter (Aglient 81635). The spectral response of the fabricated submicron-disk resonator is measured when it is covered by the NaCl solution with different concentrations, i.e., c 0–25%. The corresponding refractive index of NaCl solution changes from 1.316 to 1.361. Before switching to measure the new salt solution with a different concentration, DI water was used to clean the sensor chip, which helps to remove the residual salt solution inside the sample reservoir [7]. Figure 5 shows the measured spectral response of the fabricated submicron-disk resonator when covered by the NaCl solution with different concentrations. The overall insertion loss is about 22 dB, which is mainly from the coupling loss of the grating coupler and the mode conversion loss. These spectral responses are normalized by the transmission of the straight waveguide. The wavelength range shown here is limited by the bandwidth of the grating coupler, while the theoretical FSR of the submicron disk is estimated to be around 130 nm to enable a broad measurement range. Figure 5 shows a major resonant wavelength λMRR0 with a high extinction ratio of >20 dB in the range of 1520–1535 nm. The loaded Q-value is about 100, which is comparable with the theoretical value of 112 obtained from a 3D-FDTD simulation. The extinction ratio and Q-value decrease with the increase of the NaCl solution concentration, which should be due to increased radiation loss and over-coupling of the disk-resonator caused by the increment of ambient refractive index. The inset of Fig. 5 shows the resonance wavelength λMRR0 shifts as the concentration varies. It can be seen that the sensitivity of the suspended submicron-disk resonator is about 130 nm∕RIU, which is close to the theoretical value of 150 nm∕RIU. As a comparison, the ring resonator-based optical sensor demonstrated in [19] has a sensitivity of 70 nm∕RIU. In [20], the detection limit (DL) is given as DL r∕S, where r is the sensor resolution (the minimal detectable change of the resonant wavelength). According to [19], a wavelength shift of 1∕15 of the peak width Δλpeak is usually measurable. In our case, one has Δλpeak 1.5 nm and S 130 nm∕RIU. Thus the estimated detection limit for the refractive index is about 8 × 10−4 RIU
Fig. 5. Measured transmission spectrums of the suspended submicron-disk resonator covered by NaCl solution with different concentrations (0%–25%). Inset: the resonant wavelength shifts as the concentration varies.
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OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
organic liquids. It has been shown that the present suspended submicron-disk optical sensor has an improved sensitivity (∼130 nm∕RIU) due to the enhanced evanescent field. This project was supported by a 973 project (2011CB503700), NSFC project (61077040), Zhejiang provincial NSF (LY12A04010), and the Fundamental Research Funds for the Central Universities.
Fig. 6. Measured transmission spectrums of the suspended submicron-disk sensor covered with different organic liquids. Inset: the resonant wavelength shifts as the refractive index of organic liquid varies.
(corresponding to the concentration change of 0.4%). An improved DL can be achieved when using the present suspended submicron-disk resonator for gas sensing because of the improved Q-value. The present submicron-disk sensor also is used to measure the refractive indices of different organic liquids, including DI water (n 1.316) [7], methanol (n 1.326) [21], ethanol (n 1.354) [7], and isopropyl alcohol (IPA, n 1.364) [22]. The measured transmission responses are shown in Fig. 6. It can be seen that the Q-value and the extinction ratio are similar to those shown in Fig. 5. The inset of Fig. 6 shows the resonance wavelength λMRR0 shifts as the refractive index varies. It can be seen that the sensitivity of the suspended submicron-disk resonator is about 130 nm∕RIU, which is consistent with the measurement result for a salt solution. In summary, we have demonstrated an ultra-small optical sensor by using a suspended-silicon submicron-disk resonator sitting on an SiO2 pedestal. An asymmetrical suspended rib waveguide is introduced to work as the access waveguide of the suspended submicron-disk resonator, so that it is convenient to couple light into/out from the resonator. We have designed and fabricated a suspended submicron-disk sensor with R 0.8 μm, which has a very high extinction ratio (>30 dB) and a moderate Q-value (∼102 ). It also has an ultra-large FSR so that a large measurement range is available. The present submicron-disk sensor has been used to measure the concentration of NaCl aqueous solutions and some
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