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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 2 — Jan. 17, 2011
  • pp: 861–869
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Fabrication and characterization of High Q polymer micro-ring resonator and its application as a sensitive ultrasonic detector

Tao Ling, Sung-Liang Chen, and L. Jay Guo  »View Author Affiliations


Optics Express, Vol. 19, Issue 2, pp. 861-869 (2011)
http://dx.doi.org/10.1364/OE.19.000861


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Abstract

Smooth sidewall silicon micro-ring molds have been fabricated using resist reflow and thermal oxidation method. High Q factor polymer micro-ring resonators have been fabricated using these molds. Quality factors as high as 105 have been measured at telecommunication wavelength range. By carefully examining the different loss mechanisms in polymer micro-ring, we find that the surface scattering loss can be as low as 0.23 dB/cm, much smaller than the absorption loss of the polystyrene polymer used in our devices. When used as an ultrasound detector such a high Q polymer micro-ring device can achieve an acoustic sensitivity around 36.3 mV/kPa with 240 μW operating power. A noise equivalent pressure (NEP) is around 88 Pa over a bandwidth range of 1–75 MHz. We have improved the NEP by a factor of 3 compared to our previous best result.

© 2011 OSA

1. Introduction

Recently, an optical cavity based ultrasound detection platform has attracted increasing interest [3

3. H. Nakano, Y. Matsuda, and S. Nagai, “Ultrasound detection by using a confocal Fabry-Perot interferometer with phase-modulated light,” Ultrasonic 37(3), 257–259 (1999). [CrossRef]

6

6. E. Z. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]

,17

17. S. W. Huang, S. L. Chen, T. Ling, A. Maxwell, M. O’Donnell, L. J. Guo, and S. Ashkenazi, “Low-noise wideband ultrasound detection using polymer microring resonators,” Appl. Phys. Lett. 92(19), 193509 (2008). [CrossRef]

]. Compared with conventional piezoelectric transducers, the new detection platform provides several advantages, such as preserving high sensitivity with reduced element sizes, high-frequency and wideband response with simple fabrication. In this new optical cavity based detection platform, optically transparent polymer material was used because of its high optical elastic coefficient and high deformability, which can provide a sensitive response under acoustic pressure. A polymer based fiber tip Fabry-Perot (F-P) cavity ultrasonic hydrophone has been demonstrated with comparable sensitivity and noise equivalent pressure (NEP) to current piezoelectric PVDF ultrasonic sensing devices [5

5. P. C. Beard and T. N. Mills, “Miniature optical fibre ultrasonic hydrophone using a Fabry-Perot polymer film interferometer,” Electron. Lett. 33(9), 801–803 (1997). [CrossRef]

]. An improved sensitivity has been realized in a polymer based plane F-P device by increasing the device’s Q factor, which results from an increased reflection of the multi-layer stacks based 1-dimension photonic crystal mirror based on multi-layer stacks [6

6. E. Z. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]

]. Additional improvement of the sensitivity can be achieved by further increasing the reflectivity of the 1-D photonic crystal mirror, but this requires a sophisticated system to precisely control the thickness and uniformity of the deposited multilayer film. Except for the photonic crystal based light confining mechanism, the total internal reflection (TIR) mechanism has been widely used to confine the light in the cavity to achieve a much higher Q factor. High Q resonators using the TIR mechanism include micro-spheres [7

7. M. Cai, O. Painter, K. J. Vahala, and P. C. Sercel, “Fiber-coupled microsphere laser,” Opt. Lett. 25(19), 1430–1432 (2001). [CrossRef]

,8

8. F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett. 80(21), 4057–4069 (2002). [CrossRef]

], micro-disks [9

9. M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85(17), 3693–3695 (2004). [CrossRef]

,10

10. T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83(4), 797–799 (2003). [CrossRef]

], micro-rings [11

11. A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express 17(14), 11366–11370 (2009), http://www.opticsinfobase.org/abstract.cfm?uri=oe-17-14-11366. [CrossRef] [PubMed]

14

14. H. S. Sun, A. T. Chen, B. C. Olbricht, J. A. Davies, P. A. Sullivan, Y. Liao, and L. R. Dalton, “Direct electron beam writing of electro-optic polymer microring resonators,” Opt. Express 16(9), 6592–6599 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-9-6592. [CrossRef] [PubMed]

] and micro-tubes [15

15. I. M. White, H. Oveys, and X. D. Fan, “Liquid-core optical ring-resonator sensors,” Opt. Lett. 31(9), 1319–1321 (2006). [CrossRef] [PubMed]

,16

16. T. Ling and L. J. Guo, “A unique resonance mode observed in a prism-coupled micro-tube resonator sensor with superior index sensitivity,” Opt. Express 15(25), 17424–17432 (2007), http://www.opticsinfobase.org/abstract.cfm?uri=oe-15-25-17424. [CrossRef] [PubMed]

]. By combining a polymer material’s high optical elastic coefficient and high deformability with the TIR-based high Q factor micro-resonators, we have previously demonstrated wideband ultrasound detection with a NEP 20 times better than the best piezoelectric transducer of similar size and bandwidth by using polymer micro-ring devices with a Q of around 6000 [17

17. S. W. Huang, S. L. Chen, T. Ling, A. Maxwell, M. O’Donnell, L. J. Guo, and S. Ashkenazi, “Low-noise wideband ultrasound detection using polymer microring resonators,” Appl. Phys. Lett. 92(19), 193509 (2008). [CrossRef]

].

Ultrasound detectors with improved sensitivity can directly benefit ultrasound and PA imaging by allowing imaging at increased depth. Smaller micro-rings have larger angular sensitivity, enabling higher spatial resolution in imaging applications such as beam-forming and PA tomography. Higher sensitivity can be obtained by using the micro-ring resonator with a higher Q factor, which normally requires a ring waveguide with smooth sidewalls to minimize the surface roughness induced scattering loss. Smooth sidewalls are even more important for small size micro-ring devices due to increased scattering loss as compared to large size micro-ring devices. To achieve a smooth sidewall in waveguides, several methods have been used. A thermal reflow method has been used to fabricate ultra-high Q micro-toroids [18

18. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]

], but the cavity size can shrink by tens of microns during the reflow process which makes it very difficult to fabricate the on chip integrated waveguides coupled to such high Q micro-cavities. A smooth buffer layer modified stamp [19

19. D. H. Kim, J. G. Im, S. S. Lee, S. W. Ahn, and K. D. Lee, “Polymeric microring resonator using nanoimprint technique based on a stamp incorporating a smoothing buffer layer,” IEEE Photon. Technol. Lett. 17(11), 2352–2354 (2005). [CrossRef]

] has been used to fabricate an embedded polymer race-track micro-ring with a Q factor as high as 105; however, it would be challenging to apply this to micro-rings with small diameters because a very narrow gap from the bus waveguide of few hundred nanometers is required for efficient coupling.

2. Device fabrication and measurement technique

In order to fabricate high Q factor polymer micro-rings using nano-imprint technique, a mold with smooth sidewalls is needed. Many materials such as glass, polymer, metal, and silicon have been examined for mold applications. Among these materials, silicon is the most attractive material candidate due to the excellent processibility of silicon and a wide range of fabrication processes developed by the integrated circuit (IC) industry. In particular dry etching of silicon has been shown to achieve smooth and vertical sidewalls [22

22. H. C. Liu, Y. H. Lin, and W. Hsu, “Sidewall roughness control in advanced silicon etch process,” Microsyst. Technol. 10(1), 29–34 (2003). [CrossRef]

], which is ideal for our application. Our silicon mold is fabricated using electron beam lithography followed by reactive ion etch (RIE). Two important modifications are used to further smooth the silicon mold sidewalls. The process starts with a silicon wafer with an initial 400 nm thermal oxide layer. Electron beam lithography is used to create a pattern with a micro-ring and a straight bus waveguide on an 800 nm thick positive electron-beam resist (950k PMMA). The patterns are then transferred to the silicon oxide layer using RIE with PMMA as the etch mask. After this step the PMMA mask is removed in hot acetone. The micro-ring pattern is further transferred onto the silicon wafer by deep silicon etching with the silicon oxide layer as etch mask. After deep silicon etching, the silicon oxide masking layer is removed by buffered hydrofluoric acid (BHF), which completes the master mold fabrication. Two important steps have been added to this silicon mold fabrication process to achieve smooth sidewalls.

The first important step is the PMMA resist reflow, which is applied before the silicon oxide etch step. By choosing a suitable temperature and time duration, this reflow process can greatly reduce imperfections in the PMMA resist patterns and harden the edge of the PMMA resist. Too low of a temperature will not cause the resist to reflow, while too high of a temperature will cause deformation in the coupling gap region, which could significantly increase the optical loss. After a number of experiments, the appropriate PMMA resist reflowing temperature and time duration was determined to be 115°C for 90 seconds. Figure 1
Fig. 1 Sidewall SEM image of the polymer micro-ring fabricated from the mold: (a) without resist reflow process, (b) with resist reflow process, (c) with resist reflow and thermal oxidation process.
show the SEM pictures of the imprinted polystyrene micro-ring waveguide sidewalls by using the silicon molds with (1b) and without (1a) the PMMA resist reflow process. We can clearly see from Fig. 1(a) that there is a great amount of roughness on the sidewall of the polymer micro-ring, which is thought to be due to the damage caused by RIE on the edge of the PMMA pattern during etching of SiO2, which eventually gets transferred onto the silicon mold. However, in Fig. 1(b), the sidewall of the polymer micro-ring fabricated from the mold with the PMMA reflowing process has relatively small vertical roughness, which means that the edges of the PMMA are well protected during the RIE process. The spectrum characterization of these two devices will be shown in Section 3.

The second important step is the thermal oxidation followed by the BHF etching step, which was used to smooth out the roughness after deep silicon etching. The oxidation step is performed in a high temperature furnace that grows around 100 nm of thermal oxide on the silicon surface. During the oxidation process the amount of Si consumed is 44% of final oxide thickness, the rough Si surface layer is converted to SiO2 in this step and removed by BHF etching after the oxidation step. Figure 1(c) shows the sidewall of the imprinted polymer micro-ring using the silicon mold after the thermal oxidation and BHF etching step. We see a major improvement in the sidewall roughness compared to Fig. 1(b). Most of the sharp ripple-like roughness and small holes on the sidewall seen in Fig. 1b have disappeared in Fig. 1(c).

After fabrication, the polymer micro-ring transmission spectra were characterized using a tunable laser (Santec TSL-220) with a wavelength range of 1530-1610nm. The laser’s line-width is much smaller than the resonance peak line-width of the polymer micro-ring. The input light polarization was controlled by a fiber based optical polarization controller. In order to make our measurements consistent, we fixed the input light as TE polarization.

3. Polymer micro-ring’s loss characterization

4. Acoustic sensitivity characterization

5. Summary

Acknowledgements

The authors acknowledge the support by National Institutes of Health (NIH) grant EB007619.

References and links

1.

S. Srinivasan, H. S. Baldwin, O. Aristizabal, L. Kwee, M. Labow, M. Artman, and D. H. Turnbull, “Noninvasive, in utero imaging of mouse embryonic heart development with 40-MHz echocardiography,” Circulation 98(9), 912–918 (1998). [PubMed]

2.

H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]

3.

H. Nakano, Y. Matsuda, and S. Nagai, “Ultrasound detection by using a confocal Fabry-Perot interferometer with phase-modulated light,” Ultrasonic 37(3), 257–259 (1999). [CrossRef]

4.

S. Ashkenazi, Y. Hou, T. Buma, and M. O’Donnell, “Optoacoustic imaging using thin polymer etalon,” Appl. Phys. Lett. 86(13), 134102 (2005). [CrossRef]

5.

P. C. Beard and T. N. Mills, “Miniature optical fibre ultrasonic hydrophone using a Fabry-Perot polymer film interferometer,” Electron. Lett. 33(9), 801–803 (1997). [CrossRef]

6.

E. Z. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]

7.

M. Cai, O. Painter, K. J. Vahala, and P. C. Sercel, “Fiber-coupled microsphere laser,” Opt. Lett. 25(19), 1430–1432 (2001). [CrossRef]

8.

F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett. 80(21), 4057–4069 (2002). [CrossRef]

9.

M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85(17), 3693–3695 (2004). [CrossRef]

10.

T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83(4), 797–799 (2003). [CrossRef]

11.

A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express 17(14), 11366–11370 (2009), http://www.opticsinfobase.org/abstract.cfm?uri=oe-17-14-11366. [CrossRef] [PubMed]

12.

P. Rabiei, W. H. Steier, Cheng Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]

13.

C. Y. Chao and L. J. Guo, “Biochemical sensors based on polymer microrings with sharp asymmetrical resonances,” Appl. Phys. Lett. 83(8), 527–529 (2003). [CrossRef]

14.

H. S. Sun, A. T. Chen, B. C. Olbricht, J. A. Davies, P. A. Sullivan, Y. Liao, and L. R. Dalton, “Direct electron beam writing of electro-optic polymer microring resonators,” Opt. Express 16(9), 6592–6599 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-9-6592. [CrossRef] [PubMed]

15.

I. M. White, H. Oveys, and X. D. Fan, “Liquid-core optical ring-resonator sensors,” Opt. Lett. 31(9), 1319–1321 (2006). [CrossRef] [PubMed]

16.

T. Ling and L. J. Guo, “A unique resonance mode observed in a prism-coupled micro-tube resonator sensor with superior index sensitivity,” Opt. Express 15(25), 17424–17432 (2007), http://www.opticsinfobase.org/abstract.cfm?uri=oe-15-25-17424. [CrossRef] [PubMed]

17.

S. W. Huang, S. L. Chen, T. Ling, A. Maxwell, M. O’Donnell, L. J. Guo, and S. Ashkenazi, “Low-noise wideband ultrasound detection using polymer microring resonators,” Appl. Phys. Lett. 92(19), 193509 (2008). [CrossRef]

18.

D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]

19.

D. H. Kim, J. G. Im, S. S. Lee, S. W. Ahn, and K. D. Lee, “Polymeric microring resonator using nanoimprint technique based on a stamp incorporating a smoothing buffer layer,” IEEE Photon. Technol. Lett. 17(11), 2352–2354 (2005). [CrossRef]

20.

C. Y. Chao and L. J. Guo, “Polymer Micro-ring Resonators Fabricated by Nanoimprint Technique,” J. Vac. Sci. Technol. B 20(6), 2862–2866 (2002). [CrossRef]

21.

K. K. Lee, D. R. Lim, L. C. Kimerling, J. Shin, and F. Cerrina, “Fabrication of ultralow-loss Si/SiO(2) waveguides by roughness reduction,” Opt. Lett. 26(23), 1888–1890 (2001). [CrossRef]

22.

H. C. Liu, Y. H. Lin, and W. Hsu, “Sidewall roughness control in advanced silicon etch process,” Microsyst. Technol. 10(1), 29–34 (2003). [CrossRef]

23.

C. Y. Chao and L. J. Guo, “Reduction of Surface Scattering Loss in Polymer Microrings Using Thermal-Reflow Technique,” IEEE Photon. Technol. Lett. 16(6), 1498–1500 (2004). [CrossRef]

24.

K. J. Vahala, Optical Microcavities (World Scientific 2004), Chapter 7.

25.

M. Oxborrow, “How to simulate the whispering gallery modes of dielectric microresonator in FEMLAB/COMSOL,” Proc. SPIE 6452, 64520J, 64520J-12 (2007). [CrossRef]

26.

R. K. Chang, and A. J. Campillo, Optical Processes in Microcavities (World scientific 1996), Chapter 6.

27.

V. R. Almeida and M. Lipson, “Optical bistability on a silicon chip,” Opt. Lett. 29(20), 2387–2389 (2004). [CrossRef] [PubMed]

28.

M. Borselli, T. J. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express 13(5), 1515–1530 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=OPEX-13-5-1515. [CrossRef] [PubMed]

29.

R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer Science and Business Media 2009), Chapter 5.

30.

J. R. Schwesyg, T. Beckmann, A. S. Zimmermann, K. Buse, and D. Haertle, “Fabrication and characterization of whispering-gallery-mode resonators made of polymers,” Opt. Express 17(4), 2573–2578 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-4-2573. [CrossRef] [PubMed]

OCIS Codes
(130.6010) Integrated optics : Sensors
(140.4780) Lasers and laser optics : Optical resonators
(170.7170) Medical optics and biotechnology : Ultrasound
(230.5750) Optical devices : Resonators

ToC Category:
Sensors

History
Original Manuscript: October 27, 2010
Revised Manuscript: December 18, 2010
Manuscript Accepted: December 23, 2010
Published: January 6, 2011

Citation
Tao Ling, Sung-Liang Chen, and L. Jay Guo, "Fabrication and characterization of High Q polymer micro-ring resonator and its application as a sensitive ultrasonic detector," Opt. Express 19, 861-869 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-2-861


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References

  1. S. Srinivasan, H. S. Baldwin, O. Aristizabal, L. Kwee, M. Labow, M. Artman, and D. H. Turnbull, “Noninvasive, in utero imaging of mouse embryonic heart development with 40-MHz echocardiography,” Circulation 98(9), 912–918 (1998). [PubMed]
  2. H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]
  3. H. Nakano, Y. Matsuda, and S. Nagai, “Ultrasound detection by using a confocal Fabry-Perot interferometer with phase-modulated light,” Ultrasonic 37(3), 257–259 (1999). [CrossRef]
  4. S. Ashkenazi, Y. Hou, T. Buma, and M. O’Donnell, “Optoacoustic imaging using thin polymer etalon,” Appl. Phys. Lett. 86(13), 134102 (2005). [CrossRef]
  5. P. C. Beard and T. N. Mills, “Miniature optical fibre ultrasonic hydrophone using a Fabry-Perot polymer film interferometer,” Electron. Lett. 33(9), 801–803 (1997). [CrossRef]
  6. E. Z. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]
  7. M. Cai, O. Painter, K. J. Vahala, and P. C. Sercel, “Fiber-coupled microsphere laser,” Opt. Lett. 25(19), 1430–1432 (2001). [CrossRef]
  8. F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett. 80(21), 4057–4069 (2002). [CrossRef]
  9. M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85(17), 3693–3695 (2004). [CrossRef]
  10. T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83(4), 797–799 (2003). [CrossRef]
  11. A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express 17(14), 11366–11370 (2009), http://www.opticsinfobase.org/abstract.cfm?uri=oe-17-14-11366 . [CrossRef] [PubMed]
  12. P. Rabiei, W. H. Steier, Cheng Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]
  13. C. Y. Chao and L. J. Guo, “Biochemical sensors based on polymer microrings with sharp asymmetrical resonances,” Appl. Phys. Lett. 83(8), 527–529 (2003). [CrossRef]
  14. H. S. Sun, A. T. Chen, B. C. Olbricht, J. A. Davies, P. A. Sullivan, Y. Liao, and L. R. Dalton, “Direct electron beam writing of electro-optic polymer microring resonators,” Opt. Express 16(9), 6592–6599 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-9-6592 . [CrossRef] [PubMed]
  15. I. M. White, H. Oveys, and X. D. Fan, “Liquid-core optical ring-resonator sensors,” Opt. Lett. 31(9), 1319–1321 (2006). [CrossRef] [PubMed]
  16. T. Ling and L. J. Guo, “A unique resonance mode observed in a prism-coupled micro-tube resonator sensor with superior index sensitivity,” Opt. Express 15(25), 17424–17432 (2007), http://www.opticsinfobase.org/abstract.cfm?uri=oe-15-25-17424 . [CrossRef] [PubMed]
  17. S. W. Huang, S. L. Chen, T. Ling, A. Maxwell, M. O’Donnell, L. J. Guo, and S. Ashkenazi, “Low-noise wideband ultrasound detection using polymer microring resonators,” Appl. Phys. Lett. 92(19), 193509 (2008). [CrossRef]
  18. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]
  19. D. H. Kim, J. G. Im, S. S. Lee, S. W. Ahn, and K. D. Lee, “Polymeric microring resonator using nanoimprint technique based on a stamp incorporating a smoothing buffer layer,” IEEE Photon. Technol. Lett. 17(11), 2352–2354 (2005). [CrossRef]
  20. C. Y. Chao and L. J. Guo, “Polymer Micro-ring Resonators Fabricated by Nanoimprint Technique,” J. Vac. Sci. Technol. B 20(6), 2862–2866 (2002). [CrossRef]
  21. K. K. Lee, D. R. Lim, L. C. Kimerling, J. Shin, and F. Cerrina, “Fabrication of ultralow-loss Si/SiO(2) waveguides by roughness reduction,” Opt. Lett. 26(23), 1888–1890 (2001). [CrossRef]
  22. H. C. Liu, Y. H. Lin, and W. Hsu, “Sidewall roughness control in advanced silicon etch process,” Microsyst. Technol. 10(1), 29–34 (2003). [CrossRef]
  23. C. Y. Chao and L. J. Guo, “Reduction of Surface Scattering Loss in Polymer Microrings Using Thermal-Reflow Technique,” IEEE Photon. Technol. Lett. 16(6), 1498–1500 (2004). [CrossRef]
  24. K. J. Vahala, Optical Microcavities (World Scientific 2004), Chapter 7.
  25. M. Oxborrow, “How to simulate the whispering gallery modes of dielectric microresonator in FEMLAB/COMSOL,” Proc. SPIE 6452, 64520J, 64520J-12 (2007). [CrossRef]
  26. R. K. Chang, and A. J. Campillo, Optical Processes in Microcavities (World scientific 1996), Chapter 6.
  27. V. R. Almeida and M. Lipson, “Optical bistability on a silicon chip,” Opt. Lett. 29(20), 2387–2389 (2004). [CrossRef] [PubMed]
  28. M. Borselli, T. J. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express 13(5), 1515–1530 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=OPEX-13-5-1515 . [CrossRef] [PubMed]
  29. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer Science and Business Media 2009), Chapter 5.
  30. J. R. Schwesyg, T. Beckmann, A. S. Zimmermann, K. Buse, and D. Haertle, “Fabrication and characterization of whispering-gallery-mode resonators made of polymers,” Opt. Express 17(4), 2573–2578 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-4-2573 . [CrossRef] [PubMed]

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