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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 9 — Apr. 23, 2012
  • pp: 10212–10217
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On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing

Jintian Lin, Shangjie Yu, Yaoguang Ma, Wei Fang, Fei He, Lingling Qiao, Limin Tong, Ya Cheng, and Zhizhan Xu  »View Author Affiliations


Optics Express, Vol. 20, Issue 9, pp. 10212-10217 (2012)
http://dx.doi.org/10.1364/OE.20.010212


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Abstract

We report on the fabrication of three-dimensional (3D) high-Q whispering gallery microcavities on a fused silica chip by femtosecond laser microfabriction, enabled by the 3D nature of femtosecond laser direct writing. The processing mainly consists of formation of freestanding microdisks by femtosecond laser direct writing and subsequent wet chemical etching. CO2 laser annealing is followed to smooth the microcavity surface. Microcavities with arbitrary tilting angle, lateral and vertical positioning are demonstrated, and the quality (Q)-factor of a typical microcavity is measured to be up to 1.07 × 106, which is currently limited by the low spatial resolution of the motion stage used during the laser patterning and can be improved with motion stages of higher resolutions.

© 2012 OSA

1. Introduction

Recently, femtosecond laser micromachining has been proved as a promising solution for high-precision and flexible fabrication of three dimensional (3D) microstructures, such as microoptics [7

7. Y. Cheng, H. L. Tsai, K. Sugioka, and K. Midorikawa, “Fabrication of 3D microoptical lenses in photosensitive glass using femtosecond laser micromachining,” Appl. Phys., A Mater. Sci. Process. 85(1), 11–14 (2006). [CrossRef]

8

8. E. Brasselet, M. Malinauskas, A. Žukauskas, and S. Juodkazis, “Photopolymerized microscopic vortex beam generators: precise delivery of optical orbital angular momentum,” Appl. Phys. Lett. 97(21), 211108 (2010). [CrossRef]

], microfluidics [9

9. A. Marcinkevicius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, and J. Nishii, “Femtosecond laser-assisted three-dimensional microfabrication in silica,” Opt. Lett. 26(5), 277–279 (2001). [CrossRef] [PubMed]

11

11. Y. Cheng, K. Sugioka, and K. Midorikawa, “Microfluidic laser embedded in glass by three-dimensional femtosecond laser microprocessing,” Opt. Lett. 29(17), 2007–2009 (2004). [CrossRef] [PubMed]

], optical waveguides [12

12. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

14

14. F. He, J. Lin, and Y. Cheng, “Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining,” Appl. Phys. B 105(2), 379–384 (2011). [CrossRef]

], microbiochips [15

15. K. Sugioka and Y. Cheng, “Integrated microchips for biological analysis fabricated by femtosecond laser direct writing,” MRS Bull. 36(12), 1020–1027 (2011). [CrossRef]

18

18. A. Schaap, Y. Bellouard, and T. Rohrlack, “Optofluidic lab-on-a-chip for rapid algae population screening,” Biomed. Opt. Express 2(3), 658–664 (2011). [CrossRef] [PubMed]

] and polymer-based microcavities [19

19. Z.-P. Liu, Y. Li, Y.-F. Xiao, B.-B. Li, X.-F. Jiang, Y. Qin, X.-B. Feng, H. Yang, and Q. Gong, “Direct laser writing of whispering gallery microcavities by two-photon polymerization,” Appl. Phys. Lett. 97(21), 211105 (2010). [CrossRef]

21

21. T. Grossmann, S. Schleede, M. Hauser, T. Beck, M. Thiel, G. von Freymann, T. Mappes, and H. Kalt, “Direct laser writing for active and passive high-Q polymer microdisks on silicon,” Opt. Express 19(12), 11451–11456 (2011). [CrossRef] [PubMed]

]. In comparison with polymer, fused silica is considered to be a very attractive substrate due to its wide transparency range and extremely low intrinsic material absorption loss [22

22. M. L. Gorodetsky, A. A. Savchenkov, and V. S. Ilchenko, “Ultimate Q of optical microsphere resonators,” Opt. Lett. 21(7), 453–455 (1996). [CrossRef] [PubMed]

]. 3D microoptics in fused silica can be fabricated by femtosecond laser direct writing, followed by wet chemical etching and a postannealing method [14

14. F. He, J. Lin, and Y. Cheng, “Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining,” Appl. Phys. B 105(2), 379–384 (2011). [CrossRef]

].

In this article, by applying femtosecond laser direct writing method, we demonstrate a new way to realize 3D high-Q microcavities on fused silica wafer that may have the light output from optical mode on an arbitrary plane respect to the substrate plane. Either the tilting angles or the heights of the microcavities are free of limitations, which, to the best of our knowledge, cannot be realized with any planar lithographic fabrication techniques. After the CO2 laser annealing process, the Q-factor of a microcavity is measured to be above 106, which can be further improved by replacing the low-resolution (~1 μm) motion stage used in the process of femtosecond laser fabrication with a better one, as explained in details in main context.

2. Experiment

In this work, commercially available fused silica glass substrates (UV grade fused silica JGS1 whose upper and bottom surfaces are polished to optical grade) with a thickness of 1 mm are used. The process flow for fabrication of the high-Q microcavity mainly consists of two steps: (1) femtosecond laser exposure followed by selective wet etching of the irradiated areas to create the microdisk structures; and (2) selective reflow of the silica cavities by CO2 laser annealing to improve the quality factors, as illustrated in Fig. 1
Fig. 1 Procedures of fabrication of 3D microcavity by femtosecond laser direct writing.
. The laser system consists of a Ti: sapphire oscillator (Coherent, Inc.) and a regenerative amplifier, which emits 800 nm, ~58 fs pulses with maximum pulse energy of ~5 μJ at 250-kHz repetition rate. The initial 8.8-mm-diameter beam is reduced to 5 mm by passing through a circular aperture to guarantee a high beam quality. Power adjustment is realized using neutral density (ND) filters. The glass samples can be arbitrarily translated in 3D space with a resolution of 1 μm by a PC-controlled XYZ stage. In the femtosecond laser direct writing, a 100 × objective with a numerical aperture (NA) of 0.9 is used to focus the beam down to a ~1 μm-dia. spot, and the average femtosecond laser power measured before the objective is ~0.05 W. To form the microdisk supported by a thin pillar, a layer-by-layer annular scanning method with the lateral scanning step set to be 1 μm is adopted, and the femtosecond laser scanning speed is chosen to be ~600 μm/s. The scanning is designed to modify the regions surrounding the areas which form a disk with a radius of 29 μm, a thickness of ~7 μm, tilted with respect to the substrate at 24°, and an underneath pillar with a radius of 12 μm, as the modified regions will be preferentially etched away in hydrofluoride (HF) acid. In addition, the angle between the disk and the pillar is set to be 57°.

3. Results and discussion

To characterize the mode structure and Q factor of the microtoroidal cavity, resonance spectra are measured via the optical fiber taper coupling method [23

23. A. Serpengüzel, S. Arnold, and G. Griffel, “Excitation of resonances of microspheres on an optical fiber,” Opt. Lett. 20(7), 654–656 (1995). [CrossRef] [PubMed]

]. For facilitating a convenient coupling, a microtoroidal cavity parallel to the substrate with a diameter of ~39 μm and a thickness of ~9 μm is fabricated using the technique mentioned in Sec. 2, as shown by its SEM image in Fig. 3(a)
Fig. 3 (a) SEM image of a microtoroidal cavity parallel to the substrate whose Q factor is to be examined. (b) An optical micrograph of the microtoroidal cavity coupled with a fiber taper. (c) Transmission spectrum of the microcavity coupled with the fiber taper. The free spectral range of 13.65nm agrees well with the numerical calculation result. (d) Lorentzian fit (red solid line) of measured spectrum around the resonant wavelength at 1534.72nm (black dotted line), showing a Q factor of 1.07 × 106.
. A swept-wavelength tunable external-cavity diode Laser (New Focus, Model: 6528-LN) and a swept spectrometer (dBm Optics, Model: 4650) are used to measure the transmission spectrum from the fiber taper with a resolution of 0.1pm. A periodic pulse signal with a power of 1 dBm is used to continuously sweep from 1530 to 1565 nm. The fiber taper formed by heating and stretching a section of a commercial optical fiber (Corning, SMF-28) has a minimum waist diameter of approximately ~1 μm, providing an evanescent excitation of WGMs of the cavity. The microcavity sample is fixed on a three-axis nanopositioning stage with a spatial resolution of 50 nm in the XYZ directions, so that the critical coupling may be realized by carefully adjusting the relative position between the cavity and the fiber taper. We use dual CCD cameras to simultaneously image microcavity and fiber taper from the side and the top, as shown in Fig. 3(b) [side view image not shown]. Please note that the thinnest portion of the fiber taper (dia. ~1 μm) is not very clear in Fig. 3(b) because of the limited resolution of the optical microscope (a 50X objective with a NA of 0.3 is used for the imaging in this case to obtain a sufficiently long working distance).

A resonance transmission spectrum of a fiber taper coupled to the microtoroidal cavity with various excited WGMs is depicted in Fig. 3(c). The experimentally measured free spectral range (ΔλFSR = 13.65 nm, defined as the wavelength spacing between modes with successive angular mode number) agrees well with the numerical calculation based on the experimentally measured cavity diameter, which is given approximately by the well-known expression [20

20. J. F. Ku, Q. D. Chen, R. Zhang, and H. B. Sun, “Whispering-gallery-mode microdisk lasers produced by femtosecond laser direct writing,” Opt. Lett. 36(15), 2871–2873 (2011). [CrossRef] [PubMed]

],
ΔλFSRλ02/2πRn
(1)
where λ0 is the wavelength in vacuum, R the radius of the microcavity, n the refractive index of the fused silica. For R = 19.5μm, n = 1.445, the theoretically predicted value of ΔλFSR is about 13.30 nm at 1534.72 nm. Figure 3(d) shows an individual WGM located at 1534.72 nm with a Lorentzian shaped dip. The linewidth is measured as 1.44 pm, and the Q factor for the mode is calculated to be 1.07 × 106, inferred from the Lorentzian fit of the spectrum, as shown in Fig. 3(d). This indicates that femtosecond laser micromachining on fused silica enables fabrication of smooth cavity surfaces with low surface-scattering loss of the WGMs..

In the current experiment, it took ~6 hours for fabricating the microcavity by femtosecond laser direct writing. This long fabrication time is mostly due to the low performance of our X-Y-Z motion stage. If a rotary stage could be used, we expect that the fabrication time can be greatly shortened to less than ~30 min. Still, our technique is slower than the traditional photolithography-based technology, however, it should be stressed here that it is very difficult, if not completely impossible, to realize such flexible 3D microcavities presented here via planar lithography method such as photolithography or electron beam lithography. Though the microtoroids still exhibit isotropic properties of the confined WGM, the femtosecond laser direct writing can easily create preferred deformation into the cavity and thus introduce directional light output into any desired solid angle. This would open a broad spectrum of applications such as micro-lasers, sensors, and so on.

4. Conclusions

To summarize, we demonstrate the fabrication of 3D microcavities on the fused silica chip by femtosecond laser direct writing. The typical Q factors of the microcavities are over 106. Further improvement of the Q-factor should be achievable by replacing the XYZ stage of relatively low resolution in the current writing system with a high-resolution XYZ stage. Though the cavity shown here is a passive device, with such high Q factors, lasing can be easily achieved by coating the cavity surface with gain medium such as semiconductor colloidal quantum dots, or by using rare-earth-doped substrate to provide optical gain. Therefore, our technique opens a new avenue for constructing high-Q microcavities with non-inplane geometries which may find important uses in both fundamental research and biological and chemical sensing applications.

Acknowledgments

We thank Prof. Yunfeng Xiao of Peking University, Dr. Xiaoshun Jiang of Nanjing University for the helpful discussion. The work is supported by National Basic Research Program of China (No. 2011CB808100), NSFC (Nos. 11134010, 60825406, 61008011, 10974213, 11104245) and Fundamental Research Funds for the Central Universities.

References and links

1.

V. S. Ilchenko and A. B. Matsko, “Optical resonators with whispering-gallery modes—part II: applications,” IEEE J. Sel. Top. Quantum Electron. 12(1), 15–32 (2006). [CrossRef]

2.

S. L. McCall, A. F. J. Levi, R. E. Slusher, S. J. Pearton, and R. A. Logan, “Whispering-gallery mode microdisk lasers,” Appl. Phys. Lett. 60(3), 289–291 (1992). [CrossRef]

3.

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]

4.

C. Gmachl, F. Capasso, E. E. Narimanov, J. U. Nöckel, A. D. Stone, J. Faist, D. L. Sivco, and A. Y. Cho, “High-power directional emission from microlasers with chaotic resonators,” Science 280(5369), 1556–1564 (1998). [CrossRef] [PubMed]

5.

A. F. J. Levi, R. E. Slusher, S. L. McCall, J. L. Glass, S. J. Pearton, and R. A. Logan, “Directional light coupling from microdisk lasers,” Appl. Phys. Lett. 62(6), 561–563 (1993). [CrossRef]

6.

L. Mahler, A. Tredicucci, F. Beltram, C. Walther, J. Faist, B. Witzigmann, H. E. Beere, and D. A. Ritchie, “Vertically emitting microdisk lasers,” Nat. Photonics 3(1), 46–49 (2009). [CrossRef]

7.

Y. Cheng, H. L. Tsai, K. Sugioka, and K. Midorikawa, “Fabrication of 3D microoptical lenses in photosensitive glass using femtosecond laser micromachining,” Appl. Phys., A Mater. Sci. Process. 85(1), 11–14 (2006). [CrossRef]

8.

E. Brasselet, M. Malinauskas, A. Žukauskas, and S. Juodkazis, “Photopolymerized microscopic vortex beam generators: precise delivery of optical orbital angular momentum,” Appl. Phys. Lett. 97(21), 211108 (2010). [CrossRef]

9.

A. Marcinkevicius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, and J. Nishii, “Femtosecond laser-assisted three-dimensional microfabrication in silica,” Opt. Lett. 26(5), 277–279 (2001). [CrossRef] [PubMed]

10.

Y. Bellouard, A. Said, M. Dugan, and P. Bado, “Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemical etching,” Opt. Express 12(10), 2120–2129 (2004). [CrossRef] [PubMed]

11.

Y. Cheng, K. Sugioka, and K. Midorikawa, “Microfluidic laser embedded in glass by three-dimensional femtosecond laser microprocessing,” Opt. Lett. 29(17), 2007–2009 (2004). [CrossRef] [PubMed]

12.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

13.

M. Ams, G. Marshall, D. Spence, and M. Withford, “Slit beam shaping method for femtosecond laser direct-write fabrication of symmetric waveguides in bulk glasses,” Opt. Express 13(15), 5676–5681 (2005). [CrossRef] [PubMed]

14.

F. He, J. Lin, and Y. Cheng, “Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining,” Appl. Phys. B 105(2), 379–384 (2011). [CrossRef]

15.

K. Sugioka and Y. Cheng, “Integrated microchips for biological analysis fabricated by femtosecond laser direct writing,” MRS Bull. 36(12), 1020–1027 (2011). [CrossRef]

16.

Y. Hanada, K. Sugioka, I. Shihira-Ishikawa, H. Kawano, A. Miyawaki, and K. Midorikawa, “3D microfluidic chips with integrated functional microelements fabricated by a femtosecond laser for studying the gliding mechanism of cyanobacteria,” Lab Chip 11(12), 2109–2115 (2011). [CrossRef] [PubMed]

17.

A. Crespi, Y. Gu, B. Ngamsom, H. J. W. M. Hoekstra, C. Dongre, M. Pollnau, R. Ramponi, H. H. van den Vlekkert, P. Watts, G. Cerullo, and R. Osellame, “Three-dimensional Mach-Zehnder interferometer in a microfluidic chip for spatially-resolved label-free detection,” Lab Chip 10(9), 1167–1173 (2010). [CrossRef] [PubMed]

18.

A. Schaap, Y. Bellouard, and T. Rohrlack, “Optofluidic lab-on-a-chip for rapid algae population screening,” Biomed. Opt. Express 2(3), 658–664 (2011). [CrossRef] [PubMed]

19.

Z.-P. Liu, Y. Li, Y.-F. Xiao, B.-B. Li, X.-F. Jiang, Y. Qin, X.-B. Feng, H. Yang, and Q. Gong, “Direct laser writing of whispering gallery microcavities by two-photon polymerization,” Appl. Phys. Lett. 97(21), 211105 (2010). [CrossRef]

20.

J. F. Ku, Q. D. Chen, R. Zhang, and H. B. Sun, “Whispering-gallery-mode microdisk lasers produced by femtosecond laser direct writing,” Opt. Lett. 36(15), 2871–2873 (2011). [CrossRef] [PubMed]

21.

T. Grossmann, S. Schleede, M. Hauser, T. Beck, M. Thiel, G. von Freymann, T. Mappes, and H. Kalt, “Direct laser writing for active and passive high-Q polymer microdisks on silicon,” Opt. Express 19(12), 11451–11456 (2011). [CrossRef] [PubMed]

22.

M. L. Gorodetsky, A. A. Savchenkov, and V. S. Ilchenko, “Ultimate Q of optical microsphere resonators,” Opt. Lett. 21(7), 453–455 (1996). [CrossRef] [PubMed]

23.

A. Serpengüzel, S. Arnold, and G. Griffel, “Excitation of resonances of microspheres on an optical fiber,” Opt. Lett. 20(7), 654–656 (1995). [CrossRef] [PubMed]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers
(160.2750) Materials : Glass and other amorphous materials

ToC Category:
Laser Microfabrication

History
Original Manuscript: March 2, 2012
Revised Manuscript: April 11, 2012
Manuscript Accepted: April 11, 2012
Published: April 19, 2012

Citation
Jintian Lin, Shangjie Yu, Yaoguang Ma, Wei Fang, Fei He, Lingling Qiao, Limin Tong, Ya Cheng, and Zhizhan Xu, "On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing," Opt. Express 20, 10212-10217 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-9-10212


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References

  1. V. S. Ilchenko and A. B. Matsko, “Optical resonators with whispering-gallery modes—part II: applications,” IEEE J. Sel. Top. Quantum Electron.12(1), 15–32 (2006). [CrossRef]
  2. S. L. McCall, A. F. J. Levi, R. E. Slusher, S. J. Pearton, and R. A. Logan, “Whispering-gallery mode microdisk lasers,” Appl. Phys. Lett.60(3), 289–291 (1992). [CrossRef]
  3. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature421(6926), 925–928 (2003). [CrossRef] [PubMed]
  4. C. Gmachl, F. Capasso, E. E. Narimanov, J. U. Nöckel, A. D. Stone, J. Faist, D. L. Sivco, and A. Y. Cho, “High-power directional emission from microlasers with chaotic resonators,” Science280(5369), 1556–1564 (1998). [CrossRef] [PubMed]
  5. A. F. J. Levi, R. E. Slusher, S. L. McCall, J. L. Glass, S. J. Pearton, and R. A. Logan, “Directional light coupling from microdisk lasers,” Appl. Phys. Lett.62(6), 561–563 (1993). [CrossRef]
  6. L. Mahler, A. Tredicucci, F. Beltram, C. Walther, J. Faist, B. Witzigmann, H. E. Beere, and D. A. Ritchie, “Vertically emitting microdisk lasers,” Nat. Photonics3(1), 46–49 (2009). [CrossRef]
  7. Y. Cheng, H. L. Tsai, K. Sugioka, and K. Midorikawa, “Fabrication of 3D microoptical lenses in photosensitive glass using femtosecond laser micromachining,” Appl. Phys., A Mater. Sci. Process.85(1), 11–14 (2006). [CrossRef]
  8. E. Brasselet, M. Malinauskas, A. Žukauskas, and S. Juodkazis, “Photopolymerized microscopic vortex beam generators: precise delivery of optical orbital angular momentum,” Appl. Phys. Lett.97(21), 211108 (2010). [CrossRef]
  9. A. Marcinkevicius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, and J. Nishii, “Femtosecond laser-assisted three-dimensional microfabrication in silica,” Opt. Lett.26(5), 277–279 (2001). [CrossRef] [PubMed]
  10. Y. Bellouard, A. Said, M. Dugan, and P. Bado, “Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemical etching,” Opt. Express12(10), 2120–2129 (2004). [CrossRef] [PubMed]
  11. Y. Cheng, K. Sugioka, and K. Midorikawa, “Microfluidic laser embedded in glass by three-dimensional femtosecond laser microprocessing,” Opt. Lett.29(17), 2007–2009 (2004). [CrossRef] [PubMed]
  12. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett.21(21), 1729–1731 (1996). [CrossRef] [PubMed]
  13. M. Ams, G. Marshall, D. Spence, and M. Withford, “Slit beam shaping method for femtosecond laser direct-write fabrication of symmetric waveguides in bulk glasses,” Opt. Express13(15), 5676–5681 (2005). [CrossRef] [PubMed]
  14. F. He, J. Lin, and Y. Cheng, “Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining,” Appl. Phys. B105(2), 379–384 (2011). [CrossRef]
  15. K. Sugioka and Y. Cheng, “Integrated microchips for biological analysis fabricated by femtosecond laser direct writing,” MRS Bull.36(12), 1020–1027 (2011). [CrossRef]
  16. Y. Hanada, K. Sugioka, I. Shihira-Ishikawa, H. Kawano, A. Miyawaki, and K. Midorikawa, “3D microfluidic chips with integrated functional microelements fabricated by a femtosecond laser for studying the gliding mechanism of cyanobacteria,” Lab Chip11(12), 2109–2115 (2011). [CrossRef] [PubMed]
  17. A. Crespi, Y. Gu, B. Ngamsom, H. J. W. M. Hoekstra, C. Dongre, M. Pollnau, R. Ramponi, H. H. van den Vlekkert, P. Watts, G. Cerullo, and R. Osellame, “Three-dimensional Mach-Zehnder interferometer in a microfluidic chip for spatially-resolved label-free detection,” Lab Chip10(9), 1167–1173 (2010). [CrossRef] [PubMed]
  18. A. Schaap, Y. Bellouard, and T. Rohrlack, “Optofluidic lab-on-a-chip for rapid algae population screening,” Biomed. Opt. Express2(3), 658–664 (2011). [CrossRef] [PubMed]
  19. Z.-P. Liu, Y. Li, Y.-F. Xiao, B.-B. Li, X.-F. Jiang, Y. Qin, X.-B. Feng, H. Yang, and Q. Gong, “Direct laser writing of whispering gallery microcavities by two-photon polymerization,” Appl. Phys. Lett.97(21), 211105 (2010). [CrossRef]
  20. J. F. Ku, Q. D. Chen, R. Zhang, and H. B. Sun, “Whispering-gallery-mode microdisk lasers produced by femtosecond laser direct writing,” Opt. Lett.36(15), 2871–2873 (2011). [CrossRef] [PubMed]
  21. T. Grossmann, S. Schleede, M. Hauser, T. Beck, M. Thiel, G. von Freymann, T. Mappes, and H. Kalt, “Direct laser writing for active and passive high-Q polymer microdisks on silicon,” Opt. Express19(12), 11451–11456 (2011). [CrossRef] [PubMed]
  22. M. L. Gorodetsky, A. A. Savchenkov, and V. S. Ilchenko, “Ultimate Q of optical microsphere resonators,” Opt. Lett.21(7), 453–455 (1996). [CrossRef] [PubMed]
  23. A. Serpengüzel, S. Arnold, and G. Griffel, “Excitation of resonances of microspheres on an optical fiber,” Opt. Lett.20(7), 654–656 (1995). [CrossRef] [PubMed]

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