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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 18091–18096
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Focused Ion Beam Engineered Whispering Gallery Mode Resonators with Open Cavity Structure

David C. Aveline, Lukas Baumgartel, Byungmin Ahn, and Nan Yu  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 18091-18096 (2012)
http://dx.doi.org/10.1364/OE.20.018091


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Abstract

We report the realization of an open cavity whispering gallery mode optical resonator, in which the circulating light traverses a free space gap. We utilize focused ion beam microfabrication to precisely cut a 10 μm wide notch into the perimeter of a crystalline disc. We have shown that this modified resonator structure supports high quality modes, and demonstrated qualify factor, Q ≃ 106, limited by the notch surface roughness due to the ion milling process. Furthermore, we investigated the spatial profile of the modes inside the open cavity with a microfabricated probe mechanism. This new type of resonator structure facilitates interaction of the cavity’s optical field with mechanical resonators as well as individual atoms or molecules.

© 2012 OSA

1. Introduction

This paper describes our effort to apply microfabrication techniques to macroscopic mechanically-polished crystalline disc resonators in order to achieve uniquely tailored high-Q resonator structures. In our studies to date, we have made use of focused ion beam (FIB) microfabrication to mill features into the WGMR. We report the realization of FIB engineered disc (FIBED) resonators with open structure, i.e. a milled notch creates a free space gap within the mode volume. The gap provides complete access to the internal fields of the resonator and therefore the full mode profile and intensity. This novel approach facilitates interaction of external mechanisms, atoms, or molecules with the full extent of the resonant light field.

2. Focused Ion Beam Engineered Discs

The process first entails sputter coating a 10–20 nm layer of gold onto a mechanically polished high-Q disc made in house. This added electrically conductive layer reduces charge collection during FIB and scanning electron microscopy (SEM), and provides consistent alignment of the ion beam throughout the milling procedure. The disc, mounted on a short brass pedestal, is installed into a multi beam SEM/FIB system (JEOL JIB-4500) such that the top surface of the disc faces the ion beam for initial coarse milling using 360 nm diameter beams with 10 nA probe current. We then mill the walls with finer precision ion beams incident from edge-on to achieve parallel walls and smooth surface finish. In order to mitigate material re-deposition, we repeatedly mill alternate sides of the notch with ion beams of increasingly smaller cross section, ending with 50 nm diameter and 500 pA probe current. Ultimately, even finer beam sizes are available (6.5 nm and 1 pA) for future efforts. After completing the ion beam milling, the gold layer is removed with a wet etchant, followed by a deionized water rinse and an alcohol cleaning step.

In several millimeter-size discs made of CaF2 and fused silica, we milled notches ranging 10–20 μm in width and approximately 50–100 μm in depth. Figure 1 shows SEM images and a photograph of a notched CaF2 disc with 1.15 mm radius. On one side of the notch, surface features of approximately 90 nm could be resolved with SEM (Fig. 2(b)). The opposite side, however, is smooth within the resolution of the SEM (< 25 μm). Barring re-deposition limitations, ultimately the same surface finish could be attained on both sides. Our FIB milling tests, along with other literature [9

9. J. Gierak, A. Madouri, A. Biance, E. Bourhis, G. Patriarche, C. Ulysse, D. Lucot, X. Lafosse, L. Auvray, L. Bruchhaus, and R. Jede, “Sub-5nm FIB direct patterning of nanodevices,” Microelectron. Eng. 84, 779–783 (2007). [CrossRef]

], indicate that we can achieve surface roughness as low as 1–5 nm in future iterations. Smooth surfaces are critical because resonator optical Q for CaF2 is dominated by scattering losses [19

19. M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scattering in high-Q microspheres,” J. Opt. Soc. Am. B 17, 1051–1057 (2000). [CrossRef]

].

Fig. 1 SEM images (a,b) of CaF2 FIBED resonator with 50 μm thickness and 1.15 mm radius, and a photograph (c) from above after partial cleaning. Also shown is a close-up SEM image (d) of the 10 μm notch viewed at a slight angle.
Fig. 2 SEM images of notch surfaces, including top view (a) after the initial coarse milling and the right wall partially milled with finer FIB precision. Images at ×10,000 (right) compare the two notch surfaces after many iterations of ion beam polishing. Left wall (b) exhibits 90 μm surface features while the right (c) has smoother surface finish than the SEM resolution (< 25 μm).

3. Optical Q

We utilize an optical analysis system with several laser sources of various wavelengths and tuning ranges to measure the Q factor and characterize the spectra of the FIBED resonators. Figure 3(a) illustrates our experimental setup using angle-polished fibers, which allow evanescent coupling to the WGMR [20

20. V. S. Ilchenko, X. S. Yao, and L. Maleki, “Pigtailing the high-Q microsphere cavity: a simple fiber coupler for optical whispering-gallery modes,” Opt. Lett. 24, 723–725 (1999). [CrossRef]

]. With two independent fiber couplers and a collection lens, our setup has the capability to simultaneously measure optical modes that are transmitted clockwise (CW) and counter-clockwise (CCW) through the resonator. Bidirectional coupling is important due to significant reflection from the notch surfaces. The reflected light is not lost, but rather it couples backwards into the WGMR generating normal mode splitting. This strong coupling creates large splitting and makes the modes well resolved, unlike the typical doublets observed due to scattering [21

21. D. S. Weiss, V. Sandoghdar, J. Hare, V. Lèfevre-Seguin, J.-M. Raimond, and S. Haroche, “Splitting of high-Q mie modes induced by light backscattering in silica microspheres,” Opt. Lett. 20, 1835–1837 (1995). [CrossRef] [PubMed]

,22

22. A. Mazzei, S. Götzinger, L. de S. Menezes, G. Zumofen, O. Benson, and V. Sandoghdar, “Controlled coupling of counterpropagating whispering-gallery modes by a single rayleigh scatterer: A classical problem in a quantum optical light,” Phys. Rev. Lett. 99, 173603 (2007). [CrossRef] [PubMed]

]. Figure 3(b) shows a photo of the FIBED resonator with red light coupled through angle-polished fibers, and the brightest scattering is apparent at the notch location.

Fig. 3 Diagram of the optical set-up (a) illustrating the two angle-polished fiber couplers and detectors for the transmitted forward (CW) and backward (CCW) light. A photograph with red light coupled (b) shows the fiber couplers and scattering from the notch on top.

Most of our optical measurements are carried out with a distributed feedback laser at 1560 nm with linewidth of several megahertz. We sweep the laser’s optical frequency by ramping the laser diode current. Although this sweep alters the output power (1–4 mW), we still observe good signal to noise across 40 GHz. Figure 4(a) shows the spectra of an over-coupled FIBED resonator across one free spectral range, FSRc/2nπr, which is 29 GHz for a CaF2 disc with radius, r = 1.15 mm, and index of refraction, n = 1.426. We acquire the optical quality factor, Q = f/Γ, by measuring the mode linewidth, Γ, while sweeping the laser’s optical frequency, f. Figure 4(b) shows a high-Q mode of the notched resonator with Q ≃ 106 at 1560 nm.

Fig. 4 Optical frequency scans of FIBED resonator transmission show (a) the entire free spectral range, FSR ≃ 29 GHz, and (b) a high-Q mode with Lorentzian fit (red), Γ = 194 MHz, corresponding to Q ≃ 106. The wavelength plot (c) shows the linewidth dependence fit to λ−4 (dashed) expected in the Rayleigh scattering limited regime.

Scattering from the notch surfaces appears to be the dominating loss mechanism. Bulk absorption and perimeter surface roughness are not significant factors for highly polished CaF2, and we measured Q ≥ 108 in the original polished discs prior to the FIB process. We investigated the optical modes at shorter wavelengths, 975–1000 nm, using a tunable external-cavity diode laser and find good agreement with the scattering expression derived in [19

19. M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scattering in high-Q microspheres,” J. Opt. Soc. Am. B 17, 1051–1057 (2000). [CrossRef]

]—the mode linewidth inversely scales with wavelength to the fourth power, as depicted in Fig. 4(c). This wavelength dependence supports the case that the quality factor is currently limited by Rayleigh scattering from the notch surfaces. Achieving smooth surfaces on these faces is critical to attain the theoretical diffraction limited Q. For a gap length of 10 μm and mode size of 25 μm, we could attain Q ≥ 108 before reaching the limit due to diffraction loss across the free space gap.

4. Probing the WGMR Interior

Fig. 5 Probing the optical modes with (a) 5μm-thick gold wire tip. This micro beam-block introduces losses as it is inserted radially (b) into the gap from just outside the perimeter. The mode amplitude plot (c) shows a mode (blue) attenuated within the first 4 μm of insertion depth, while another (red) exists deeper in the WGMR.

5. Conclusion

We have demonstrated a high-Q FIB engineered WGMR with an open cavity geometry, which provides access to the internal fields of the resonator. We fabricated a CaF2 FIBED resonator with optical quality factor exceeding one million. With our current methodology, charge collection and material re-deposition issues in the FIB process have been mitigated and we observe no effects of gallium ion contamination. We expect that the same techniques could be applied to many different types of resonator materials, such as magnesium fluoride, crystal quartz, and lithium niobate to name a few. The Q appears to be limited by Rayleigh scattering losses from the notch surfaces, currently observed to have roughness of about 90 nm. FIB techniques can perform at least 10 times better than this initial demonstration.

This novel open cavity WGMR system allows direct interaction of external mechanisms, atoms, or molecules with the full intensity of the resonant light field. The complete through-cut, relatively large gap size, high quality surfaces (high-Q), and flexibility of design and material selection make it very versatile and enable strong interactions with mesoscopic structures. For example, a mechanical resonator could be engineered within the high-Q cavity to realize a unique optomechanical device. This FIBED technology could foster development of new photonic devices, including optomechanical sensors, spectrometers, and laser sources. We are continuing to develop FIB methods to achieve smoother surface finish, and we are investigating other structural modifications, including schemes for spectral engineering and free space optical coupling.

Acknowledgments

This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration, with support from NASA’s Center Innovation Fund and JPL’s Research and Technology Development Program. FIB and SEM were performed at USC’s Center for Electron Microscopy and Microanalysis (CEMMA). The authors thank Thanh Le, I.S. Grudinin, D. Strekalov, R. J.Thompson and B. C. Regan for helpful discussions and contributions.

References and links

1.

K. J. Vahala, “Optical microcavities,” Nature 424, 839–846 (2003). [CrossRef] [PubMed]

2.

I. S. Grudinin, V. S. Ilchenko, and L. Maleki, “Ultrahigh optical Q factors of crystalline resonators in the linear regime,” Phys. Rev. A 74, 063806 (2006). [CrossRef]

3.

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

4.

M. Soltani, S. Yegnanarayanan, and A. Adibi, “Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics,” Opt. Express 15, 4694–4704 (2007). [CrossRef] [PubMed]

5.

S. Wang, K. Broderick, H. Smith, and Y. Yi, “Strong coupling between on chip notched ring resonator and nanoparticle,” Appl. Phys. Lett. 97, 051102 (2010). [CrossRef]

6.

S. Sridaran and S. Bhave, “Opto-acoustic oscillator using silicon MEMS optical modulator,” in “Solid-State Sensors, Actuators and Microsystems Conference, 2011 16th International,” 2920–2923 (2011). [CrossRef]

7.

T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip,” Appl. Phys. Lett. 85, 6113–6115 (2004). [CrossRef]

8.

A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “Kilohertz optical resonances in dielectric crystal cavities,” Phys. Rev. A 70, 051804 (2004). [CrossRef]

9.

J. Gierak, A. Madouri, A. Biance, E. Bourhis, G. Patriarche, C. Ulysse, D. Lucot, X. Lafosse, L. Auvray, L. Bruchhaus, and R. Jede, “Sub-5nm FIB direct patterning of nanodevices,” Microelectron. Eng. 84, 779–783 (2007). [CrossRef]

10.

D. J. Moss, V. G. Ta’eed, B. J. Eggleton, D. Freeman, S. Madden, M. Samoc, B. Luther-Davies, S. Janz, and D.-X. Xu, “Bragg gratings in silicon-on-insulator waveguides by focused ion beam milling,” Appl. Phys. Lett. 85, 4860–4862 (2004). [CrossRef]

11.

J. Schrauwen, D. V. Thourhout, and R. Baets, “Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides,” Appl. Phys. Lett. 89, 141102 (2006). [CrossRef]

12.

J. Schrauwen, J. Van Lysebettens, T. Claes, K. De Vos, P. Bienstman, D. Van Thourhout, and R. Baets, “Focused-Ion-Beam fabrication of slots in silicon waveguides and ring resonators,” IEEE Photonic. Tech. L. 20, 2004–2006 (2008). [CrossRef]

13.

Y. Kim, A. Danner, J. Raftery, and K. Choquette, “Focused ion beam nanopatterning for optoelectronic device fabrication,” IEEE J. Sel. Top. Quant. 11, 1292–1298 (2005). [CrossRef]

14.

H.-B. Kim, “Focused ion beam fabrication of curved structures using the concept of beam shaping and variable dwell time,” Microelectron. Eng. 88, 3365–3371 (2011). [CrossRef]

15.

C. F. Wang, Y.-S. Choi, J. C. Lee, E. L. Hu, J. Yang, and J. E. Butler, “Observation of whispering gallery modes in nanocrystalline diamond microdisks,” Appl. Phys. Lett. 90, 081110 (2007). [CrossRef]

16.

L. A. M. Barea, F. Vallini, A. R. Vaz, J. R. Mialichi, and N. C. Frateschi, “Low-roughness active microdisk resonators fabricated by focused ion beam,” J. Vac. Sci. Technol. B 27, 2979–2981 (2009).

17.

J. R. Mialichi, L. A. M. Barea, P. L. D. Souza, R. M. S. Kawabata, M. P. Pires, and N. C. Frateschi, “Resonance modes in InAs/InGaAlAs/InP quantum dot microdisk resonators,” ECS Trans. 31, 289–293 (2010). [CrossRef]

18.

M. Ding, G. S. Murugan, G. Brambilla, and M. N. Zervas, “Whispering gallery mode selection in optical bottle microresonators,” Appl. Phys. Lett. 100, 081108 (2012). [CrossRef]

19.

M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scattering in high-Q microspheres,” J. Opt. Soc. Am. B 17, 1051–1057 (2000). [CrossRef]

20.

V. S. Ilchenko, X. S. Yao, and L. Maleki, “Pigtailing the high-Q microsphere cavity: a simple fiber coupler for optical whispering-gallery modes,” Opt. Lett. 24, 723–725 (1999). [CrossRef]

21.

D. S. Weiss, V. Sandoghdar, J. Hare, V. Lèfevre-Seguin, J.-M. Raimond, and S. Haroche, “Splitting of high-Q mie modes induced by light backscattering in silica microspheres,” Opt. Lett. 20, 1835–1837 (1995). [CrossRef] [PubMed]

22.

A. Mazzei, S. Götzinger, L. de S. Menezes, G. Zumofen, O. Benson, and V. Sandoghdar, “Controlled coupling of counterpropagating whispering-gallery modes by a single rayleigh scatterer: A classical problem in a quantum optical light,” Phys. Rev. Lett. 99, 173603 (2007). [CrossRef] [PubMed]

23.

J. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, and L. Yang, “On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator,” Nat. Photonics 4, 46–49 (2010). [CrossRef]

24.

M. Ostrowski, P. Pignalosa, H. Smith, and Y. Yi, “Higher-order optical resonance node detection of integrated disk microresonator,” Opt. Lett. 36, 3042–3044 (2011). [CrossRef] [PubMed]

OCIS Codes
(140.4780) Lasers and laser optics : Optical resonators
(220.4000) Optical design and fabrication : Microstructure fabrication
(220.4880) Optical design and fabrication : Optomechanics
(230.5750) Optical devices : Resonators
(280.4788) Remote sensing and sensors : Optical sensing and sensors

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: July 17, 2012
Revised Manuscript: July 18, 2012
Manuscript Accepted: July 18, 2012
Published: July 23, 2012

Citation
David C. Aveline, Lukas Baumgartel, Byungmin Ahn, and Nan Yu, "Focused ion beam engineered whispering gallery mode resonators with open cavity structure," Opt. Express 20, 18091-18096 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-18091


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References

  1. K. J. Vahala, “Optical microcavities,” Nature424, 839–846 (2003). [CrossRef] [PubMed]
  2. I. S. Grudinin, V. S. Ilchenko, and L. Maleki, “Ultrahigh optical Q factors of crystalline resonators in the linear regime,” Phys. Rev. A74, 063806 (2006). [CrossRef]
  3. M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett.85, 3693–3695 (2004). [CrossRef]
  4. M. Soltani, S. Yegnanarayanan, and A. Adibi, “Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics,” Opt. Express15, 4694–4704 (2007). [CrossRef] [PubMed]
  5. S. Wang, K. Broderick, H. Smith, and Y. Yi, “Strong coupling between on chip notched ring resonator and nanoparticle,” Appl. Phys. Lett.97, 051102 (2010). [CrossRef]
  6. S. Sridaran and S. Bhave, “Opto-acoustic oscillator using silicon MEMS optical modulator,” in “Solid-State Sensors, Actuators and Microsystems Conference, 2011 16th International,” 2920–2923 (2011). [CrossRef]
  7. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip,” Appl. Phys. Lett.85, 6113–6115 (2004). [CrossRef]
  8. A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “Kilohertz optical resonances in dielectric crystal cavities,” Phys. Rev. A70, 051804 (2004). [CrossRef]
  9. J. Gierak, A. Madouri, A. Biance, E. Bourhis, G. Patriarche, C. Ulysse, D. Lucot, X. Lafosse, L. Auvray, L. Bruchhaus, and R. Jede, “Sub-5nm FIB direct patterning of nanodevices,” Microelectron. Eng.84, 779–783 (2007). [CrossRef]
  10. D. J. Moss, V. G. Ta’eed, B. J. Eggleton, D. Freeman, S. Madden, M. Samoc, B. Luther-Davies, S. Janz, and D.-X. Xu, “Bragg gratings in silicon-on-insulator waveguides by focused ion beam milling,” Appl. Phys. Lett.85, 4860–4862 (2004). [CrossRef]
  11. J. Schrauwen, D. V. Thourhout, and R. Baets, “Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides,” Appl. Phys. Lett.89, 141102 (2006). [CrossRef]
  12. J. Schrauwen, J. Van Lysebettens, T. Claes, K. De Vos, P. Bienstman, D. Van Thourhout, and R. Baets, “Focused-Ion-Beam fabrication of slots in silicon waveguides and ring resonators,” IEEE Photonic. Tech. L.20, 2004–2006 (2008). [CrossRef]
  13. Y. Kim, A. Danner, J. Raftery, and K. Choquette, “Focused ion beam nanopatterning for optoelectronic device fabrication,” IEEE J. Sel. Top. Quant.11, 1292–1298 (2005). [CrossRef]
  14. H.-B. Kim, “Focused ion beam fabrication of curved structures using the concept of beam shaping and variable dwell time,” Microelectron. Eng.88, 3365–3371 (2011). [CrossRef]
  15. C. F. Wang, Y.-S. Choi, J. C. Lee, E. L. Hu, J. Yang, and J. E. Butler, “Observation of whispering gallery modes in nanocrystalline diamond microdisks,” Appl. Phys. Lett.90, 081110 (2007). [CrossRef]
  16. L. A. M. Barea, F. Vallini, A. R. Vaz, J. R. Mialichi, and N. C. Frateschi, “Low-roughness active microdisk resonators fabricated by focused ion beam,” J. Vac. Sci. Technol.B 27, 2979–2981 (2009).
  17. J. R. Mialichi, L. A. M. Barea, P. L. D. Souza, R. M. S. Kawabata, M. P. Pires, and N. C. Frateschi, “Resonance modes in InAs/InGaAlAs/InP quantum dot microdisk resonators,” ECS Trans.31, 289–293 (2010). [CrossRef]
  18. M. Ding, G. S. Murugan, G. Brambilla, and M. N. Zervas, “Whispering gallery mode selection in optical bottle microresonators,” Appl. Phys. Lett.100, 081108 (2012). [CrossRef]
  19. M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scattering in high-Q microspheres,” J. Opt. Soc. Am. B17, 1051–1057 (2000). [CrossRef]
  20. V. S. Ilchenko, X. S. Yao, and L. Maleki, “Pigtailing the high-Q microsphere cavity: a simple fiber coupler for optical whispering-gallery modes,” Opt. Lett.24, 723–725 (1999). [CrossRef]
  21. D. S. Weiss, V. Sandoghdar, J. Hare, V. Lèfevre-Seguin, J.-M. Raimond, and S. Haroche, “Splitting of high-Q mie modes induced by light backscattering in silica microspheres,” Opt. Lett.20, 1835–1837 (1995). [CrossRef] [PubMed]
  22. A. Mazzei, S. Götzinger, L. de S. Menezes, G. Zumofen, O. Benson, and V. Sandoghdar, “Controlled coupling of counterpropagating whispering-gallery modes by a single rayleigh scatterer: A classical problem in a quantum optical light,” Phys. Rev. Lett.99, 173603 (2007). [CrossRef] [PubMed]
  23. J. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, and L. Yang, “On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator,” Nat. Photonics4, 46–49 (2010). [CrossRef]
  24. M. Ostrowski, P. Pignalosa, H. Smith, and Y. Yi, “Higher-order optical resonance node detection of integrated disk microresonator,” Opt. Lett.36, 3042–3044 (2011). [CrossRef] [PubMed]

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