We develop a method for fabricating very small silica microbubbles having a micrometer-order wall thickness and demonstrate the first optical microbubble resonator. Our method is based on blowing a microbubble using stable radiative CO2 laser heating rather than unstable convective heating in a flame or furnace. Microbubbles are created along a microcapillary and are naturally opened to the input and output microfluidic or gas channels. The demonstrated microbubble resonator has 370 µm diameter, 2 µm wall thickness, and a Q factor exceeding 10

Dimmick

Grellier

Kippenberg

Knight

M. Sumetsky

McLachlan

Murugan

Pöllinger

R. S. Windeler

Sumetsky

Vahala

von Klitzing

White

Y. Dulashko

English

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Optical microbubble resonator

Sumetsky, M.

Dulashko, Y.

Windeler, R.S.

Aston Publications Explorer

898 OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010Optical microbubble resonatorM. Sumetsky,* Y. Dulashko, and R. S. WindelerOFS Laboratories, 19 Schoolhouse Road, Somerset, New Jersey 08873, USA*Corresponding author: sumetski@ofsoptics.comReceived December 7, 2009; accepted January 29, 2010;posted February 18, 2010 (Doc. ID 121139); published March 17, 2010We develop a method for fabricating very small silica microbubbles having a micrometer-order wall thick-ness and demonstrate the first optical microbubble resonator. Our method is based on blowing a microbubbleusing stable radiative CO2 laser heating rather than unstable convective heating in a flame or furnace. Mi-crobubbles are created along a microcapillary and are naturally opened to the input and output microfluidicor gas channels. The demonstrated microbubble resonator has 370 m diameter, 2 m wall thickness, anda Q factor exceeding 106. © 2010 Optical Society of AmericaOCIS codes: 230.3990, 220.4000, 060.2340, 140.4780, 140.3948, 130.6010.Investigation and application of the high Q-factor op-tical microresonators has become an interesting andrapidly developing field of modern optics. Special at-tention is given to the Fabry–Perot, ring, disk,sphere, and toroid microresonators [1,2] as well asmore recently introduced bottle [3–5], coil [6,7], andcapillary [8,9] microresonators. Optical microresona-tors are demonstrated as super-perceptive physical,chemical, and biological sensors and microlasers andhave promising applications in cavity quantum elec-trodynamics, cavity optomechanics, and other areasof fundamental and applied research and develop-ment [1,2].In this Letter we experimentally demonstrate atype of optical microresonator—a microbubble reso-nator. This resonator is created from a silica mi-crobubble having a diameter of a few hundred mi-crometers and a micrometer-order wall thickness.The blowing of silica bubbles that are so small andthin is challenging. In fact, consider an open silicatube filled with pressurized gas. For a constant pres-sure, the process of blowing of a thin-wall bubblefrom a section of this tube, heated in a flame or fur-nace, is unstable and, eventually, will result in break-age. For a very small tube diameter and bubble di-mensions, the accurate control of pressure becomesextremely difficult. Here, we suggest a method thatstabilizes the expansion of a bubble.Our approach is based on application of the stableradiative heating by a CO2 laser beam rather thanthe unstable convective heating in a flame or furnace.This approach originates from the idea developed inmicrofiber tapering using a CO2 laser [10,11]. It hasbeen found that a fiber heated with a laser beam ofcertain power can be tapered down to a certainthreshold diameter, dth, only. This effect is never ob-served for conventional fiber tapering in a flame andcan be used for controlling the tapered fiber diameterwith the laser beam power [11]. Freezing a microfiberat the diameter dth has a simple explanation [10,11].The CO2 laser beam penetration depth into theheated silica is a few tens of micrometers [12]. There-fore, for a thin silica fiber having a diameter d10 m, the beam power is acquired by the silica0146-9592/10/070898-3/$15.00 ©volume almost uniformly [Fig. 1(a)]. For this reason,in the process of fiber tapering, the acquired powerdecreases proportionally to the heated volume or tothe d2. On the other hand, the dissipated power is al-ways proportional to the fiber surface area that de-creases proportionally to the fiber diameter d, i.e.,slower than the acquired power. Thus, the steady-state temperature of the fiber heated with the CO2laser decreases with its diameter.The blowing of a silica microbubble can be con-trolled similarly. The thickness of the bubble wall, w,decreases in process of the bubble expansion. For agiven incident laser beam power, the wall thicknesscannot be less than the threshold value, wth, for thefollowing reason: the wall with thickness w10 mis heated almost uniformly [12]. Ignoring, for simplic-ity, the gas dynamics inside the microcapillary, weconclude that the acquired power is proportional tow, while the dissipated power is independent of w[Fig. 1(b)]. As the result, the total heating effect de-creases with w and the microbubble freezes at athreshold wall thickness wth.Generally, if the characteristic dimensions of an ob-ject are less than the depth of radiation penetration,then the acquired power decreases proportionally tothe volume of this object. The dissipated power of theobject is always proportional to its surface area. Forthis reason, the heating rate decreases with volumefaster than the cooling rate so that the steady tem-perature of a small object heated by radiation de-creases with its dimensions. Alternatively, the poweracquired through the convective heating, similar toFig. 1. (Color online) Illustration of radiative heating ac-companied by convective cooling (a) for a thin fiber and (b)for a thin wall.2010 Optical Society of AmericaApril 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS 899the dissipated power, is proportional to the surfacearea and the cooling effect is not observed.A microbubble is blown from a microcapillary thatis fabricated from a silica tube preform at a conven-tional fiber-drawing station. The microcapillary ispressurized and rotated inside the incident beam of aCO2 laser. The optical microscope image of the fabri-cated sample silica microbubble is shown in Fig. 2(a).This bubble is created from a capillary with diameterDcap=120 m and wall thickness wcap=6.3 m. Themicrobubble wall thickness wb is estimated from itsaverage diameter Db370 m by the relation wb=wcapDcap /Db, yielding wb2 m. The developedmethod allows sequential blowing of microbubbleswith similar and different diameters and wall thick-ness. This can be achieved by changing the powerand uniformity of the input beam as well as the di-ameter and wall thickness of the capillary. As an ex-ample, Fig. 2(b) shows an image of three mi-crobubbles spaced by 500 m having a wall thicknessof 1.5 m. By increasing the laser beam power weare able to fabricate larger microbubbles having asubmicrometer wall thickness.Following experiments with microsphere resona-tors [1,2,13], we investigate the resonant propertiesof a microbubble by coupling it to a microfiber. A bi-conical taper with a microfiber waist is fabricatedfrom a conventional single-mode fiber by adiabatictapering using the CO2 laser indirect heating method[14]. The input and output ends of the taper are con-nected to the JDS Uniphase tunable laser source anddetector, respectively. The diameter of the microfibersection coupled to the microbubble is 2 m. As anexample, the black curve 1 in Fig. 3 plots the trans-mission spectrum of the microbubble shown in Fig.2(a) (microfiber position A) measured in the wave-Fig. 2. (a) Optical microscope image of a microbubble; (b)optical microscope image of three successive microbubbles.length interval from 1530 to 1550 nm. For compari-son, the blue curve 2 shows the transmission spec-trum of the capillary (microfiber position B). Theinset in Fig. 3 magnifies the microbubble spectrumnear a resonance. This inset determines the lowestestimate of the resonance Q factor, Q=1.5106,which is obtained by Lorentzian fit of the experimen-tal data limited by the 3 pm resolution of our mea-surement system. The cyan curve 3 in Fig. 3 charac-terizes the density of the microbubble transmissionspectrum. It is obtained by averaging of the blackcurve 1 over 100 measurement points (0.3 nm inter-val). While the distribution of peaks of the blackcurve 1 looks stochastic, the cyan curve 3 has a pe-riod b determined by the microbubble circumferencelength. The ratio of this period and the period c ofthe capillary spectrum (blue curve 2) determines theratio of the microcapillary and the microbubble diam-eters c /b0.32, in good agreement with the mea-surement taken from Fig. 2(b). Finally, the greencurve 4 in Fig. 3 determines the oscillations of themicrobubble transmission spectrum after filling itwith ethanol (refractive index 1.36). As expected, theperiod of these oscillations coincide with the period ofthe spectrum density oscillations of the empty mi-crobubble. In this case, the disappearance of high-Qresonances is caused by the ethanol transmissionloss.Compared with other microresonators, there areimportant advantages of a microbubble resonator inthe application for microfluidic sensing. In fact, asopposed to microsphere or microtoroid resonatorsand similar to a microcapillary resonator [8,9], thetested liquid or gas flows inside the microbubble anddoes not disturb the microfiber alignment. Further-Fig. 3. (Color online) Transmission spectra of the mi-crobubble and microcapillary shown in Fig. 2(a): 1, mi-crobubble; 2, capillary; 3, spectrum of the microbubble av-eraged over 0.3 nm interval; 4, microbubble filled withethanol. Inset, magnified measurement data near a reso-nant peak (dots) fitted with a Lorentzian curve (dashed).more, as opposed to the microcapillary, the mi-900 OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010crobubble possesses real localized eigenmodes anddoes not exhibit noticeable insertion loss (compareblack curve 1 and blue curve 2). Several other appli-cations of a microbubble resonator can be considered.For example, it will be interesting to investigate thetunability of a microbubble resonator under mechani-cal deformations [15,5] as well as its resonant optom-echanical behavior [16].In summary, a novel type of optical microresonator,a microbubble resonator, is created. The developedmethod of silica microbubble fabrication is based onradiative heating with the CO2 laser. It allowed us tofabricate microbubbles that are naturally opened tothe input and output microcapillary channels. Thedemonstrated microbubble resonator has 370 m di-ameter, 2 m wall thickness, and a Q factor exceed-ing 106. Potentially, it is possible to create mi-crobubbles of different shapes and wall thicknessesby variation of the power and uniformity of the laserbeam, asymmetry and speed of rotation, and param-eters of the capillary. We expect applications of themicrobubble resonator as a microfluidic bio andchemical sensor as well as a tunable laser, qualita-tively improving the performance of microcapillary-based microfluidic devices demonstrated so far. Po-tential applications of the demonstrated silicamicrobubble can go far beyond utilizing its highQ-factor resonance behavior.The authors are grateful to D. J. DiGiovanni fornumerous discussions and suggestions.References1. K. J. Vahala, Nature 424, 839 (2003).2. A. B. Matsko, ed., Practical Applications of Microreso-nators in Optics and Photonics (CRC Press, 2009).3. M. Sumetsky, Opt. Lett. 29, 8 (2004).4. G. S. Murugan, J. S. Wilkinson, and M. N. Zervas, Opt.Express 17, 11916 (2009).5. M. Pöllinger, D. O’Shea, F. Warken, and A. Rauschen-beutel, Phys. Rev. Lett. 103, 053901 (2009).6. M. Sumetsky, Opt. Express 12, 2303 (2004).7. M. Sumetsky, J. Lightwave Technol. 26, 21 (2008).8. I. M. White, H. Oveys, and X. Fan, Opt. Lett. 31, 1319(2006).9. I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, Y. Sun,and X. Fan, in Advanced Photonic Structures for Bio-logical and Chemical Detection, X. Fan, ed. (Springer,2009), pp. 377–393.10. A. J. C. Grellier, N. K. Zayer, and C. N. Pannell, Opt.Commun. 152, 324 (1998).11. T. E. Dimmick, G. Kakarantzas, T. A. Birks, and P. St.J. Russell, Appl. Opt. 38, 6845 (1999).12. A. D. McLachlan and F. P. Meyer, Appl. Opt. 26, 1728(1987).13. J. C. Knight, G. Cheung, F. Jacques, and T. A. Birks,Opt. Lett. 22, 1129 (1997).14. M. Sumetsky, Y. Dulashko, and A. Hale, Opt. Express12, 3521 (2004).15. W. von Klitzing, R. Long, V. S. Ilchenko, J. Hare, and V.Lefévre-Seguin, Opt. Lett. 26, 166 (2001).16. T. J. Kippenberg and K. J. Vahala, Opt. Express 15,17172 (2007).

We develop a method for fabricating very small silica microbubbles having a micrometer-order wall thickness and demonstrate the first optical microbubble resonator. Our method is based on blowing a microbubble using stable radiative CO2 laser heating rather than unstable convective heating in a flame or furnace. Microbubbles are created along a microcapillary and are naturally opened to the input and output microfluidic or gas channels. The demonstrated microbubble resonator has 370 µm diameter, 2 µm wall thickness, and a Q factor exceeding 10. © 2010 Optical Society of America

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Optical microbubble resonator

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