Silicon-waveguide-coupled high-Q chalcogenide microspheres

We fabricate high-Q arsenic triselenide glass microspheres through a three-step resistive heating process. We demonstrate quality factors greater than 2×10 6 at 1550 nm and achieve efficient coupling via a novel scheme utilizing index-engineered unclad silicon nanowires. We find that at powers above 1 mW the microspheres exhibit high thermal instability, which limits their application for resonator-enhanced nonlinear optical processes. 2009 Optical Society of America OCIS codes: (220.4000) Microstructure fabrication; (160.2750) Glass and other amorphous materials; (230.5750) Resonators. References and links 1. K. J. Vahala, “Optical microcavities,” Nature 424, 839-846 (2003). 2. I. H. Agha, J. E. Sharping, M. A. Foster, and A. L. Gaeta, “Optimal sizes of silica microspheres for linear and nonlinear optical interactions,” Appl. Phys. B 83, 303-309 (2006). 3. M. Gorodetsky and V. Ilchenko, “Optical microsphere resonators: Optimal coupling to high-Q whisperinggallery modes,” J. Opt. Soc. Am. B 16, 147-154 (1999). 4. D. Vernooy, V. Ilchenko, H. Mabuchi, E. Streed, and H. Kimble, “High-Q measurements of fused-silica microspheres in the near infrared,” Opt. Lett. 23, 247-249 (1998). 5. B. Little, J. Laine, and H. Haus, “Analytic theory of coupling from tapered fibers and half-blocks into microsphere resonators,” J. Lightwave Technol. 17, 704-715 (1999). 6. M. Cai, O. Painter, and K. J. Vahala, “Observation of critical coupling in a fiber taper to a silica-microsphere whispering-gallery mode system,” Phys. Rev. Lett. 85, 74-77 (2000). 7. T. Kippenberg, S. Spillane, D. Armani, and K. Vahala, “Ultralow-threshold microcavity raman laser on a microelectronic chip,” Opt. Lett. 29, 1224-1226 (2004). 8. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Kerr-nonlinearity optical parametric oscillation in an ultrahigh-Q toroid microcavity,” Phys. Rev. Lett. 93, 083904 (2004). 9. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-threshold raman laser using a spherical dielectric microcavity,” Nature 415, 621-623 (2002). 10. I. H. Agha, Y. Okawachi, M. A. Foster, J. E. Sharping, and A. L. Gaeta, “Four-wave-mixing parametric oscillations in dispersion-compensated high-Q silica microspheres,” Phys. Rev. A 76, 043837-4 (2007). 11. P. Del'Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450, 1214-1217 (2007). 12. A. Hilton, “Optical properties of chalcogenide glass,” J. Non-Cryst. Solids 2, 28-39 (1970). 13. J. S. Sanghera, L. B. Shaw, L. E. Busse, V. Q. Nguyen, P. C. Pureza, B. C. Cole, B. B. Harrison, I. D. Aggarwal, R. Mossadegh, F. Kung, D. Talley, D. Roselle, and R. Miklos, “Development and infrared applications of chalcogenide glass optical fibers,” Fiber Integr. Opt. 19, 251-274 (2000). 14. B. J. Eggleton, V. G. Ta'eed, and B. Luther-Davies, “Chalcogenide glass advanced for all-optical processing,” Photonics Spectra 41, (2007). http://www.photonics.com/content/spectra/2007/September/features/88885.aspx. 15. V. Ta'eed, N. Baker, L. Fu, K. Finsterbusch, M. Lamont, D. Moss, H. Nguyen, B. Eggleton, D. Choi, S. Madden, and B. Luther-Davies, “Ultrafast all-optical chalcogenide glass photonic circuits,” Opt. Express 15, 9205-9221 (2007). 16. M. Asobe, H. Itoh, T. Miyazawa, and T. Kanamori, “Efficient and ultrafast all-optical switching using high ∆n, small core chalcogenide glass fibre,” Electron. Lett. 29, 1966-1968 (1993). (C) 2009 OSA 13 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 5998 #107493 $15.00 USD Received 11 Feb 2009; revised 23 Mar 2009; accepted 25 Mar 2009; published 30 Mar 2009 17. V. Ta'eed, L. Fu, M. Pelusi, M. Rochette, I. Littler, D. Moss, and B. Eggleton, “Error free all optical wavelength conversion in highly nonlinear As-Se chalcogenide glass fiber,” Opt. Express 14, 10371-10376 (2006). 18. L. Fu, M. Rochette, V. Ta'eed, D. Moss, and B. Eggleton, “Investigation of self-phase modulation based optical regeneration in single mode As2Se3 chalcogenide glass fiber,” Opt. Express 13, 7637-7644 (2005). 19. R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. B 21, 1146-1155 (2004). 20. O. Kulkarni, C. Xia, D. Lee, M. Kumar, A. Kuditcher, M. Islam, F. Terry, M. Freeman, B. Aitken, S. Currie, J. McCarthy, M. Powley, and D. Nolan, “Third order cascaded Raman wavelength shifting in chalcogenide fibers and determination of Raman gain coefficient,” Opt. Express 14, 7924-7930 (2006). 21. E. C. Magi, L. B. Fu, H. C. Nguyen, M. R. Lamont, D. I. Yeom, and B. J. Eggleton, “Enhanced kerr nonlinearity in sub-wavelength diameter As2Se3 chalcogenide fiber tapers,” Opt. Express 15, 10324-10329 (2007). 22. L. Fu, A. Fuerbach, I. Littler, and B. Eggleton, “Efficient optical pulse compression using chalcogenide single-mode fibers,” Appl. Phys. Lett. 88, (2006). 23. A. Husakou and J. Herrmann, “Supercontinuum generation in photonic crystal fibers made from highly nonlinear glasses,” Appl. Phys. B 77, 227-234 (2003). 24. D. Yoem, E. Magi, M. Lamont, M. Roelens, L. Fu, and B. J. Eggleton, “Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires,” Opt. Lett. 33, 660-662 (2008). 25. V. Ta'eed, M. Shokooh-Saremi, L. Fu, D. Moss, M. Rochette, I. Littler, B. Eggleton, Y. Ruan, and B. Luther-Davies, “Integrated all-optical pulse regenerator in chalcogenide waveguides,” Opt. Lett. 30, 29002902 (2005). 26. N. D. Psaila, R. R. Thomson, H. T. Bookey, S. Shen, N. Chiodo, R. Osellame, G. Cerullo, A. Jha, and A. K. Kar, “Supercontinuum generation in an ultrafast laser-inscribed chalcogenide glass waveguide,” Opt. Express 15, 15776-15781 (2007). 27. G. Boudebs, S. Cherukulappurath, M. Guignard, J. Troles, F. Smektala, and F. Sanchez, “Linear optical characterization of chalcogenide glasses,” Opt. Commun. 230, 331-336 (2004). 28. G. R. Elliott, D. W. Hewak, G. S. Murugan, and J. S. Wilkinson, “Chalcogenide glass microspheres; their production, characterization and potential,” Opt. Express 15, 17542-17553 (2007). 29. Y. Ruan, M. Kim, Y. Lee, B. Luther-Davies, and A. Rode, “Fabrication of high-Q chalcogenide photonic crystal resonators by e-beam lithography,” Appl. Phys. Lett. 90, 071102-3 (2007). 30. J. Hu, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Demonstration of chalcogenide glass racetrack micro-resonators,” Opt. Lett. 33, 761-763 (2008). 31. C. Grillet, S. Bian, E. Magi, and B. Eggleton, “Fiber taper coupling to chalcogenide microsphere modes,” Appl. Phys. Lett. 92, 171109 (2008). 32. J. Hu, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Planar waveguidecoupled, high-index-contrast, high-Q resonators in chalcogenide glass for sensing,” Opt. Lett. 33, 2500-2502 (2008). 33. A. Snyder and J. Love, Optical Waveguide Theory, (Kluwer Academic Publishers, 1983). 34. V. Almeida, R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28, 13021304 (2003). 35. V. Almeida, C. Barrios, R. Panepucci, and M. Lipson, “All-optical control of light on a silicon chip,” Nature 431, 1081-1084 (2004). 36. H. Naito, H. Amii, M. Okuda, and T. Matsushita, “Structural changes in amorphous arsenic triselenide below the glass-transition temperature,” J. Non-Cryst. Solids 164-166, 1239-1242 (1993). 37. E. D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” Am. J. Phys. 69, 79-87 (2001).

In addition, As 2 Se 3 has a large refractive index of 2.83 and an absorption of 5 m -1 at 1550 nm [27].
Here we fabricate high-Q As 2 Se 3 microspheres and overcome phase matching challenges by coupling to the microspheres with index-tailored silicon nanowires.We demonstrate a Q > 2×10 6 , which to our knowledge is the highest Q achieved in a chalcogenide microresonator.We also explore thermal instabilites in these microspheres and show that these instablilites are present at incident laser powers of 1 mW.Ultimately, these instabilities will limit the viability of As 2 Se 3 as a material for nonlinear-optical microresonators.

Theory
High-Q resonator systems require a means of coupling light into the cavity.For optimal buildup in the resonator cavity, the interaction between the cavity and the coupling mechanism must be accurately controlled.Evanescent coupling from a fiber taper has been shown to provide sufficient degrees of freedom to achieve critical coupling to silica microspheres [5,6].Futhermore, waveguide coupler geometries such as fiber tapers offer ease of integration with external systems.
Controlling phase matching in waveguide coupling schemes is a key parameter for achieving efficient coupling into the high index chalcogenide microspheres.The propagation constant mismatch ∆β between the waveguide and the microsphere mode is ), ( where n s is the effective index of the sphere mode, and n w is the effective index of the waveguide mode.Coupling efficiency falls off exponentially with the square of ∆β [5].Appropriate phase matching can only be achieved via a waveguide with equal or greater material index than the microsphere's effective index, since the effective index of a guided mode is always less than that of the material index of the core for waveguides based on total internal reflection [33].Our simulations show that silicon nanowaveguides with a cross sectional area less than 0.3 µm 2 can achieve phase matching with such microspheres.These waveguides offer a robust coupling platform, a convenient waveguide coupling geometry, and easy integration into both on-chip and fiber systems [34,35].

Fabrication
We fabricate the high-Q As 2 Se 3 microspheres using resistive heating and a three-step process.We utilize a platinum heater, which allows for fabrication under normal lab conditions, including humidity and O 2 concentration.Oxidation on the surface of the sphere may contribute to overall loss; however, the effects were not investigated in this paper.Using this process, we fabricate microspheres with high surface quality over a size range of 55-400 µm.We begin with IRT-SE-06-01 As 2 Se 3 step-index fiber available commerically from CorActive.We then taper the fiber down to a waist size of 15 µm with a sharp transition.We taper at a temperature of 200°C, which is slightly above the glass transition temperature (177°C) [36], and then translate 400-1200 µm along the fiber axis and taper until we cinch off the end of the fiber, which leaves a bulge.We control the size of the bulge by changing the distance translated, which determines the volume of the sphere.We then heat the bulge above the melting point (320°C) of the chalcogenide glass, and the surface tension of the molten chalcogenide glass molds the bulge into a spherical shape.Due to the rotational symmetry of the fiber and fabrication process the microspheres have low on-axis eccentricity.Figure 2 shows a mockup of the three steps, and Fig. 3 shows three completed spheres.

Experiment
Figure 4 shows a schematic of our coupling and detection scheme.To characterize the microspheres, light from an external-cavity diode laser is coupled into the sphere, and the frequency is repeatedly tuned over several GHz via piezo control.An erbium-doped fiber amplifier (EDFA) amplifies the signal, which is then coupled into an unclad silicon rib waveguide via a tapered lens fiber.The waveguides have cross sectional dimensions of 900 nm by 250 nm and are fabricated on a SiO 2 substrate.At the point of coupling to the microsphere, the waveguides taper down to a cross section of 250 nm by 250 nm.This corresponds to n eff ranging from 2.9 to 1.1 in the waveguides.We control coupling to the microsphere by bringing the microsphere in contact with the photonic chip and translate it transversely to the waveguide over the tapered region until optimal coupling is achieved.We then detect the output signal with a photodiode and view its output on an oscilloscope.

Results
As we tune over the resonances, transmission dips are observed when coupling to the sphere is achieved, and we measure the width of these transmission dips to obtain the cavity linewidth.We observe a linewidth of 80 MHz in an isolated mode with 7 dB of extinction, as shown in Fig. 5.The Q of the system can then be estimated through the relation, , where ∆ω is the cavity linewidth, and ω 0 is the center frequency.From Eq. 2 we obtain Q = 2.3×10 6 , which we believe to be the highest Q achieved in a chalcogenide microresonator by a factor of 10.In addition to characterizing the Q of the microspheres, we also explore their thermal stability.
Comparing As 2 Se 3 to silica, we find that thermal instability, stemming from comparatively high linear absorption at 1550 nm and a comparatively low melting temperature, severely limits the viability of As 2 Se 3 as a material for NLO oscillation in a microresonator geometry.This instability is manifested in two ways: resonance shifting at low incident powers and melting.We demonstrate the resonance shifts by locating a cavity resonance and increasing the incident power in the waveguide.In Fig. 6(a) we show that with 1 mW of incident power large shifts in the position of the resonance are observed.This shift is due to light being coupled into the sphere mode and heating of the sphere via absorption, which results in a thermal change in the refractive index and a shift of the cavity resonance.If this shift corresponds to the direction of the laser scan, more light will be coupled in, causing further shifting of the resonance.This process will continue until the maximum coupling for the system is reached.At that time, no more light can be coupled into the microsphere, and the laser will scan past the resonance.This leads to the characteristic sharkfin shape seen in Fig. 6(a).
At slightly higher incident powers of 2-3 mW, melting occurs, and material may be violently ejected from the sphere.Figure 6(b) shows debris ejected from a microsphere.While thermal resonance shifting can be mitigated with laser locking [37], the melting cannot be solved without cumbersome active cooling.This places a hard limit on useable incident powers and precludes experiments such as broadband cascaded FWM [11] that require incident powers of ~50 mW.

Conclusions
We fabricated microspheres with Q's > 2×10 6 and demonstrated efficient coupling to high-Q chalcogenide microspheres via silicon nanowires.We showed that thermal effects limit nonlinear optical performance and found that these thermal effects are present with ≥ 1 mW of incident laser power.

Figure 1 (Fig. 1 .
Fig. 1.Effective indices of As2Se3 microspheres, silicon nanowires, and silica nanowires.(a) Effective index vs.sphere diameter for fundamental TE (red) and TM (blue) modes in an As2Se3 microsphere.(b) Effective index vs.material index multiplied by cross sectional area of silicon (red) and silica (blue) waveguides.

Fig. 2 .
Fig. 2. Mockup of the three-step sphere fabrication process.(a) Taper with 15-µm waist.(b) Bulge formed by cinching off end of fiber.(c) Sphere formed using surface tension of molten AssSe3.

Fig. 5 .
Fig. 5. Isolated As2Se3 microsphere mode.The linewidth of the mode is 80 MHz, which corresponds to a Q of 2.3×10 6 at 1550 nm.