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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 20 — Sep. 24, 2012
  • pp: 22012–22017
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Flexible coupling of high-Q goblet resonators for formation of tunable photonic molecules

Torsten Beck, Steffen Schloer, Tobias Grossmann, Timo Mappes, and Heinz Kalt  »View Author Affiliations


Optics Express, Vol. 20, Issue 20, pp. 22012-22017 (2012)
http://dx.doi.org/10.1364/OE.20.022012


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Abstract

We report on a method for a highly flexible arrangement of polymeric high-Q whispering gallery mode resonators. Parallel on-chip fabricated goblet resonators are detached from the substrate by bonding a gold wire to the field-free center of their polymeric cavity. This enables the precise control of the resonator’s spatial position. The modal spectrum of the detached resonator reveals preservation of its high optical quality. Manipulation of the resonators’ position allows for designing coupled resonators geometries and tuning the coupling properties dynamically after batch fabrication. The properties of the modal spectrum evidence the successful optical coupling.

© 2012 OSA

1. Introduction

Even more desirable is the possibility to flexibly arrange and dynamically control the individual cavities. In particular the adjustability of the coupling gap between two resonators and therefore the determination of the inter-cavity photon tunneling rate adds an important additional degree of freedom. In photonic applications this could be used for tuning of the lasing characteristics of active resonant systems during operation or for the realization of adjustable filtering devices.

But a couple of obstacles have to be overcome to achieve efficient resonator coupling. Such systems are typically defined by lithographic processing [15

15. M. A. Popovíc, T. Barwicz, M. R. Watts, P. T. Rakich, L. Socci, E. P. Ippen, F. X. Kärtner, and H. I. Smith, “Multistage high-order microring-resonator add-drop filters,” Opt. Lett. 31(17), 2571–2573 (2006). [CrossRef] [PubMed]

17

17. S. Ishii and T. Baba, “Bistable lasing in twin microdisk photonic molecules,” Appl. Phys. Lett. 87(18), 181102 (2005). [CrossRef]

] and are therefore inherently inflexible. After fabrication the system is fixed and tuning of the coupling conditions or changes in geometry are not feasible. Furthermore, the coupling of two or more WGM resonators requires a spatial separation of the resonators in the order of the wavelength. However, a thermal treatment of the resonators will result in shrinkage of each single resonator. Thus the gap between two resonators would enlarge and coupling is annihilated. As a consequence a thermal surface modification, which is required to reduce surface-scattering losses and to improve optical qualities, may not be applied to the coupled structures after lithography.

One possibility to couple on-chip resonators is to approach one substrate to another one, where the resonators on both substrates are fabricated at the edge. This is technically demanding and restricted to coupling of only two resonators [18

18. I. S. Grudinin, H. Lee, O. Painter, and K. J. Vahala, “Phonon laser action in a tunable two-Level system,” Phys. Rev. Lett. 104(8), 083901 (2010). [CrossRef] [PubMed]

]. In an alternative method the resonators are detached with a microfork and placed at the desired location. In this case the control of the resonator gets lost once the resonators are positioned [19

19. M. Hossein-Zadeh and K. J. Vahala, “Free ultra-high-Q microtoroid: a tool for designing photonic devices,” Opt. Express 15(1), 166–175 (2007). [CrossRef] [PubMed]

]. So, dynamic manipulation of the coupling conditions is not possible.

We report here on a novel method to realize post-fabrication tunable photonic molecules based on thermally treated, high-Q polymeric resonators fabricated parallel on-chip. By a subsequent detachment and mounting procedure we are able to controllably position detachedresonators with nanometer resolution with respect to substrate-bound cavities. Optical coupling of such resonator pairs is demonstrated by the modifications of the modal properties.

2. Fabrication

In order to realize an adjustable system of resonators, polymeric goblet-resonators with a diameter of 90 µm are fabricated using lithography, isotropic etching and a subsequent thermal reflow-step. The process parameters are provided in [20

20. T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-Q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]

]. Such WGM resonators made out of PMMA on silicon pedestals are depicted schematically in Fig. 1(a)
Fig. 1 Schematic of resonator detaching method: Starting with parallel on-chip fabricated polymeric goblet resonators on silicon pedestals (a). A gold wire is approached to the center of a resonator and bonded with UV-curing adhesive (b). By XeF2 etching the silicon pedestal is removed and the polymer resonator is detached (c), freely movable and can be coupled to another resonator on a different substrate (d).
. This type of goblet resonator shows a Q-factor in the order of 106 in the near infra-red band (1300-1400 nm). Large numbers of these resonators can be processed in parallel on one substrate.

3. Optical characterization of portable polymer resonators

4. Realization of coupled-resonator systems

Because of the high degree of flexibility, various coupling schemes, in terms of number of resonators and geometry, are conceivable. In the setup shown in Fig. 3(a)
Fig. 3 (a) Microscope picture of two evanescently coupled resonators: The lower resonator is fixed to the substrate, the upper one is freely movable. The tapered fiber optical waveguide is aligned to the on-chip resonator. (b) Tapered fiber transmission: The red and the blue curve represent the spectra of the individual resonators, the black line shows the coupled system. Evanescently coupled modes are highlighted in red and blue. In violet the vanishing of single modes due to destructive inference is marked.(c) Transmission spectra for different inter cavity coupling gaps. From (i) to (ii) the gap size was reduced by 250 nm. For spectrum (iii) the air gap is further decreased by 500 nm. If the air gap is sufficiently small mode splitting occurs. With decreasing gap width the spectral separation of the modes increases.
a double-cavity geometry, often referred to as photonic molecule, is built from a detached and a substrate-bound resonator. The tapered fiber is placed in the vicinity the on-chip resonator. The detached resonator is then approached to the substrate bound resonator in the configuration shown in Fig. 3(a). When the gap and the resonator planes are well aligned coupling of the resonators can be observed.

To investigate the photonic molecule, the spectra of the individual resonators are compared to the spectrum of the coupled system. In Fig. 3(b) the spectrum of the substrate bound resonator (blue) as well as the spectrum of the detached resonator (red) is depicted. In both spectra resonances with Q-factors in the order of 105 can be observed. The spectrum of the double-cavity is shown in black. In this spectrum modes of the on-chip resonator can be found. In addition, modes of the detached resonator appear. The wavelength of a mode localized on one of the resonators is slightly red-shifted in presence of the second resonator, compared to its spectral position in the isolated cavity. This is caused by the increased optical path length, due to the change of the dielectric environment, induced by the presence of the second resonator. Because the critical coupling gap-size is different for each individual mode, the Q-factors of some modes are reduced. When interacting resonances overlap spectrally, splitting of the cavity eigenstates occurs [21

21. D. Smith, H. Chang, and K. Fuller, “Whispering-gallery mode splitting in coupled microresonators,” J. Opt. Soc. Am. B 20(9), 1967–1974 (2003). [CrossRef]

]. In the violet highlighted spectral region, a mode in both isolated resonators is visible. However, in the coupled system this mode vanishes. The two new minima that appear aside (black arrows in Fig. 3(b)) suggest a splitting of about 320 pm. By variation of the air gap width between the resonators, the splitting of the super-modes can be controlled (Fig. 3(c)). These observations clearly demonstrate that the presented coupling method is suitable for the formation of photonic molecules.

5. Conclusion and outlook

The presented method is adaptable to other on-chip produced resonators like disks or toroids.

Even geometries with more than two resonators are feasible. By the combination of several on-chip resonators with one or more detached resonators more complex adjustable photonic molecules can be built. One further advantage is the production of the coupled resonators on different substrates. A composition of resonators that are pretreated with different processes, for example bio-functionalization or dye-doping, may add further functionalities. Coupling of photon emitters to optical modes of tunable photonic molecules enables cavity quantum electro-dynamic experiments and applications in quantum information processing.

This work has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe, by a grant from the Ministry of Science, Research, and the Arts of Baden-Württemberg (Grant No. Az:7713.14-300) and by the German Federal Ministry for Education and Research BMBF (Grant No. FKZ 13N8168A). T.M.’s Young Investigator Group (YIG 08) received financial support from the Concept for the Future of the Karlsruhe Institute of Technology (KIT) within the framework of the German Excellence Initiative. T.G. gratefully acknowledges financial support of the Deutsche Telekom Stiftung and the Karlsruhe House of Young Scientists (KHYS). T.G. and T.B. are pursuing their Ph.D. within the Karlsruhe School of Optics and Photonics (KSOP). We acknowledge support by the Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology.

Acknowledgments

References and links

1.

T. Aoki, B. Dayan, E. Wilcut, W. P. Bowen, A. S. Parkins, T. J. Kippenberg, K. J. Vahala, and H. J. Kimble, “Observation of strong coupling between one atom and a monolithic microresonator,” Nature 443(7112), 671–674 (2006). [CrossRef] [PubMed]

2.

K. J. Vahala, “Optical microcavities,” Nature 424(6950), 839–846 (2003). [CrossRef] [PubMed]

3.

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

4.

T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klink-hammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett. 97(6), 063304 (2010). [CrossRef]

5.

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(1), 46–49 (2010). [CrossRef]

6.

C.-Y. Chao, W. Fung, and L. Guo, “Polymer microring resonators for biochemical sensing applications,” IEEE J. Sel. Top. Quantum Electron. 12(1), 134–142 (2006). [CrossRef]

7.

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), 4057–4059 (2002). [CrossRef]

8.

T. J. Kippenberg and K. J. Vahala, “Cavity optomechanics: back-action at the mesoscale,” Science 321(5893), 1172–1176 (2008). [CrossRef] [PubMed]

9.

F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods 5(7), 591–596 (2008). [CrossRef] [PubMed]

10.

M. J. Hartmann, F. G. S. L. Brandao, and M. B. Plenio, “Strongly interacting polaritons in coupled arrays of cavities,” Nat. Phys. 2(12), 849–855 (2006). [CrossRef]

11.

S. Boriskina, “Spectrally engineered photonic molecules as optical sensors with enhanced sensitivity: a proposal and numerical analysis,” J. Opt. Soc. Am. B 23(8), 1565–1573 (2006). [CrossRef]

12.

J. V. Hryniewicz, P. P. Absil, B. E. Little, R. A. Wilson, and P.-T. Ho, “Higher order filter response in coupled microring resonators,” IEEE Photonic. Tech. L. 12(3), 320–322 (2000). [CrossRef]

13.

J. K. Poon, L. Zhu, G. A. DeRose, and A. Yariv, “Transmission and group delay of microring coupled-resonator optical waveguides,” Opt. Lett. 31(4), 456–458 (2006). [CrossRef] [PubMed]

14.

E. I. Smotrova, A. I. Nosich, T. M. Benson, and P. Sewell, “Threshold reduction in a cyclic photonic molecule laser composed of identical microdisks with whispering-gallery modes,” Opt. Lett. 31(7), 921–923 (2006). [CrossRef] [PubMed]

15.

M. A. Popovíc, T. Barwicz, M. R. Watts, P. T. Rakich, L. Socci, E. P. Ippen, F. X. Kärtner, and H. I. Smith, “Multistage high-order microring-resonator add-drop filters,” Opt. Lett. 31(17), 2571–2573 (2006). [CrossRef] [PubMed]

16.

X. Wu, H. Li, L. Liu, and L. Xu, “Unidirectional single-frequency lasing from a ring-spiral coupled microcavity laser,” Appl. Phys. Lett. 93(8), 081105 (2008). [CrossRef]

17.

S. Ishii and T. Baba, “Bistable lasing in twin microdisk photonic molecules,” Appl. Phys. Lett. 87(18), 181102 (2005). [CrossRef]

18.

I. S. Grudinin, H. Lee, O. Painter, and K. J. Vahala, “Phonon laser action in a tunable two-Level system,” Phys. Rev. Lett. 104(8), 083901 (2010). [CrossRef] [PubMed]

19.

M. Hossein-Zadeh and K. J. Vahala, “Free ultra-high-Q microtoroid: a tool for designing photonic devices,” Opt. Express 15(1), 166–175 (2007). [CrossRef] [PubMed]

20.

T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-Q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]

21.

D. Smith, H. Chang, and K. Fuller, “Whispering-gallery mode splitting in coupled microresonators,” J. Opt. Soc. Am. B 20(9), 1967–1974 (2003). [CrossRef]

OCIS Codes
(140.4780) Lasers and laser optics : Optical resonators
(160.5470) Materials : Polymers
(220.4000) Optical design and fabrication : Microstructure fabrication
(140.3945) Lasers and laser optics : Microcavities
(230.4555) Optical devices : Coupled resonators

ToC Category:
Optical Devices

History
Original Manuscript: June 29, 2012
Revised Manuscript: August 27, 2012
Manuscript Accepted: September 6, 2012
Published: September 11, 2012

Citation
Torsten Beck, Steffen Schloer, Tobias Grossmann, Timo Mappes, and Heinz Kalt, "Flexible coupling of high-Q goblet resonators for formation of tunable photonic molecules," Opt. Express 20, 22012-22017 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-20-22012


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References

  1. T. Aoki, B. Dayan, E. Wilcut, W. P. Bowen, A. S. Parkins, T. J. Kippenberg, K. J. Vahala, and H. J. Kimble, “Observation of strong coupling between one atom and a monolithic microresonator,” Nature443(7112), 671–674 (2006). [CrossRef] [PubMed]
  2. K. J. Vahala, “Optical microcavities,” Nature424(6950), 839–846 (2003). [CrossRef] [PubMed]
  3. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol.20(11), 1968–1975 (2002). [CrossRef]
  4. T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klink-hammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett.97(6), 063304 (2010). [CrossRef]
  5. 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(1), 46–49 (2010). [CrossRef]
  6. C.-Y. Chao, W. Fung, and L. Guo, “Polymer microring resonators for biochemical sensing applications,” IEEE J. Sel. Top. Quantum Electron.12(1), 134–142 (2006). [CrossRef]
  7. 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), 4057–4059 (2002). [CrossRef]
  8. T. J. Kippenberg and K. J. Vahala, “Cavity optomechanics: back-action at the mesoscale,” Science321(5893), 1172–1176 (2008). [CrossRef] [PubMed]
  9. F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods5(7), 591–596 (2008). [CrossRef] [PubMed]
  10. M. J. Hartmann, F. G. S. L. Brandao, and M. B. Plenio, “Strongly interacting polaritons in coupled arrays of cavities,” Nat. Phys.2(12), 849–855 (2006). [CrossRef]
  11. S. Boriskina, “Spectrally engineered photonic molecules as optical sensors with enhanced sensitivity: a proposal and numerical analysis,” J. Opt. Soc. Am. B23(8), 1565–1573 (2006). [CrossRef]
  12. J. V. Hryniewicz, P. P. Absil, B. E. Little, R. A. Wilson, and P.-T. Ho, “Higher order filter response in coupled microring resonators,” IEEE Photonic. Tech. L.12(3), 320–322 (2000). [CrossRef]
  13. J. K. Poon, L. Zhu, G. A. DeRose, and A. Yariv, “Transmission and group delay of microring coupled-resonator optical waveguides,” Opt. Lett.31(4), 456–458 (2006). [CrossRef] [PubMed]
  14. E. I. Smotrova, A. I. Nosich, T. M. Benson, and P. Sewell, “Threshold reduction in a cyclic photonic molecule laser composed of identical microdisks with whispering-gallery modes,” Opt. Lett.31(7), 921–923 (2006). [CrossRef] [PubMed]
  15. M. A. Popovíc, T. Barwicz, M. R. Watts, P. T. Rakich, L. Socci, E. P. Ippen, F. X. Kärtner, and H. I. Smith, “Multistage high-order microring-resonator add-drop filters,” Opt. Lett.31(17), 2571–2573 (2006). [CrossRef] [PubMed]
  16. X. Wu, H. Li, L. Liu, and L. Xu, “Unidirectional single-frequency lasing from a ring-spiral coupled microcavity laser,” Appl. Phys. Lett.93(8), 081105 (2008). [CrossRef]
  17. S. Ishii and T. Baba, “Bistable lasing in twin microdisk photonic molecules,” Appl. Phys. Lett.87(18), 181102 (2005). [CrossRef]
  18. I. S. Grudinin, H. Lee, O. Painter, and K. J. Vahala, “Phonon laser action in a tunable two-Level system,” Phys. Rev. Lett.104(8), 083901 (2010). [CrossRef] [PubMed]
  19. M. Hossein-Zadeh and K. J. Vahala, “Free ultra-high-Q microtoroid: a tool for designing photonic devices,” Opt. Express15(1), 166–175 (2007). [CrossRef] [PubMed]
  20. T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-Q conical polymeric microcavities,” Appl. Phys. Lett.96(1), 013303 (2010). [CrossRef]
  21. D. Smith, H. Chang, and K. Fuller, “Whispering-gallery mode splitting in coupled microresonators,” J. Opt. Soc. Am. B20(9), 1967–1974 (2003). [CrossRef]

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