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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15853–15858
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Wide tuning of SiN microring resonators by auto-realigning nematic liquid crystal

Tzyy-Jiann Wang, Shun-Chen Yang, Tien-Jung Chen, and Bo-Yu Chen  »View Author Affiliations


Optics Express, Vol. 20, Issue 14, pp. 15853-15858 (2012)
http://dx.doi.org/10.1364/OE.20.015853


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Abstract

We present wide electrical tuning of microring resonators with auto-realigned nematic liquid crystal (NLC) cladding. By applying electric field, homeotropically-aligned negative Δε NLC with non-rubbed alignment layers is auto-realigned along the microring waveguide due to the protruding of the ridge structure. The consistent cladding index distribution along the microring waveguide produces effective tuning of the resonant wavelength. It achieves a large tuning range of 13nm for TM mode and 2.1nm for TE mode. The NLC reorientation characteristics are investigated by minimizing Oseen-Frank energy. The proposed microring resonator owns the features of large tuning range and bi-polarization wavelength tuning.

© 2012 OSA

1. Introduction

Microring resonators possess excellent wavelength-selective characteristics in a compact structure. They have been widely applied in many advanced applications, including high-speed intensity modulation [1

1. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

], wavelength add-drop [2

2. K. Takahashi, Y. Kanamori, Y. Kokubun, and K. Hane, “A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator,” Opt. Express 16(19), 14421–14428 (2008). [CrossRef] [PubMed]

], dispersion compensation [3

3. G. Lenz and C. K. Madsen, “General optical all-pass filter structures for dispersion control in WDM systems,” J. Lightwave Technol. 17(7), 1248–1254 (1999). [CrossRef]

], biochemical sensing [4

4. C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett. 33(7), 708–710 (2008). [CrossRef] [PubMed]

], laser resonator [5

5. S. Mikroulis, E. Roditi, and D. Syvridis, “Direct modulation properties of 1.55-μm InGaAsP/InP microring lasers,” J. Lightwave Technol. 26(2), 251–256 (2008). [CrossRef]

], and wavelength conversion [6

6. M. Gandomkar and V. Ahmadi, “Design and analysis of enhanced second harmonic generation in AlGaAs/AlO(x) microring waveguide,” Opt. Express 19(10), 9408–9418 (2011). [CrossRef] [PubMed]

]. The resonant wavelength of the microring resonator can be dynamically tuned by varying the index of waveguide core or cladding through physical effects, such as electro-optic effect [7

7. A. Guarino, G. Poberaj, D. Rezzonico, R. Degl’innocenti, and P. Günter, “Electro-optically tunable microring resonators in lithium niobate,” Nat. Photonics 1(7), 407–410 (2007). [CrossRef]

, 8

8. T.-J. Wang, C.-H. Chu, and C.-Y. Lin, “Electro-optically tunable microring resonators on lithium niobate,” Opt. Lett. 32(19), 2777–2779 (2007). [CrossRef] [PubMed]

], thermo-optic effect [9

9. M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett. 89(7), 071110 (2006). [CrossRef]

], free-carrier plasma dispersion effect [10

10. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

], and nematic liquid crystal (NLC) reorientation [11

11. B. Maune, R. Lawson, G. Gunn, A. Scherer, and L. Dalton, “Electrically tunable ring resonators incorporating nematic liquid crystals as cladding layers,” Appl. Phys. Lett. 83(23), 4689–4691 (2003). [CrossRef]

15

15. W. De Cort, J. Beeckman, T. Claes, K. Neyts, and R. Baets, “Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding,” Opt. Lett. 36(19), 3876–3878 (2011). [CrossRef] [PubMed]

]. Among them, NLC reorientation produces the largest index change and has the potential to achieve the widest tuning of resonant wavelength. NLC has been used in various photonic applications [16

16. J. L. D. Bougrenet and D. L. Tocnaye, “Engineering liquid crystals for optimal uses in optical communication systems,” Liq. Cryst. 31(2), 241–269 (2004). [CrossRef]

, 17

17. J. Beeckman, K. Neyts, and P. J. M. Vanbrabant, “Liquid-crystal photonic applications,” Opt. Eng. 50(8), 081202 (2011). [CrossRef]

]. Several device structures are proposed to reorient NLC molecules in the cladding for effectively tuning the resonant wavelength. The spin-coated NLC film is realigned azimuthally by using two hemi-circular-ended strip electrodes. It produces a resonant wavelength shift of 0.22nm for TE mode in the Si microring resonator [11

11. B. Maune, R. Lawson, G. Gunn, A. Scherer, and L. Dalton, “Electrically tunable ring resonators incorporating nematic liquid crystals as cladding layers,” Appl. Phys. Lett. 83(23), 4689–4691 (2003). [CrossRef]

]. The reorientation of a NLC layer sandwiched between a SOI chip and an ITO glass plate achieves a tuning range of 0.6nm for TE mode in the Si microring resonator [12

12. W. De Cort, J. Beeckman, R. James, F. A. Fernández, R. Baets, and K. Neyts, “Tuning of silicon-on-insulator ring resonators with liquid crystal cladding using the longitudinal field component,” Opt. Lett. 34(13), 2054–2056 (2009). [CrossRef] [PubMed]

]. The NLC in-plane switching has been used to tune the polymer [13

13. T. Cai, Q. Liu, Y. Shi, P. Chen, and S. He, “An effectively tunable microring resonator using a liquid crystal-cladded polymer waveguide,” Appl. Phys. Lett. 97(12), 121109 (2010). [CrossRef]

] and Si [14

14. W. De Cort, J. Beeckman, R. James, F. A. Fernandez, R. Baets, and K. Neyts, “Tuning silicon-on-insulator ring resonators with in-plane switching liquid crystals,” J. Opt. Soc. Am. B 28(1), 79–85 (2011). [CrossRef]

] microring resonators. Their tuning ranges for TE mode are 0.73nm and 1nm, respectively. The Si ring resonator using four straight sections and input/output grating couplers produces the resonant wavelength shifts of 31nm for TM mode and 4.5nm for TE mode [15

15. W. De Cort, J. Beeckman, T. Claes, K. Neyts, and R. Baets, “Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding,” Opt. Lett. 36(19), 3876–3878 (2011). [CrossRef] [PubMed]

].

2. Device design

In order to have the same anisotropic cladding index distribution along the microring waveguide, the proposed microring resonator is designed as follows. The microring has the silicon nitride (SiN) ridge waveguide structure and the NLC cladding is formed by using negative Δε NLC combined with non-rubbed homeotropic alignment layers. As the applied voltage is zero, negative Δε NLC molecules are perpendicular to the substrate surface due to the action of homeotropic alignment layer. As a voltage sufficient for NLC reorientation is applied, negative Δε NLC molecules tilt toward the substrate surface. Because of using non-rubbed alignment layers, the NLC molecules tilt freely with arbitrary azimuth angles except those near the SiN waveguides. They are affected by the protruding of the ridge structure from the substrate surface and auto-reorient themselves such that their azimuth angle distribution follows the circular path turning of the microring waveguide. Therefore, the same cladding index distribution is sensed by the optical field propagating at any position of the microring waveguide, no matter whether applying electric field or not.

3. Simulation results

The reorientation of NLC molecules in Fig. 1(b) under various voltages has been calculated based on the minimization of Oseen-Frank energy [19

19. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford U. Press, 1995).

]. Figure 2
Fig. 2 Tilt angle distribution of NLC molecules for the applied voltage: (a) V = 0; (b) V = 5V, (c) V = 10V; (d) V = 100V.
shows the tilt angle distribution of NLC molecules for the voltage V = 0V, 5V, 10V, and 100V. When V = 0V, the tilt angles of the NLC molecules are 90° except those near the sidewalls of the ridge waveguides. Their orientations are perpendicular to the sidewall surface and have the tilt angles around 0°. As the voltage increases to 5V, the NLC molecules in the middle of the gap rotate to the tilt angles of ~30°. The orientation of the NLC molecules near the substrate surface is fixed due to the surface anchoring effect. When the voltage increases from 10V to 100V, most of the NLC molecules orient parallel to the substrate surface (θ = 0°). It is found that the orientations of the NLC molecules near the waveguide sidewalls are less affected by the applied voltage. During the electrical tuning process, the variation of the effective index of the guided mode is mainly contributed by the cladding index change induced by the NLC reorientation on the top of the SiN ridge waveguide.

In order to understand the voltage effect on the resonant wavelength, the full-vectorial mode solver considering the full anisotropy of NLC is used to calculate the effective index of the guided mode. The anisotropic optical property of the NLC directors with the specified tilt angle and azimuth angle is represented by dielectric tensor. The resonant wavelength of the microring resonator is determined by the effective index neff through the equation λm = (2πR + 2L) neff /m, where L is the length of the zero-gap coupler and m is the order number. In the numerical calculation, the boundary condition needs to be specified for obtaining the solution. Because the homeotropic alignment layer anchors the NLC directors along the direction perpendicular to the surface, the simulation uses the boundary conditions of θ = 90° and a certain ϕ value on the horizontal surface of the homeotropic alignment layer. The preset ϕ value affects the calculated director orientation distribution and thus the effective index. In the following simulation, the preset ϕ values of 0°, 45°, and 90° are considered. When the voltage increases from 0 to 100V, the effective index changes of TM mode are −1.27 × 10−2, −1.38 × 10−2, and −1.52 × 10−2 respectively for ϕ = 0°, 45°, and 90°. For TE mode, the effective index changes are 2.10 × 10−4, 3.11 × 10−3, and 6.40 × 10−3. Figure 3
Fig. 3 Dependence of the simulated resonant wavelength for (a) TM mode; (b) TE mode; on the voltage for the preset azimuth angle ϕ = 0°, 45°, and 90°. (Inset: the field distribution of guided mode).
shows the dependence of the resonant wavelength on the voltage for TM and TE modes. For both polarizations, the threshold voltage for NLC reorientation is 4V. As the voltage increases further, the resonant wavelength has a blue shift for TM mode and a red shift for TE mode. The resonant wavelength shifts for TM mode are −11.71nm, −12.73nm, and −14.03nm, for ϕ = 0°, 45°, and 90°. As to TE mode, the corresponding resonant wavelength shifts are 0.19nm, 2.78nm, and 5.72nm. Consider the cladding index variation Δn experienced by the main electric field on the top of the ridge waveguide for the tilt angles of 90° and 0°. The Δn value depends on the azimuth angle. For TM mode, the Δn experienced by Ey is always (no-ne) in the range of ϕ = 0°~90°. The dependence of resonant wavelength shift on the azimuth angle is due to the index sensed by the longitudinal electric field Ez. For TE mode, the Δn experienced by Ex changes from 0 to (ne-no) as ϕ varies from 0° to 90°. The resonant wavelength shift for ϕ = 0° is due to the index change sensed by the longitudinal electric field Ez. The difference in Δn causes the obvious variation of resonant wavelength shift with the azimuth angle. Because the main electric field of TM mode (or TE mode) feels the decreasing (or increasing) index change, the resonant wavelength has a blue (or red) shift. The resonant wavelength shift for TM mode is larger than that for TE mode due to the larger |Δn| and more evanescent field extending to the NLC cladding, as shown in the inset of Fig. 3.

4. Experimental results and discussions

Figure 4
Fig. 4 Photographs of polarized optical microscope for the NLC-cladded microring resonator with the applied voltage of (a) 0V; (b) 50V. The directions of polarizer (P) and analyzer (A) are indicated in the figure.
shows the photographs of polarized optical microscope (POM) of the microring resonator with the applied voltage of 0V and 50V. As the voltage is zero, the incident linearly polarized light regards the homeotropically aligned NLC molecules as an isotropic medium with index no. Because the polarizations of the incident and reflected light are identical, the complete dark image is observed. As the voltage of 50V is applied, the tilt NLC molecules make the NLC layer act as an anisotropic medium. The phase difference between ordinary and extraordinary rays alters the polarization state of the reflected light. A 180° phase difference rotates the input polarization by 90° such that the brightest line (or highest intensity) is observed. It is noted in Fig. 4(b) that two bright lines appear on two sides of the microring waveguide and the straight waveguides. The difference in the gradual variation of brightness levels observed on the inner and outer sides of the microring waveguide is inferred due to the symmetric distribution of azimuth angles, which are ϕ for the NLC molecules on the outer side and -ϕ for those on the inner side, referred to Fig. 1(a). The POM photographs confirm that the protruding of the ridge waveguide structure produce the consistent azimuth angle distribution along the microring waveguide.

5. Conclusion

We have presented the widely tunable SiN microring resonator by auto-realigning nematic liquid crystal. Under the action of electric field, the use of the SiN ridge waveguide and the homeotropically aligned negative Δε NLC cladding produce auto-realignment of nematic liquid crystal along the microring waveguide. No matter whether applying electric field or not, the propagating field at any position of the microring waveguide always senses the consistent cladding index distribution. The wide tuning ranges of 13nm for TM mode and 2.1 nm for TE mode are achieved due to the consistent NLC azimuth angle distribution along the microring waveguide and the strong interaction between NLC and guided mode. Further improvement in the tuning range can be made by designing the waveguide structure.

Acknowledgments

This work was supported by National Science Council of the Republic of China under grants NSC100-2221-E-027-052 and NSC100-2221-E-027-060.

References and links

1.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

2.

K. Takahashi, Y. Kanamori, Y. Kokubun, and K. Hane, “A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator,” Opt. Express 16(19), 14421–14428 (2008). [CrossRef] [PubMed]

3.

G. Lenz and C. K. Madsen, “General optical all-pass filter structures for dispersion control in WDM systems,” J. Lightwave Technol. 17(7), 1248–1254 (1999). [CrossRef]

4.

C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett. 33(7), 708–710 (2008). [CrossRef] [PubMed]

5.

S. Mikroulis, E. Roditi, and D. Syvridis, “Direct modulation properties of 1.55-μm InGaAsP/InP microring lasers,” J. Lightwave Technol. 26(2), 251–256 (2008). [CrossRef]

6.

M. Gandomkar and V. Ahmadi, “Design and analysis of enhanced second harmonic generation in AlGaAs/AlO(x) microring waveguide,” Opt. Express 19(10), 9408–9418 (2011). [CrossRef] [PubMed]

7.

A. Guarino, G. Poberaj, D. Rezzonico, R. Degl’innocenti, and P. Günter, “Electro-optically tunable microring resonators in lithium niobate,” Nat. Photonics 1(7), 407–410 (2007). [CrossRef]

8.

T.-J. Wang, C.-H. Chu, and C.-Y. Lin, “Electro-optically tunable microring resonators on lithium niobate,” Opt. Lett. 32(19), 2777–2779 (2007). [CrossRef] [PubMed]

9.

M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett. 89(7), 071110 (2006). [CrossRef]

10.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

11.

B. Maune, R. Lawson, G. Gunn, A. Scherer, and L. Dalton, “Electrically tunable ring resonators incorporating nematic liquid crystals as cladding layers,” Appl. Phys. Lett. 83(23), 4689–4691 (2003). [CrossRef]

12.

W. De Cort, J. Beeckman, R. James, F. A. Fernández, R. Baets, and K. Neyts, “Tuning of silicon-on-insulator ring resonators with liquid crystal cladding using the longitudinal field component,” Opt. Lett. 34(13), 2054–2056 (2009). [CrossRef] [PubMed]

13.

T. Cai, Q. Liu, Y. Shi, P. Chen, and S. He, “An effectively tunable microring resonator using a liquid crystal-cladded polymer waveguide,” Appl. Phys. Lett. 97(12), 121109 (2010). [CrossRef]

14.

W. De Cort, J. Beeckman, R. James, F. A. Fernandez, R. Baets, and K. Neyts, “Tuning silicon-on-insulator ring resonators with in-plane switching liquid crystals,” J. Opt. Soc. Am. B 28(1), 79–85 (2011). [CrossRef]

15.

W. De Cort, J. Beeckman, T. Claes, K. Neyts, and R. Baets, “Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding,” Opt. Lett. 36(19), 3876–3878 (2011). [CrossRef] [PubMed]

16.

J. L. D. Bougrenet and D. L. Tocnaye, “Engineering liquid crystals for optimal uses in optical communication systems,” Liq. Cryst. 31(2), 241–269 (2004). [CrossRef]

17.

J. Beeckman, K. Neyts, and P. J. M. Vanbrabant, “Liquid-crystal photonic applications,” Opt. Eng. 50(8), 081202 (2011). [CrossRef]

18.

T.-J. Wang, Y.-H. Huang, and H.-L. Chen, “Resonant-wavelength tuning of microring filters by oxygen plasma treatment,” IEEE Photon. Technol. Lett. 17(3), 582–584 (2005). [CrossRef]

19.

P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford U. Press, 1995).

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(160.3710) Materials : Liquid crystals
(130.7408) Integrated optics : Wavelength filtering devices

ToC Category:
Integrated Optics

History
Original Manuscript: April 27, 2012
Revised Manuscript: June 12, 2012
Manuscript Accepted: June 18, 2012
Published: June 27, 2012

Citation
Tzyy-Jiann Wang, Shun-Chen Yang, Tien-Jung Chen, and Bo-Yu Chen, "Wide tuning of SiN microring resonators by auto-realigning nematic liquid crystal," Opt. Express 20, 15853-15858 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15853


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References

  1. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature435(7040), 325–327 (2005). [CrossRef] [PubMed]
  2. K. Takahashi, Y. Kanamori, Y. Kokubun, and K. Hane, “A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator,” Opt. Express16(19), 14421–14428 (2008). [CrossRef] [PubMed]
  3. G. Lenz and C. K. Madsen, “General optical all-pass filter structures for dispersion control in WDM systems,” J. Lightwave Technol.17(7), 1248–1254 (1999). [CrossRef]
  4. C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett.33(7), 708–710 (2008). [CrossRef] [PubMed]
  5. S. Mikroulis, E. Roditi, and D. Syvridis, “Direct modulation properties of 1.55-μm InGaAsP/InP microring lasers,” J. Lightwave Technol.26(2), 251–256 (2008). [CrossRef]
  6. M. Gandomkar and V. Ahmadi, “Design and analysis of enhanced second harmonic generation in AlGaAs/AlO(x) microring waveguide,” Opt. Express19(10), 9408–9418 (2011). [CrossRef] [PubMed]
  7. A. Guarino, G. Poberaj, D. Rezzonico, R. Degl’innocenti, and P. Günter, “Electro-optically tunable microring resonators in lithium niobate,” Nat. Photonics1(7), 407–410 (2007). [CrossRef]
  8. T.-J. Wang, C.-H. Chu, and C.-Y. Lin, “Electro-optically tunable microring resonators on lithium niobate,” Opt. Lett.32(19), 2777–2779 (2007). [CrossRef] [PubMed]
  9. M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett.89(7), 071110 (2006). [CrossRef]
  10. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature435(7040), 325–327 (2005). [CrossRef] [PubMed]
  11. B. Maune, R. Lawson, G. Gunn, A. Scherer, and L. Dalton, “Electrically tunable ring resonators incorporating nematic liquid crystals as cladding layers,” Appl. Phys. Lett.83(23), 4689–4691 (2003). [CrossRef]
  12. W. De Cort, J. Beeckman, R. James, F. A. Fernández, R. Baets, and K. Neyts, “Tuning of silicon-on-insulator ring resonators with liquid crystal cladding using the longitudinal field component,” Opt. Lett.34(13), 2054–2056 (2009). [CrossRef] [PubMed]
  13. T. Cai, Q. Liu, Y. Shi, P. Chen, and S. He, “An effectively tunable microring resonator using a liquid crystal-cladded polymer waveguide,” Appl. Phys. Lett.97(12), 121109 (2010). [CrossRef]
  14. W. De Cort, J. Beeckman, R. James, F. A. Fernandez, R. Baets, and K. Neyts, “Tuning silicon-on-insulator ring resonators with in-plane switching liquid crystals,” J. Opt. Soc. Am. B28(1), 79–85 (2011). [CrossRef]
  15. W. De Cort, J. Beeckman, T. Claes, K. Neyts, and R. Baets, “Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding,” Opt. Lett.36(19), 3876–3878 (2011). [CrossRef] [PubMed]
  16. J. L. D. Bougrenet and D. L. Tocnaye, “Engineering liquid crystals for optimal uses in optical communication systems,” Liq. Cryst.31(2), 241–269 (2004). [CrossRef]
  17. J. Beeckman, K. Neyts, and P. J. M. Vanbrabant, “Liquid-crystal photonic applications,” Opt. Eng.50(8), 081202 (2011). [CrossRef]
  18. T.-J. Wang, Y.-H. Huang, and H.-L. Chen, “Resonant-wavelength tuning of microring filters by oxygen plasma treatment,” IEEE Photon. Technol. Lett.17(3), 582–584 (2005). [CrossRef]
  19. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford U. Press, 1995).

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