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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 8 — Dec. 1, 2011
  • pp: 1471–1477
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Liquid crystal infiltrated waveguide with distributed Bragg reflectors

Dong-Po Cai, Hung-Yi Pan, Ji-Fang Tsai, Hua-Kung Chiu, Shan-Chi Nian, Sheng Hsiung Chang, Chii-Chang Chen, and Chien-Chieh Lee  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 8, pp. 1471-1477 (2011)
http://dx.doi.org/10.1364/OME.1.001471


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Abstract

The electrically tunable band-pass filter in the visible light region is demonstrated by the liquid-crystal infiltrated waveguide formed by the distributed Bragg reflectors with the length of 3 mm. As the white light source is launched in the waveguide, by applying the external voltages from 0 to 30 Vrms, the dynamic control of filter characteristics can be achieved to tune the color of the output light from white light to red, yellow or green. The intensity of the output light can also be attenuated by applying the voltage. The 25 dB attenuation can be achieved as the applied voltage is as low as 9 Vrms.

© 2011 OSA

1. Introduction

The electro-optical or thermo-optical materials such as liquid crystal (LC) can be infiltrated into the hollow core waveguide or photonic crystal fibers (PCFs) to alter the optical properties of waveguide or PCFs by tuning the applying voltage or temperature. The waveguides with different optical characteristics have be studied such as Bragg optical waveguides [1

1. J. B. Shellan, P. Agmon, P. Yeh, and A. Yariv, “Statistical analysis of Bragg reflectors,” J. Opt. Soc. Am. 68(1), 18–27 (1978). [CrossRef]

], anti-resonant reflecting optical waveguides [2

2. M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2-Si multilayer structures,” Appl. Phys. Lett. 49(1), 13–15 (1986). [CrossRef]

5

5. R. Bernini, G. Testa, L. Zeni, and P. M. Sarro, “Integrated optofluidic Mach-Zehnder interferometer based on liquid core waveguides,” Appl. Phys. Lett. 93(1), 011106 (2008). [CrossRef]

], liquid-liquid waveguides (L2s) [6

6. D. B. Wolfe, R. S. Conroy, P. Garstecki, B. T. Mayers, M. A. Fischbach, K. E. Paul, M. Prentiss, and G. M. Whitesides, “Dynamic control of liquid-core/liquid-cladding optical waveguides,” Proc. Natl. Acad. Sci. U.S.A. 101(34), 12434–12438 (2004). [CrossRef] [PubMed]

], nanoporous cladding waveguides [7

7. W. Risk, H. Kim, R. Miller, H. Temkin, and S. Gangopadhyay, “Optical waveguides with an aqueous core and a low-index nanoporous cladding,” Opt. Express 12(26), 6446–6455 (2004). [CrossRef] [PubMed]

], Liquid-core waveguide (LCW) [8

8. A. D’Alessandro, B. D. Donisi, R. Beccherelli, and R. Asquini, “Nematic liquid crystal optical channel waveguides on silicon,” IEEE J. Quantum Electron. 42(10), 1084–1090 (2006). [CrossRef]

10

10. H. Schmidt and A. R. Hawkins, “Optofluidic waveguides: I. Concepts and implementations,” Microfluid. Nanofluid. 4(1-2), 3–16 (2008). [CrossRef] [PubMed]

] and the LC filled PCFs [11

11. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11(20), 2589–2596 (2003). [CrossRef] [PubMed]

15

15. T. R. Woliński, S. Ertman, A. Czapla, P. Lesiak, K. Nowecka, A. W. Domanski, E. Nowinowski-Kruszelnicki, R. Dabrowski, and J. Wojcik, “Polarization effects in photonic liquid crystal fibers,” Meas. Sci. Technol. 18(10), 3061–3069 (2007). [CrossRef]

], Liquid-crystal optical channel waveguide [16

16. D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “Switchable liquid-crystal optical channel wavguide on silicon,” IEEE J. Quantum Electron. 46(5), 762–768 (2010). [CrossRef]

],the electrically tunable LC waveguide [17

17. D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express 19(12), 11890–11896 (2011). [CrossRef] [PubMed]

], LC in waveguides for tuning and sensing [18

18. K. Neyts, W. Decort, H. Azarinia, P. Vanbrabant, R. James, and J. Beeckman, “Liquid crystals in waveguides for tuning and sensing,” Photonics Lett. Poland 3(1),17–19 (2011).

]. By infiltrating the LC or liquid into the hollow core waveguide or PCFs, the waveguide and PCFs can be the versatile devices such as optical modulator, optical attenuator [11

11. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11(20), 2589–2596 (2003). [CrossRef] [PubMed]

,12

12. F. Du, Y. Q. Lu, and S. T. Wu, “Electrically tunable liquid-crystal photonic crystal fiber,” Appl. Phys. Lett. 85(12), 2181–22183 (2004). [CrossRef]

,16

16. D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “Switchable liquid-crystal optical channel wavguide on silicon,” IEEE J. Quantum Electron. 46(5), 762–768 (2010). [CrossRef]

,17

17. D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express 19(12), 11890–11896 (2011). [CrossRef] [PubMed]

] or biosensor [19

19. V. K. Gupta, J. J. Skaife, T. B. Dubrovsky, and N. L. Abbott, “Optical amplification of ligand-receptor binding using liquid crystals,” Science 279(5359), 2077–2080 (1998). [CrossRef] [PubMed]

21

21. V. J. Cadarso, A. Llobera, C. Fernandez-Sanchez, M. Darder, and C. Dominguez, “Hollow waveguide-based full-field absorbance biosensor,” Sens. Actuators B Chem. 139(1), 143–149 (2009). [CrossRef]

]. Recently, we demonstrated the LC filled hollow waveguide formed by SiO2 cladding [17

17. D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express 19(12), 11890–11896 (2011). [CrossRef] [PubMed]

]. The maximum optical attenuation of the device with the length of 0.4 cm is over 30 dB for the driving voltage of 5 volt. The corresponding optical attenuation per unit length and per voltage is 18.75(dB/cm/volt). The device is operated at the wavelength of 1550 nm. The color tunable device in visible has been proposed by T. T. Larsen et al. [11

11. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11(20), 2589–2596 (2003). [CrossRef] [PubMed]

]. By heating the LC filled PCFs with the length of 2 cm between the temperature of 26.5 oC and 26.9 oC, the maximum optical extinction ratio is in excess of 60 dB. The color of the output light can be changed by varying the temperature to shift the photonic band gap (PBG) of the PCFs. The color of the output light can be tuned from green, yellow to blue.

In this paper, we demonstrate the LC filled waveguide based on the distributed Bragg reflectors (DBRs). The optical attenuation over 25 dB can be achieved for the driving voltage as low as 9 volt. The PBG of LC filled waveguide can be altered by changing the external voltage to varying the effective index of LC (ncore). The color of the output light can be tuned from white light to red, yellow or green.

2. Simulation

The DBRs used to form the hollow waveguide consist of six pairs of thin films in SiO2 and Si3N4. The structure is schematically shown in Figs. 1(a)
Fig. 1 (color online) (a) Schematic structure of LC filled waveguide based on DBRs. The direction of applied voltage is along x-direction. (b) Cross-section view of LC filled waveguide based on DBRs. (c) PBG of DBRs obtained by the Bloch theorem.
and 1(b). By using the photonic band gap of the thin film, the light can be confined in the core between the multilayer stacks. To design the multilayer structure, the transfer matrix method [22

22. H. A. Macleod, Thin-Film Optical Filters (Macmillan, 1986), Chap. 2.

] is used. The refractive index of SiO2 and Si3N4 at the wavelength of 532 nm is nSiO2 = 1.5469 and nSi3N4 = 2.0268, respectively. The substrate material is silicon with the index, nSi, of 4.139 - 0.034i. The guiding material is liquid crystal, E7 (Merck), with the ordinary index of 1.52 and the extraordinary index of 1.75 denoted as no and ne, respectively. For simplifying the calculation process, the indices of the materials at the wavelength of 532 nm are used for the calculation in the wavelength range from 500 to 800 nm. In Fig. 1(b), θ is the incident angle of the propagating light on the multilayer stack. The definition of the TE and TM polarization states is shown in Fig. 1(b). The thickness of SiO2 and Si3N4 is calculated to be dSiO2 = 335 nm and dSi3N4 = 95 nm, respectively to obtain the largest photonic band gap. The thickness of one pair of SiO2 and Si3N4 corresponds to the period Λ of DBRs is 430 nm.

The width and the height of the hollow core are chosen to be dcore = 5 μm. The thickness of indium tin oxide (ITO), dITO, which is 0.5 μm, serves as the electrodes. We use the finite difference method to solve the Helmholtz equation to obtain the waveguide mode profile and the propagation constant as the refractive index of the LC core is 1.52. The propagation constant of the fundamental mode is used to calculate the incident angle of the light on the multilayer stack to be 88o. By using the Bloch theorem, the complete band gap for the TE and TM modes can be obtained for different indices of LC as shown in Fig. 1(c), as the incident angle of the propagating light on the multilayer stack is 88o. The blue region represents the complete band gap for the TE and TM modes in which the light can be confined in the LC filled waveguide. The result shows that the wavelength region for the light confinement can be altered due to the reorientation of LC director. The range of the index change of the LC is between 1.52 and 1.75 by varying the applied voltage. For the index range from 1.52 to 1.622, the light at short wavelength such as the yellow and the green light can be confined. For the index range from 1.622 to 1.654, no light at the wavelength region between 500 nm to 800 nm can be confined indicating that no light will be outputted (dark state). As the core index is changed from 1.694 to 1.75, the output light is changed from red to white light. The result shows that by applying the external voltage, the color of the output light can be changed from green to red. White light and no light at the output of the waveguide can also be obtained.

3. Sample fabrication

The hollow waveguide based on DBRs is fabricated on the (100) silicon wafer. The six pairs of multilayer stack (Si3N4/SiO2) are deposited as the bottom cladding layer by employing the plasma-enhanced chemical vapor deposition (PECVD). The photo-resist, SU-8 is coated as sacrificial layer on the bottom cladding layer and patterned by the conventional photolithography process. Both of the width and the height of the sacrificial layer are 5 μm. The six pairs of multilayer stack (Si3N4/SiO2) are deposited again as the top cladding layer by PECVD. The SiO2 cover layer with the thickness of 2.5 μm is deposited on the top of the cladding layer to fortify the structure of device. The photo-resist remover is used to remove the photo-resist to form the hollow core. The transparent conductive film, indium-tin-oxide (ITO), with thickness of 0.5 μm is deposited as the electrodes on the top SiO2 layer and the rear of silicon wafer for applying the voltage on the device. The image of scanning electron microscope of the end of the waveguide is shown in Fig. 2
Fig. 2 SEM image of the cleaved waveguide facet.
. For infiltrating the LC into the hollow waveguide, the LC is heated to be the isotropic phase at 75 oC on the hot plate. By submerging the end of hollow waveguide into the LC, the LC can be infiltrated into the hollow waveguide by the capillary effect.

4. Results and discussion

To characterize the LC filled waveguide based on the DBRs, the supercontinuum broad-band light source by coupling the YAG laser into the PCFs is launched into the device. The wavelength range of the broad-band light source is from 500 nm to 1700 nm. Simultaneously, the external AC voltage with the frequency of 1 kHz is applied on the structure as shown in Fig. 1(a). The output light is collimated by the microscope objective with the magnification of 60X and the numerical aperture of 0.85 and is collected by the convex lens with the focal length of 12 cm. Finally, the spectrum of the output light is measured by the monochromator and the optical image is captured by the CCD. The average propagation loss is 1.15 ± 0.07 dB/mm. The propagation loss can be diminished by increasing the pair number of multilayer stack. The average coupling loss is 22.2 ± 4.7 dB. The large coupling loss is due to the residual LC on the facet of the waveguide which scatters the input light inducing the decrease of the light coupling.

Figure 4(a)
Fig. 4 (color online) (a) Far-field image and transmission spectra of output light for different applied voltages. (b) Extinction ratio and output image of LC filled waveguide at the wavelength of 532 nm for different applied voltages.
shows the output images and the transmission spectra by varying the external voltage from 0 to 3.5 Vrms and from 25 to 30 Vrms. Since the intensity of the output light between 3.5 and 25 Vrms is significantly attenuated (dark state), the spectrum is not shown in Fig. 4(a). This state can be observed in the simulation result as shown in Fig. 1(c) when the refractive index is between 1.622 and 1.654. In Fig. 4(a), the color of the output light can be altered such as yellow, green, tangerine and white. As the applied voltage is 30 Vrms, the waveguide output shows white light. This corresponds to the simulation result as the refractive index is around 1.75 as shown in Fig. 1(c) where the light in the wavelength range from 500 to 800nm can be confined in the waveguide. The results show that the output color of the waveguide can be controlled by the applied voltage to obtain the green light to red light as well as the white light and the dark state.

To investigate the polarization dependence of the waveguide, the linearly polarized laser light source at the wavelength of 532 nm is launched to the LC filled waveguide. The polarization of the input light is controlled by a polarizer. The power of incident light is 6 dBm. Simultaneously, the external AC voltage with the frequency of 1 kHz is applied on the structure. The applied voltage is varied from 0 to 15 Vrms. The power of the output light is measured by the power meter. The extinction ratio and the output image are shown in Fig. 4(b). The ϕ in Fig. 4(b) represents the angle between the x-direction [shown in Fig. 1(a)] and the linear polarization direction. We can observe that the attenuation of LC filled waveguide is almost independent of the polarization state. This is due to the fact that the linearly polarized light propagating in the LC filled waveguide can be depolarized by the multi-reflection in the rectangle waveguide [17

17. D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express 19(12), 11890–11896 (2011). [CrossRef] [PubMed]

].

5. Conclusion

In this study, the electrically tunable broad-band filter and attenuator has been fabricated by infiltrated the LC into the hollow core waveguide based on the DBRs. By applying the voltage from 0 to 30 Vrms, the wavelength of light confined in the LC filled waveguide can be varied. The color of the far-field output image is changed from green, yellow to white. Specifically, the guide mode for the whole visible range can be transformed to the leaky mode as ncore is between 1.622 and 1.654 for applying voltage from 3.5 to 25 Vrms. This property can be used to fabricate the broad-band tunable filter. Additionally, the light at the wavelength of 532 nm is also launched into the LC filled waveguide. By applying voltage between 9 Vrms and 15 Vrms, the light can radiate out of waveguide. The maximum extinction ratio of the LC filled waveguide of 25 dB can be achieved. This characterization can be employed to fabricate the optical attenuators or modulators. The device might be potentially applied for the scanning projection display or tunable dye-doped LC laser.

References and links

1.

J. B. Shellan, P. Agmon, P. Yeh, and A. Yariv, “Statistical analysis of Bragg reflectors,” J. Opt. Soc. Am. 68(1), 18–27 (1978). [CrossRef]

2.

M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2-Si multilayer structures,” Appl. Phys. Lett. 49(1), 13–15 (1986). [CrossRef]

3.

D. Yin, D. W. Deamer, H. Schmidt, J. P. Barber, and A. R. Hawkins, “Integrated optical waveguides with liquid cores,” Appl. Phys. Lett. 85(16), 3477–3479 (2004). [CrossRef]

4.

H. Schmidt, D. Yin, J. P. Barber, and A. R. Hawkins, “Hollow-core waveguides and 2-D waveguide arrays for integrated optics of gas and liquids,” IEEE J. Sel. Top. Quantum Electron. 11(2), 519–527 (2005). [CrossRef]

5.

R. Bernini, G. Testa, L. Zeni, and P. M. Sarro, “Integrated optofluidic Mach-Zehnder interferometer based on liquid core waveguides,” Appl. Phys. Lett. 93(1), 011106 (2008). [CrossRef]

6.

D. B. Wolfe, R. S. Conroy, P. Garstecki, B. T. Mayers, M. A. Fischbach, K. E. Paul, M. Prentiss, and G. M. Whitesides, “Dynamic control of liquid-core/liquid-cladding optical waveguides,” Proc. Natl. Acad. Sci. U.S.A. 101(34), 12434–12438 (2004). [CrossRef] [PubMed]

7.

W. Risk, H. Kim, R. Miller, H. Temkin, and S. Gangopadhyay, “Optical waveguides with an aqueous core and a low-index nanoporous cladding,” Opt. Express 12(26), 6446–6455 (2004). [CrossRef] [PubMed]

8.

A. D’Alessandro, B. D. Donisi, R. Beccherelli, and R. Asquini, “Nematic liquid crystal optical channel waveguides on silicon,” IEEE J. Quantum Electron. 42(10), 1084–1090 (2006). [CrossRef]

9.

A. R. Hawkins and H. Schmidt, “Optofluidic waveguides: II. Fabrication and structures,” Microfluid. Nanofluid. 4(1-2), 17–32 (2008). [CrossRef] [PubMed]

10.

H. Schmidt and A. R. Hawkins, “Optofluidic waveguides: I. Concepts and implementations,” Microfluid. Nanofluid. 4(1-2), 3–16 (2008). [CrossRef] [PubMed]

11.

T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11(20), 2589–2596 (2003). [CrossRef] [PubMed]

12.

F. Du, Y. Q. Lu, and S. T. Wu, “Electrically tunable liquid-crystal photonic crystal fiber,” Appl. Phys. Lett. 85(12), 2181–22183 (2004). [CrossRef]

13.

M. W. Haakestad, T. T. Alkeskjold, M. D. Nielsen, L. Scolari, J. Riishede, H. E. Engan, and A. Bjarklev, “Electrically tunable photonic bandgap guidance in a liquid-crystal-filled photonic crystal fiber,” IEEE Photon. Technol. Lett. 17(4), 819–821 (2005). [CrossRef]

14.

T. R. Wolinski, K. Szaniawska, S. Ertman, P. Lesiak, A. W. Domanski, R. Dabrowski, E. Nowinowski-Kruszelnicki, and J. Wojcik, “Influence of temperature and electrical field on propagation properties of photonic liquid-crystal fibres,” Meas. Sci. Technol. 17(5), 985–991 (2006). [CrossRef]

15.

T. R. Woliński, S. Ertman, A. Czapla, P. Lesiak, K. Nowecka, A. W. Domanski, E. Nowinowski-Kruszelnicki, R. Dabrowski, and J. Wojcik, “Polarization effects in photonic liquid crystal fibers,” Meas. Sci. Technol. 18(10), 3061–3069 (2007). [CrossRef]

16.

D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “Switchable liquid-crystal optical channel wavguide on silicon,” IEEE J. Quantum Electron. 46(5), 762–768 (2010). [CrossRef]

17.

D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express 19(12), 11890–11896 (2011). [CrossRef] [PubMed]

18.

K. Neyts, W. Decort, H. Azarinia, P. Vanbrabant, R. James, and J. Beeckman, “Liquid crystals in waveguides for tuning and sensing,” Photonics Lett. Poland 3(1),17–19 (2011).

19.

V. K. Gupta, J. J. Skaife, T. B. Dubrovsky, and N. L. Abbott, “Optical amplification of ligand-receptor binding using liquid crystals,” Science 279(5359), 2077–2080 (1998). [CrossRef] [PubMed]

20.

J. B. Jensen, L. H. Pedersen, P. E. Hoiby, L. B. Nielsen, T. P. Hansen, J. R. Folkenberg, J. Riishede, D. Noordegraaf, K. Nielsen, A. Carlsen, and A. Bjarklev, “Photonic crystal fiber based evanescent-wave sensor for detection of biomolecules in aqueous solutions,” Opt. Lett. 29(17), 1974–1976 (2004). [CrossRef] [PubMed]

21.

V. J. Cadarso, A. Llobera, C. Fernandez-Sanchez, M. Darder, and C. Dominguez, “Hollow waveguide-based full-field absorbance biosensor,” Sens. Actuators B Chem. 139(1), 143–149 (2009). [CrossRef]

22.

H. A. Macleod, Thin-Film Optical Filters (Macmillan, 1986), Chap. 2.

23.

M. Green and S. J. Madden, “Low loss nematic liquid crystal cored fiber waveguides,” Appl. Opt. 28(24), 5202–5203 (1989). [CrossRef] [PubMed]

24.

T. T. Alkeskjold, J. Lægsgaard, A. Bjarklev, D. S. Hermann, A. Anawati, J. Broeng, J. Li, and S. T. Wu, “All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers,” Opt. Express 12(24), 5857–5871 (2004). [CrossRef] [PubMed]

25.

G. P. Bryan-Brown, E. L. Wood, and I. C. Sage, “Weak surface anchoring of liquid crystals,” Nature 399(6734), 338–340 (1999). [CrossRef]

26.

M. Barón, “Definitions of basic terms relating to low-molar-mass and polymer liquid crystals,” Pure Appl. Chem. 73(5), 845–895 (2001). [CrossRef]

OCIS Codes
(230.2090) Optical devices : Electro-optical devices
(230.3720) Optical devices : Liquid-crystal devices
(230.7370) Optical devices : Waveguides

ToC Category:
Liquid Crystals

History
Original Manuscript: September 7, 2011
Revised Manuscript: September 25, 2011
Manuscript Accepted: September 25, 2011
Published: November 3, 2011

Virtual Issues
Liquid Crystal Materials for Photonic Applications (2011) Optical Materials Express

Citation
Dong-Po Cai, Hung-Yi Pan, Ji-Fang Tsai, Hua-Kung Chiu, Shan-Chi Nian, Sheng Hsiung Chang, Chii-Chang Chen, and Chien-Chieh Lee, "Liquid crystal infiltrated waveguide with distributed Bragg reflectors," Opt. Mater. Express 1, 1471-1477 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-8-1471


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References

  1. J. B. Shellan, P. Agmon, P. Yeh, and A. Yariv, “Statistical analysis of Bragg reflectors,” J. Opt. Soc. Am.68(1), 18–27 (1978). [CrossRef]
  2. M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2-Si multilayer structures,” Appl. Phys. Lett.49(1), 13–15 (1986). [CrossRef]
  3. D. Yin, D. W. Deamer, H. Schmidt, J. P. Barber, and A. R. Hawkins, “Integrated optical waveguides with liquid cores,” Appl. Phys. Lett.85(16), 3477–3479 (2004). [CrossRef]
  4. H. Schmidt, D. Yin, J. P. Barber, and A. R. Hawkins, “Hollow-core waveguides and 2-D waveguide arrays for integrated optics of gas and liquids,” IEEE J. Sel. Top. Quantum Electron.11(2), 519–527 (2005). [CrossRef]
  5. R. Bernini, G. Testa, L. Zeni, and P. M. Sarro, “Integrated optofluidic Mach-Zehnder interferometer based on liquid core waveguides,” Appl. Phys. Lett.93(1), 011106 (2008). [CrossRef]
  6. D. B. Wolfe, R. S. Conroy, P. Garstecki, B. T. Mayers, M. A. Fischbach, K. E. Paul, M. Prentiss, and G. M. Whitesides, “Dynamic control of liquid-core/liquid-cladding optical waveguides,” Proc. Natl. Acad. Sci. U.S.A.101(34), 12434–12438 (2004). [CrossRef] [PubMed]
  7. W. Risk, H. Kim, R. Miller, H. Temkin, and S. Gangopadhyay, “Optical waveguides with an aqueous core and a low-index nanoporous cladding,” Opt. Express12(26), 6446–6455 (2004). [CrossRef] [PubMed]
  8. A. D’Alessandro, B. D. Donisi, R. Beccherelli, and R. Asquini, “Nematic liquid crystal optical channel waveguides on silicon,” IEEE J. Quantum Electron.42(10), 1084–1090 (2006). [CrossRef]
  9. A. R. Hawkins and H. Schmidt, “Optofluidic waveguides: II. Fabrication and structures,” Microfluid. Nanofluid.4(1-2), 17–32 (2008). [CrossRef] [PubMed]
  10. H. Schmidt and A. R. Hawkins, “Optofluidic waveguides: I. Concepts and implementations,” Microfluid. Nanofluid.4(1-2), 3–16 (2008). [CrossRef] [PubMed]
  11. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express11(20), 2589–2596 (2003). [CrossRef] [PubMed]
  12. F. Du, Y. Q. Lu, and S. T. Wu, “Electrically tunable liquid-crystal photonic crystal fiber,” Appl. Phys. Lett.85(12), 2181–22183 (2004). [CrossRef]
  13. M. W. Haakestad, T. T. Alkeskjold, M. D. Nielsen, L. Scolari, J. Riishede, H. E. Engan, and A. Bjarklev, “Electrically tunable photonic bandgap guidance in a liquid-crystal-filled photonic crystal fiber,” IEEE Photon. Technol. Lett.17(4), 819–821 (2005). [CrossRef]
  14. T. R. Wolinski, K. Szaniawska, S. Ertman, P. Lesiak, A. W. Domanski, R. Dabrowski, E. Nowinowski-Kruszelnicki, and J. Wojcik, “Influence of temperature and electrical field on propagation properties of photonic liquid-crystal fibres,” Meas. Sci. Technol.17(5), 985–991 (2006). [CrossRef]
  15. T. R. Woliński, S. Ertman, A. Czapla, P. Lesiak, K. Nowecka, A. W. Domanski, E. Nowinowski-Kruszelnicki, R. Dabrowski, and J. Wojcik, “Polarization effects in photonic liquid crystal fibers,” Meas. Sci. Technol.18(10), 3061–3069 (2007). [CrossRef]
  16. D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “Switchable liquid-crystal optical channel wavguide on silicon,” IEEE J. Quantum Electron.46(5), 762–768 (2010). [CrossRef]
  17. D. P. Cai, S. C. Nien, H. K. Chiu, C. C. Chen, and C. C. Lee, “Electrically tunable liquid crystal waveguide attenuators,” Opt. Express19(12), 11890–11896 (2011). [CrossRef] [PubMed]
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