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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 16 — Aug. 1, 2011
  • pp: 14999–15008
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Preventing occurrence of disclination lines in liquid crystal lenses with a large aperture by means of polymer stabilization

Che Ju Hsu and Chia Rong Sheu  »View Author Affiliations


Optics Express, Vol. 19, Issue 16, pp. 14999-15008 (2011)
http://dx.doi.org/10.1364/OE.19.014999


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Abstract

Liquid crystal (LC) lenses with a circularly hole-patterned electrode possess excellent characteristics in optical performance, especially for the capability of tunable focal lengths. But, non-uniformly symmetrical electric fields in LC lenses usually induce disclination lines when operating. In general, the occurrence of disclination lines not only degrades their optical capability such as imaging performance, but also spends more time for tuning focal lengths. In this paper, we use a way of polymer stabilization to successfully prevent the disclination lines in LC lenses. Even arbitrarily adjusting the applied voltages in LC lenses, it seems no occurrence of disclination lines again. In addition, we compare the basic optical performance for LC lenses with or without polymer stabilization. From experimental results, it shows that they almost have the same optical performance.

© 2011 OSA

1. Introduction

Among most of LC lenses, the typical one with a circularly hole-patterned electrode possesses some characteristics including easy fabrication, excellent capabilities of tunable focal lengths and large effective apertures etc. However, this kind of LC lenses also has a serious drawback, that is, occurrence of disclination lines when operating. Disclination lines not only degrade optical performance of the LC lenses, but also spend more time to tune focal lengths. Many researches had pointed out the cause of disclination lines [8

8. F. D. Pasquale, F. A. Fernández, S. E. Day, and J. B. Davies, “Two-dimensional finite-element modeling of nematic liquid crystal devices for optical communications and displays,” IEEE J. Sel. Top. Quantum Electron. 2(1), 128–134 (1996). [CrossRef]

,9

9. M. Ye, B. Wang, and S. Sato, “Liquid-crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43(35), 6407–6412 (2004). [CrossRef] [PubMed]

] and proposed some methods to resolve this problem such as controlling LC lenses with two individually applied voltages [9

9. M. Ye, B. Wang, and S. Sato, “Liquid-crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43(35), 6407–6412 (2004). [CrossRef] [PubMed]

], symmetrical structure of electrodes [10

10. N. Fraval and J. L. de la Tocnaye, “Low aberrations symmetrical adaptive modal liquid crystal lens with short focal lengths,” Appl. Opt. 49(15), 2778–2783 (201l0). [CrossRef] [PubMed]

] and adding an extra in-plane electric field in LC lenses [11

11. M. Ye, B. Wang, and S. Sato, “Driving of liquid crystal lens without disclination occurring by applying an in-plane electric field,” Jpn. J. Appl. Phys. 42(Part 1, No. 8), 5086–5089 (2003). [CrossRef]

,12

12. M. Ye and S. Sato, “New method of voltage application for improving response time of a liquid crystal lens,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 433(1), 229–236 (2005). [CrossRef]

]. Generally speaking, using an extra electric field not only efficiently prevent the disclination lines, but also provide more flexibility to achieve better optical performance in LC lenses. Certainly, more fabricating processes or complicated controlling circuits are necessary.

In this work, we demonstrate that a way of polymer stabilization is successfully used in LC lenses with a circularly hole-patterned electrode of 7 mm in diameter to prevent the occurrence of disclination lines when operating. We also compare the optical performance of LC lenses with or without polymer stabilization. From experimental results, it shows that the major characteristics of LC lenses are very consistent for LC lenses with or without polymer stabilization.

2. The structure of PSLC lens with a circularly hole-patterned electrode

Although the LC lens with a circularly hole-patterned electrode is easily fabricated and has good optical performance, disclination lines usually occur to affect its optical functions when operating. The major cause of disclination line comes from the interaction between the electric fields and LC directors in cells. In general, hole-patterned electrode provides a non-uniformly axially-symmetrical electric field so that LC molecules near the surface of upper substrate will be reorientation with reverse directions when applying voltages as shown in Fig. 2
Fig. 2 Scheme of interaction between the electric field and LC directors in a LC lens. When applying voltages in the cell, a non-uniformly axially-symmetrical electric field is established. The LCs near surface of upper glass substrate will be reorientation with reverse directions (zoom in chart) so that the disclination line occurs.
. A simple way of applying voltages with a slowly increasing rate is efficient to prevent disclination lines, but it will spend more time when operating, especially for the LC lenses with large apertures.

3. Processes of polymer stabilization in the PSLC lens

Polymer stabilization is usually used to improve the performance of LC optical devices. Here, we try to use it to prevent disclination lines in LC lenses. The preparation of PSLC lenses is described as follows: We made one material mixed with reactive mesogen (RM257, purchased from Merck), photo initiator (Iragcure 651 purchased from Chiba) and LCs (E7 purchased form Merck) with a weight ratio of 1.40: 0.14: 98.46. The completely mixed material was injected with capillarity into the empty cells on the 100°C hot plate. The completely injected LC lenses were applied voltages with a slowly increasing rate from 0 Vrms to 140 Vrms in order to prevent occurrence of disclination lines. The applied voltage in the LC lenses was a square waveform with frequency of 1 kHz. And then, the injected LC lenses were exposed with ultraviolet light (7 mW/cm2) for 4 minutes when applying voltages of 140 Vrms. After UV exposure, there were some polymer structures formed near the surfaces of glass substrates, which was as well as most illustration shown in papers [6

6. S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G. D. Lee, J. J. Lyu, and K. H. Kim, “Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen,” Appl. Phys. Lett. 90(26), 261910-1-261910-3 (2007). [CrossRef]

,14

14. V. V. Sergan, T. A. Sergan, and P. J. Bos, “Control of the molecular pretilt angle in liquid crystal devices by using a low-density localized polymer network,” Chem. Phys. Lett. 486(4-6), 123–125 (2010). [CrossRef]

]. The formed polymer structures affected LC molecules near the surface of glass substrates. But, they did not affect other LCs away from surfaces of glass substrates in the cells, which could be illustrated with the proof of no interference patterns in the PSLC lens without applied voltage. Figure 3
Fig. 3 The processes of polymer stabilization for a PSLC lens. (a) The completely injected cell contains the mixture of LCs, RM and photo initiator. (b) The cell with applied voltages of 140 Vrms is exposed with UV light (7 mW/cm2) for 4 minutes. The polymer structures are formed. (c) A PSLC lens is completed. When removing the applied voltage, the polymer structures sustain the LC directors near the glass surface, which are efficient to prevent disclination lines.
shows the processes of polymer stabilization with UV exposure in a PSLC lens with applied voltages. The polymer structures are generated from the injected mixture after UV exposure. When removing the applied voltages, the LC directors near the glass surface are stabilized in the directions as same as the reoriented LC directors by electrical fields.

4. Measurement and optical performance of PSLC lenses

Using polymer stabilization, we have successfully prevented the occurrence of disclination lines in LC lenses. But, does the polymer stabilization affect no disadvantage for optical performance in LC lenses? In order to verify it, we did the following experiments. We compared the characteristics of focal lengths versus applied voltages for two LC lens possessing the same cell conditions, except with or without RM dopant and UV exposure. Figure 5(a)
Fig. 5 Comparisons of tunable focal lengths and interference patterns in LC lenses without/with polymer stabilization. (a) Comparisons of focal lengths versus applied voltages between two LC lenses without and with RM dopant/UV exposure. They are very consistent each other. (b) The global interference patterns and extraction in radial direction from center to edge of circular hole-pattern in the LC lens without RM dopant/UV exposure. (c) The global interference patterns and extraction in radial direction from center to edge of circular hole-pattern in the PSLC lens with RM dopant/UV exposure. From (b) and (c), it is obvious that two LC lenses have same rings in interference patterns, which means the same optical performance for them.
shows the comparisons of focal lengths with respect to applied voltages between two LC lenses. Obviously, both of them have almost same characteristics of tunable focal lengths from ∞ to 26 cm. Simultaneously, we also compared their interference patterns extracted in the radial direction from center to edge of circular hole-pattern in LC lenses with the CCD camera. Figure 5(b) and 5(c) shows the individual interference patterns with applied voltages of 180 Vrms in LC lenses, which have the same rings each other. The same rings in interference patterns mean the same optical performance of LC lenses.

f=r22λN=πr2λ2πN=πr2λΔδ.
(1)

In Eq. (1), r is the effective radius of LC lenses in which of circular area the interference patterns exist; the λ is the wavelength of incident laser beam; the N is the number of rings in interference patterns; the Δδ is the total phase retardation from center to edge in the cells. When the PSLC lens with applied voltages of 60 Vrms, we got the 31 rings in interference patterns with the CCD camera, which was inputted in Eq. (1) to get a focal length of 31.2 cm for the incident He-Ne laser. In the same status of PSLC lens, the focal length was 30 cm by means of measurement of focused light spots. In Fig. 6(c), it shows the comparisons of tunable focal lengths of a PSLC lens from rings N in interference patterns and experimental measurement, respectively. Obviously, they are very consistent each other except for the regions with higher and lower applied voltages. We think that the inconsistency of focal lengths is caused from different distribution of LC directors in the cells. In general, a well-behaved LC lens possesses an ideal distribution of LC directors to achieve a profile of quadratic refractive indices in radial direction. When applying lower voltages, LC directors have not completely reoriented in central area so that there is no distribution of gradient refractive indices here. Moreover, when applying higher voltages, the rings in larger radial positions disappear so that its optical intensity becomes more uniform in this area. This phenomenon is shown in Fig. 4. It is also no distribution of gradient refractive indices here. Therefore, an optimal applied voltage about 60 Vrms induces an ideal distribution of LC directors to achieve a profile of quadratic refractive indices in radial direction for this PSLC lens.

The imaging performance of PSLC lenses was also observed. Figure 7(a)
Fig. 7 Observation of tunable focal lengths in a PSLC lens. (a) The setup for observing capabilities of tunable focal lengths in PSLC lenses. (b) The photo of target was taken by the CCD camera with the applied voltage of 0 Vrms in the lens. (c) The clearer photo of target was taken by the CCD camera with the applied voltage of 60 Vrms in the lens.
shows the setup for observing imaging performance of the PSLC lens. A polarizer with its polarization parallel to the rubbing direction of PSLC lens was placed in front of the CCD camera. The target was placed in front of 23 cm away from the PSLC lens. In Fig. 7(b) and 7(c), they show the target was taken photos by the CCD camera when the PSLC lens with applied voltages 0 and 60 Vrms, respectively. A clearer photo was achieved when tuning focal lengths of the PSLC lens with applied voltages.

For another case, the PSLC lens with 1.3 wt.% (or lower concentration than 1.3 wt.%) RM dopant was still observed occurrence of disclination lines even if it was processed with UV exposure for longer time (about half or two hours) as shown in Fig. 9
Fig. 9 Interference patterns of a PSLC lens with 1.3 wt.% RM dopant after UV exposure. (a) No interference pattern exists in the cell without applied voltage. (b) The disclination line occurs when directly applying a voltage of 50 Vrms. It is similar to the LC lenses without RM dopant.
. Figure 9(a) shows there is no interference pattern in the cell without applied voltage after polymer stabilization. In Fig. 9(b), when directly applying a voltage of 50 Vrms, the disclinations line occurs in the cell as similar as the LC lenses without RM dopant.

5. Conclusion

We have demonstrated a way of polymer stabilization to prevent occurrence of disclination lines in LC lenses, so that their optical characteristics can be maintained as well as the LC lenses without polymer stabilization. Both of RM dopant and UV exposure are very sensitive to final optical performance in the PSLC lenses. We have successfully achieved the operated PSLC lenses without disclination lines when using 1.4 wt.% RM dopant and UV exposure (7 mW/cm2) for 4 minutes in the cells.

References and links

1.

B. Wang, M. Ye, M. Honma, T. Nose, and S. Sato, “Liquid crystal lens with spherical electrode,” Jpn. J. Appl. Phys. 41(Part 2, No. 11A), L1232–L1233 (2002). [CrossRef]

2.

H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, “Tunable-focus flat liquid crystal spherical lens,” Appl. Phys. Lett. 84(23), 4789–4791 (2004). [CrossRef]

3.

H. Ren and S. T. Wu, “Tunable electronic lens using a gradient polymer network liquid crystal,” Appl. Phys. Lett. 82(1), 22–24 (2003). [CrossRef]

4.

M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41(Part 2, No. 5B), L571–L573 (2002). [CrossRef]

5.

S. H. Lee, S. M. Kim, and S. T. Wu, “Emerging vertical-alignment liquid-crystal technology associated with surface modification using UV-curable monomer,” J. Soc. Inf. Disp. 17(7), 551–559 (2009). [CrossRef]

6.

S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G. D. Lee, J. J. Lyu, and K. H. Kim, “Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen,” Appl. Phys. Lett. 90(26), 261910-1-261910-3 (2007). [CrossRef]

7.

Y. W. Kim, J. Jeong, S. H. Lee, J.-H. Kim, and C.-J. Yu, “Improvement in switching speed of nematic liquid crystal microlens array with polarization independence,” Appl. Phys. Express 3(9), 094102, 094102–094103 (2010). [CrossRef]

8.

F. D. Pasquale, F. A. Fernández, S. E. Day, and J. B. Davies, “Two-dimensional finite-element modeling of nematic liquid crystal devices for optical communications and displays,” IEEE J. Sel. Top. Quantum Electron. 2(1), 128–134 (1996). [CrossRef]

9.

M. Ye, B. Wang, and S. Sato, “Liquid-crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43(35), 6407–6412 (2004). [CrossRef] [PubMed]

10.

N. Fraval and J. L. de la Tocnaye, “Low aberrations symmetrical adaptive modal liquid crystal lens with short focal lengths,” Appl. Opt. 49(15), 2778–2783 (201l0). [CrossRef] [PubMed]

11.

M. Ye, B. Wang, and S. Sato, “Driving of liquid crystal lens without disclination occurring by applying an in-plane electric field,” Jpn. J. Appl. Phys. 42(Part 1, No. 8), 5086–5089 (2003). [CrossRef]

12.

M. Ye and S. Sato, “New method of voltage application for improving response time of a liquid crystal lens,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 433(1), 229–236 (2005). [CrossRef]

13.

C. J. Hsu, C. Y. Huang, and C. R. Sheu, “Experimental analysis to avoid migrating zigzag lines occurring in homogeneously aligned liquid crystal lenses with a hole-patterned electrode,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 544(1), 185–191 (2011). [CrossRef]

14.

V. V. Sergan, T. A. Sergan, and P. J. Bos, “Control of the molecular pretilt angle in liquid crystal devices by using a low-density localized polymer network,” Chem. Phys. Lett. 486(4-6), 123–125 (2010). [CrossRef]

15.

H. Ren, D. W. Fox, B. Wu, and S. T. Wu, “Liquid crystal lens with large focal length tunability and low operating voltage,” Opt. Express 15(18), 11328–11335 (2007). [CrossRef] [PubMed]

16.

T. Nose, S. Masuda, S. Sato, J. Li, L. C. Chien, and P. J. Bos, “Effects of low polymer content in a liquid-crystal microlens,” Opt. Lett. 22(6), 351–353 (1997). [CrossRef] [PubMed]

OCIS Codes
(160.3710) Materials : Liquid crystals
(220.3620) Optical design and fabrication : Lens system design
(230.3720) Optical devices : Liquid-crystal devices

ToC Category:
Optical Devices

History
Original Manuscript: May 23, 2011
Revised Manuscript: July 8, 2011
Manuscript Accepted: July 11, 2011
Published: July 20, 2011

Citation
Che Ju Hsu and Chia Rong Sheu, "Preventing occurrence of disclination lines in liquid crystal lenses with a large aperture by means of polymer stabilization," Opt. Express 19, 14999-15008 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-16-14999


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References

  1. B. Wang, M. Ye, M. Honma, T. Nose, and S. Sato, “Liquid crystal lens with spherical electrode,” Jpn. J. Appl. Phys. 41(Part 2, No. 11A), L1232–L1233 (2002). [CrossRef]
  2. H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, “Tunable-focus flat liquid crystal spherical lens,” Appl. Phys. Lett. 84(23), 4789–4791 (2004). [CrossRef]
  3. H. Ren and S. T. Wu, “Tunable electronic lens using a gradient polymer network liquid crystal,” Appl. Phys. Lett. 82(1), 22–24 (2003). [CrossRef]
  4. M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41(Part 2, No. 5B), L571–L573 (2002). [CrossRef]
  5. S. H. Lee, S. M. Kim, and S. T. Wu, “Emerging vertical-alignment liquid-crystal technology associated with surface modification using UV-curable monomer,” J. Soc. Inf. Disp. 17(7), 551–559 (2009). [CrossRef]
  6. S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G. D. Lee, J. J. Lyu, and K. H. Kim, “Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen,” Appl. Phys. Lett. 90(26), 261910-1-261910-3 (2007). [CrossRef]
  7. Y. W. Kim, J. Jeong, S. H. Lee, J.-H. Kim, and C.-J. Yu, “Improvement in switching speed of nematic liquid crystal microlens array with polarization independence,” Appl. Phys. Express 3(9), 094102, 094102–094103 (2010). [CrossRef]
  8. F. D. Pasquale, F. A. Fernández, S. E. Day, and J. B. Davies, “Two-dimensional finite-element modeling of nematic liquid crystal devices for optical communications and displays,” IEEE J. Sel. Top. Quantum Electron. 2(1), 128–134 (1996). [CrossRef]
  9. M. Ye, B. Wang, and S. Sato, “Liquid-crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43(35), 6407–6412 (2004). [CrossRef] [PubMed]
  10. N. Fraval and J. L. de la Tocnaye, “Low aberrations symmetrical adaptive modal liquid crystal lens with short focal lengths,” Appl. Opt. 49(15), 2778–2783 (201l0). [CrossRef] [PubMed]
  11. M. Ye, B. Wang, and S. Sato, “Driving of liquid crystal lens without disclination occurring by applying an in-plane electric field,” Jpn. J. Appl. Phys. 42(Part 1, No. 8), 5086–5089 (2003). [CrossRef]
  12. M. Ye and S. Sato, “New method of voltage application for improving response time of a liquid crystal lens,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 433(1), 229–236 (2005). [CrossRef]
  13. C. J. Hsu, C. Y. Huang, and C. R. Sheu, “Experimental analysis to avoid migrating zigzag lines occurring in homogeneously aligned liquid crystal lenses with a hole-patterned electrode,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 544(1), 185–191 (2011). [CrossRef]
  14. V. V. Sergan, T. A. Sergan, and P. J. Bos, “Control of the molecular pretilt angle in liquid crystal devices by using a low-density localized polymer network,” Chem. Phys. Lett. 486(4-6), 123–125 (2010). [CrossRef]
  15. H. Ren, D. W. Fox, B. Wu, and S. T. Wu, “Liquid crystal lens with large focal length tunability and low operating voltage,” Opt. Express 15(18), 11328–11335 (2007). [CrossRef] [PubMed]
  16. T. Nose, S. Masuda, S. Sato, J. Li, L. C. Chien, and P. J. Bos, “Effects of low polymer content in a liquid-crystal microlens,” Opt. Lett. 22(6), 351–353 (1997). [CrossRef] [PubMed]

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