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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 10 — Oct. 1, 2012
  • pp: 1353–1358
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Bistable state in polymer stabilized blue phase liquid crystal

Zhi-gang Zheng, Dan Zhang, Xiao-wen Lin, Ge Zhu, Wei Hu, Dong Shen, and Yan-qing Lu  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 10, pp. 1353-1358 (2012)
http://dx.doi.org/10.1364/OME.2.001353


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Abstract

The bistable phenomenon is found in polymer stabilized blue phase liquid crystal. Two stable phases, blue phase and chiral nematic state, are obtained and we succeeded in the switch between these two states through different operation procedures. The mechanism of the bistable phenomenon is discussed, according to which we consider the bistability may widely exist in various polymer stabilized blue phase liquid crystal systems. This work provides some useful insights into the application of polymer stabilized blue phase liquid crystal in bistable devices.

© 2012 OSA

1. Introduction

The bistable liquid crystal (LC) is a very interesting topic, which has attracted more and more attentions all over the world for its potential application in information storages and displays. Since the beginning of this century, the bistable phenomena in ferroelectric LC, dual-frequency LC, and polymer stabilized cholesteric texture have been studied [1

1. K. E. Maly, M. D. Wand, and R. P. Lemieux, “Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant,” J. Am. Chem. Soc. 124(27), 7898–7899 (2002). [CrossRef] [PubMed]

5

5. R. Bao, C. M. Liu, and D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 2(11), 112401 (2009). [CrossRef]

]; bistable state induced by substrate anchoring has also been noticed [6

6. J. K. Kim, F. Araoka, S. M. Jeong, S. Dhara, K. Ishikawa, and H. Takezoe, “Bistable device using anchoring transition of nematic liquid crystals,” Appl. Phys. Lett. 95(6), 063505 (2009). [CrossRef]

]. In addition, many bistable devices involve display, light shutter, intensity modulator, lens and photonic crystal have been fabricated in these years [7

7. J. I. Nitsuma, M. Yoneya, and H. Yokoyama, “Contact photolithographic micropatterning for bistable nematic liquid crystal displays,” Appl. Phys. Lett. 92(24), 241120 (2008). [CrossRef]

11

11. C. Y. Wu, Y. H. Zou, I. Timofeev, Y. T. Lin, V. Y. Zyryanov, J. S. Hsu, and W. Lee, “Tunable bi-functional photonic device based on one-dimensional photonic crystal infiltrated with a bistable liquid-crystal layer,” Opt. Express 19(8), 7349–7355 (2011). [CrossRef] [PubMed]

].

Another hot topic that has attracted a lot of attentions is the research on blue phase liquid crystals (BPLCs), especially polymer stabilized blue phase liquid crystals (PSBPLCs) [12

12. Z. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys. 12(11), 113018 (2010). [CrossRef]

15

15. J. Yan and S. T. Wu, “Polymer-stabilized blue phase liquid crystals: a tutorial [Invited],” Opt. Mater. Express 1(8), 1527–1535 (2011). [CrossRef]

]. The unique structure of this material permits two major desiring features: no need for alignment layers and a fast response time down to the submillisecond range. That makes it an emerging promising candidate for display and photonic applications [16

16. Z. Ge, S. Gauza, M. Jiao, H. Xianyu, and S.-T. Wu, “Electro-optics of polymer-stabilized blue phase liquid crystal displays,” Appl. Phys. Lett. 94(10), 101104 (2009). [CrossRef]

19

19. G. Zhu, J. Li, X. W. Lin, H. F. Wang, W. Hu, Z. Zheng, H. Q. Cui, D. Shen, and Y. Q. Lu, “Polarization-independent blue-phase liquid-crystal gratings driven by vertical electric field,” J. Soc. Inf. Disp. 20(6), 341–346 (2012). [CrossRef]

]. Whether the bistable phenomenon exists in BPLCs? This question stimulates the interests of researchers. Recently, Wang et al. reported the bistable state of BP I and BP II that found in BPLC system, and their phase transition behaviors were also studied. The bistable phenomenon could exist in the range from 47 to 50 °C [20

20. C. T. Wang, H. Y. Liu, H. H. Cheng, and T. H. Lin, “Bistable effect in the liquid crystal blue phase,” Appl. Phys. Lett. 96(4), 041106 (2010). [CrossRef]

]. However, the bistable phenomenon in PSBPLC system has not been revealed.

In this paper, we report experimental results on the bistable effect in PSBPLC and phase transition between BP and N*. In the voltage-off state, two stable phase, BP and N*, can be observed respectively with different operation conditions. In addition, the bistable state can exist in a relative wide temperature range. The phase transition behaviors and electro-optical performance are studied and discussed in the following text.

2. Materials and experiments

The electrically induced phase transition behaviors of PSBPLC are observed under a polarizing optical microscope (POM). Transmission and reflection modes of POM are selectively used. As depicted in Fig. 1
Fig. 1 Setup for phase transition observation and electro-optical performance testing.
, Light Source-1 is turned on at transmission mode, and adjusts Polarizer-1 to cross with Analyzer; at the reflection mode, Light Source-2 is turned on, Polarizer-2 and Analyzer are crossed with each other. Two light sources are the wide-band halogen. The light paths at transmission and reflection modes are labeled in blue and red color respectively. The BP texture is observed through reflection mode (Figs. 2(a)
Fig. 2 Textures before exposure (a) and those of bistable PSBPLC as the voltage increases gradually, shown from (b) to (e); the voltage decreases slowly (f) and the voltage removed suddenly (g). The applied voltage corresponds to textures (b), (c), (d), (e) are 0, 0.33, 3 and 5.3 V/μm, respectively; and the voltage of texture (f) is 3 V/μm. (a), (b), (c) and (g) are observed using reflection mode, the others are observed using transmission mode. The spectra of BP (red line) and N* (green line) are shown in (h).
, 2(b), 2(c) and 2(g)) for better observation; while the other phases are observed through transmission mode (Figs. 2(d), 2(e) and 2(f)). The reflection spectrum is detected by an optical fiber connected spectroscope. The sample is settled on the Hot stage at the temperature of 25°C, and a 1 kHz-square wave is applied through two planar ITO electrodes to trigger the phase transition. The phase transition time is tested by applying a voltage on the cell and measuring the response time through oscillator. The applied voltage for testing the transition time of BP-Homeotropic is 5.3 V/μm; and that for BP-N* is 3 V/μm. The rise time is defined as the time for the reflection changes from 10% to 90% of the maximum; and the decay time is that the reflection changes from 90% to 10% of the maximum.

3. Results and discussions

Electric induced phase transition behaviors are studied. The initial texture of the sample is shown in Fig. 2(b). As the applied voltage is weak, 0.33 V/μm, no evident change of the texture is found (Fig. 2(c)). When the voltage increases, the BP disappears gradually while some bright and small balls appear. These balls coalesce together and form the focal conic (the N*) textures. Figure 2(d) shows the typical N* texture at the voltage of 3 V/μm. The whole field changes to dark state (Fig. 2(e)) when the voltage reaches saturation value (5.3 V/μm), because the LCs align along the direction of electric field and form a homeotropic alignment. As the voltage is decreased slowly from saturation, the homeotropic state transits to N* again, and the N* phase can exist even after the voltage is totally removed, rather than transit back to the initial BP state. Figure 2(f) shows the N* texture when the voltage gradually decrease to 3 V/μm. On the contrary, when a saturation voltage (5.3 V/μm is selected in the experiment) is applied suddenly, the BP directly transits to dark state, and would reappear if the voltage is suddenly removed, as shown in Fig. 2(g). We ascribe the formation of N* to the competition between electric field force and polymer elastic force. During the slowly changing of voltage, N* forms when the electric field force exceeds the elastic force. However, the elastic force is dominated when the voltage suddenly removed, thus the N* is suppressed. In order to prove that the texture shown in Fig. 2(d) is N*, we measured the reflection spectra when the sample is under a 3 V/μm voltage as well as that at zero voltage state. As shown in Fig. 2(h), the reflection band of zero voltage state (corresponding to BP) ranges from 532 to 598 nm, in accordance with that observed by POM, and the peak position locates at 570 nm with the reflection about 33%. However, there is no obvious reflection peak of green line and the reflection is low because of the light scattering due to the focal conic texture, proving this is N* state without the existence of BP.

The mechanism of the bistable state in PSBPLC is discussed as follows. BP is a frustration system. It always coexists with defects, which are formed by isotropic aligned LCs. The stability of BP depends on a large twisted power and a stable defect [22

22. H. Kikuchi, “Liquid crystalline blue phases,” Struct. Bonding 128(12), 99–117 (2008). [CrossRef]

]. The essence of the traditional PSBPLC is to stabilize the isotropic aligned LCs in defects by polymer network [14

14. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]

]. The polymer network is usually strong enough to maintain the defects against external stimuli. However, in our experiment, the long-chain end group monomers are selected and photo-polymerized to form the low-crossing-density polymer, which leads to the decrease of polymer elastic power. The low elastic power of polymer network indicates that a relatively low electric field force could damage the stable defects of our sample. Therefore, the sample would present different phase transitions through different procedures i.e. suddenly and slowly applying/removing voltage; while traditional PSBPLC only possesses BP-Homeotropic transition in spite of different procedures either in voltage applying or removing processes. When the sample is suddenly applied a saturation voltage, LCs align along the direction of electric field and changes to Homeotropic state, the result of which is the same as that of traditional PSBPLC. If voltage is slowly increased, N* state would occur when electric field force exceeds the polymer elastic force because the LCs in defects are realigned by external fields. In addition, the electric induced polymer network distortion may also lead to the alignment change of LCs in defect, that further lead to the formation of N* [23

23. J. Yan and S. T. Wu, “Effect of polymer concentration and composition on blue-phase liquid crystals,” J. Displ. Technol. 7(9), 490–493 (2011). [CrossRef]

]. The N* state would transit to Homeotropic state when applied voltage reaches saturation. For voltage removing process, two different procedures are also carried out. If the voltage is suddenly removed, the LCs restored to original BP state due to the recovery force by polymer network, which is similar with that of traditional PSBPLC. However, when the voltage is slowly decreased, there are some differences. N* appears due to the existence of strong electric field. Keeping decreasing even totally removing the voltage, N* is also very stable, because the single twisted defect-free system prevails the frustration system without the aid of external energy. Thus, the bistability is achieved. We suppose the bistable phenomenon may widely exist in PSBPLC system, however in the traditional PSBPLC, the polymer elastic force is so strong that the applied electric field force is difficult to destroy the defects. Therefore the bistability is hard to be observed. If we appropriately decrease the polymer elastic force through the reasonable molecular design and the preparation condition selections, the bistability may appear. In fact, we have found similar phenomena in other material systems. Further researches are ongoing.

4. Conclusion

In summary, the bistable effect of PSBPLC is found. As the increasing of applied voltage, the BP firstly transits to N* phase, and then forms the homeotropic alignment at a saturation voltage. As the voltage is decreased slowly, the N* reappears, and it is stable even when the voltage is totally removed. However, if the voltage is suddenly applied to or removed from saturation, the N* state is suppressed and the BP appears. The electro-optical performances are tested, and the results indicate that the drive voltage for these transformations is lower. The mechanism of this phenomenon is discussed, according to which we consider the bistability may widely exist in polymer stabilized blue phase liquid crystal systems.

Acknowledgments

References and links

1.

K. E. Maly, M. D. Wand, and R. P. Lemieux, “Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant,” J. Am. Chem. Soc. 124(27), 7898–7899 (2002). [CrossRef] [PubMed]

2.

E. P. Pozhidaev and V. G. Chigrinov, “Bistable and multistable states in ferroelectric liquid crystals,” Crystallogr. Rep. 51(6), 1030–1040 (2006). [CrossRef]

3.

M. Xu and D. K. Yang, “Electrooptical properties of dual-frequency cholesteric liquid crystal reflective display and drive scheme,” Jpn. J. Appl. Phys. 38(Part 1, No. 12A), 6827–6830 (1999). [CrossRef]

4.

Y. Q. Lu, X. Liang, Y. H. Wu, F. Du, and S. T. Wu, “Dual-frequency addressed hybrid-aligned nematic liquid crystal,” Appl. Phys. Lett. 85(16), 3354–3356 (2004). [CrossRef]

5.

R. Bao, C. M. Liu, and D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 2(11), 112401 (2009). [CrossRef]

6.

J. K. Kim, F. Araoka, S. M. Jeong, S. Dhara, K. Ishikawa, and H. Takezoe, “Bistable device using anchoring transition of nematic liquid crystals,” Appl. Phys. Lett. 95(6), 063505 (2009). [CrossRef]

7.

J. I. Nitsuma, M. Yoneya, and H. Yokoyama, “Contact photolithographic micropatterning for bistable nematic liquid crystal displays,” Appl. Phys. Lett. 92(24), 241120 (2008). [CrossRef]

8.

H. H. Liang, C. C. Wu, P. H. Wang, and J. Y. Lee, “Electrothermal switchable bistable reverse mode polymer stabilized cholesteric texture light shutter,” Opt. Mater. 33(8), 1195–1202 (2011). [CrossRef]

9.

Y. C. Hsiao, C. Y. Tang, and W. Lee, “Fast-switching bistable cholesteric intensity modulator,” Opt. Express 19(10), 9744–9749 (2011). [CrossRef] [PubMed]

10.

C. Y. Huang, Y. J. Huang, and Y. H. Tseng, “Dual-operation-mode liquid crystal lens,” Opt. Express 17(23), 20860–20865 (2009). [CrossRef] [PubMed]

11.

C. Y. Wu, Y. H. Zou, I. Timofeev, Y. T. Lin, V. Y. Zyryanov, J. S. Hsu, and W. Lee, “Tunable bi-functional photonic device based on one-dimensional photonic crystal infiltrated with a bistable liquid-crystal layer,” Opt. Express 19(8), 7349–7355 (2011). [CrossRef] [PubMed]

12.

Z. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys. 12(11), 113018 (2010). [CrossRef]

13.

G. Zhu, X. W. Lin, W. Hu, Z. G. Zheng, H. F. Wang, H. Q. Cui, D. Shen, and Y. Q. Lu, “Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups,” Opt. Mater. Express 1(8), 1478–1483 (2011). [CrossRef]

14.

H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]

15.

J. Yan and S. T. Wu, “Polymer-stabilized blue phase liquid crystals: a tutorial [Invited],” Opt. Mater. Express 1(8), 1527–1535 (2011). [CrossRef]

16.

Z. Ge, S. Gauza, M. Jiao, H. Xianyu, and S.-T. Wu, “Electro-optics of polymer-stabilized blue phase liquid crystal displays,” Appl. Phys. Lett. 94(10), 101104 (2009). [CrossRef]

17.

W. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater. 1(2), 111–113 (2002). [CrossRef] [PubMed]

18.

J. Yan, Y. Li, and S. T. Wu, “High-efficiency and fast-response tunable phase grating using a blue phase liquid crystal,” Opt. Lett. 36(8), 1404–1406 (2011). [CrossRef] [PubMed]

19.

G. Zhu, J. Li, X. W. Lin, H. F. Wang, W. Hu, Z. Zheng, H. Q. Cui, D. Shen, and Y. Q. Lu, “Polarization-independent blue-phase liquid-crystal gratings driven by vertical electric field,” J. Soc. Inf. Disp. 20(6), 341–346 (2012). [CrossRef]

20.

C. T. Wang, H. Y. Liu, H. H. Cheng, and T. H. Lin, “Bistable effect in the liquid crystal blue phase,” Appl. Phys. Lett. 96(4), 041106 (2010). [CrossRef]

21.

Z. Zheng, H. F. Wang, G. Zhu, X. W. Lin, J. Li, W. Hu, H. Q. Cui, D. Shen, and Y. Q. Lu, “Low-temperature-applicable polymer-stabilized blue-phase liquid crystal and its Kerr effect,” J. Soc. Inf. Disp. 20(6), 326–332 (2012). [CrossRef]

22.

H. Kikuchi, “Liquid crystalline blue phases,” Struct. Bonding 128(12), 99–117 (2008). [CrossRef]

23.

J. Yan and S. T. Wu, “Effect of polymer concentration and composition on blue-phase liquid crystals,” J. Displ. Technol. 7(9), 490–493 (2011). [CrossRef]

OCIS Codes
(160.2100) Materials : Electro-optical materials
(160.3710) Materials : Liquid crystals

ToC Category:
Liquid Crystals

History
Original Manuscript: July 18, 2012
Revised Manuscript: August 21, 2012
Manuscript Accepted: August 31, 2012
Published: September 4, 2012

Citation
Zhi-gang Zheng, Dan Zhang, Xiao-wen Lin, Ge Zhu, Wei Hu, Dong Shen, and Yan-qing Lu, "Bistable state in polymer stabilized blue phase liquid crystal," Opt. Mater. Express 2, 1353-1358 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-10-1353


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References

  1. K. E. Maly, M. D. Wand, and R. P. Lemieux, “Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant,” J. Am. Chem. Soc.124(27), 7898–7899 (2002). [CrossRef] [PubMed]
  2. E. P. Pozhidaev and V. G. Chigrinov, “Bistable and multistable states in ferroelectric liquid crystals,” Crystallogr. Rep.51(6), 1030–1040 (2006). [CrossRef]
  3. M. Xu and D. K. Yang, “Electrooptical properties of dual-frequency cholesteric liquid crystal reflective display and drive scheme,” Jpn. J. Appl. Phys.38(Part 1, No. 12A), 6827–6830 (1999). [CrossRef]
  4. Y. Q. Lu, X. Liang, Y. H. Wu, F. Du, and S. T. Wu, “Dual-frequency addressed hybrid-aligned nematic liquid crystal,” Appl. Phys. Lett.85(16), 3354–3356 (2004). [CrossRef]
  5. R. Bao, C. M. Liu, and D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express2(11), 112401 (2009). [CrossRef]
  6. J. K. Kim, F. Araoka, S. M. Jeong, S. Dhara, K. Ishikawa, and H. Takezoe, “Bistable device using anchoring transition of nematic liquid crystals,” Appl. Phys. Lett.95(6), 063505 (2009). [CrossRef]
  7. J. I. Nitsuma, M. Yoneya, and H. Yokoyama, “Contact photolithographic micropatterning for bistable nematic liquid crystal displays,” Appl. Phys. Lett.92(24), 241120 (2008). [CrossRef]
  8. H. H. Liang, C. C. Wu, P. H. Wang, and J. Y. Lee, “Electrothermal switchable bistable reverse mode polymer stabilized cholesteric texture light shutter,” Opt. Mater.33(8), 1195–1202 (2011). [CrossRef]
  9. Y. C. Hsiao, C. Y. Tang, and W. Lee, “Fast-switching bistable cholesteric intensity modulator,” Opt. Express19(10), 9744–9749 (2011). [CrossRef] [PubMed]
  10. C. Y. Huang, Y. J. Huang, and Y. H. Tseng, “Dual-operation-mode liquid crystal lens,” Opt. Express17(23), 20860–20865 (2009). [CrossRef] [PubMed]
  11. C. Y. Wu, Y. H. Zou, I. Timofeev, Y. T. Lin, V. Y. Zyryanov, J. S. Hsu, and W. Lee, “Tunable bi-functional photonic device based on one-dimensional photonic crystal infiltrated with a bistable liquid-crystal layer,” Opt. Express19(8), 7349–7355 (2011). [CrossRef] [PubMed]
  12. Z. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys.12(11), 113018 (2010). [CrossRef]
  13. G. Zhu, X. W. Lin, W. Hu, Z. G. Zheng, H. F. Wang, H. Q. Cui, D. Shen, and Y. Q. Lu, “Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups,” Opt. Mater. Express1(8), 1478–1483 (2011). [CrossRef]
  14. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater.1(1), 64–68 (2002). [CrossRef] [PubMed]
  15. J. Yan and S. T. Wu, “Polymer-stabilized blue phase liquid crystals: a tutorial [Invited],” Opt. Mater. Express1(8), 1527–1535 (2011). [CrossRef]
  16. Z. Ge, S. Gauza, M. Jiao, H. Xianyu, and S.-T. Wu, “Electro-optics of polymer-stabilized blue phase liquid crystal displays,” Appl. Phys. Lett.94(10), 101104 (2009). [CrossRef]
  17. W. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater.1(2), 111–113 (2002). [CrossRef] [PubMed]
  18. J. Yan, Y. Li, and S. T. Wu, “High-efficiency and fast-response tunable phase grating using a blue phase liquid crystal,” Opt. Lett.36(8), 1404–1406 (2011). [CrossRef] [PubMed]
  19. G. Zhu, J. Li, X. W. Lin, H. F. Wang, W. Hu, Z. Zheng, H. Q. Cui, D. Shen, and Y. Q. Lu, “Polarization-independent blue-phase liquid-crystal gratings driven by vertical electric field,” J. Soc. Inf. Disp.20(6), 341–346 (2012). [CrossRef]
  20. C. T. Wang, H. Y. Liu, H. H. Cheng, and T. H. Lin, “Bistable effect in the liquid crystal blue phase,” Appl. Phys. Lett.96(4), 041106 (2010). [CrossRef]
  21. Z. Zheng, H. F. Wang, G. Zhu, X. W. Lin, J. Li, W. Hu, H. Q. Cui, D. Shen, and Y. Q. Lu, “Low-temperature-applicable polymer-stabilized blue-phase liquid crystal and its Kerr effect,” J. Soc. Inf. Disp.20(6), 326–332 (2012). [CrossRef]
  22. H. Kikuchi, “Liquid crystalline blue phases,” Struct. Bonding128(12), 99–117 (2008). [CrossRef]
  23. J. Yan and S. T. Wu, “Effect of polymer concentration and composition on blue-phase liquid crystals,” J. Displ. Technol.7(9), 490–493 (2011). [CrossRef]

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