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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 1 — Jan. 14, 2013
  • pp: 724–729
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Laser-driven microflow-induced bistable orientation of a nematic liquid crystal in perfluoropolymer-treated unrubbed cells

V. S. R. Jampani, M. Sǩarabot, H. Takezoe, I. Muševič, and S. Dhara  »View Author Affiliations


Optics Express, Vol. 21, Issue 1, pp. 724-729 (2013)
http://dx.doi.org/10.1364/OE.21.000724


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Abstract

We demonstrate laser-driven microflow-induced orientational change (homeotropic to planar) in a dye-doped nematic liquid crystal. The homeotropic to planar director alignment is achieved in unrubbed cells in the thermal hysteresis range of a discontinuous anchoring reorientation transition due to the local heating by light absorption in dye-doped sample. Various bistable patterns were recorded in the cell by a programmable laser tweezers. The width of the patterns depend on the scanning speed of the tightly focussed laser beam and the minimum width obtained is ≃0.57μm which is about 35 times smaller than the earlier report in the rubbed cells. We show that the motion of the microbeam spot causes local flow as a result the liquid crystal director is aligned along that direction.

© 2012 OSA

1. Introduction

2. Experimental Results and Discussion

We used a liquid crystal 4′-butyl-4-hepty-bicyohexyl-4-carbonitrile (CCN-47) obtained from Merck. It exhibits the following phase transitions: Cr. 25.6 °C SmA°C N 57.3 °C I. It has a large transverse dipole moment and possesses negative dielectric anisotropy (Δε = −5.7 at 30 °C) [19

19. S. Dhara and N. V. Madhusudana, “Physical characterisation of 4-butyl-4-heptyl-bicyclohexyl- 4-carbonitrile,” Phase Transitions 81, 561–569 (2008). [CrossRef]

]. The laser dye 4-dicyanomethylene-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM) was purchased from Exciton and 0.1wt% was mixed with the CCN-47 liquid crystal. An amorphous perfluoropolymer, poly[perfluoro (4-vinyloxy-1-butene)] (CYTOP) obtained from Asahi Glass., Ltd was spin coated onto the ITO coated glass plates. These plates were cured at 100 °C for about 30 min. Epoxy glue was used to make cells and Mylar spacer of thickness 5 μm was used to maintain the cell thickness. The samples were filled in the cells at the isotropic phase by capillary action. An ITO coated glass plate of thickness 1.2 mm was used as a heater. We used a laser tweezers setup that was built around an inverted microscope (Nikon Eclipse, TE2000-U) with an argon laser operating at 514 nm as a light source and a pair of acousto-optic deflectors driven by a computerized system (Aresis, TWEEZ 70) for trap manipulation. The experiments were performed using a 60X high numerical aperture water immersion objective.

We first show the textures of the sample taken in optical polarizing microscope in heating and cooling in Fig. 1. In case of cooling the sample exhibits planar degenerate texture with many half strength defects in the nematic phase. At 47 °C the texture changes to homeotropic state indicating an anchoring transition. In this case the optical axis is uniform and oriented vertical to the substrates [14

14. S. Dhara, J. K. Kim, S. M. Jeong, R. Kogo, F. Araoka, K. Ishikawa, and H. Takezoe, “Anchoring transitions of transversely polar liquid-crystal molecules on perfluoropolymer surfaces,” Phys. Rev. E 79, 060701R (2009). [CrossRef]

]. In case of heating the sample remains homeotropic up to the temperature 52 °C and beyond that it changes to a planar state with characteristic umbilic and wall defects [20

20. T. A. Kumar, V. S. S. Sastry, K. Ishikawa, H. Takezoe, N.V. Madhusudana, and S. Dhara, “Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring,” Liq. Cryst. 38, 971–979 (2011). [CrossRef]

]. Thus there is about 5 °C temperature range of thermal hysteresis in which either planar or homeotropic state can be achieved depending on the heating and cooling. The details of the measurement of transmitted intensity as a function of temperature in rubbed cells and defects in unrubbed cells are reported by us previously [20

20. T. A. Kumar, V. S. S. Sastry, K. Ishikawa, H. Takezoe, N.V. Madhusudana, and S. Dhara, “Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring,” Liq. Cryst. 38, 971–979 (2011). [CrossRef]

]. The inclusion of small percentage of DCM dyes does not appreciably change the transition temperatures [15

15. 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, 063505 (2009). [CrossRef]

]. Its absorption peak matches with the emission of Argon-ion laser wavelength (514nm) and thus can increase the local temperature by a few degrees and hence can change the orientation.

Fig. 1 Representative textures of the sample as observed under optical polarizing microscope in cooling (top) and heating (bottom). The bistable (hysteresis) temperature range is indicated below.

Fig. 2 (a) Recorded lines at various scanning speed of the laser tweezer beam at a temperature 51°C. The scanning speeds are (a)8 μm/s (b)19 μm/s (c)45 μm/s, respectively. The line widths are mentioned in each figures. Laser power 120 mW. Cell thickness ≃ 5μm. The intensity at each pixel across the bistable line is shown on the right side.

To understand the director orientation inside the bistable region we created two orthogonal lines in the same cell. In Fig. 3 we show two orthogonal lines which are created at a scanning rate of 11 μm/s. The lines are not seen when they are oriented parallel to the polarizers / analyzer whereas they are observed with good contrast and without any defects when the line direction is changed to 45° with respect to one of the polarizers (Fig. 3 (b)). This suggests that the director is uniformly orientated in both regions (planar and homeotropic). We used a full wave retardation plate (530 nm) i.e., a“red plate” between the sample and the analyzer to find out the director orientation inside two orthogonal lines. Interestingly we found that the two lines show different colors namely yellow and blueish and vice versa depending on the orientation of the fast axis of the red plate. In Fig. 3(c) the yellow line is parallel to the fast axis and the bluish line is perpendicular to it. This means the director is aligned along the direction of the fast axis considering the birefringence of the sample is positive.

Fig. 3 Two orthogonal recorded lines under polarizing microscope at temperature 51°C (a) the beam scanning direction is parallel to the polarizer/analyser (b) 45° with respect to the polarizer/analyzer (c) with λ-plate and fast axis orientated parallel to the vertical line (d) with λ-plate and fast axis orientated parallel to the horizontal line. Cell thickness 5.2 μm.

Fig. 4 Square grid patterns prepared at 51°C. (a) Pattern for a square box generated in the computer using Origin Software. Each square box is composed of parallel line of equal width and spacing. (b) the beam scanning direction is parallel to the polarizer/analyser. (c) 45° with respect to the polarizer/analyzer (d) with λ-plate and fast axis orientated parallel to the vertical line (e) with λ-plate and fast axis orientated parallel to the horizontal line. Cell thickness 5.2 μm.
Fig. 5 Schematic representation of the molecular orientation in planar and homeotropic regions (top view). The DCM dye molecules are indicated in red colour. The green arrows indicate the direction of the motion of the beam spot in two orthogonal directions.

3. Conclusion

In conclusion, we have shown that various bistable patterns can be generated easily by using a programmable laser tweezers in the thermal hysteresis region of CCN-47 liquid crystal on perfluoropolymer-treated unrubbed surfaces. The line width decreases with increasing speed and the minimum width obtained is 0.57μm. The orientational transition is induced by local heating generated by a laser in dye-doped samples. The orientation of the director is achieved by a micro-flow in unrubbed cells. Such systems can be used as data storage devices and the storage density can be increased due to the reduction in the line width. Further such patters could also be used as channels for guiding plane polarized light as it sees a larger refractive index (ne) in the planar region than the homeotropic region (no) and thus is useful for “Soft Chip” applications.

Acknowledgments

S.D. gratefully acknowledges the financial support from DST ( SR/NM/NS-134/2010), CSIR ( 03(1207)/12/EMR-II) and UPE-II, UoH. V.S.R.J acknowledges (EC)7OP Marie Curie ITN Project HIERARCHY Grant No. PITN-GA-2008-215851 and the Slovenian Research Agency (ARRS) Contract No. J1-3612. We also acknowledge Merck Japan Ltd. for supplying CCN-47.

References and links

1.

L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag Inc., 1994). [CrossRef]

2.

D. Andrienko, A. Dyadyusha, Y. Kurioz, V. Reshetnyak, and Y. Reznikov, “Light-induced anchoring transitions and bistable nematic alignment on polysiloxane-based aligning surface,” Mol. Cryst. Liq. Cryst. 321, 299–307 (1998). [CrossRef]

3.

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

4.

J. S. Gwag, J. H. Kim, M. Yoney, and H. Yokoyama, “Surface nematic bistability at nanoimprinted topography,” Appl. Phys. Lett. 92, 153110 (2008). [CrossRef]

5.

G. D. Boyd, J. Cheng, and P. D. T. Ngo, “Liquid crystal orientational bistability and nematic storage effects,” Appl. Phys. Lett. 36, 556–558 (1980). [CrossRef]

6.

D. W. Berreman and W. R. Heffner, “New bistable liquid crystal twist cell,” J. Appl. Phys. 52, 3032–3039 (1981). [CrossRef]

7.

R. Barberi, M. Boix, and G. Durand, “Electrically controlled surface bistability in nematic liquid crystals,” Appl. Phys. Lett. 55, 2506–2508 (1989). [CrossRef]

8.

J. H. Kim, M. Yoneya, J. Yamamoto, and H. Yokoyama, “Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns,” Appl. Phys. Lett. 78, 3055–3057 (2001). [CrossRef]

9.

M. Yoneya, J. H. Kim, and H. Yokoyama, “Multistable nematic liquid crystal device using nanoscopically patterned surface alignment,” Appl. Phys. Lett. 80, 1034–1035 (2002).

10.

J. H. Kim, M. Yoney, and H. Yokoyama, “Tristable nematic liquid-crystal device using micropatterned surface alignment,” Nature 420, 159–162 (2002). [CrossRef] [PubMed]

11.

J. H. Kim, M. Yoney, J. Yamamoto, and H. Yokoyama, “Nano-rubbing of a liquid crystal alignment layer by an atomic force microscope: a detailed characterization,” Nanotechnology 13, 133–137 (2002). [CrossRef]

12.

Y. Kurioz, D. Kurysh, V. Reshetnyak, and Y. Reznikov, “Temperature induced anchoring transition in nematic liquid crystal cell,” Proc. SPIE 5257, 128–131 (2003). [CrossRef]

13.

J. S. Patel and H. Yokoyama, “Continuous anchoring transition in liquid crystals,” Nature (London) 362, 525–527 (1993). [CrossRef]

14.

S. Dhara, J. K. Kim, S. M. Jeong, R. Kogo, F. Araoka, K. Ishikawa, and H. Takezoe, “Anchoring transitions of transversely polar liquid-crystal molecules on perfluoropolymer surfaces,” Phys. Rev. E 79, 060701R (2009). [CrossRef]

15.

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, 063505 (2009). [CrossRef]

16.

J. K. Kim, K. V. Le, S. Dhara, F. Araoka, K. Ishikawa, and H. Takezoe, “Heat-driven and electric-field-driven bistable devices using dye-doped nematic liquid crystals,” J. Appl. Phys. 107, 123108 (2010). [CrossRef]

17.

T. A. Kumar, K. V. Le, S. Aya, S. Kang, F. Araoka, K. Ishikawa, S. Dhara, and H. Takezoe, “Anchoring transition in a nematic liquid crystal doped with chiral agents,” Phase Transitions 85, 888–899 (2012). [CrossRef]

18.

S. M. Jeong, J. K. Kim, Y. Shimbo, F. Araoka, S. Dhara, N. Y. Ha, K. Ishikawa, and H. Takezoe, “Perfluoropolymer surface for shock-free homeotropic alignment of smectic liquid crystals,” Adv. Mater. 22, 34–38 (2010). [CrossRef] [PubMed]

19.

S. Dhara and N. V. Madhusudana, “Physical characterisation of 4-butyl-4-heptyl-bicyclohexyl- 4-carbonitrile,” Phase Transitions 81, 561–569 (2008). [CrossRef]

20.

T. A. Kumar, V. S. S. Sastry, K. Ishikawa, H. Takezoe, N.V. Madhusudana, and S. Dhara, “Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring,” Liq. Cryst. 38, 971–979 (2011). [CrossRef]

OCIS Codes
(160.3710) Materials : Liquid crystals
(230.3720) Optical devices : Liquid-crystal devices
(350.4855) Other areas of optics : Optical tweezers or optical manipulation

ToC Category:
Optical Devices

History
Original Manuscript: November 15, 2012
Revised Manuscript: December 5, 2012
Manuscript Accepted: December 5, 2012
Published: January 7, 2013

Citation
V. S. R. Jampani, M. Sǩarabot, H. Takezoe, I. Muševič, and S. Dhara, "Laser-driven microflow-induced bistable orientation of a nematic liquid crystal in perfluoropolymer-treated unrubbed cells," Opt. Express 21, 724-729 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-724


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References

  1. L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag Inc., 1994). [CrossRef]
  2. D. Andrienko, A. Dyadyusha, Y. Kurioz, V. Reshetnyak, and Y. Reznikov, “Light-induced anchoring transitions and bistable nematic alignment on polysiloxane-based aligning surface,” Mol. Cryst. Liq. Cryst.321, 299–307 (1998). [CrossRef]
  3. J. Niitsuma, M. Yoneya, and H. Yokoyama, “Contact photolithographic micropatterning for bistable nematic liquid crystal displays,” Appl. Phys. Lett.92, 241120 (2008). [CrossRef]
  4. J. S. Gwag, J. H. Kim, M. Yoney, and H. Yokoyama, “Surface nematic bistability at nanoimprinted topography,” Appl. Phys. Lett.92, 153110 (2008). [CrossRef]
  5. G. D. Boyd, J. Cheng, and P. D. T. Ngo, “Liquid crystal orientational bistability and nematic storage effects,” Appl. Phys. Lett.36, 556–558 (1980). [CrossRef]
  6. D. W. Berreman and W. R. Heffner, “New bistable liquid crystal twist cell,” J. Appl. Phys.52, 3032–3039 (1981). [CrossRef]
  7. R. Barberi, M. Boix, and G. Durand, “Electrically controlled surface bistability in nematic liquid crystals,” Appl. Phys. Lett.55, 2506–2508 (1989). [CrossRef]
  8. J. H. Kim, M. Yoneya, J. Yamamoto, and H. Yokoyama, “Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns,” Appl. Phys. Lett.78, 3055–3057 (2001). [CrossRef]
  9. M. Yoneya, J. H. Kim, and H. Yokoyama, “Multistable nematic liquid crystal device using nanoscopically patterned surface alignment,” Appl. Phys. Lett.80, 1034–1035 (2002).
  10. J. H. Kim, M. Yoney, and H. Yokoyama, “Tristable nematic liquid-crystal device using micropatterned surface alignment,” Nature420, 159–162 (2002). [CrossRef] [PubMed]
  11. J. H. Kim, M. Yoney, J. Yamamoto, and H. Yokoyama, “Nano-rubbing of a liquid crystal alignment layer by an atomic force microscope: a detailed characterization,” Nanotechnology13, 133–137 (2002). [CrossRef]
  12. Y. Kurioz, D. Kurysh, V. Reshetnyak, and Y. Reznikov, “Temperature induced anchoring transition in nematic liquid crystal cell,” Proc. SPIE5257, 128–131 (2003). [CrossRef]
  13. J. S. Patel and H. Yokoyama, “Continuous anchoring transition in liquid crystals,” Nature (London)362, 525–527 (1993). [CrossRef]
  14. S. Dhara, J. K. Kim, S. M. Jeong, R. Kogo, F. Araoka, K. Ishikawa, and H. Takezoe, “Anchoring transitions of transversely polar liquid-crystal molecules on perfluoropolymer surfaces,” Phys. Rev. E79, 060701R (2009). [CrossRef]
  15. 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, 063505 (2009). [CrossRef]
  16. J. K. Kim, K. V. Le, S. Dhara, F. Araoka, K. Ishikawa, and H. Takezoe, “Heat-driven and electric-field-driven bistable devices using dye-doped nematic liquid crystals,” J. Appl. Phys.107, 123108 (2010). [CrossRef]
  17. T. A. Kumar, K. V. Le, S. Aya, S. Kang, F. Araoka, K. Ishikawa, S. Dhara, and H. Takezoe, “Anchoring transition in a nematic liquid crystal doped with chiral agents,” Phase Transitions85, 888–899 (2012). [CrossRef]
  18. S. M. Jeong, J. K. Kim, Y. Shimbo, F. Araoka, S. Dhara, N. Y. Ha, K. Ishikawa, and H. Takezoe, “Perfluoropolymer surface for shock-free homeotropic alignment of smectic liquid crystals,” Adv. Mater.22, 34–38 (2010). [CrossRef] [PubMed]
  19. S. Dhara and N. V. Madhusudana, “Physical characterisation of 4-butyl-4-heptyl-bicyclohexyl- 4-carbonitrile,” Phase Transitions81, 561–569 (2008). [CrossRef]
  20. T. A. Kumar, V. S. S. Sastry, K. Ishikawa, H. Takezoe, N.V. Madhusudana, and S. Dhara, “Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring,” Liq. Cryst.38, 971–979 (2011). [CrossRef]

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