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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 5 — Mar. 11, 2013
  • pp: 6243–6248
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Complementarity between fluorescence and reflection in photoluminescent cholesteric liquid crystal devices

Jang-Kyum Kim, Suk-Hwan Joo, and Jang-Kun Song  »View Author Affiliations


Optics Express, Vol. 21, Issue 5, pp. 6243-6248 (2013)
http://dx.doi.org/10.1364/OE.21.006243


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Abstract

The combination of photoluminescence (PL) and cholesteric liquid crystal (CLC) provides interesting complementary features for an optimized display application. Distortion of the Bragg lattice of CLCs decreases selective reflection but increases fluorescence intensity; recovery of a uniform lattice in turn results in increased reflection and decreased fluorescence. This complementary relationship between the fluorescence and the Bragg reflection gives rise to self-compensations for color shifts due to either dynamic slow response of CLC helix or mismatch of oblique incidence of light with respect to the helical axis. These color shifts have long been intrinsic unsolved limitations of conventional CLC devices. Thus, the complementary coupling between the fluorescence and the CLC Bragg reflections plays an important role in improving the color performance and the quality of moving images.

© 2013 OSA

1. Introduction

Cholesteric liquid crystal (CLC) devices can display the images with vivid color by using the selective reflection without the aid of a polarizer or color filter [1

1. D. K. Yang, J. W. Doane, Z. Yaniv, and J. Glasser, “Cholesteric reflective display: Drive scheme and contrast,” Appl. Phys. Lett. 64(15), 1905–1907 (1994). [CrossRef]

3

3. M. Mitov and N. Dessaud, “Going beyond the reflectance limit of cholesteric liquid crystals,” Nat. Mater. 5(5), 361–364 (2006). [CrossRef] [PubMed]

]. This feature enables the CLC devices to have a few significant advantages as portable display devices that require low power consumption and excellent outdoor image quality. However, the CLC device has several drawbacks arising from its one dimensional photonic crystals structure. It has been reported that the slow helical axis reorientation occurs over several seconds during the transition from homeotropic to planar alignments, and it causes slow color changes when displaying dynamic images [4

4. D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, “Control of reflectivity and bistability in displays using cholesteric liquid crystals,” J. Appl. Phys. 76(2), 1331–1333 (1994). [CrossRef]

6

6. Y. C. Yang, M. H. Lee, J. E. Kim, H. Y. Park, and J. C. Lee, “Theoretical Study on the Homeotropic-Transient Planar Transition of Cholesteric Liquid Crystals,” Jpn. J. Appl. Phys. 40(Part 1, No. 2A), 649–653 (2001). [CrossRef]

]. CLC devices also exhibit large color shift with viewing angle because the wavelength of Bragg reflection from the CLC photonic band-gap sensitively depends on the angle of incidence with respect to the helical axis of CLCs [7

7. W. D. St. John, Z. J. Lu, and J. W. Doane, “Characterization of reflective cholesteric liquid-crystal displays,” J. Appl. Phys. 78, 5253–5265 (1995). [CrossRef]

, 8

8. W. D. S. John, W. J. Fritz, Z. J. Lu, and D. K. Yang, “Bragg reflection from cholesteric liquid crystals,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 51(2), 1191–1198 (1995).

]. These color shifts in dynamic images and in off-axis viewing conditions are intrinsic drawbacks of CLC devices and thus have so far not overcome.

In this paper, we report a complementary interference phenomenon between the fluorescence from PL molecules and the selective reflection from CLC band-gap, which can provide solutions to the two intrinsic limitations of normal CLC cells via self-compensation effect for temporal and directional spectral variations.

2. Experiments

A CLC mixture was prepared by mixing a positive nematic LC mixture (ZSM0000, Merck) with 30 wt% chiral dopant (R-811, Merck), which reflects green light with a center wavelength of 525 nm. Two PL molecules, coumarin6 (C6) and 2,5-bis(5-tertbutyl-2-benzoxazolyl) thiophen (BBOT) which have rod-like shape similar to usual nematic LC, were added into the CLC mixture in concentrations of 0.6 wt%. The chemical structure and the absorption and luminescence spectra of C6 and BBOT are shown in Fig. 1(A)
Fig. 1 (A) Chemical structures and spectral absorption and luminescence of C6 and BBOT. (B) Cell structure: thickness of the cell is approximately 3.8 μm.
. Here, BBOT, blue PL dopant, was used for an energy transfer dopant, which absorbs UV light and transfers the energy to C6 via Förster transfer process [16

16. R. Yamaguchi, H. Nagato, H. Hafiz, and S. Sato, “Sensitized Fluorescence of Dichroic Dye in Emissive Type Liquid Crystal Displays,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 410(1), 495–504 (2004). [CrossRef]

]. A PL-CLC cell was fabricated by sandwiching the PL-CLC mixture with two patterned indium tin oxide (ITO) substrates as shown in Fig. 1(B). The inner surfaces of the two substrates were coated by a homeotropic alignment layer to reduce the specular reflection on the CLC surface.

The optical measurements for the cells were carried out using an SR-3A, camera type spectrophotometer (Topcon) for both reflection and fluorescence measurements. For the reflection measurement, external D65 illumination was used along with SR-3A, which illuminates the sample from a 23° oblique direction. For the fluorescence measurement, a UV backlight (60 mW/cm2, VL-4.LC, Vilber Lourmat, France) was placed under PL-CLC cells as shown in Fig. 1(B).

3. Results and discussion

Figures 2(A)
Fig. 2 (A) Reflection luminance of CLC and PL-CLC cells with increasing applied voltage. (B) Fluorescence of a PL-CLC cell on a UV backlight with the applied voltages. (C-D) Spectral reflectance of CLC and PL-CLC cells with increasing applied voltage. (E) Fluorescence of a PL-CLC cell on a UV backlight with the applied voltages. Inset images are microscopic images under crossed polarizers at different applied voltages.
and 2(B) show the reflection and the fluorescence, respectively, with applied voltage in the CLC and PL-CLC cells, which exhibits three different regions corresponding to planar, focal conic and homeotropic states. Enhanced reflection of the PL-CLC compared to pure CLC cell in Fig. 2(A) is due to the fluorescence of C6 after absorbing shorter wavelength light as illustrated in Fig. 1(B). The square data points in Fig. 2(A) represent the reflection difference between the PL-CLC and the pure CLC cells, and serves to indicate the fluorescent light from C6 after absorbing the external D65 illumination. Interestingly, the maximum peak in the difference curve was observed in the intermediate state between the planar and the focal conic states. The similar peak was observed in the pure fluorescence on the UV backlight measured in a dark room, as shown in Fig. 2(B). The PL molecules align along the host LC molecules; accordingly, the fluorescence also varies with the LC alignment states. In the intermediate region, the pure CLC has low reflectance as shown in the solid line of Fig. 1(A), indicating that the CLC was nearly in the focal conic state.

In the intermediate state, the applied electric voltage tilts the liquid crystal molecules, deforming the planar helical state. The distortion creates locally distorted helical clusters, as indicated in the inset microscopic image in Fig. 2(E), a transitional state before complete transition to the focal conic state. The distorted helical structure diminished the Bragg reflection, as shown in Fig. 2(C), instead, giving rise to a broad, weak peak in the shorter wavelength region. The peak shift is due to the tilting of the helical axis during the helical deformation. Note that the CLC band-gap suppresses approximately half of the C6 fluorescence when the spectral range coincides with the band-gap. The fluorescence suppressed by the band-gap is no longer suppressed when the helix is deformed. The majority of the molecules are still parallel to the substrate in the intermediate state, leading to a greater fluorescence intensity in the intermediate state than in the planar state, as indicated in Fig. 2(E). Due to the increasing fluorescence, the reflection spectrum in a PL-CLC cell (Fig. 2(D)) exhibited a clear difference from that in a CLC cell (Fig. 2(C)). Thus, the large fluorescence peak in the state intermediate between the planar and focal conic states is caused by the coupling-to-decoupling transition between the fluorescence and the CLC band-gap.

Another well-known weakness of CLC devices is the shift in color with varying viewing directions [8

8. W. D. S. John, W. J. Fritz, Z. J. Lu, and D. K. Yang, “Bragg reflection from cholesteric liquid crystals,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 51(2), 1191–1198 (1995).

], which is detailed in the upper images in Fig. 4(A)
Fig. 4 Color shift with viewing direction in the CLC and PL-CLC cells: (A) Photos taken at different viewing angles of CLC and PL-CLC cells. (B) Optical simulation for selective reflection with incident angle. (C) Changing color values with viewing angle in u'v' color space. (D) Fluorescence of a PL-CLC cell with UV backlight as a function of viewing angle.
. The color variation is due to the sensitivity of Bragg reflections to incident direction, which can be simply simulated by using 4 × 4 matrix method [18

18. J. K. Song, J. K. Vij, and B. K. Sadashiva, “Conoscopy of chiral smectic liquid crystal cells,” J. Opt. Soc. Am. A 25(7), 1820–1827 (2008). [CrossRef] [PubMed]

, 19

19. P. Yeh, “Electromagnetic propagation in birefringent layered media,” J. Opt. Soc. Am. 69(5), 742–756 (1979). [CrossRef]

], as shown in Fig. 4(B). Interestingly, the shift in color with viewing angle was significantly reduced in PL-CLC cells, as indicated in the second row images in Fig. 4(A). While the color values changed substantially in the conventional CLC cell, the PL-CLC cell showed very stable color performance with varying viewing direction, which is also indicated in the color values in Fig. 4(C). This phenomenon has the same color compensation mechanism as the dynamic response. Figure 4(D) shows the fluorescence spectrum from the PL-CLC cell according to viewing direction, in which the fluorescence intensity increases as the viewing angle increases. This is because the CLC band-gap shifts to shorter wavelengths with increasing incident light angle [8

8. W. D. S. John, W. J. Fritz, Z. J. Lu, and D. K. Yang, “Bragg reflection from cholesteric liquid crystals,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 51(2), 1191–1198 (1995).

], causing decoupling between the fluorescence and the band-gap.

Thus, the complementary interrelationship between the fluorescence and the band-gap exhibits self-compensation during temporal and geometrical color shifts in Bragg reflections. In order to accomplish a full color gamut display application, the same compensation mechanisms should be investigated in red and blue colors as well by combining red and blue CLCs with red and blue PL materials.

4. Conclusion

The fluorescence has a complementary relationship with the selective reflection in PL-CLC cells; in other words, when the selective reflection decreases due to the distortion of the helical structure, the fluorescence increases, compensating for the reduction in reflectance and vice versa. This complementary compensation occurs in the homeotropic-to-planar transition in which the helical axes reorient over several seconds. It is also observed in off-axis viewing condition, in which the band-gap shifts with viewing angle due to oblique incidence. Under these conditions, the complementary compensation significantly reduces the color shifts due to moving picture images and changing viewing angle, which previously have been fundamental limitations of the conventional CLC devices.

Thus, our finding can pave the way to new display applications with fast response times and wide viewing angle using PL-CLC materials, although further studies are required in order to optimize its electro-optical properties and material aspects.

Acknowledgments

We thank Merck Co. for providing materials. This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012R1A1A1012167) and by the Technology Innovation Program (No.10041596) funded by the Ministry of Knowledge Economy (MKE, Korea).

References and links

1.

D. K. Yang, J. W. Doane, Z. Yaniv, and J. Glasser, “Cholesteric reflective display: Drive scheme and contrast,” Appl. Phys. Lett. 64(15), 1905–1907 (1994). [CrossRef]

2.

D. M. Makow, “Peak reflectance and color gamut of superimposed left and right-handed cholesteric liquid crystals,” Appl. Opt. 19(8), 1274–1277 (1980). [CrossRef] [PubMed]

3.

M. Mitov and N. Dessaud, “Going beyond the reflectance limit of cholesteric liquid crystals,” Nat. Mater. 5(5), 361–364 (2006). [CrossRef] [PubMed]

4.

D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, “Control of reflectivity and bistability in displays using cholesteric liquid crystals,” J. Appl. Phys. 76(2), 1331–1333 (1994). [CrossRef]

5.

P. Watson, V. Sergan, J. E. Anderson, J. Ruth, and P. J. Bos, “Characteristic times in the homeotropic to planar transition in cholesteric liquid crystals,” Liq. Cryst. 26(5), 731–736 (1999). [CrossRef]

6.

Y. C. Yang, M. H. Lee, J. E. Kim, H. Y. Park, and J. C. Lee, “Theoretical Study on the Homeotropic-Transient Planar Transition of Cholesteric Liquid Crystals,” Jpn. J. Appl. Phys. 40(Part 1, No. 2A), 649–653 (2001). [CrossRef]

7.

W. D. St. John, Z. J. Lu, and J. W. Doane, “Characterization of reflective cholesteric liquid-crystal displays,” J. Appl. Phys. 78, 5253–5265 (1995). [CrossRef]

8.

W. D. S. John, W. J. Fritz, Z. J. Lu, and D. K. Yang, “Bragg reflection from cholesteric liquid crystals,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 51(2), 1191–1198 (1995).

9.

L. J. Yu and M. M. Labes, “Fluorescent liquid-crystal display utilizing an electric-field-induced cholesteric-nematic transition,” Appl. Phys. Lett. 31(11), 719–720 (1977). [CrossRef]

10.

R. W. Filas and M. M. Labes, “Homogeneous-homeotropic fluorescent liquid crystal cells,” J. Appl. Phys. 52(6), 3949–3953 (1981). [CrossRef]

11.

S. Sato and M. M. Labes, “Multicolor fluorescent display by scattering states in liquid crystals,” J. Appl. Phys. 52(6), 3941–3948 (1981). [CrossRef]

12.

M. Grell and D. D. C. Bradley, “Polarized luminescence from oriented molecular materials,” Adv. Mater. 11(11), 895–905 (1999). [CrossRef]

13.

S. Chen, D. Katsis, A. Schmid, J. Mastrangelo, T. Tsutsui, and T. Blanton, “Circularly polarized light generated by photoexcitation of luminophores in glassy liquid-crystal films,” Nature 397(6719), 506–508 (1999). [CrossRef]

14.

K. L. Woon, M. O'Neill, G. J. Richards, M. P. Aldred, S. M. Kelly, and A. M. Fox, “Highly Circularly Polarized Photoluminescence over a Broad Spectral Range from a Calamitic, Hole‐Transporting, Chiral Nematic Glass and from an Indirectly Excited Dye,” Adv. Mater. 15(18), 1555–1558 (2003). [CrossRef]

15.

Y. Inoue, H. Yoshida, K. Inoue, Y. Shiozaki, H. Kubo, A. Fujii, and M. Ozaki, “Tunable Lasing from a Cholesteric Liquid Crystal Film Embedded With a Liquid Crystal Nanopore Network,” Adv. Mater. 23(46), 5498–5501 (2011). [CrossRef] [PubMed]

16.

R. Yamaguchi, H. Nagato, H. Hafiz, and S. Sato, “Sensitized Fluorescence of Dichroic Dye in Emissive Type Liquid Crystal Displays,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 410(1), 495–504 (2004). [CrossRef]

17.

N. Ohta and A. R. Robertson, Colorimetry, Fundamentals and Applications (John Wiley & Sons, Ltd, Chichester, England, 2005).

18.

J. K. Song, J. K. Vij, and B. K. Sadashiva, “Conoscopy of chiral smectic liquid crystal cells,” J. Opt. Soc. Am. A 25(7), 1820–1827 (2008). [CrossRef] [PubMed]

19.

P. Yeh, “Electromagnetic propagation in birefringent layered media,” J. Opt. Soc. Am. 69(5), 742–756 (1979). [CrossRef]

OCIS Codes
(230.3720) Optical devices : Liquid-crystal devices
(230.5298) Optical devices : Photonic crystals

ToC Category:
Optical Devices

History
Original Manuscript: January 30, 2013
Revised Manuscript: February 25, 2013
Manuscript Accepted: February 25, 2013
Published: March 5, 2013

Citation
Jang-Kyum Kim, Suk-Hwan Joo, and Jang-Kun Song, "Complementarity between fluorescence and reflection in photoluminescent cholesteric liquid crystal devices," Opt. Express 21, 6243-6248 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-5-6243


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References

  1. D. K. Yang, J. W. Doane, Z. Yaniv, and J. Glasser, “Cholesteric reflective display: Drive scheme and contrast,” Appl. Phys. Lett.64(15), 1905–1907 (1994). [CrossRef]
  2. D. M. Makow, “Peak reflectance and color gamut of superimposed left and right-handed cholesteric liquid crystals,” Appl. Opt.19(8), 1274–1277 (1980). [CrossRef] [PubMed]
  3. M. Mitov and N. Dessaud, “Going beyond the reflectance limit of cholesteric liquid crystals,” Nat. Mater.5(5), 361–364 (2006). [CrossRef] [PubMed]
  4. D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, “Control of reflectivity and bistability in displays using cholesteric liquid crystals,” J. Appl. Phys.76(2), 1331–1333 (1994). [CrossRef]
  5. P. Watson, V. Sergan, J. E. Anderson, J. Ruth, and P. J. Bos, “Characteristic times in the homeotropic to planar transition in cholesteric liquid crystals,” Liq. Cryst.26(5), 731–736 (1999). [CrossRef]
  6. Y. C. Yang, M. H. Lee, J. E. Kim, H. Y. Park, and J. C. Lee, “Theoretical Study on the Homeotropic-Transient Planar Transition of Cholesteric Liquid Crystals,” Jpn. J. Appl. Phys.40(Part 1, No. 2A), 649–653 (2001). [CrossRef]
  7. W. D. St. John, Z. J. Lu, and J. W. Doane, “Characterization of reflective cholesteric liquid-crystal displays,” J. Appl. Phys.78, 5253–5265 (1995). [CrossRef]
  8. W. D. S. John, W. J. Fritz, Z. J. Lu, and D. K. Yang, “Bragg reflection from cholesteric liquid crystals,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics51(2), 1191–1198 (1995).
  9. L. J. Yu and M. M. Labes, “Fluorescent liquid-crystal display utilizing an electric-field-induced cholesteric-nematic transition,” Appl. Phys. Lett.31(11), 719–720 (1977). [CrossRef]
  10. R. W. Filas and M. M. Labes, “Homogeneous-homeotropic fluorescent liquid crystal cells,” J. Appl. Phys.52(6), 3949–3953 (1981). [CrossRef]
  11. S. Sato and M. M. Labes, “Multicolor fluorescent display by scattering states in liquid crystals,” J. Appl. Phys.52(6), 3941–3948 (1981). [CrossRef]
  12. M. Grell and D. D. C. Bradley, “Polarized luminescence from oriented molecular materials,” Adv. Mater.11(11), 895–905 (1999). [CrossRef]
  13. S. Chen, D. Katsis, A. Schmid, J. Mastrangelo, T. Tsutsui, and T. Blanton, “Circularly polarized light generated by photoexcitation of luminophores in glassy liquid-crystal films,” Nature397(6719), 506–508 (1999). [CrossRef]
  14. K. L. Woon, M. O'Neill, G. J. Richards, M. P. Aldred, S. M. Kelly, and A. M. Fox, “Highly Circularly Polarized Photoluminescence over a Broad Spectral Range from a Calamitic, Hole‐Transporting, Chiral Nematic Glass and from an Indirectly Excited Dye,” Adv. Mater.15(18), 1555–1558 (2003). [CrossRef]
  15. Y. Inoue, H. Yoshida, K. Inoue, Y. Shiozaki, H. Kubo, A. Fujii, and M. Ozaki, “Tunable Lasing from a Cholesteric Liquid Crystal Film Embedded With a Liquid Crystal Nanopore Network,” Adv. Mater.23(46), 5498–5501 (2011). [CrossRef] [PubMed]
  16. R. Yamaguchi, H. Nagato, H. Hafiz, and S. Sato, “Sensitized Fluorescence of Dichroic Dye in Emissive Type Liquid Crystal Displays,” Mol. Cryst. Liq. Cryst. (Phila. Pa.)410(1), 495–504 (2004). [CrossRef]
  17. N. Ohta and A. R. Robertson, Colorimetry, Fundamentals and Applications (John Wiley & Sons, Ltd, Chichester, England, 2005).
  18. J. K. Song, J. K. Vij, and B. K. Sadashiva, “Conoscopy of chiral smectic liquid crystal cells,” J. Opt. Soc. Am. A25(7), 1820–1827 (2008). [CrossRef] [PubMed]
  19. P. Yeh, “Electromagnetic propagation in birefringent layered media,” J. Opt. Soc. Am.69(5), 742–756 (1979). [CrossRef]

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