Polymer-stabilized blue phase liquid crystals: a tutorial [Invited]
Jin Yan and Shin-Tson Wu*
College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA *swu@mail.ucf.edu

Abstract: Blue phase liquid crystals exhibit several attractive features, such as self-assembled three-dimensional cubic structures, optically-isotropic in the voltage-off state, no need for alignment layers, and submillisecond response time. This tutorial gives step-by-step introduction on basic bluephase materials and properties, monomers and polymerization processes, and key device performance criteria for display and photonics applications.
©2011 Optical Society of America
OCIS codes: (160.3710) Liquid crystals; (160.5470) Polymers.

References and links
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. F. Reinitzer, “Beiträge zur Kenntniss des Cholestherins,” Monatsh. Chem. 9(1), 421–441 (1888). A. Saupe, “On molecular structure and physical properties of thermotropic liquid crystals,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 7(1), 59–74 (1969). S. A. Brazovskii and S. G. Dmitriev, “Phase transitions in cholesteric liquid crystals,” Zh. Eksp. Teor. Fiz. 69, 979–989 (1975). R. M. Hornreich and S. Shtrikman, Liquid Crystals of One- and Two- Dimensional Order (Springer-Verlag, Berlin, 1980). S. Meiboom, J. P. Sethna, W. P. Anderson, and W. F. Brinkman, “Theory of the blue phase cholesteric liquid crystals,” Phys. Rev. Lett. 46(18), 1216–1219 (1981). E. Dubois-Violette and B. Pansu, “Frustration and related topology of blue phases,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 165, 151–182 (1988). P. P. Crooker, Chirality in Liquid Crystals (Springer, New York, 2001), Chap. 7. D. L. Johnson, J. H. Flack, and P. P. Crooker, “Structure and properties of the cholesteric blue phases,” Phys. Rev. Lett. 45(8), 641–644 (1980). P. E. Cladis, T. Garel, and P. Pieranski, “Kossel diagrams show electric-field-induced cubic-tetragonal structural transition in frustrated liquid-crystal blue phases,” Phys. Rev. Lett. 57(22), 2841–2844 (1986). R. J. Miller and H. F. Gleeson, “Order parameter measurements from the Kossel diagrams of the liquid-crystal blue phases,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 52(5), 5011–5016 (1995). H. S. Kitzerow, H. Schmid, A. Ranft, G. Heppke, R. A. M. Hikmet, and J. Lub, “Observation of blue phases in chiral networks,” Liq. Cryst. 14(3), 911–916 (1993). H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). H. J. Coles and M. N. Pivnenko, “Liquid crystal ‘blue phases’ with a wide temperature range,” Nature 436(7053), 997–1000 (2005). H. Kikuchi, Liquid Crystalline Blue Phases (Springer Berlin / Heidelberg, 2008), pp. 99–117. G. Heppke, H.-S. Kitzerow, and M. Krumrey, “Electric field induced variation of the refractive index in cholesteric blue phases,” Mol. Cryst. Liq. Cryst. Lett. 2, 59–65 (1985). J. Yan, L. Rao, M. Jiao, Y. Li, H. C. Cheng, and S. T. Wu, “Polymer-stabilized optically-isotropic liquid crystals for next-generation display and photonics applications,” J. Mater. Chem. 21(22), 7870–7877 (2011). G. Heppke, B. Jérôme, H.-S. Kitzerow, and P. Pieranski, “Electrostriction of the cholesteric blue phases BPI and BPII in mixtures with positive dielectric anisotropy,” J. Phys. (France) 50(19), 2991–2998 (1989). H. Stegemeyer and F. Porsch, “Electric field effect on phase transitions in liquid-crystalline blue-phase systems,” Phys. Rev. A 30(6), 3369–3371 (1984). H.-S. Kitzerow, “The effect of electric fields on blue phases,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 202(1), 51–83 (1991). K. M. Chen, S. Gauza, H. Xianyu, and S. T. Wu, “Hysteresis effects in blue-phase liquid crystals,” J. Disp. Technol. 6(8), 318–322 (2010). J. Yan, H. C. Cheng, S. Gauza, Y. Li, M. Jiao, L. Rao, and S. T. Wu, “Extended Kerr effect of polymer-stabilized blue-phase liquid crystals,” Appl. Phys. Lett. 96(7), 071105 (2010). J. Yan, M. Jiao, L. Rao, and S. T. Wu, “Direct measurement of electric-field-induced birefringence in a polymerstabilized blue-phase liquid crystal composite,” Opt. Express 18(11), 11450–11455 (2010).

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23. T. Seshadri and H. J. Haupt, “Novel ferrocene-based chiral Schiff's base derivative with a twist-grain boundary phase (TGBA) and a blue phase,” Chem. Commun. (Camb.) 7(7), 735–736 (1998). 24. J. Buey, P. Espinet, H. S. Kitzerow, and J. Strauss, “Metallomesogens presenting blue phases in a glassy state and in metallomesogen/nematic mixtures,” Chem. Commun. (Camb.) 5(5), 441–442 (1999). 25. G. Heppke, D. Krüerke, C. Löhning, D. Lötzsch, D. Moro, M. Müller, and H. Sawade, “New chiral discotic triphenylene derivatives exhibiting a cholesteric blue phase and a ferroelectrically switchable columnar mesophase,” J. Mater. Chem. 10(12), 2657–2661 (2000). 26. Y. Haseba and T. Kuninobu, “Optically isotropic liquid crystal medium and optical device,” U.S. patent 7,722,783 B2 (2010). 27. 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). 28. P. R. Gerber, “Electro-optical effects of a small-pitch blue-phase system,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 116(3-4), 197–206 (1985). 29. H. F. Gleeson and H. J. Coles, “Dynamic properties of blue-phase mixtures,” Liq. Cryst. 5(3), 917–926 (1989). 30. L. Rao, J. Yan, S. T. Wu, S. Yamamoto, and Y. Haseba, “A large Kerr constant polymer-stabilized blue phase liquid crystal,” Appl. Phys. Lett. 98(8), 081109 (2011). 31. M. Wittek, N. Tanaka, M. Bremer, D. Pauluth, K. Tarumi, M. C. Wu, D. M. Song, and S. E. Lee, “New materials for polymer-stabilized blue phase,” SID Int. Symp. Digest Tech. Papers 42(1), 292–293 (2011). 32. J. Yan and S. T. Wu, “Effect of polymer concentration and composition on blue-phase liquid crystals,” J. Disp. Technol. 7(9), 490–493 (2011). 33. Y. Haseba and H. Kikuchi, “Optically isotropic chiral liquid crystals induced by polymer network and their electro-optical behavior,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 470(1), 1–9 (2007). 34. C. Y. Fan, C. T. Wang, T. H. Lin, F. C. Yu, T. H. Huang, C. Y. Liu, and N. Sugiura, “Hysteresis and residual birefringence free polymer-stabilized blue phase liquid crystal,” SID Int. Symp. Digest Tech. Papers 42(1), 213– 215 (2011). 35. L. Rao, Z. Ge, S. T. Wu, and S. H. Lee, “Low voltage blue-phase liquid crystal displays,” Appl. Phys. Lett. 95(23), 231101 (2009). 36. M. Jiao, Y. Li, and S. T. Wu, “Low voltage and high transmittance blue-phase liquid crystal displays with corrugated electrodes,” Appl. Phys. Lett. 96(1), 011102 (2010). 37. H. C. Cheng, J. Yan, T. Ishinabe, and S. T. Wu, “Vertical field switching for blue-phase liquid crystal devices,” Appl. Phys. Lett. 98(26), 261102 (2011). 38. L. Rao, J. Yan, S. T. Wu, Y. H. Chiu, H. Y. Chen, C. C. Liang, C. M. Wu, P. J. Hsieh, S. H. Liu, and K. L. Cheng, “Critical field for a hysteresis-free blue-phase liquid crystal device,” J. Disp. Technol. 7(12), 627–629 (2011). 39. 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). 40. S. Yokoyama, S. Mashiko, H. Kikuchi, K. Uchida, and T. Nagamura, “Laser emission from a polymer-stabilized liquid-crystalline blue phase,” Adv. Mater. (Deerfield Beach Fla.) 18(1), 48–51 (2006). 41. 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). 42. Y. H. Lin, H. S. Chen, H. C. Lin, Y. S. Tsou, H. K. Hsu, and W. Y. Li, “Polarizer-free and fast response microlens arrays using polymer-stabilized blue phase liquid crystals,” Appl. Phys. Lett. 96(11), 113505 (2010). 43. Y. Li and S. T. Wu, “Polarization independent adaptive microlens with a blue-phase liquid crystal,” Opt. Express 19(9), 8045–8050 (2011). 44. C. H. Lin, Y. Y. Wang, and C. W. Hsieh, “Polarization-independent and high-diffraction-efficiency Fresnel lenses based on blue phase liquid crystals,” Opt. Lett. 36(4), 502–504 (2011).

1. Introduction Blue phases (BPs) appear as a highly twisted chiral nematic liquid crystal (LC) cools down from isotropic phase. The most obvious feature of blue phases is selective reflection of incident light. In early investigations, blue phases exhibit blue colors; that is how blue phases got their name. Blue phases are not always blue, however; they may reflect light of other colors, depending on the chirality of the liquid crystals. Another important characteristic of blue phases is frustration. They only exist in such a narrow temperature range (0.5-2K) that further investigation of blue phases’ true nature was not pursued for over 80 years ever since the discovery by Reinitzer in 1888 [1]. Not until 1970s, the study of blue phases became popular and tremendous progresses had been made. It was discovered that blue phase is optically isotropic while exhibiting unusually strong optical activity. Based on this phenomenon, Saupe proposed that blue phases have cubic superstructures, which is the most foresighted proposal in the history of blue phase studies [2]. Later, much effort was devoted to explore the structure of blue phases, both experimentally and theoretically [3–10]. There are

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up to three types of blue phases: BPI, BPII, and BPIII, in the order of increasing temperature. While BPIII possesses the same symmetry as isotropic phase, BPI and BPII are comprised of double-twist cylinders arranged in cubic lattices, as shown in Figs. 1(a) and 1(c). Inside each cylinder, the LC director rotates spatially about any radius of the cylinder. These double-twist cylinders are then fitted into a three-dimensional structure. However, they cannot fill the full space without defects. Therefore, blue phase is a coexistence of double-twist cylinders and disclinations. Defects occur at the points where the cylinders are in contact, as illustrated in Figs. 1(b) and 1(d). These defects tend to make the structure less stable which in turn results in a narrow temperature range. Although the delicate structures of blue phases greatly attract scientific attentions, their applications are limited by the narrow temperature range.

Fig. 1. Cubic structures of BP-I [(a) and (b)] and BP-II [(c) and (d)] filled with double twist cylinders. The black lines in (b) and (d) represent disclination lines.

Several methods have been proposed to widen the temperature range [11–13]. The one that attracts the most interest is the polymer-stabilization method developed by Kikuchi et al. [12]. It is considered that the cross-linked polymer network is selectively concentrated in the disclination lines to stabilize the lattice structure of blue phases. As a result, the temperature range of BPLC has been extended to more than 60K, including room temperature (260-326K). Moreover, the polymer-stabilized blue phase exhibits fast electro-optic switching property. This approach opens a new gateway for display and photonic applications of blue phases. Another wide-temperature-range blue phase liquid crystal is reported by Coles et al. [13]. The large localized flexoelectricity in dimer liquid crystals is considered as the reason for the wide temperature range. In this tutorial, we give step-by-step introduction on how to prepare and characterize polymer-stabilized BPLCs for photonics and display applications. We emphasize on the optical and electro-optical properties of both pure blue phases and polymer-stabilized blue phases. The optimization of each component in a polymer-stabilized blue phase system is discussed from the application viewpoint. Several examples are given to demonstrate the promising features of the emerging BPLC devices. 2. Blue phase properties 2.1 Bragg Scattering It is truly remarkable that a liquid can have self-assembled periodic structures, like a photonic crystal. The periodic structure of blue phases results in selective Bragg reflections in the range of light spectrum. It is well-known that selective reflection occurs in cholesteric (chiral

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nematic) liquid crystal, the reflection wavelength λ = n⋅P, where n is the average refractive index and P is the pitch length. The reflection band is relatively broad with Δλ = Δn⋅P, where Δn is the birefringence of the liquid crystal host. Different from the chiral nematic phase, blue phases do not need any alignment layer and have several reflection wavelengths, corresponding to various crystal planes. The reflection wavelength can be expressed as:

λ=

2na h + k2 + l2
2

,

(1)

where n and a denote average refractive index and lattice constant of blue phases, and h, k, and l are the Miller indices. In BPI, the lattice constant corresponds to one pitch length and diffraction peaks appear at (110), (200), and (211), etc. The summation of Miller indices h + k + l is an even number. In BPII, the lattice constant corresponds to half a pitch length and diffraction peaks appear at (100), (110), etc. [14]. The pitch length of a BPLC is slightly different from that of chiral nematic phase. The reflection bandwidth is also much narrower than that of chiral nematic phase. Figure 2 shows the platelet textures of two BPLCs under crossed polarizers. The two photos exhibit different colors because of their different pitch lengths. The multiple colors in one photo correspond to the different crystal planes.

Fig. 2. BPLC platelet textures under polarizing optical microscope with different chiral concentrations.

2.2. Electric field effects At off-resonance wavelengths, blue phases are optically isotropic. When an electric field (E) is applied, liquid crystal molecules tend to align with the electric field if the dielectric anisotropy Δε > 0 (or perpendicular to the electric field if Δε < 0). As a result, birefringence is induced. In the low field region, the induced birefringence can be described by Kerr effect as:

∆nind = 2 , λ KE

(2)

where λ is the wavelength of incident light, K is Kerr constant which depends on wavelength, temperature and driving frequency [15,16]. Kerr effect exhibits a fast response time (80%) at low voltage (Von