Manipulation of Structural Colors in Liquid-Crystal Helical Structures

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Manipulation of Structural Colors in Liquid-Crystal Helical Structures Deformed by Surface Controls Chaeri Lim, Sooyeon Bae, Soon Moon Jeong, and Na Young Ha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03456 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Applied Materials & Interfaces

Manipulation of Structural Colors in Liquid-Crystal Helical Structures Deformed by Surface Controls Chaeri Lim,† Sooyeon Bae,† Soon Moon Jeong, ‡ and Na Young Ha*,†, § †

Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea

‡

Division of Nano and Energy Convergence Research, DGIST, Daegu 42988, Republic of Korea

§

Department of Physics, Ajou University, Suwon 16499, Republic of Korea

KEYWORDS: liquid crystals, photonic crystals, perfluoropolymer, surface anchoring energy, photonics.

ABSTRACT: Structural colors from cholesteric liquid crystals (CLCs) are manipulated by changing the only surface anchoring energy of an alignment layer. This behavior comes from the fact that weak surface energy of the perfluoropolymer induces the tilting of the cholesteric helix. Such deformed CLC structures with durability are successfully demonstrated without both any external field applications and additional solidification processes. In addition, electrical tunings of structural colors from the deformed CLCs occur at very low operating voltages, compared with those of conventional CLC structures. Based on easy and simple fabrication, high

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durability, electrical tunability at low operating voltages, and the unique optical characteristics, the new deformed CLC structure could lead to extension in applications of CLCs, including multifunctional sensors, displays, and lasers.

1. INTRODUCTION CLCs consisting of nematic liquid crystals (LCs) and chiral dopants form self-assembled helical structures of rodlike molecules. From its periodic helical structure, the CLC is a chiral photonic crystal that exhibits a structural color, corresponding to a photonic band gap (PBG), by selectively reflecting circularly polarized light of the same handedness as the helix.1 This unique optical property of the CLC has been attractive for various photonic applications such as biosensors,2,3 reflective displays, polarizers, color filters,4-19 lasing devices,20-25 and microlenses.26,27 Selective reflection of the CLC occurs at the wavelength equal to an optical pitch λp = np cosθ, where n is the average refractive index of the ordinary and extraordinary indices, no and ne, p is the physical pitch of the CLC helix, and θ is the angle between the propagation direction of incident light and the helix axis. In practice, the physical pitch of the CLC can be controlled by adjusting the concentration of a chiral dopant, temperature,1 pressure,2,3 and irradiation.4-6 The reflection color and the surface topography also have been tuned by varying the annealing conditions of cholesteric oligomer films with hybrid anchoring.26-29 The reflection bandwidth of the CLC is proportional to the difference of refractive indices ∆n = ne - no and is in the range of few tens of nanometers.


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Manipulation of such optical functionality in the CLC has been considered as a promising approach in terms of applications for control of white or multicolored light. To date, broadening of the PBG has been demonstrated by introducing a pitch gradient into the CLC gel7-10 and by stacking of the CLC layers with different pitches.9,11,12 Also, multiple PBGs, that is, optical pitches have been reported in multilayered films consisting of a single-pitched CLC layer and an isotropic polymer layer13,14 and in microfluidically-encapsulated CLCs.15 However, such fabrication processes to achieve broad or multiple PBGs have limitations with regards to their complex and delicate steps, additional solidification processes to obtain sustainable states, or necessary external stimuli of electric fields, temperature, and light. Thus, it is highly desirable to develop a simple process to durably tune the PBG from the CLC in the intended spectral region. In this study, we show the simultaneous manipulation of spectral position and width of its structural color in a single CLC mixture condition by employing an amorphous perfluoropolymer poly[perfluoro(4-vinyloxy-l-butene)] (PPFVB, called CYTOP) as an alignment layer. The PPFVB was chosen because of its strong hydrophobicity and high transmittance.30 Generally, the hydrophobic surface is preferred to obtain homeotropic alignment of liquid crystals due to its weak surface energy, indicating that the minimized energy of the system is possible when LC molecules are in contact with each other rather than the surface. Based on this, PPFVB films have provided the shock-free homeotropic alignment of smectic LCs,30 the planar alignment of nematic LCs with very weak anchoring energy,31 the micropatterns of nematic LCs,32 and various molecular orientations of the smectic and nematic LCs doped with low concentration of the chiral agent below ~ 5 wt.%.33 Here, we introduced the rubbed PPFVB alignment layer into the CLC cell with a short pitch, namely high concentration of the chiral dopant. Because of the helical structure, the molecular orientation on the surface cannot be


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compatible with that in nematic LCs. Optical characteristics of CLC cells with PPFVB alignment layers clearly showed different behaviors from those of planar textures with helical axes perpendicular to the cell surface,1 chiral smectic LCs with molecular tiltng,23 focal conic textures with axes oriented randomly,16,17 and uniform lying helix in the plane of the surface.17,34,35 From these results, it is shown that weak surface anchoring energy of the PPFVB film induces new deformation of the periodic CLC structure as the tilting of helical axes. Hence, by adjusting only surface anchoring energy with the CLC thickness, we present successful tunings of structural colors from the CLC molecules on the PPFVB surface without introductions of any polymer matrix or multilayered systems, and applications of external fields. Furthermore, we also examine electro-optic properties of CLC cells with rubbed PPFVB films and confirm hydrophobic effects of them to electrical tunings of selective reflection.

2. RESULTS AND DISCUSSION Figure 1a and b display schematic views of two CLC cell structures fabricated with a conventional alignment layer, that is, a polyimide (PI) film and the PPFVB layer, respectively. The PPFVB with the chemical structure of Figure 1c forms a highly transparent film on an indium time oxide (ITO) glass substrate due to its antireflection characteristic30 which originates from its low refractive index of 1.34 and high transmittance (Figure 1d). The rubbing direction of the PI- and the PPFVB-coated films is defined as the y-axis, with the x- and z-axes shown in Figure 1a. Only difference between two CLC cells of Figure 1a and b is the alignment layer. CLCs on the PI film have typical planar alignments with helical axes perpendicular to the cell surface or the alignment layer (Figure 1a). On the other hand, weak surface anchoring energy of


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the PPFVB film with its strongly hydrophobic surface can induce deformations of periodic helical structures of CLCs such as the tilting of the helical axes (Figure 1d) or molecular tilting in the helical structures, that is, a chiral smectic LC structure.23 To investigate these deformations of CLC structures on the PPFVB surface, we measured optical transmission spectra of two kinds of CLC cells with different alignment layers (Figure 1e and f). All transmission spectra for normal incidence of left-circularly polarized (L-CP) light show clear PBGs, corresponding to selective reflections of left-handed CLC structures. The cell thickness dCLC were changed from 2.1 µm to 12.7 µm for CLC cells with PI layers (Figure 1e), and from 2.5 µm to 12.9 µm for those with PPFVB layers (Figure 1f), respectively. An increase in the thickness dCLC corresponds to a decrease of the PPFVB surface effect, because the surface anchoring energy strength of LCs usually depends on the cell thickness.36,37 In Figure 1e, the transmittance of transmission dips decreases and the bandwidth of each PBG becomes narrower with increasing thickness of the CLC on the PI film, while the optical pitch of λp = 662 nm remains essentially the same over the entire range of the cell thickness. This behavior is similar to that of a general PBG material and corresponds to an inherent optical property of the planar alignment of CLCs (Figure 1a).1,13 In CLC structures on PPFVB alignment layers (Figure 1f), we clearly observed transmission spectra with different spectral positions and widths of PBGs by changing the cell thickness. Here, it is noted that all optical pitches are smaller than λp = 662 nm for planar-aligned CLC cells (Figure 1e), indicating the tilting of cholesteric helix to the propagation direction of incident light.1,14 We also found that an increase in the cell thickness results in the spectral shift of the transmission dip positions from λp = 555 nm (dCLC = 2.5 µm) to λp = 632 nm (dCLC = 12.9 µm).


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Hence, the spectral position of the PBGs shifts gradually to longer wavelengths, as large as ~ 77 nm, with increasing the cell thickness. This behavior comes from the reduction of the PPFVB surface effect by increasing thickness of the CLC layer. As shown in Figure S1, the tilting angle

θ of the cholesteric helix on the rubbed PPFVB, calculated from the equation of the optical pitch λp, decreases from 33.0° to 17.3° with increasing the thickness of CLCs from dCLC = 2.5 µm to dCLC = 12.9 µm. This result indicates that we can control the tilt angle of the CLC helix on the PPFVB surface by adjusting the cell thickness or resultant surface anchoring energy of the alignment layer. In the comparison with PBGs of planar-aligned CLCs (Figure 1e), PBG widths of CLCs on PPFVB films become broader and transmittance of PBGs show almost same behaviors. For example, the full width at half maximum increased by 1.8 times from 65 nm (dCLC = 6.6 µm in Figure 1e) to 116 nm (dCLC = 7.1 µm in Figure 1f). The small deviation in the spatial distribution of the tilting angle leads to the slight blue-shift and broadening of the PBG,18,19 while the focal conic texture with helical axes oriented randomly throughout the cell shows low optical transparency over the whole spectral region due to the scattering of light.16,17 The CLC cell with the PPFVB alignment layer represents both high transmittance above ~ 50 % except for the spectral regions of PBGs and large spectral broadening of ~ 1.8 times. We could not directly observe cross-sectional structures of deformed CLC structures because those are not in solid states. Thus, it is difficult to completely rule out contribution of the unwinding of cholesteric helices10 to such spectral broadening of the PBGs. Based on this, we suppose that this spectral broadening of the PBG originates from small differences in tilting angles of the CLC helices associated with scattering and unwinding of helical structure with reflection.


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Next, the textures of the deformed CLC structure on the PPFVB surface (dCLC = 7.1 µm) were examined using crossed-polarizer microscopy. The upper-left, upper-right, lower-left, and lower-right micrographs of Figure 2a correspond to textures measured at rotation angles φ of 0°, 45°, 90°, and 135°, respectively, where the rotation angle is the angle between the polarization direction of the polarizer and the rubbing direction of the PPFVB film (y-axis). No Grandjean planar textures but also the dark and bright states, indicating the CLC textures of the uniform lying helices, were not seen in the CLCs on the PPFVB films at any rotation angles φ. In all textures of Figure 2a, domains aligned parallel to the rubbing direction (y-axis) are observed and are clearly different from those of the focal conic textures with random orientations of cholesteric helices. Hence, CLCs on PPFVB films form two-dimensional domain structures with tilting of the cholesteric helices that are oriented in the directions of the rubbing. Because the nematic LC director usually tilts up in the direction of rubbing,38 two tilting angles of θn and 180 º - θn for the rubbing direction (y-axis) are possible as shown in Figure 2b. Here, θn is the angle between the CLC helical axis and the normal direction of the cell surface (z-axis). We also investigated the viewing angles of selective reflections from deformed CLC structures on PPFVB surfaces. For oblique incidence of L-CP light, transmission spectra from CLC cells with PI (dCLC = 6.6 µm, Figure 3a) and PPFVB (dCLC = 7.1 µm, Figure 3b) films were measured, respectively. Here, the angle α is defined as the angle between the propagation direction of incident light (z-axis) and the normal direction of the cell surface. An increase in the angle α from 0º to 22º results in spectral blue-shift of PBGs from planar aligned CLCs on the PI layer (Figure 3a). However, PBGs from deformed CLCs on the PPFVB surface go through a continuous red-shift, as large as ~ 24 nm (square symbol in Figure 3b), while transmittance in the


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wavelength region shorter than the optical pitch decreases with increasing the angle α (circle symbol in Figure 3b). This behavior results from the fact that the tilting angle θ of the cholesteric helix for the direction of incident light (z-axis) increases from the angle θn to θn + α in some cholesteric domains (left, Figure 3c), and simultaneously decreases from θn to θn - α in others (right, Figure 3c). In other words, some CLC domains with increasing θ exhibit decreases of transmittance in the shorter wavelength region than λp owing to light diffractions like those of grating or lying helix structures, while the spectral positions of reflection bands move to longer wavelength region in others with decreasing the tilting angle θ. We could also find similar behaviors in transmission spectra from deformed CLC structures with different thickness (Figure S2). Thus, the aligned cholesteric domains with tilting helices along the rubbing direction are confirmed. Figure 3d displays photographs of structural reflection colors from two kinds of CLC cells with the PPFVB (left) and the PI (right) layers for obliquely incidence of white light. Both of CLC cells show spectral blue-shift with increasing the angle α (from upper to lower in Figure 3d). Due to tilting of CLC helices on the PPFVB surface, reflection colors from deformed CLC structures are located at shorter wavelength regions than those of planar aligned CLCs on the PI films. As expected in transmission spectra of Figure 3b, the hydrophobic PPFVB surface with weak anchoring energy provides low incident and viewing angle dependence of reflection colors at small angle α below θn. From Figure S1, the calculated angle θn for the CLC cell of Figure 3d is 26.6°. However, for large angle α, we observed spectral blue-shift of selective reflection from the CLC on the PPFVB surface. Similar results with these spectral shift and broadening of PBGs have been observed in transmission spectra of the electrical switching modes of CLC gels10 and


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the transient state with two-dimensional periodic undulations of helical structures induced by electric field applications, that is, the Helfrich deformation.16,39-41 It is noted that deformed CLC structures on PPFVB surfaces are durable without additional solidification process and can be obtained from only surface controls without applications of external fields. In the present system, it was difficult to determine a threshold value of surface energy for cholesteric deformations on the PPFVB surface. When we consider different kinds of alignment layers with relatively weak surface anchoring energies in the cells, we may obtain the threshold value for given CLC molecules. Next, the electro-optical performance of CLCs on the PPFVB alignment layer was investigated by applying an external field (1 kHz, rectangular voltage) across two ITO electrodes of the cells. For planar-aligned CLCs on PI films, spectral positions of transmission dips (selective reflection) begin to shift at ~ 10 V for the cell with dCLC = 6.6 µm (Figure 4a) and ~ 14 V for that with dCLC = 12.7 µm (Figure 4b), respectively. Then, PBGs disappear beyond ~ 18 V in the former and ~ 34 V in the latter for normally incident L-CP light. Therefore, the increase in the cell thickness results in increasing the applied voltage to obtain such tunings of selective reflections. This thickness dependence of electrical tuning behaviors can be found in general CLC cells.38 As shown in Figure 4c and d, deformed CLC structures on PPFVB surfaces show clear differences in electro-optic performances, compared with those of planar textures of CLCs on PI films. The spectral shift of selective reflection occurs at very low voltages of ~ 1 V for the cell with dCLC = 7.1 µm and ~ 3 V for that with dCLC = 11.9 µm, respectively. From ~ 10 V (Figure 4c) and ~ 12 V (Figure 4d) on, PBGs vanish for each deformed CLC structure with different thickness. We found that the CLC structure on the PPFVB surface has low operating voltage and weak thickness dependence for electrical tuning of selective reflection, differing


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from those of the conventional CLC. These results originate from weak surface anchoring energy of the PPFVB films.

3. CONCLUSION To summarize, we investigated the CLC alignment on PPFVB films with high transmittance, strong hydrophobicity, and weak surface anchoring. From the optical measurements for CLC cells with PPFVB films, it was proven that the PPFVB layer with weak surface energy induced the tilting of the cholesteric helix. Moreover, deformed CLC structures on the PPFVB surface were durable without applying any external fields and show very low operating voltage for tuning of selective reflections. From these results, it is shown that the new CLC combined with the PPFVB layer is an important system, which provides possibilities in photonic applications such as active components of reflective display devices, photonic sensors, and tunable lasers.

4. EXPERIMENTAL METHODS 4.1 Sample preparation: The PPFVB alignment layer was fabricated on an indium tin oxide (ITO) glass substrate by spin-coating a PPFVB solution (CTX-809A, Asahi Glass Co.) and annealing at 100 °C for 30 min. The thickness of the PPFVB film was 75.6 ± 6.2 nm, as determined by a surface profiler (P-10, KLS-Tencor). A pair of PPFVB-coated substrates was rubbed unidirectionally at room temperature, stacked face to face with a spacer in between, and sealed to prepare an empty cell. Then, a left-handed CLC mixture consisting of a nematic LC (ZLI-2293, Merck) and a chiral dopant (S-811, Merck) was introduced into the cell by capillary


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action at the temperature of the isotropic phase. The fabricated CLC cell was then cooled down to room temperature. In all experiments, we fixed the concentration of the chiral dopant in the CLC mixture as 23.05 wt. % to achieve a single physical pitch of the cholesteric helix. The average refractive index n of the CLC mixture is 1.57 from ne = 1.63 and no = 1.50 of ZLI2293 used.25 As a reference, we prepared another cell with a conventional alignment layer, i.e., a polyimide (PI; AL22620, JSR) film instead of the PPFVB film. 4.2 Characterization: Optical transmittance measurements were performed with a spectrometer (V-670, Jasco) for normal and oblique incidence of light, respectively. From the equation of the optical pitch λp, we calculated the tilting angle θ of Figure S1, where the average refractive index is taken to be n = 1.57 and the physical pitch is fixed as p = 423 nm, obtained from the optical pitch λp = 662 nm of the CLC cell with the PI alignment layer. Textures of CLCs contained within cells were examined by using a polarizing microscope (LV100POL, Nikon).

ASSOCIATED CONTENT SUPPORTING INFOMATION The cell thickness dependence of optical pitches and calculated tilt angles; Transmission spectra from deformed CLC structures with different thickness AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the NRF funded by the Ministry of Science, ICT & Future Planning (2012R1A1A1014948) and the Ministry of Education (2015R1D1A1A01058747) and “Human Resources Program in Energy Technology” of the KETEP, granted financial resource from the Ministry of Trade, Industry & Energy (20164030201380). REFERENCES (1)

De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Oxford University

Press: New York, 1993. (2)

Woltman, S. J.; Jay, G. D.; Crawford, G. P. Liquid-Crystal Materials Find a New Order

Biomedical Applications. Nature Mater. 2007, 6, 929 – 938. (3)

Kamita, G.; Frka-Petesic, B.; Allard, A.; Dargaud, M.; King, K.; Dumali, A. G.;

Vignolini; S.

Biocompatible and Sustainable Optical Strain Sensors for Large-Area

Applications. Adv. Optical Mater. 2016, 4, 1950–1954. (4)

Qin, L.; Gu, W.; Wei, J.; Yu, Y. Piecewise Phototuning of Self-Organized Helical

Superstructures. Adv. Mater. DOI: 10.1002/adma.201704941 (5)

Li, L.; Urbas, A.; Li, Q. Reversible Light-Directed Red, Green, and Blue Reflection with

Thermal Stability Enabled by a Self-Organized Helical Superstructure. J. Am. Chem. Soc. 2012, 134, 9573 – 9576. (6)

Bisoyi, H. K.; Li, Q. Light-Driven Liquid Crystalline Materials: From Photo-Induced

Phase Transition and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089 – 15166.


12

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7)

Broer, D. J.; Lub, J.; Mol G. N. Wide-Band Reflective Polarizers from Cholesteric

Polymer Networks with a Pitch Gradient. Nature 1995, 378, 467 – 469. (8)

Relaix S.; Bourgerette, C.; Mitov. M. Broadband Reflective Liquid Crystalline Gels due

to the Ultraviolet Light Screening made by the Liquid Crystal. Appl. Phys. Lett. 2006, 89, 251907. (9)

Mitov, M. Cholesteric Liquid Crystals with a Broad Light Reflection Band. Adv. Mater.

2012, 24, 6260 – 6276. (10)

Hikmet, R. A. M.; Kemperman, H. Electrical Switchable Mirrors and Optical

Components Made from Liquid-Crystal Gels. Nature 1998, 392, 476 – 479. (11)

Makow, D. M.; Sanders, C. L. Additive Colour Properties and Colour Gamut of

Cholesteric Liquid Crystals. Nature 1978, 276, 48 – 50. (12)

Jeong, S. M.; Ohtsuka, Y.; Ha, N. Y.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.;

Nishimura, S.; Suzaki, G. Highly Circularly Polarized Electroluminescence from Organic LightEmitting Diodes with Wide-Band Reflective Polymeric Cholesteric Liquid Crystal Films. Appl. Phys. Lett. 2007, 90, 211106. (13)

Ha, N. Y.; Ohtsuka, Y.; Jeong, S. M.; Nishimura S.; Suzaki G.; Takanishi, Y.; Ishikawa,

K.; Takezoe, H. Fabrication of a Simultaneous Red–Green–Blue Reflector Using Single-Pitched Cholesteric Liquid Crystals. Nature Mater. 2008, 7, 43 – 47. (14)

Ha, N. Y.; Jeong, S. M.; Nishimura S.; Takezoe, H. Color-Temperature Tunable White

Reflector Using Bichiral Liquid Crystal Films. Opt. Express 2010, 18, 26339 – 26344. (15)

Lee, S. S.; Seo, H. J.; Kim, Y. H.; Kim, S. –H. Structural Color Palettes of Core–Shell

Photonic Ink Capsules Containing Cholesteric Liquid Crystals. Adv. Mater. 2017, 29, 1606894.


13


(16)

Page 14 of 21

Yang, D.‐K.; West, J. L.; Chien L.‐C. Chien, and J. W. Doane, “Control of Reflectivity

and Bistability in Displays using Cholesteric Liquid Crystals,” J. Appl. Phys. 1994, 76, 13311333. (17)

Wang, C.-T.; Wang, W.-Y.; Lin, T.-H. A Stable and Switchable Uniform Lying Helix

Structure in Cholesteric Liquid Crystals. Appl. Phys. Lett. 2011, 99, 041108. (18)

Lu, S.-Y.; Chien, L.-C. A Polymer-Stabilized Single-Layer Color Cholesteric Liquid

Crystal Display with Anisotropic Reflection. Appl. Phys. Lett. 2007, 91, 131119. (19)

Hsiao, Y. –C.; Timofeev, I. V.; Zyranov, V. Y.; Lee, W. Hybrid Anchoring for a Color-

Reflective Dual-Frequency Cholesteric Liquid Crystal Device Switched by Low Voltages. Opt. Express 2015, 5, 251394. (20)

ll’chishin, I. P.; Tikhonov, E. A.; Shpak, M. T., Doroshkin, A. A. Stimulated Emission

Lasing by Organic Dyes in a Nematic Liquid Crystal. Pis’ma Zh. Eksp. Teor. Fiz. 1976, 24, 336339. (21)

ll'chishin I. P.; Tikhonov E. A.; Tishchenko V. G.; Shpak M. T. Generation of a Tunable

Radiation by Impurity Cholesteric Liquid Crystals. Pis’ma Zh. Eksp. Teor. Fiz. 1980, 32, 27-30. (22)

Kopp, V. I.; Fan, B.; Vithana, H. K. M.; Genack, A. Z. Low-Threshold Lasing at the

Edge of a Photonic Stop Band in Cholesteric Liquid Crystals. Opt. Lett. 1998, 23, 1707-1709. (23)

Ozaki, M.; Kasano, M.; Ganzke, D.; Haase, W.; Yoshino, K. Mirrorless Lasing in a Dye-

Doped Ferroelectric Liquid Crystal. Adv. Mater. 2002, 14, 306 – 309. (24)

Coles, H.; Morris, S. Liquid-Crystal Lasers. Nat. Photonics 2010, 4, 676 – 685.

(25)

Ha, N. Y.; Jeong, S. M.; Nishimura S.; Suzaki G.; Ishikawa, K.; Takezoe H.

Simultaneous Red, Green, and Blue Lasing Emissions in a Single-Pitched Cholesteric LiquidCrystal System. Adv. Mater. 2008, 20, 2503 – 2507.


14

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Bayon, C.; Agez, G.; Mitov, M. Wavelength-Tunable Light Shaping with Cholesteric

Liquid Crystal Microlenses. Lab. Chip. 2014, 14, 2063 - 2071. (27)

Bayon, C.; Agez, G.; Mitov, M. Size-Effect of Oligomeric Cholesteric Liquid-Crystal

Microlenses on the Optical Specifications. Opt. Lett. 2015, 40, 4763 - 4766. (28)

Agez, G.; Bitar R.; Mitov, M. Color Selectivity Lent to a Cholesteric Liquid Crystal by

Monitoring Interface-Induced Deformations. Soft Matter 2011, 7, 2841 - 2847. (29)

Sinitsyna, O. V.; Bobrovsky, A. Y.; Meshkov, G. B.; Yaminsky, I. V.; Shibaev, V. P.

Surface Relief Changes in Cholesteric Cyclosiloxane Oligomer Films at Different Temperatures. J. Phys. Chem. B 2015, 119, 12708 - 12713. (30)

Jeong, S. M.; Kim, J. K.; Shimbo Y.; Araoka, F.; Dhara, S.; Ha, N. Y.; Ishikawa, K.;

Takezoe H. Perfluoropolymer Surface for Shock-Free Homeotropic Alignment of Smectic Liquid Crystals. Adv. Mater. 2010, 22, 34 – 38. (31)

Kumar, T. A.; Takezoe H.; Dhara, S. Perfluoropolymer as Planar Alignment Layer for

Liquid Crystal Mixtures. Jpn. J. Appl. Phys. 2011, 50, 040203. (32)

Sasaki Y.; Ueda M.; Le K. V.; Amano R.; Sakane, S.; Fujii S.; Araoka F.; Orihara H.,

Polymer-Stabilized Micropixelated Liquid Crystals with Tunable Optical Properties Fabricated by Double Templating. Adv. Mater. 2017, 29, 1703054. (33)

Kumar, T. A.; Le, K. V.; Aya, S.; Kang, S.; Araoka, F.; Ishikawa, K.; Dhara, S.; Takezoe

H. Anchoring Transition in a Nematic Liquid Crystal Doped with Chiral Agents. Phase Transit. 2012, 85, 888 – 899. (34)

Zheng, Z.-g.; Li, Y.; Bisoyi, H. K.; Wang, L.; Bunning, T. J.; Li, Q. Three-Dimensional

Control of the Helical Axis of a Chiral Nematic Liquid Crystal by Light. Nature 2016, 531, 352 – 356.


15


(35)

Page 16 of 21

Zhang, L.; Wang L.; Hiremath U. S.; Bioyi H. K.; Nair G. G.; Yelamaggad, C. V.; Urbas

A. M.; Bunning T. J.; Li Q. Dynamic Orthogonal Swiching of a Thermoresponsive SelfOrganized Helical Superstructure. Adv. Mater. 2017, 29, 1700676. (36)

AtaalLa, R. M. S.; Barbero, G.; Komitov, L. Thickness Dependence of the Anchoring

Energy of a Nematic Cell. J. Appl. Phys. 2013, 113, 164501. (37)

Bryan-Brown, G. P.; Wood, E.L.; Sage, I. C. Weak Surface Anchoring of Liquid

Crystals. Nature 1999, 399, 338-340. (38)

Yeh, P.; Gu, C. Optics of Liquid Crystal Displays, John Wiley & Sons, Inc.: New York,

1999. (39)

Helfrich, W. Deformation of Cholesteric Liquid Crystals with Low Threshold Voltage.

Appl. Phys. Lett. 1970, 17, 531-532. (40)

Nemati, H.; Yang, D. –K.; Cheng, K. -L.; Liang, C. –C.; Shiu, J. W.; Tsai, C. –C.; Zola,

R. S. Zola, Effect of Surface Alignment Layer and Polymer Network on the Helfrich Deformation in Cholesteric Liquid Crystals. J. Appl. Phys. 2012, 112, 124513. (41)

Inoue, Y.; Moritake, H. Dynamic Control of Colorful Reflection toward Practical

Cholesteric Liquid Crystal Displays. Opt. Express 2016, 24, 272679.


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Figure 1. Schematic illustrations of CLC cell structures with (a) the PI and (b) the PPFVB alignment layers. The rubbing direction of the alignment layer is defined as the y-axis, and the xaxis is perpendicular to the y-axis in the plane of the substrate. (c) Chemical structure of the alignment layer, PPFVB, used in this study. (d) Transmission spectra of the ITO glass and the PPFVB-coated ITO glass for normal incidence of unpolarized light. Transmission spectra of CLC cells with (e) the PI and (f) the PPFVB films as a function of cell thickness. The normally incident light of L-CP light is parallel to the z-axis.


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Figure 2. (a) Crossed-polarized micrographs of the CLC cell with the PPFVB layer for various rotation angles of φ = 0°, 45°, 90°, and 135°, where φ is the angle between the polarization direction of the polarizer and the rubbing direction of the PPFVB film. Arrow with R refers to the direction of the rubbing. (b) Schematic representation of the tilting behaviors of cholesteric helices on the PPFVB film. The normal direction of the cell surface corresponds to the z-axis.


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Figure 3. Transmission spectra of the CLC structures on (a) the PI (dCLC = 6.6 µm) and (b) the PPFVB films (dCLC = 7.1 µm) with different angles α, corresponding to the angle between the propagation direction of incident L-CP light and the normal direction of the cell surface. (c) Schematic illustrations of cholesteric domains on the PPFVB layers with (left) increase of the angle θ from the tilting angle for normally incident light θn to θn + α and (right) decrease of them from θn to θn - α. (d) Photographs of selective reflections of the PPFVB- (left) and the PI- (right) coated cells at various angles α.


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Figure 4. Transmission spectra as a function of applied voltage for CLC cells with the PI ((a) dCLC = 6.6 µm, (b) dCLC = 12.7 µm) and the PPFVB ((c) dCLC = 7.1 µm, (d) dCLC = 11.9 µm) alignment layers. The L-CP light is normally incident on cell surfaces. Colors represent the transmittance obtained from the CLC cell.


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Table of Contents


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Manipulation of Structural Colors in Liquid-Crystal Helical Structures

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