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 [1–3]. 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 [4–6]. 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, 8]. 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.

Yu and Labes suggested a fluorescent display device using the photoluminescence (PL)-doped CLC in 1977 [9–11], but the relevant study was not continued due to a lack of advantages as compared to normal LCDs. Studies related to PL-LCs have mainly focused on the fluorescence characteristics and contrast ratio improvement between the bright and dark fluorescent states [12]. Recently, the PL-CLC has attracted much interest in other emerging fields that use polarized light emission [13, 14], and tunable reflection or lasing [15], but not in display applications.

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) . 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]. 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.

 

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.

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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) 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.

 

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.

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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.

The opposite process was observed during the homeotropic-to-planar transition process, in which a locally distorted helix is formed initially within a few milliseconds, which then reorients to form a stable planar alignment for the next several seconds [4–6]. This helical reorientation process occurs in all CLC cells and is accompanied by a large luminance change and color shift in the second time scale due to the shift of the Bragg reflection peak. Figure 3(B) shows the optical response of CLC and PL-CLC cells during the homeotropic-to-planar transition, which was induced by suddenly decreasing the applied AC voltage (60 Hz) from 12 V to 0 V. The reflectance of CLC and PL-CLC cells were measured with external D65 illumination, and the fluorescence of PL-CLC was measured with UV backlight illumination. All the optical response curves exhibit a sharp jump within 20 msec after the signal change, indicating the fast initial formation of distorted helices. The CLC reflectance curve exhibits a slow increase over the next several seconds due to the reorientation of the helices, whereas the fluorescent luminance decreases inversely because of the decoupling-to-coupling transition. Thus, the fluorescent light automatically compensates for the slow selective reflectance change during the helical reorientation; as a result, the reflectance of a PL-CLC cell has a much more stable luminance curve than a CLC cell. The color shift during the helical reorientation is more of a problem than the variation in luminance because the color shift is more clearly perceived in moving pictures, as indicated in the upper images in Fig. 3(A). In contrast, the PL-CLC cell has much less color change during the same transition process. Figure 3(C) shows the color trajectories of the CLC cell (blue closed data sets) and the PL-CLC cell (red open data set) during the homeotropic-to-planar transition in the u’v’ uniform color space [17]. While the u’ of the pure CLC cell exhibits a large change during the initial second, the u’ of the PL-CLC cell shows a much smaller change. The difference can be better seen in the video clip (see Fig. 3(A) (Media 1)).

 

Fig. 3 Complementary coupling between fluorescence and band-gap during dark to bright switching: (A) Photo images captured from the supporting video clip during the switching (Media 1). (B) CLC reflectance increases slowly due to the helical reorientation during homeotropic to planar transition, but the fluorescence exhibits the opposite trend. As a result, the PL-CLC reflectance exhibits rather stable luminance, though further optimization may be required. (C) Color trajectories during the transition in u'v' color space.

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Another well-known weakness of CLC devices is the shift in color with varying viewing directions [8], which is detailed in the upper images in Fig. 4(A) . 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, 19], 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], causing decoupling between the fluorescence and the band-gap.

 

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.

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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.