1. Introduction

For illumination applications, a color temperature as well as the Commission Internationale de L'Eclairage (CIE) chromaticity value is essential [1–3]. Therefore, color-temperature tunable lighting devices have been studied in various white organic light-emitting diodes (OLEDs) [3–5] and semiconductor-based white light-emitting diodes (LEDs) [6–8]. Here, for such illuminant devices we demonstrate a white reflector with the color-temperature tunability using cholesteric liquid crystal (CLC) films and isotropic polymer films.

The CLC with a periodic helical structure of rodlike molecules is a chiral photonic crystal that exhibits a photonic band gap (PBG) for circularly polarized light with the same handedness as the CLC helix [9]. The spectral position of the PBG, called selective reflection, is determined by an optical pitch λ _p = nP of the CLC, where P is the structural pitch of the CLC helix and n is the average refractive index of the ordinary and extraordinary indices, n _o and n _e. Hence, a single-pitched CLC has a single PBG at a wavelength of the optical pitch with a 50% reflectance limit for normal incidence of unpolarized light. Recently, we demonstrated polarization-independent multicolored reflections or multiple PBGs from a bichiral liquid crystal (LC) film consisting of left-handed polymeric CLC (L-PCLC) and right-handed polymeric CLC (R-PCLC) films with a single pitch, and a poly(vinyl alcohol) (PVA) film [10].

2. Tunable reflection spectra of CLCs and bichiral LC films

In case of oblique incidence as shown in Fig. 1(a) , the specific spectral range with polarization-independent reflection can emerge in the middle of the selective reflection region of CLCs [11, 12]. In Fig. 1(a), the linear polarization angle ϕ of a polarizer or an analyzer is defined as an angle between the vertical direction and the axis of the polarizer or analyzer. The vertical direction is also parallel to the local director of the PCLC director defined by the rubbing direction at the alignment layer. The counter-clockwise (clockwise) rotation along the light propagation direction is defined as positive for the polarizer (analyzer).

 

Fig. 1 (a) Schematic illustrations of the measurements setup of an L-PCLC film for various linear polarization directions ϕ of obliquely incident and reflected lights. (b) Simulated reflection spectra of the L-PCLC film with n _o = 1.56, n _e = 1.78, λ _p = 800 nm, and d _L-PCLC = 20.0P using R- and L-CP incident lights (upper), LP incident and reflected lights with ϕ = 0° and 90° (middle), and ϕ = 45° and 135° (lower). The incident angle of various polarized lights is fixed as θ = 52°. (c) Schematic illustrations of the measurements setup of a bichiral LC film for various ϕ of normally incident and reflected lights. The bichiral LC film M ₃ = {ABABA} consisting of A layer (L-PCLC, PVA, and R-PCLC films) and B layer (PVA film). (d) Simulated reflection spectra of M ₃ with n _o = 1.56, n _e = 1.78, λ _p = 480 nm for L-PCLC, d _L-PCLC = 2.00P, λ _p = 610 nm for R-PCLC, d _R-PCLC = 2.00P, and d _PVA = 0.250 μm, using R- and L-CP incident lights (upper) and LP incident and reflected lights with ϕ = 0° and 90° (middle), and ϕ = 45° and 135° (lower).

Download Full Size | PDF

Figure 1(b) shows reflection spectra for various light polarizations at an incident angle θ = 52°, simulated numerically by means of a 4 × 4 Berreman matrix [13]. Here we used an L-PCLC film with n _o = 1.56, n _e = 1.78, λ _p = 800 nm, and a thickness d _L-PCLC = 20.0P. The reflection spectra of Fig. 1(b), which shift toward the shorter wavelength side with increasing θ, can be divided into three spectral regions. First, the central region from 525 nm to 550 nm shows ~95% reflectance for both R-CP and L-CP incident lights (without the analyzer), i.e., total reflection in contrast to selective reflection because of polarization-independent reflection. For linearly polarized (LP) lights with ϕ = 0° and 90° (both in polarizer and analyzer), the reflectance decreases to ~10%. Actually, Takezoe et al. reported that CLCs act as a 90 degrees rotator of the incident linear polarization for ϕ = 0° and 90° [12]. By contrast the reflectance for ϕ = 45° and 135° amounts to ~95%. Second, the shorter wavelength region from 480 nm to 525 nm shows high reflectance only for L-CP incident light, i.e., selective reflection, and becomes dominant in the reflectance for LP lights with ϕ = 90°. Third, the longer wavelength region from 550 nm to 610 nm also exhibits selective reflection, where high reflectance is mainly observed for LP lights with ϕ = 0° in addition to L-CP.

In this paper, we designed and fabricated a bichiral LC film consisting of an L-PCLC film with a blue-colored pitch, an R-PCLC film with a red-colored pitch, and an isotropic PVA film in between. Using the bichiral LC film various white reflections with both polarization-independent and selective reflections are demonstrated experimentally for normally incident light. By adjusting incident and reflected polarizations, we can tune color-temperatures of the white reflections from ~4000 K to ~10000 K.

Bichiral LC films were assembled by stacking a bichiral structure A with L-PCLC, PVA, and R-PCLC films and a B layer (a PVA film) sequentially, as shown in Fig. 1(c). The lower order sequences of the bichiral LC are M ₁ = {A} with 3 layers, M ₂ = {ABA} with 7 layers, M ₃ = {ABABA} with 11 layers, and M _k with (4k-1) layers. In the A structure, optical pitches and thicknesses of L-PCLC and R-PCLC films were so chosen that the reflection spectrum for L-CP incident light overlapped with that for R-CP light at the green-colored region. Thus, the polarization-independent reflection is expected to occur in this overlapped region. The PVA films used in A and B act as alignment layers for PCLC as well as defect layers. In order to investigate such bichiral LC films, we employed normally incident light with various polarizations. The polarizer of Fig. 1(c) adjusts the LP direction ϕ of incident and reflected lights simultaneously.

Figure 1(d) shows the reflection spectra simulated for M ₃ using the L-PCLC film with λ _p = 480 nm and d _L-PCLC = 2.00P, the R-PCLC film with λ _p = 610 nm and a thickness d _R-PCLC = 2.00P, and the PVA film with a refractive index n _PVA = 1.50 and a thickness d _PVA = 250 nm. Multiple PBGs or multicolored reflections are clearly observed in all reflection spectra of Fig. 1(d), whose formation mechanism and characteristics such as reflectance, spectral position, width of the multiple PBGs, and the number of each PBG can be understood from those of the bichiral and homochiral LC films with a single pitch [10, 14]. As in obliquely incident case of Fig. 1(b), the multiple PBGs in Fig. 1(d) for normally incident lights can be also divided into three spectral regions such as 1) the central region (510 nm - 550 nm) with the polarization-independent reflection, and 2) the shorter (405 nm - 510 nm) and 3) longer (550 nm - 750 nm) wavelength regions with selective reflections. Hence, in the central region of Fig. 1(d), M ₃ reflects ~90% of both L-CP and R-CP incident lights. For LP incident and reflected lights, Fig. 1(d) shows dominant red-colored (longer wavelength) reflections at ϕ = 0°, blue-colored (longer wavelength) reflections at ϕ = 90°, and green-colored reflections at ϕ = 45° and 135°. These behaviors are similar to those of the CLC for the obliquely incident light except for multiple PBGs or multicolored reflections covering full-colored regions.

3. Fabrication and characterization of bichiral LC films with tunable whitish reflections

Based on such optical properties of the bichiral LC films with two different pitches for normally incident light, we experimentally fabricated the color-temperature tunable CLC system, and measured multiple PBGs using various polarizations of incident and reflected lights. Two PCLC films (JX Nippon Oil & Energy Co.) of an L-PCLC film [14, 15] with λ _p = 480 nm (blue color) and an R-PCLC film [16, 17] with λ _p = 620 nm (red color) were fabricated by spin-coating, rubbing, and curing processes. The handedness of the PCLCs is determined by a type of a chiral dopant, and the pitch of the PCLCs is controlled by changing the content of the chiral dopant and the curing temperature. A PVA aqueous solution was spin-coated onto glass substrate coated with the PCLC film. After baking at 100 °C for 30 min, the PVA film was rubbed unidirectionally at room temperature [10, 14, 15]. The thickness of each film was controlled by adjusting the concentration of each solute and rotational speed of the spin coater. By repeating this procedure, we could obtain bichiral LC films with desired structures. Reflection and transmission spectra were measured by using a microscope spectrometer (TFM-120AFT-PCM, ORC) for various polarizations of normally incident lights. The thickness of each film was measured by using a stylus surface profiler (Sloan Technology, Dektak³ST).

Figure 2(a) shows experimental (solid line) and theoretical (dashed line) transmission spectra for M ₃ for L-CP and R-CP incident lights. These transmission spectra clearly indicate the presence of multiple PBGs from M ₃; low transmittance both for L-CP and R-CP incidence occurs from 520 nm to 560 nm, corresponding to the polarization-independent reflection region or the total reflection region of Fig. 1(d). The best fit of the experimental results to the simulated ones by the Berreman 4 × 4 matrix is not satisfactory, although the correspondence of multiple PBG positions can be seen. The disagreement is due to the difficulty in ideal film fabrication; i.e., owing to limited solubility of chiral dopants in the CLC, it is difficult to obtain PBGs located in the blue-colored or ultraviolet region. In the simulations, we assumed perfectly the same thickness and planar alignment of R- and L-PCLC films without any defects. In addition, the neglect of the refractive index dispersion used for the calculations can be attributed to the disagreements. From the fittings, we obtained physical parameters of the fabricated sample such as λ _P = 480 nm for L-PCLC, d _L-PCLC = 2.50P (0.718 μm), λ _P = 620 nm for R-PCLC, d _R-PCLC = 2.50P (0.928 μm), and d _PVA = 0.260 μm. The overall thickness of the sample M ₃ with 11 layers was 6.24 μm as calculated from the parameters, and is similar to the thickness 6.19 ± 0.27 μm experimentally measured. As expected in the simulations of Fig. 1(d) for LP-in and LP-out polarizations, we observed mainly reddish reflections (520 nm - 730 nm) for ϕ = 0°, bluish reflections (400 nm - 520 nm) for ϕ = 90°, and greenish reflections (520 nm - 560 nm) for ϕ = 45° and 135° among multiple PBGs covering a whole visible region, as shown in Fig. 2(b).

 

Fig. 2 (a) Measured (solid line) and simulated (dashed line) transmission spectra of M ₃ (black) for normal incidences of L-CP (upper) and R-CP lights. (b) Measured reflection spectra of M ₃ for linear polarization angles ϕ = 0° (upper, black solid line), 90° (upper, gray solid line), 45° (upper, black solid line), and 135° (lower, gray solid line) of normally incident and reflected LP lights.

Download Full Size | PDF

Next, we investigated the dependence of measured (Fig. 3(a) ) and simulated reflection spectra (Fig. 3(b)) on various linear polarization directions ϕ of normally incident and reflected lights. In the simulation of Fig. 3(b) for M ₃, we used same physical parameters as those of dashed lines in Fig. 2(a). Although some differences in reflectance and spectral positions of multiple PBGs in Fig. 3(a) and (b), reflection spectra of both cases show the spectral blue-shift of PBGs with high reflectance from ϕ = 0° to 90° and the spectral red-shift from ϕ = 90° to 180° .

 

Fig. 3 (Color online) (a) Measured and (b) simulated reflection spectra of the sample M ₃ for various polarization angles ϕ of normally incident and reflected lights. White dashed lines are eye guidelines. Colors represent the reflectance obtained from the bichiral LC film.

Download Full Size | PDF

The CIE chromaticity values [1, 2] were also calculated from the reflection spectra of M ₃ in Fig. 3(a) and (b), and all are close to a whitish color (Fig. 4 ) because of simultaneous multicolored reflections. Here, color changes from the spectral shifts of multiple PBGs can occur by increasing reflected or incident angle of lights [14]. The CIE coordinates shift from (0.39, 0.32) at ϕ = 0° to (0.27, 0.31) at ϕ = 90°, as shown by open circles in Fig. 4, in which the color temperature values are close to ~4000 K for ϕ = 0° and ~10000 K for ϕ = 90°. The circle symbol at (0.36, 0.36) for ϕ = 30° in Fig. 4 is close to a color temperature of ~4874 K, corresponding to a direct solar sunlight at noon [1]. For ϕ = 60°, the circle at (0.30, 0.35) is close to a color temperature of ~6500 K, corresponding to average daylight [1].

 

Fig. 4 Measured (circles) and simulated (squares) reflection colors in the CIE chromaticity diagram calculated from the reflection spectra of M ₃ in Fig. 3. Also shown are the coordinates corresponding to 2500 K, 4000 K, 5000 K, 6500 K, 7500 K, and 10000 K (triangles). The solid line indicates different color temperatures.

Download Full Size | PDF

4. Conclusion

In summary, we theoretically designed the bichiral LC films with two different pitches and handednesses and experimentally demonstrated color-temperature tunable multiple reflections by changing linear polarization directions of normally incident and reflected lights. Such bichiral LC films using L- and R-PCLC films and an isotropic PVA film were fabricated by an all-solution process. Our systems can extend practical applications of CLCs to illuminant devices such as OLEDs and LEDs.