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Polymer/cholesteric liquid crystal composites (ChLCs) exhibit unique characteristics such as sub-millisecond response and deformation-free electro-optic tuning of the selective reflection band, and have thus been studied for the prospect of developing polarization-independent phase modulators. Here, we propose a diffusion-based method to replace the liquid crystal in the composite and hence improve the threshold characteristics. Because there is no rinsing process, the polymer network is retained almost perfectly, and can improve the threshold without deteriorating the light scattering or response characteristics. Simply substituting the chiral liquid crystal in the composite to an achiral liquid crystal without the chiral dopant was found to cause a 25% reduction in the threshold voltage, from 57 V to 41 V.
© 2016 Optical Society of America
1. Introduction
Cholesteric liquid crystals (ChLCs) are a phase in which the rod-like molecules self-organize into a helical structure [1]. The helical modulation of the dielectric tensor leads to the appearance of a selective reflection (SR) band, in which circularly polarized light with the same handedness as the helix is Bragg reflected [2]. In conventional ChLCs, electrical tuning of the SR band is not a trivial task, because the so-called focal conic texture appears above a certain threshold voltage, which causes light scattering and depolarization. To suppress the appearance of the focal conic texture, polymer/ChLC composites, in which a nano-sized polymer network is introduced to stabilize the helical structure, have been actively studied [3]. These composites enable SR band tuning without deforming the macroscopic helix structure (i.e., only through modulation of the effective refractive index), making them potentially useful for switchable optical devices such as reflectors [4], lasers [5,6], and displays [7–9]. A polarization-independent phase modulator has been also demonstrated by shifting the optical pitch to the ultraviolet [10], using the fact that the composite becomes quasi-isotropic at wavelengths longer than the SR band.
The ‘deformation-free’ switching behavior of polymer/ChLC composite, along with the sub-millisecond fast response attributed to the confinement of LC molecules in nano-sized domains [3], make these materials attractive for a variety of applications. A problem that needs to be solved for practical applications, however, is the high threshold voltage. Current approaches to improve the composite performance relies either on improving the properties of the host LC [11] and monomer [12], or on optimizing the fabrication procedure [13,14]. Although it has been shown that the polymer network structure and host LC properties greatly affect the electro-optic properties, there is an on-going search for the combination of materials that give a low enough driving voltage. On the other hand, an approach that has recently been attracting attention is the washout refill process, in which the unpolymerized LC in the polymer network is washed out by an organic solvent and another LC is refilled into the residual polymer network [15,16]. The refilled sample has been reported to exhibit new or improved properties, such as the reflection of both left- and right-circularly polarized light, and decrease in threshold voltage [17,18]. However, this method is only effective when the polymer network is strong enough that it is unaffected by the rinsing process; in some cases, an irreversible decrease in the transmittance and/or shift of the SR band have been reported [16,19]. Given the potential of the washout refill method, an improved method with a smaller effect on the optical quality is much in need.
Here, we propose a new fabrication process that can substitute the LC in the polymer/ChLC composite without damaging the polymer network. The “substitution method” is a process where the polymer/LC composite is replaced with another LC by gradual diffusion. Unlike the washout refill method, there is no rinsing by an organic solvent, and so damage to the polymer originating from solvation and solvent evaporation can be avoided. In this study, substitution is performed with the same host LC as the composite but without the chiral dopant, to decrease the electrostatic energy required to reorient the LC. A reduction in threshold voltage was observed without deterioration in the transmittance and response characteristics, showing the effectiveness of the approach.
2. Experimental procedure
A composite precursor was prepared by mixing a host nematic LC (HCCH, HTG-135200-100), photo-polymerizable LC monomer (Merck, RM257), chiral dopant (HCCH, R-5011) and photoinitiator (Ciba, Irgacure 819) at a weight ratio of 85: 7.5: 5.5: 2. The helical pitch length after polymerization, evaluated from Grandjean-Cano method, was approximately 136 nm [20]. Two substrates with ITO electrodes were coated with polyimide alignment layers, and rubbed unidirectionally. The overlapping area of the two substrates was approximately 1 × 1 cm2. A small drop of the precursor was placed on the center of one of the substrates using a capillary tube (internal diameter: 400 μm), and the two substrates were assembled using a photo-curable resin containing bead spacers (6 μm) (Fig. 1(a)). The droplet was heated to 100 °C and cooled to 10 °C to obtain a planar texture, after which it was photo-polymerized by irradiating UV light with intensity of 220 mW/cm2 for 40 min (Fig. 1(b)). After confirming that a droplet-shaped polymer/ChLC composite was formed, an additional host nematic LC (HTG-135200-100) was injected into the cell, and left for 1 day at a temperature of 100 °C to cause molecular diffusion (Fig. 1(c)).
Fig. 1 Schematic illustration of the samples (a) before polymerization, (b) after polymerization and (c) after substitution.
Sample observation was conducted using a polarizing optical microscope (POM; Nikon, Eclipse LV100-POL) with magnification of × 10, at a temperature of 30 °C (controlled using a thermal controller (Linkam, LTS350)). The transmittance spectrum of the polymer/ChLC composite was measured upon applying various square-wave voltages of 1 kHz frequency, using a spectrometer (Hamamatsu Photonics PMA-11, fiber core 1 mm) coupled to the POM via an objective lens with × 20 magnification. Because the sample has short pitch, it possesses an effective isotropic refractive index in the visible range, nLC = {(ne2 + no2)/2}1/2, where no and ne are the ordinary and extraordinary refractive indices. The isotropic refractive index is tunable by an electric field because ne decreases as the LC molecules reorient along the field. The transmittance spectra were analyzed according to the following equation to evaluate the voltage-dependent refractive index and scattering loss of the composite:
where Imax is the maximum transmittance, k is the coefficient of the scattering loss, R is the Fresnel reflection coefficient at the boundary between LC and ITO (assumed to have a refractive index of 2.0 [21,22]), ϕ = 2πnLCd/λ, nLC is the refractive index of the composite, d is the cell-gap, and λ is the wavelength. The dispersion of the refractive index and the scattering loss of the LC was taken into account via a Cauchy-type dependence.The response time was measured using a Mach-Zehnder interferometric microscope with a laser diode (532 nm) as the light source. The shift in interference fringes induced by the refractive index change was monitored as an intensity change on an avalanche photodiode (Thorlabs, APD110A), coupled to the system by an × 10 objective lens and optical fiber with 1 mm core diameter.
3. Results and discussion
Figure 2(a) shows the microscopic image of the sample after UV irradiation. The droplet at the center of the image is the photo-polymerized composite surrounded by air, and is isotropic and transparent, due to the short helical pitch. Figure 2(b) shows POM images after the substitution process. The textures of the droplet before and after substitution were almost the same, but between crossed polarizers, the surrounding nematic LC appeared pink, even when the rubbing direction was parallel to the polarizer orientation. The cross-polarized transmittance of the surrounding nematic LC was analyzed according to the following equation, which gives the transmittance of a nematic LC with arbitrary twist angle between crossed polarizers:
where φ is the twist angle of the surrounding nematic LC, u = πΔnd/λφ where Δn is the birefringence of the host LC, and d is the cell-gap. As shown in Fig. 2(c), the LC director twist was found to be 3π, providing evidence that the chiral dopant in the ChLC in the composite had diffused out of the composite by the substitution process. In this process, the complete removal of the chiral dopant is difficult because of the molecular diffusion. However, from the cell-gap (6 μm) and the LC director twist (3π), the helical pitch of the surrounding LC is estimated to be approximately 4000 nm, giving an approximate concentration of the chiral dopant after substitution as 5.5 × 136/4000 = 0.19 wt%, which is a significant reduction. Figure 2(d) shows the transmittance spectra of the composite before and after substitution, and Fig. 2(e) shows the transmittance spectra of the substituted composite for various incident linear polarization angles. The transmittance spectra before and after substitution were almost the same, and the polarization-independence was also retained after substitution, implying that there is no deformation of the polymer network. It is believed that the nematic LC forms a short-pitch helical structure inside composite even when the chiral dopant concentration is decreased, because of the strong anchoring imposed by the polymer network.Fig. 2 (a), (b) POM images after UV irradiation and after the substitution process. The area surrounded by dotted line is the electrode. (c) Transmittance spectrum of the surrounding nematic LC between crossed polarizers. (d) Transmittance spectra without polarizers before and after substitution. (e) Transmittance spectra of the substituted composite for various incident linear polarization (LP) angles.
Figure 3(a) shows the transmittance spectra of the sample before and after substitution, taken at various voltages. As the applied voltage increased above a threshold voltage, the interference fringe in the spectra blue-shifted, corresponding to the LC molecules in the composite reorienting along the electric field and causing a reduction of the refractive index. At intermediate voltages, light scattering also occurred, possibly originating from the refractive index mismatch between LC domains [23]. The voltage-dependent refractive index and phase shift at 532 nm before and after substitution are shown in Fig. 3(b). In the substituted sample, the threshold voltage reduced from 57 V to 41 V, enabling 2π phase modulation at 532 nm by a voltage of 123 V; this is a clear improvement compared to the unsubstituted sample, where only a phase change of 1.9π was achieved at 150 V. Figure 3(c) shows the voltage-dependent scattering loss at 532 nm. The scattering loss showed a peak immediately above the threshold voltage and decreased at high voltages. After substitution, the maximum loss decreased, and the peak width broadened. Figure 3(d) and (e) shows the hysteresis of the refractive index before and after substitution. The modulation in refractive indices at which the difference between the forward and backward-scan voltages were maximum were 0.019 and 0.052 before and after substitution process, respectively, and the voltage differences, ΔV, were 17 V and 16 V. Although the voltages are similar, the hysteresis of the substituted device is smaller compared to the original device, since the tuning range is larger. Overall, the substituted sample showed improved performance, with reduced threshold, optical loss, and hysteresis.
Fig. 3 (a) Transmittance spectra at various voltages without polarizers before (left) and after (right) substitution. (b) Voltage-dependent refractive index and phase shift of the composite at 532 nm. (c) Voltage-dependent scattering loss of the composite at 532 nm. (d), (e) Hysteresis in refractive index before and after substitution process.
To explain the reduction in threshold voltage, we consider the Frederiks transition of twisted nematic LC molecules confined in a small domain of size D in the composite. A one-dimensional model is assumed where anchoring exists at the two ends of the domain in a twisted configuration that matches the helical structure of the composite. The domain is lengthwise parallel to the cell-normal direction and hence the electric field. Assuming that the polymerized composite has a pitch P, the anchoring direction at the two boundaries differs by Φ = 2πD/P. The elastic energy density of the LC is given by
where K11, K22, K33 are the splay, twist and bend elastic constants, respectively, n is the director vector, and q0 = 2π/p0, where p0 is the helical pitch of the unpolymerized LC and depends on the chiral dopant concentration. Taking the z axis in the cell normal direction, the director n can be expressed as n = (cosθ cosϕ, cosθ sinϕ, sinθ), where θ and ϕ are the tilt and twist angles. By substituting n into Eq. (3), we obtainBy substituting the total free energy density, which is the sum of the elastic energy density and the electrostatic energy density, into the Euler-Lagrange equation and considering the case where θ << 1, the threshold voltage of the confined LC (Vth,D) can be approximated bywhere Δε is the dielectric anisotropy. In Eq. (5), Φ and D are constant, whereas p0 is initially P, but increases as the diffusion process proceeds. Therefore, the threshold voltage reduces as D/p0 decreases.Numerical simulations according to this model were performed using a commercial LC display (LCD) simulation software (Shintech, LCD Master). The physical properties of HTG-135200-100 and the domain size were obtained experimentally as follows: Δε = 72, K11 = 4.6 pN, K22 = 6.5 pN, K33 = 21 pN, and D = 50 nm. Δε was evaluated from capacitance measurements of host LC cells with perpendicular and parallel alignment, and K11 − K33 were evaluated by analyzing the voltage dependent transmittance of LC cells with anti-parallel and twisted alignments [24]. D was determined from scanning electron microscopy of the polymer network after rinsing the composite with super-critical CO2 [25].
Figure 4 shows the voltage-dependence of the maximum tilt angle within the domain (at z = D/2) for D/p0 values of 0 to 0.35. It is seen that the threshold voltage depends on D/p0, and is smaller for smaller D/p0, as expected from Eq. (5). Judging from the nematic twist angle after substitution, the helical pitch of the unpolymerized LC increases from 136 nm to approximately 4000 nm, corresponding to D/p0 of 0.37 and 0.01. A crude estimate of the threshold voltage of the composite (Vth), assuming the composite to be composed of a one-dimensional stack of nano-sized domains to thickness d, gives [26] Vth = Vth,D × d/D = 55 V (D/p0 = 0.37) and 40 V (D/p0 = 0.01), which are surprisingly close to the measured values of 57 V and 41 V. Although this model does not take into account the anchoring imposed from the side walls of the polymer network, the model can provide a semi-quantitative explanation for the reduction of threshold voltage.
For the response characteristics of the composite, we focus on the decay response, which is known to be independent of the applied field intensity and governed by material parameters [3]. Figure 5 shows the transient response curve upon cessation of 80 V voltage from the sample. The response curves can be well fitted by an exponential function, with relaxation times of 18 μs and 20 μs for the sample before and after substitution, respectively (corresponding to 0 – 90% response times of 41 μs and 46 μs). The fast response of polymer/ChLC composites has been attributed to the confinement of LC molecules in nano-sized domains, and the elasticity-mediated relaxation from the unwound state to the helical state. Considering that a comparable response time was obtained in the substituted sample with a smaller amount of chiral dopant, the influence of three-dimensional anchoring from the polymer network is believed to be the dominant factor contributing to the fast response, rather than the elasticity of the helical twist.
Fig. 5 Transient response curve upon cessation of 80 V voltage from the sample (a) before substitution and (b) after substitution.
4. Conclusion
In conclusion, we demonstrated a diffusion-based substitution method to replace the LC in a polymer/ChLC composite, and showed that the threshold voltage can be reduced without deteriorating the response or the scattering characteristics. Because the process hardly damages the polymer network, it is particularly effective for replacing the LC in composites with low monomer concentrations and thus a fragile network. On the other hand, the degree and speed of diffusion relies on the relative sizes of the composite and the substituting LC, and for this reason, it is better suited for single-electrode devices with small dimensions, such as LC lenses. With an engineering of the device structure, however, it may also be possible to apply the process to an active matrix device. Finally, although we used the same host LC as in the composite, only without the chiral dopant, further improvement may be possible by substituting with a different nematic LC. This new method should be useful for improving the characteristics of polymer/LC composites in general, and also for studying the basic properties of polymer/LC composites, since the host LC properties can be varied without changing the polymer network morphology.
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI (25630125); Photonics Advanced Research Center Program (PARC) in Osaka University.
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