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Abstract
Bistable technology has played a vital role in the development of optical elements. Bistable technology enables zero power consumption unless the optical state needs to be changed. Salt-doped cholesteric liquid crystals (SDCLCs) have been implemented as light valves. However, the orientation mechanism of SDCLCs under different operating conditions has not been elucidated in detail. Herein, the disturbance and relaxation of SDCLCs were comprehensively investigated based on the interactions between the electrohydrodynamic and dielectric effects under different voltages, frequencies, and cell gaps. By controlling the balance between the electrohydrodynamic and dielectric effects, the bistable optical performance of SDCLC devices can be optimized for practical applications.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light valves have been widely studied with the aim of controlling the light transmission more quickly, precisely, and conveniently. Many types of light valves, such as micromirror devices (DMDs) [1], grating light valves (GLVs) [2] and liquid crystal devices (LCDs) [3], have been developed. Liquid crystals (LCs) have excellent electro-optical properties, allowing easy modulation of the light by an external field, and have been widely used in flat-panel displays. In addition to polarizers, which allow birefringence-induced intensity control, scattering-type LCDs are excellent candidates for light valves [4].
Scattering control can be used not only for optical switches but also for privacy control, projection, and smart window applications. Scattering control is usually achieved by controlling the mismatch between the refractive indices of the LC and the polymer structure. For example, polymer-dispersed liquid crystals [5,6], polymer network liquid crystals [7], and polymer-stabilized cholesteric textures [8–10] can be switched between transparent and light-scattering modes by applying an external voltage. In addition to scattering caused by the dielectric effect of LCs, light scattering induced by the electrohydrodynamic (EHD) effect has been discussed extensively [11–13]. The orientational instability of ionic impurity-doped LCs, also referred to as dynamic scattering (DS), can be reduced by applying a direct current field or a low-frequency alternating current field [14,15].
In recent years, power consumption in electronic devices has become increasingly important. Compared with common monostable electronic devices, electronic devices that can switch between multiple stable states, also referred to as multistable devices, are highly desirable. Multistable devices enable lower power consumption, especially when the devices do not require to switch states frequently. For example, electrowetting displays [16] are based on controlling the shape of the water–oil interface using an external electric field; if no voltage is applied, the water droplet remains in a bistable condition. Surface-controlled nematic bistable technologies [17] have also been proposed in the past, wherein the texture can be altered by a strong shear flow (backflow) generated by applying a voltage. Cholesteric liquid crystals (CLCs), which are versatile bistable materials, can switch between two states without the continuous application of an external field: the well-aligned planar (P) state and the scattering focal conic (FC) state [18,19]. The multistability of light states allows for energy-saving smart windows [20], reflective electronic paper displays [21,22], and transparent display applications [23]. In the conventional CLC driving method [24], the switchability between the two states is highly dependent on the alignment, pitch, and cell gap, causing low tolerance in the fabrication conditions.
By integrating the dielectric and EHD effects with negative dielectric anisotropy through the addition of ionic impurities to CLCs, CLCs can be quickly switched between the scattering FC state and the highly ordered P state through dual-frequency driving [25–27]. The helical axis of the CLC is well aligned and perpendicular to the substrate when a high-frequency voltage is applied. When a low-frequency voltage is applied, the instability induced by the EHD effect disturbs the LC and randomizes the orientation of the helical axis. As such, CLCs can be bistable between transparent and scattering states and yield desirable electro-optical performance. Moheghi et al. developed and revealed the advantages of salt-doped cholesteric liquid crystals (SDCLCs) [25].
In this study, the influence of the EHD and dielectric effects on ion-doped CLCs with negative dielectric anisotropy was investigated. In addition, the operating mechanism was analyzed to study the orientation of disturbance and relaxation in SDCLCs. The EHD and dielectric effects at different frequencies, applied voltages, and cell gaps were investigated. Based on these results, the optical performance of bistable CLCs can be further optimized.
2. Material preparation, device fabrication and measurement
The CLCs were composed of a 96 wt.% negative dielectric anisotropy nematic LC DNM-9528 (ne ≈ 1.5792, no ≈ 1.4808, Δε = ε// − ε⊥ ≈ −4.8, γ1 = 96 mPa*s, K11 = 12.4 pN, and K33 = 12.8 pN) and 4 wt.% chiral agent R-5011 (HCCH). The pitch of the CLCs were approximately 250 nm which can avoid reflection in visible wavelength. The CLCs were doped with 0.35 wt.% tetrabutylammonium tetrafluoroborate (Sigma Aldrich). The mixture was then sandwiched between two indium tin oxide glass substrates without alignment layer.
A light source He–Ne laser (632 nm) and photodetector were connected to an oscilloscope (Teledyne LeCroy HDO4034) to measure the variation of the transmittance before and after removing the voltage. The measurement setup is in free-space and the collection angle was approximately 3.5 degrees. The haze values were measured using a haze meter NDH7000 (Nippon Denshoku). It is defined as the ratio of (total transmittance - parallel transmittance) to total transmittance. Here, the parallel transmittance is measured with the collection angle less than 2.5 degrees. Therefore, the haze value will be little different to the free space measurement.
The voltage signals applied on the samples were generated using a function generator (Agilent 33220A) with a 20× amplifier (A400DI) and programed using LabVIEW. This enabled precise control of the amplitude, duration, and frequency of the applied voltage. The variation in normalized transmittance over time after the application and release of voltage at different frequencies was measured using the same driving methods. Initially, the SDCLCs were switched to the P state by applying a high-frequency (5 kHz) voltage (100 V at 12 µm, 120 V at 20 µm, and 200 V at 26 µm).
3. Results and discussion
The operating principle of CLCs is shown in Fig. 1(a), where the CLC texture is in the P state because a high-frequency (5 kHz) voltage is applied. In this state, the CLC mixture appears transparent because it reflects ultraviolet light. When a low-frequency voltage (60 V at 60 Hz) was applied, EHD instability occurred and the CLC was disturbed. Once the low-frequency voltage was removed, the CLC texture switched from the disturbed EHD state to the FC state. Both the P and FC states were stable when no voltage was applied. As shown by the black line in Fig. 1(b), the transmittance was low when a low-frequency voltage (60 V at 60 Hz) was applied. In addition, the haze increased to >90%, indicating that considerable disturbance occurred. However, the transmittance increased considerably after the voltage was removed, with the haze decreasing to <70% after 15 s. The optical performance was not sufficiently high to blur the background image, as illustrated in Fig. 1(d). By contrast, as shown by the red line, when a medium-frequency voltage (60 V at 1 kHz) was applied, the disorder of the LC was not as strong as that caused by the low-frequency disturbance (60 Hz); however, the transmittance decreased and haze increased to ∼80% after the voltage was removed. Thus, a stable scattering state can be achieved, as shown in Fig. 1(e). To conclude, the violent relaxation after the application of a low-frequency voltage (60 V at 60 Hz) was removed reorientated the CLC to a highly ordered state, thereby increasing the transmittance. In contrast, a suitable disturbance can transform the CLC to a more randomly distributed FC state with a desirable scattering performance. To study the underlying mechanism, the disturbance and relaxation of the CLC under various voltages, frequencies, and cell gaps were further investigated.
Fig. 1. (a) Operating principle. (b) Normalized transmittance as a function of time upon the application of a voltage pulse. (c) Optical performance in the transparent state. (d) Optical performance in the scattering state upon the application of a low-frequency voltage (60 V at 60 Hz). (e) Optical performance in the scattering state upon the application of 60 V at 1 kHz.
To investigate the orientation of liquid crystal molecules in the CLC upon the application of a voltage pulse at different frequencies, a polarized optical microscope (POM) equipped with cross-polarizers was employed. The voltage was maintained at 60 V. When the voltage pulse was applied at a low frequency (60 Hz), random, fragmented domains were observed, as shown in Fig. 2(a), indicating that considerable disturbance occurred, giving rise to considerable scattering. However, the domain size gradually increased after the voltage was removed, as shown in Fig. 2(b). Thus, the transmittance increased and the haze decreased. The brightness of the enlarged domains differed notably when the sample was rotated under a cross-polarized optical microscope.
Fig. 2. (a)–(d) POM images obtained with the application of a low-frequency voltage (60 V at 60 Hz) (see Visualization 1): (a) POM image obtained upon the application of a voltage pulse; (b) POM image obtained when the voltage was removed for 1 s; (c) POM image obtained when the voltage had been removed for 20 s; and (d) POM image obtained when the voltage had been removed for 20 s and the sample was rotated 45° counterclockwise. (e)–(h) POM images obtained with the application of a medium-frequency voltage (60 V at 1 kHz) (see Visualization 2): (e) POM image obtained upon the application of a voltage pulse; (f) POM image obtained when the voltage was removed for 1 s; (g) POM image obtained when the voltage had been removed for 20 s; and (h) POM image obtained when the voltage had been removed for 20 s and the sample was rotated 45° counterclockwise.
As shown in Fig. 2(c) and (d), the brightness of the marked region changes when the sample is rotated by 45° owing to the birefringence change of the uniaxial crystal. This indicated that the relaxation of the EHD effect caused the CLC molecules to align to the lying helix (LH) state. When a medium-frequency voltage was applied (60 V at 1 kHz), the dynamic disturbance weakened, as indicated in Fig. 2(e). However, the CLC transformed into fragmented domains and remained stable after the voltage was removed, as shown in Fig. 2(f)–(h). No noticeable changes in the brightness of the POM images were observed when the sample was rotated by 45°, indicating that the CLC molecules were highly disordered. Therefore, the transmittance decreased considerably whereas the haze increased. In order to demonstrate the phenomenon more clearly, the videos before and after removing the voltages under 60Hz and 1kHz are shown in Visualization 1 and Visualization 2, respectively.
These results indicate that the interaction between the EHD and dielectric effects of CLCs is strongly dependent on the frequency of the applied voltage. When a low-frequency voltage is applied, the ions in the CLC are strongly disturbed. Although, high haze value can be observed, the flow of ions tend to align the LC molecules along their movement direction, which is perpendicular to the substrate [25]. After the voltage is removed, the LC molecules align vertically owing to the motion of the ions; moreover, the relaxation of the orientation caused by the EHD effect gradually switches the CLC to the LH state [28,29], with the helical axis parallel to the substrate. Therefore, the transmittance increases. As a medium-frequency (1kHz) voltage is applied, lower disturbance caused by the EHD effect resulting in larger domains. Owing to the existing dielectric effect, the morphology of the initial P state with some oil streaks are observed. After the voltage is removed, the CLC transforms into a randomly distributed FC and is highly disordered. Therefore, the transmittance decreases.
The performance of the CLC can be varied by controlling the frequency. Therefore, the dependence of the CLC performance on frequency must be investigated. Figure 3(a) shows the variation in transmittance upon the application of voltage (at 2–5 s) at different frequencies. As the frequency was increased, the transmittance increased upon the application of voltage, indicating that the disturbance due to the EHD effect decreased; however, the transmittance exhibited interesting variations upon the removal of the voltage. As the driving frequency increases, the trend of the transmittance change has an inflection point.
Fig. 3. (a) Variation in normalized transmittance upon the application and removal of voltage at different frequencies. (b) Normalized transmittance versus voltage frequency.
Figure 3(b) shows the variation in the normalized transmittance with respect to the driving frequency. The transmittance at t = 4.8 and 20 s in Fig. 3(a) are referred to as Ton and Toff, respectively. To explain the variation in the normalized transmittance more clearly, EHD-dominated regime (EDR) and dielectric-aligned regime (DAR) are defined. EDR occurs when Toff is higher than Ton, as indicated by the blue region in Fig. 3(b). In contrast, DAR occurs when Ton is higher than Toff, as indicated by the red region in Fig. 3(b). The frequency at which EDR changes to DAR is referred to as the transition frequency (ftr). Relaxation refers to the process of the change in the CLC texture from Ton to Toff. When the frequency of the applied voltage is higher than 1600 Hz, the ion-doped CLC device is completely dominated by the dielectric effect and the EHD effect almost disappears. The high-frequency field directly drives the CLC to a perfectly aligned planar state.
To verify that the disturbance and relaxation of the CLC were due to the interaction between the EHD and dielectric effects, the dependence of the transmittance on the frequency was investigated under different electric fields (Fig. 4). As the electric field was increased from 3 to 4.5 V/µm, ftr decreased from ∼500 Hz to a nontransition frequency. The relaxation of the EDR decreased, whereas the relaxation of the DAR increased considerably. EDR was not observed when the electric field was >5 V/µm. The dielectric effect was more obvious than the EHD effect at high voltages.
Fig. 4. Normalized transmittance versus frequency under different electric fields: (a) 3, (b) 3.5, (c) 4, (d) 4.5, and (e) 5 V/µm. The cell gap is 20 µm.
Unlike the dielectric effect, the threshold voltage of the DS is independent of the cell gap. The threshold voltage did not vary significantly when the cell gap was varied from 6 to 127 µm [30]. To further verify the interaction between the EHD and dielectric effects, the optical performance at various cell gaps was evaluated. Upon the application of higher voltages under the same electric field, larger cell gaps generated larger disturbances because of the EHD effect. The influence of the dielectric effect was similar under the same electric field. Therefore, the disturbance of the EHD effect can be controlled by varying the cell gap. Figure 5 shows the variation in transmittance with respect to driving frequency for a cell gap of 12 µm. The disturbance due to the EHD effect was smaller than that in the sample with a cell gap of 20 µm. However, the influence of the dielectric effect of the LC was similar because the applied electric field was the same. As indicated in Fig. 5(a), EDR was not observed even at a frequency of 60 Hz when the cell gap was 12 µm.
Fig. 5. (a) Variation in normalized transmittance upon the application and removal of a 4-V/µm electric field; the cell gap is 12 µm. Normalized transmittance versus frequency under different electric fields: (b) 3.5, (c) 4, and (d) 5 V/µm.
No ftr was observed. When the electric field was strengthened, the relaxation of the DAR increased, resulting in a slightly low transmittance of the CLC. When the cell gap was 12 µm, only DAR was observed at different voltages. The influence of the dielectric effect is always stronger than that of the EHD effect. Although the disturbance due to the EHD effect was slight, randomizing the CLC to the chaotic FC state (instead of aligning it to the LH state due to the strong relaxation of the EDR) was beneficial. Therefore, a higher scattering performance was achieved even with a thin cell gap (12 µm) under a low driving frequency. The haze was ∼80% in the scattering state (FC state), and <5% in the transparent state (P state).
When the cell gap was 26 µm, the disturbance due to the EHD effect should have been larger than that for the sample with a cell gap of 20 µm. However, the influence of the dielectric effect of the LC was similar because of the fixed applied electric field. Figure 6(a) shows the variation in normalized transmittance for different frequencies when the cell gap is 26 µm. Although DAR can be observed under this cell condition, it was only observed under a high driving frequency. Under an electric field of 3.5 V/µm, ftr increased from 600 Hz at 20 µm (Fig. 4(b)) to ∼1300 Hz at 26 µm. As the electric field was strengthened, ftr decreased owing to the enhancement of the dielectric effect.
Fig. 6. (a) Variation in normalized transmittance upon the application and removal of a 3.5-V/µm electric field; the cell gap is 26 µm. Normalized transmittance versus frequency under different electric fields: (b) 3.5, (c) 4, and (d) 5 V/µm.
In summary, the operation mechanism and optimization of SDCLCs depend on the EHD and dielectric effects. When the cell gap was large (20 µm), the EHD effect was very strong at low-frequency voltages, causing strong relaxation of the EDR after the voltage was removed. The haze decreased to ≤70%. Therefore, the EHD effect must be weakened by applying voltages with suitable frequencies. When the cell gap was small (12 µm), the EHD effect was intrinsically weak. The interaction between the EHD and dielectric effects is appropriate, resulting in a better scattering state at low frequencies. The haze in transparent and scattering state are approximately 80% and 1.5%. There is no apparent change in haze in both stable states after 12 hours. Although the haze increased in the scattering state when the cell gap was ≥26 µm, the haze in the P state was not acceptable (>20%) because of the longer optical scattering length.
4. Conclusions
In this study, the disturbance and relaxation of SDCLCs were investigated. The interaction between the EHD and dielectric effects in the LC under various frequencies, electric fields, and cell gaps was discussed in detail. EDR and DAR of the CLC were defined to explain relaxation. When the frequency of the applied voltage was lower than the transition frequency (EDR phenomenon), the strong disturbance caused by the EHD effect reorientated the CLC to the LH state. The relaxation upon releasing the voltage decreased the haze, which does not benefit the scattering state. Instead, when the frequency of the applied voltage is appropriate, the EHD effect can be decreased, causing a highly disordered FC state. The haze in the scattering state increased to ∼80% without sacrificing the transparency in the P state (haze: <5%) when the cell gap was 20 µm. This phenomenon was also verified for cell gaps of 12 and 26 µm. The operation mechanism of SDCLCs is strongly dependent on the cell gap, electric field, and driving frequency. Based on the results of this study, further research can be conducted to optimize the optical performance of SDCLCs under different application conditions.
Funding
Ministry of Science and Technology, Taiwan (MOST 109-2112-M-110-013-MY3, MOST 110-2223-E-110-001-).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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