1. Introduction

Chiral nematic liquid crystal (N*LC) cells have attracted much attention in both academic studies and applications because they have several unique properties such as periodic helical structures, wavelength selective reflection of circularly polarized light, and tunable reflection wavelength [1]. The self-assembled helical pitch length is a critical factor which significantly affects the optoelectronic cell properties of the cell. Various approaches for tuning the helical pitch length of N*LCs have been proposed such as mixing different chiral concentrations [2–4], changing the operating temperature [5], using photo-tunable chiral dopants [6], and applying external electric fields [7]. The current trend in the fabricated helical pitch length of N*LC cells is to shorten it so as to reduce the light leakage (i.e., reduce the transmitted light) in the dark state when the cell is placed between the crossed polarizers [8–15]. This kind of cell possesses a high contrast ratio and fast-switch response, but the operating voltage will be increased [14,16].

A conventional N*LC cell with a positive dielectric anisotropic nematic LC (NLC) usually has standing-helix (SH) and focal conic molecular configurations [17,18]. For the SH configuration, the N*LCs are aligned horizontally (also referred to as planar alignment), making the helical axis perpendicular to the substrate surface, and producing a stronger Bragg reflection. For focal conic configuration, the alignment of the N*LC is isotropic (also referred to as no alignment), and the direction of the helical axis is randomly distributed in the cell. Several methods to make N*LC cells have bistable configurations have been demonstrated [19–26]. Among the various methods, the dual-frequency N*LC applied to the cell is often selected due to its easy access and the fact that it is beneficial to cell fabrication. However, the operating frequency and voltage require simultaneous control. In addition, a high operating voltage [26] and low contrast ratio [24] are necessary for improvement. Furthermore, in order to keep each molecular configuration stable, applying a specific frequency and voltage to the cell is necessary at all times; however, this will increase power consumption.

To improve the electro-optical performance of the cell, some kinds of dopant materials have been proposed and mixed with the N*LCs [10,13,15,22,26–42]. Of all these materials, photosensitive polymer materials are the most commonly employed in N*LC cells to stabilize the N*LC configuration and to reduce the response time, while additional electric potential energy is required to address the N*LC molecules [30–33]. Recently, fast-response N*LC cells with a positive LC material, addressed by the three-terminal-electrode scheme, have been demonstrated [43–45] based on the two response mechanisms, the helical molecular director reorientation attributed to the dielectric effect on the positive N*LCs [8], and the helical-axis rotation attributed to the flexoelectric effect [9,10]. However, focal conic defects on the electrode surface will be revealed due to the fringe electric field effect. To eliminate this phenomenon, N*LC cells using an in-plane switching (IPS) electric field and negative N*LCs are explored [46–49]; good electro-optic properties are also exhibited. However, specific material, long fabrication time (~24 hr) and high driving voltage (> 200 V) are all necessary.

In this paper, the electro-optical properties of N*LC cells are investigated with IPS and non-uniform vertical electric fields. The color and threshold-voltage behavior are associated with the chiral dopant concentration. Under the applied IPS electric fields, the helical structure is changed from the SH to the lying-helix (LH) configuration. Of all the proposed N*LC cells, the short-pitch one (in blue) has the lowest light leakage and has the most uniform helical structure at high operating voltages. In the reverse measurement, the blue N*LC cell shows a stable helical structure, and the LH configurations are sustained when the IPS electric fields are released. With the three-electrode driving architecture, the non-uniform vertical electric field makes the LH configuration return to the original SH configuration. The two bistable states are successfully achieved without applying a holding voltage to the cell. The voltage- and temperature-dependent reflection wavelengths are also measured, while blueshift and structure transition are observed. Regarding the cell performance, the rise time for the helical structure to switch from SH to LH is around 2.1 ms at a low in-plane switching voltage (V_IPS), and the fall time for changing the driven helical structure to the original fabricated SH structure is around 0.1 ms when applying a low vertical voltage. When the V_IPS is released, the LH configuration can be kept, and the light transmittance above 0.95 can be maintained for over 6 hr. This proposed N*LC cell and the design driving scheme are suitable for optoelectronic device applications.

2. Cell fabrication and measurement

In the experiment, the indium-tin-oxide (ITO) interdigital electrode arrays (labeled V₁ and V₂) with a width of 4 μm and separation of 8 μm are fabricated on the bottom glass substrate surface to produce IPS electric fields. The uniform ITO thin film is deposited on the top glass substrate surface as the sheet electrode (labeled V₁) to create non-uniform vertical electric fields. Both substrates are coated with the AL-12G polyimide as the alignment layer and then heated at 200 °C for 1 hr. The top and bottom alignment layers are rubbed in an antiparallel direction perpendicular to the IPS electrodes. After that, 4-μm spacers are used to control the cell gap. The N*LC is composed of the NLC with negative dielectric anisotropy (Merck 95-465, Δn = 0.0829, and Δε = −4.2) and the chiral dopant (Merck ZLI-3786) has helical twisting power (HTP = 10.8 μm⁻¹). ZLI-3786 generates the right-handed helical structure of LCs in the cell, and the applicable dopant concentration, defined as the ratio of the dopant weight to the sum of the dopant and NLC weight, is selected to obtain the reflected light wavelength within the visible region. The cell architecture and molecular configurations of a fabricated N*LC cell are illustrated in Fig. 1(a).Without the external force, the cell shows the SH structure with planar molecular alignment. During the electro-optical measurement, the IPS voltage (V_IPS) is applied to the cell, sandwiched between the crossed polarizers and orientated to 45° with respect to the polarizer. Transmittance-voltage (T-V) curves, voltage-dependent reflection behavior, and optical switch responses are demonstrated using a laser light with a 650-nm wavelength and ac voltage with 1 kHz square waveform. In addition, the temperature-dependent Bragg reflection and helical structure transition are also obtained.

 

Fig. 1 Schematic illustrations of the driving mode and N*LC molecular orientation for an N*LC cell. (a) a fabricated N*LC cell at the zero electric field state and the helical axis perpendicular to the substrate surface. The helical configuration is standing-helix. (b) A small V_IPS (V_IPS < V_th) applied to the N*LC cell and the reorientation of the helical molecular director between the IPS electrodes. As the V_IPS is more than V_th, this phenomenon is more remarkable, as shown in (c). (d) An N*LC cell addressed by a large electric field and the helical axis parallel to the substrate surface. The lying-helix configuration of the cell between the IPS electrodes is constructed. Using the three-terminal-electrode architecture, the induced non-uniform vertical electric field makes the driven helical configuration return to the original, as shown in (e). The N*LC configuration of (e) is the same as that of (a).

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Before inspecting the N*LC cell performance, we firstly demonstrate the N*LC molecular configurations in the different operating modes, as illustrated in Figs. 1(b)–1(e). When the cell is operated around the threshold voltage (V_th) of the IPS mode (also called a small electric field state), the helical configurations of the LC molecules near the IPS-electrode edge start tilting towards the substrate surface, whereas those located on the IPS electrode area are perpendicular to the substrate surface, as shown in Figs. 1(b) and 1(c). This LC molecular configuration will change the light transmittance observed by conoscope and polarized optical microscope (POM) under the crossed polarizers, as will be shown later. In contrast, when a large electric field is applied to the cell, as illustrated in Fig. 1(d), the helical configurations of the LC molecules near the IPS-electrode edge will be changed and the helical axis will be parallel to the substrate surface, forming the LH configuration. When the IPS electric field is released and the non-uniform vertical electric field is applied to the cell, the helical configuration of the LC molecules will be standing, as illustrated in Fig. 1(e), which means that the helical structure is the same as the fabricated one, as illustrated in Fig. 1(a). This cell will have two stable structures, the SH and LH structures, when the driving force is removed, as will also be shown later.

3. Results and discussion

The schematic Bragg reflection of an N*LC cell is presented in Fig. 2(a). One can find that the reflected wavelength is related to the optical anisotropic refractive index and equivalent pitch, which is dependent on the incident angle, chiral dopant concentration, external electric force, and measured temperature [50]. The normalized reflectance spectra of three kinds of chiral dopant concentrations in N*LC cells at the normal incident (θ = 90°) are shown in Fig. 2(b), and the centrally-reflected wavelengths for these cells are located at 456 nm, 512 nm, and 659 nm, corresponding to the blue, green, and red lights, respectively. The reflection values for these three kinds of N*LC cells, in practice, are different, which is associated with the chiral dopant concentration and effective refractive index. The blue cell has the highest reflection value (~60%) while the red cell has the lowest (~30%). According to the Bragg reflection, the effective pitch length of each N*LC cell can be obtained, 300.6 nm for blue N*LC cells, 337.5 nm for green, and 434.4 nm for red. In addition, based on the reduced equation of HTP [51], which is equal to the inverse of the product of the pitch length and chiral dopant concentration labeled in Fig. 2(b), the experimental value is close to the theoretical one (10. 77 μm⁻¹ for blue, 10.72 μm⁻¹ for green, and 10.85 μm⁻¹ for red N*LC cells). The dependence of the reflectance of an N*LC cell on the electric force and temperature will be demonstrated later.

 

Fig. 2 (a) A schematic illustration of the Bragg reflection of an N*LC cell. (b) Reflectance spectra of three kinds of N*LC cells with different chiral dopant concentrations. The center reflection wavelengths of these three cells are at 456 nm, 512 nm, and 659 nm, which are measured at room temperature.

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Figure 3(a) shows the normalized T-V curves for three different types of N*LC cells. The labeled voltage value on the horizontal axis is the peak to peak voltage (V_pp) value of the applied V_IPS. Of all the N*LC cells, the blue one has the highest V_th (50.0 V), whereas the red one has the lowest V_th (30.0 V), which is attributed to the N*LC pitch length at zero field (P₀; V_th ∝ 1/P₀) [16]. The V_th value of each cell is determined by the maximum tangent line’s slope. The blue N*LC cell, due to its short pitch length, possesses more helical structures between the cell gap, which means that additional electric potential energy is required to change the N*LC molecular configuration from SH to LH. Thus, the T-V curve of the blue N*LC cell is located on the rightmost side. As the applied V_IPS is more than the V_th, the helical structures will gradually lie on the substrate surface, and the helical axis gradually becomes parallel to the substrate surface, making the light transmittance increase until it reaches the maximum-transmittance level. It is worth mentioning that the absolute maximum light transmission of each cell measured under the crossed polarizers is around 0.5 or 50%. This is due to the molecular structure of a cell, as shown in Fig. 1(d). Figure 3(b) shows the voltage-dependent POM images for these three kinds of N*LC cells, recorded under the crossed polarizers. At the initial state (V_IPS = 0 V), the three kinds of cells display individual colors because the N*LC molecular helical axis is normal to the substrate surface. When the applied V_IPS is increased to the V_th, the brightness around the IPS-electrode edge will become obvious, indicating that the helical structure of the N*LC cell is being transformed. The transformation of helical structures between the IPS electrodes is indicated by the red arrow. Once the applied V_IPS is increased sufficiently (like V_IPS = V_pp = 80 V), this transformation will be clearly observed and the light brightness will also be remarkable, which is attributed to the fact that the helical axis is subsequently parallel to the substrate surface. Although the red N*LC cell with longer pitch length has the lowest V_th, the contrast ratio, response time, and helical-structure stability need to be improved. Thus, based on the above considerations, the blue N*LC cell is selected for further investigation of its electro-optical properties and applications.

 

Fig. 3 (a) T-V curves for N*LC cells doped with different chiral concentrations. The represented value of the applied voltage labeled on the horizontal axis is the peak-to-peak value, and the inserted image shows the driving scheme of an N*LC cell. (b) Voltage-dependent POM images for these three N*LC cells. The helical molecular transition is labeled by the red arrows. The N*LC cell in blue shows a good dark state and good stability of the molecular structure at a high operating voltage. (c) Different driving modes used for the blue N*LC cell. From the T-V curves, the two bistable states without a holding voltage are obtained. (d) POM images, conoscopic images, and helical structure illustrations for the blue N*LC cell addressed at different IPS voltages and by different driving modes.

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Figure 3(c) shows the variation in light transmittance behavior with the different driving modes of the blue N*LC cell. It can be found that as the applied V_IPS is increased from 0 V to 200 V, the light transmittance will increase from 0 (T_min) to 1 (T_max), indicating that the helical structure changes from SH to LH. However, after that, when the applied V_IPS decreases from 200 V to 0 V (reverse measurement), the light transmittance cannot decrease to 0 (T_min) and the original SH structure cannot be restored. This implies that the proposed N*LC cell could have bistable and memory properties after being driven. To let the driven molecular configuration return to the SH configuration, the three-terminal-electrode driving method producing a non-uniform vertical electric field is employed in the N*LC cell. This method can effectively align the LC molecules as well as make the helical axis reorient and stand on the substrate surface. With this driving scheme, the light transmittance is reduced significantly and the SH structure leading to the dark state is existent at the applied voltage of 50 V. Then the two bistable states without applying a holding voltage can be obtained (the LH-structure state by means of the IPS reverse driving, and the SH structure state by means of the three-terminal-electrode driving). The helical structure and the light brightness images of the blue N*LC cell are present in Fig. 3(d), recorded by the conoscope and POM, respectively. The conoscopic images and helical configurations are inserted into the POM images, which are related with the driving modes, and which also show bistable properties.

Figure 4(a) presents the voltage-dependent Bragg reflection behavior of the blue N*LC cell. The incident light is normal to the cell surface, and the normalized reflectance (Norm. R.) labeled on the vertical axis is defined as the R/R₀, where R₀ is the reflectance of the blue N*LC cell at the applied V_IPS equal to 0 V. As the applied V_IPS gradually rises, the light reflectance decreases and the central reflection wavelength (λ_B) becomes shorter (blue shift), which is attributed to the N*LC helical variation. The change in helical-axis direction makes the original SH tilt, and effectively shrinks the pitch length for the normal incident light. This will decrease the Bragg reflection and lead to the blue shift phenomenon of λ_B. When the applied V_IPS is more than 150 V, the moderate change behavior of reflectance is observed because the SH structures existing between the IPS electrodes mostly become LH structures. However, the variation in λ_B is relatively dramatic, and is affected by the average refractive index and the effective pitch length effects. The changed region of λ_B at the specific applied V_IPS is within 5 nm, which is suitable for wavelength tuning cells. In addition, it is also found that the value of reflectance at the applied V_IPS of 200 V is around 0.5, which is associated with the interdigital-electrode pattern. Since the SH structures located on the interdigital-electrode area are always sustained independently of the magnitude of the applied V_IPS, the change in helical structure from SH to LH only occurs in the IPS-electrode space. The thermodynamic behavior of the blue N*LC cell is shown in Fig. 4(b). When the temperature rises, the reflection wavelength of the cell will become shorter due to the thermodynamic vibrations effect on the original SH structure. The increased thermal vibration strength results in increased twisting power variation between the neighboring molecular structures, and the helical pitch length of the N*LC cell will be reduced [5,52]. Such a thermal vibration effect also decreases the light intensity of the Bragg reflection, which is attributed to the fact that the director of the helical axis is disturbed (i.e., it is no longer perpendicular to the substrate surface), leading to the light scattering. Once the temperature increases to the critical temperature (~46 °C), the Bragg reflection will vanish, indicating that the helical structure is destroyed, and molecular configuration transition will occur, forming the isotropic alignment to the N*LC cell. The inserted conoscopic images under the crossed polarizers show the temperature-dependent N*LC configuration. At room temperature, the cross-line reticles are observed clearly, indicating that the helical axis is perpendicular to the substrate surface. As the temperature gradually increases, the cross-line reticles will become blurred and will eventually vanish, meaning that a definite direction of the cell helical axis does not exist, and isotropic alignment will occur. To extend the operating temperature and stabilize the molecular configuration of the blue N*LC cell, the cross-linking polymer networks with appropriate monomer concentration are applied to it. A detailed description of the polymer-network effect on the N*LC cell will be reported elsewhere.

 

Fig. 4 (a) Voltage-dependent Bragg reflection behavior of the blue N*LC cell. As the voltage is increased, the reflectance is reduced and the central reflected wavelength becomes shorter. (b) Temperature-dependent Bragg reflection behavior of the blue N*LC cell. As the temperature rises, blueshift of the central reflected wavelength is observed and the reflectance due to the thermodynamic effect is also decreased. Moreover, the full width at half maximum is gradually broadened. The inserted conoscopic images show that when the temperature is increased, the reticle and lobe edge will vanish.

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Figure 5(a) shows the variation in light transmittance of an N*LC cell with dispay time as the V_IPS is released. The N*LC molecular configuration under the influence of the IPS electric field becomes the LH configuration. Due to the bistable property and the SH and LH structures without applying a holding voltage, one can find that the proposed cell can keep the light transmittance at a high transmittance level (≥ 0.95) for over 6 hr. The inserted photos demonstrate the light intensity of the blue N*LC cell with and without the applied V_IPS, showing that this cell can display for a long time because the LH structures are maintained by the cell itself.

 

Fig. 5 (a) Time-dependent light transmittance of the blue N*LC cell at the zero applied voltage state. After the applied V_IPS is released, the value of normalized light transmittance is kept above 0.95 for over 6 hr. The inserted pictures show the variation in light transmittance of the blue N*LC cell before and after the voltage drops. (b)-(d) Bistable-state responses of the blue N*LC cell through the three-terminal-electrode driving scheme. (b)-(c) Rise-time and fall-time responses of the blue N*LC cell addressed by the IPS electric field. The helical structure is changed from SH to LH and is sustained in the LH state when the IPS electric field is turned off. Through the fitting curves, the time for the helical configuration transition and the variation in light transmittance are demonstrated. (d) Fast bistable switching response (fall time) by using the non-uniform vertical electric field on the blue N*LC cell.

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Figure 5(b)-5(d) show the response time of the blue N*LC cell, operated by IPS and non-uniform vertical electric fields. The rise and fall times are measured from the 10%–90% and 90%–10% transmission levels, respectively. In Fig. 5(b), as the V_IPS ( = V_pp = 160 V) is applied to the blue N*LC cell, some of the SH directors at the beginning of the whole response start reorienting, leading to increased light transmittance. This transient time of the structure transition (also called the dielectric effect) is very fast, approximately tens of sub-nanoseconds. Then the helical axis will gradually lie on the cell surface, which is attributed to the fact that the applied V_IPS rises to a stable voltage level and the larger electric field effect on the N*LC molecules occurs. The total rise time of the blue N*LC cell is about 2.1 ms. When the IPS electric field is released, a part of the LH N*LC molecular structure affected by the anchoring effect will start reorientating to return to the original SH N*LC molecular structure, and the parallel helical axis will be slightly tilted [53–55]. This causes the light transmittance at t > 0 to decrease to 0.98. Without the stronger anchoring effect or other external force being applied, this light transmittance level can be kept for a period of time and then will gradually reduce. The fall-time response (which can also be called the gray-level response) is shown in Fig. 5(c); the fall time is about 11 ms. To make the LH structure return to the original SH structure rapidly, a non-uniform electric field can be applied to the cell, which will significantly reduce the fall time to less than 0.1 ms at a driving voltage of around 100 V, as shown in Fig. 5(d). This driving architecture can have the helical axis rapidly reorient and switch to the other stable state (the SH structure). In order to investigate the response time and light transmittance contributed by the change in N*LC molecular structures, the rise-time and fall-time curves are also demonstrated by the following fitting equations, Eq. (1) and Eq. (2), respectively, and are plotted in Fig. 5(b)-5(c):

4. Conclusion

The electro-optical properties of the N*LC cell, composed of the chiral dopant and the NLC with a negative dielectric anisotropy, are investigated. The chiral dopant gives rise to a helical structure of the NLC. This type of cell has Bragg reflection corresponding to the effective helical pitch length of the N*LC molecules. The Bragg reflection, V_th, and helical configuration are significantly related with the chiral dopant concentration. Of all the N*LC cells, the blue cell shows the highest V_th in the IPS driving mode because of the shortest pitch length. However, this cell exhibits lower light leakage and has a more stable helical structure. Through the two-terminal-electrode reverse address (IPS electric field) and the three-terminal-electrode address (non-uniform vertical electric field), the two stable helical structures can be obtained, and can be maintained without applying a holding voltage. The voltage-dependent wavelength shift is observed by employing the IPS electric fields in the N*LC cell, and the reflection wavelength becomes shorter since the original SH structure is changed and the effective pitch length is shrunk. In addition, the temperature-dependent wavelength shift, due to the effect of thermodynamic vibrations on the helical N*LC molecules, is also exhibited, and the critical temperature of the N*LC structure transition is approximately 46 °C. The fast rise-time response of the N*LC cell is achieved through the IPS driving mode, changing the helical structure from SH to LH. Most of the light transmittance is built up within 85 μs, which is attributed to the dielectric effect. When applying the non-uniform vertical electric field, the driven helical structure (LH) can rapidly return to the original fabricated helical structure (SH), making the fall-time response faster and the light transmittance reduce dramatically within 180 μs. With these fast bistable switching characteristics, the cell can be used as a light shutter. Moreover, the bistability and memory function of the N*LC cell lead to a display time of over 6 hr in the unplugged power state. This designed N*LC cell not only benefits the electro-optical performance, but can also be used as another optoelectronic component in optical systems.