Operation of a light shutter usually requires that an electric addressing field is applied continuously to switch between a transparent and an extinct or opaque scattering state. To achieve low-power operation, a bistable light shutter, which consumes power only when switching between the two stable states, is preferred. Here, switching between the two stable states is induced with a short pulse rather than by applying a continuous field. In this work, we report bistable switching of an ion-doped chiral nematic liquid crystal between the transparent homeotropic and the scattering focal-conic states. Owing to ionic dopants, the use of complicated patterned electrodes or dual-frequency liquid crystals was not required for switching of the device. The light shutter exhibited an opaque state, which has higher scattering than previously reported bistable light shutters. Furthermore, doping of the reported device with a dichroic-dye enabled simultaneous control of light scattering and absorption. A new candidate for both smart window and see-through display applications is presented.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Light shutters for smart window and see-through display applications have been studied actively [1–11]. A light shutter can be used for the selective operation of a see-through display. It can replace the curtains in windows because it can control the light intensity passing through the window and protect an individual’s privacy by presenting an opaque surface. Various light shutter technologies, such as electro-chromic , suspended particle , and liquid crystal (LC) [12–24] devices, have been reported. In the scattering state, dichroic-dye-doped LC devices can hide objects behind them and present a black color, because they enable simultaneous control of light scattering and absorption. Recently, electrically induced scattering states with fast response times were observed in nematic LC doped with polymer [9,10]. They are thus suitable for both see-through displays and smart window applications.
A dichroic-dye-doped LC cell is switchable between two states: the transparent state which can be obtained with vertically aligned LC and dye molecules, and the opaque state, which can be obtained with randomly oriented LC and dye molecules [14–24]. The operation of a light shutter usually requires that an electric field be applied continuously to maintain either the transparent or the opaque state. To reduce the power consumption of a light shutter, bistable operation, which consumes power only when switching between the two states, is preferred.
To achieve bistable operation of a light shutter, a chiral nematic LC (CLC) cell switchable between the planar and focal-conic states has been studied [25–27]. However, it is not suitable for simultaneous control of light scattering and absorption because doped dichroic dye molecules absorb the incident light in the planar state. To control light scattering and absorption at the same time, the homeotropic state is used because vertically aligned LC and dye molecules can minimize both scattering and absorption of the incident light in this transparent state.
Bistable CLC cells that are switchable between homeotropic and focal-conic states have also been studied [28–31]. A stable homeotropic state can be realized by polymerization of the LC mixture, and the focal-conic state is used as a stable opaque state. However, these devices require a high fabrication cost due to the complicated electrode structure or the use of dual-frequency LC material [29–31]. Moreover, their optical characteristics in the opaque state may not be sufficient for practical applications. For practical applications, a bistable device with a simple structure and better optical characteristics is necessary.
In this paper, we propose a bistable CLC light shutter with a simple structure. The reported device is switchable between the two stable states: the homeotropic and focal-conic states. It can be switched from the homeotropic to the focal-conic state using the electro-hydrodynamic effect [32–38]. We can dope dichroic dye for simultaneous control of light scattering and absorption. The device is proposed as a new candidate for the selective operation of see-through displays and smart window applications.
2. Principles of operation
The schematic and operating principle of the reported device are shown schematically in Fig. 1. In the homeotropic state, vertically aligned LC and dye molecules minimize both scattering and absorption of the incident light so that the light shutter is transparent. Because the homeotropic state is stabilized with the polymer structure, it is stable without requiring a constantly applied electric field. It may be noted that the transmittance in the transparent state will decrease as the viewing angle is increased because, for oblique incidence, the long axis of the dyes is no longer perpendicular to the polarization direction of the incident light.
For switching from the homeotropic to focal-conic state, we can use the electro-hydrodynamic effect, which can be achieved by doping ionic materials in the CLCs. As shown in previous studies on CLC cells with ionic dopants [26,27], a cell can be switched to the focal-conic state by the electro-hydrodynamic effect. In our experiment, the turbulence caused by the electro-hydrodynamic effect in a CLC cell changed the ordering of the LC and dye molecules when an electric field was applied. Once the applied voltage is removed, the turbulence no longer exists. However, the CLC cell remains in the focal-conic state. This state produces a black color and hides the background behind the cell through simultaneous control of light scattering and absorption. To produce the electro-hydrodynamic effect, it is necessary to choose the frequency of the applied electric field carefully . In this work, we choose a direct current (DC) voltage because it can provide sufficient turbulence to cause the necessary change in the LC ordering.
The reported cell can be switched from the focal-conic back to the initial homeotropic state with a 100-Hz voltage wave. Because the aligning effect induced by the applied electric field is dominant and turbulence is negligible, the positive LC and dye molecules tend to be oriented perpendicular to the substrates. When the applied field is removed, the homeotropic state is maintained, owing to the sufficiently strong aligning effect of the polymer structures, which can hold the LCs in the homeotropic state without consuming power.
3. Cell fabrication
To find the stable condition in both states, we fabricated LC cells with various UV-curable monomer concentrations. The parameters of the LC mixture for the fabrication of LC cells are as follows. Positive nematic LCs (E7, Δn: 0.223, Δε: 13.5, Merck) were mixed with 10 wt% of chiral dopant (S811, pitch: 1 μm), 0.1 wt% of ionic dopant (Hexadecyltrimethylammonium bromide, Sigma-Aldrich), 4, 5, 6 wt% of UV-curable monomer (RM257, Merck), a small amount of photo-initiator (Irgacure 651, BASF), and 1 wt% of black dichroic-dye mixture (S-428, Mitsui). The dichroic ratio of the black dye used in this work was 10.96.
The LC mixtures were injected into empty cells with a thickness of 10 μm. For polymerization, the LC cells were exposed to UV light with an intensity of 25 mW/cm2 for 30 min. During polymerization, an electric field was applied between the top and bottom electrodes, to maintain the homeotropic state. Bistable switching between the homeotropic and focal-conic states required accurate control of the concentration of UV-curable monomer and the curing UV intensity. In our experiment, the concentration of UV-curable monomer was chosen to be 5 wt%. When it was lower than 5 wt%, the homeotropic state was not stable. On the other hand, when it was higher than 5 wt%, the focal-conic state was not stable.
4. Experimental results and discussion
To demonstrate the optical performance of the fabricated LC cells, a haze meter (HM-65W, Murakami Color Research Laboratory) was used. Using the haze meter, we can measure the total transmittance, specular transmittance, and haze. The specular [diffuse] transmittance, Ts [Td], refers to the ratio of the power of the beam that emerges from a sample cell, which is parallel (within a small range of angles of 2.5°) [not parallel] to a beam entering the cell, to the power carried by the beam entering the cell, as shown in Fig. 2. The total transmittance Tt is the sum of the specular transmittance Ts and the diffuse transmittance Td. The haze H can be calculated as H = Td/Tt.
To investigate the electro-hydrodynamic effect in an ion-doped CLC cell, we examined the polarized optical microscope (POM) images carefully while increasing the applied voltage. We observed the initial dark state independent of the ion dopants because the cells were polymerized in the homeotropic state and no voltage was applied, as shown in Fig. 3(a) (undoped cell) and 3(c) (doped cell). When a DC voltage was applied to the cells, we observed the domains of randomly oriented LC and dye molecules only if the cell was doped with ionic materials, as shown in Fig. 3(b) (undoped cell, remains dark) and Fig. 3(d) (doped cell, becomes clear). The electro-hydrodynamic effect [32–38] can generate turbulence, which may have changed the ordering of the LC and dye molecules in the reported cell [26,27].
The switching characteristics of the fabricated ion-doped CLC cell are shown in Fig. 4. The stable homeotropic and focal-conic states were observed with a monomer concentration of 5 wt%. Switching between the homeotropic and focal-conic states of the ion-doped CLC cell was achieved by applying a DC voltage of 90 V and a 100-Hz voltage wave of ± 90 V, as shown in Fig. 4. By applying a DC voltage, the homeotropic state can be switched to the focal-conic state. When a 100-Hz voltage wave was applied to the cell, the focal-conic state switched to the homeotropic state. Either state was maintained after the applied voltage was removed. It may be noted that there was a slight change in the transmittance when the applied voltage was removed.
We also measured the specular transmittance as we varied the monomer concentration. The aligning effect caused by the polymer structures played an important role as discussed in previous research [28–31]. A strong aligning force breaks the stable condition in the focal-conic state , whereas a weak aligning force cannot support the stable homeotropic state. In Fig. 5, we show the response of the ion-doped CLC cells to a DC applied voltage for different monomer concentrations. The specular transmittance was measured after the applied voltage was removed.
The cell with a monomer concentration of 4 wt% exhibited a lower initial specular transmittance than others because the polymer structure was not strong enough to hold the LCs in the homeotropic state. The cell cannot obtain the stable transparent state. In the cell with a monomer concentration of 5 wt%, the measured initial specular transmittance was 71.4%. In this case, the aligning effect produced by the polymer structure was sufficiently strong to maintain the homeotropic state.
The fabricated LC cell can be switched to the opaque state by applying a DC voltage. Bistable operation of an ion-doped CLC cell can be achieved with a monomer concentration of 5 wt%. However, when the monomer concentration was higher than 5 wt%, the scattering effect in the opaque state was not sufficient because the aligning effect of the polymer structure was too strong. We measured the electro-optical characteristics for applied voltages less than 90 V because there was degradation in the transparent state when the applied voltage was higher than 90 V.
Figure 6 shows the specular transmittance of the fabricated ion-doped CLC cell. To switch from the homeotropic to focal-conic state, a DC voltage was applied. The ion-doped CLC cell in the initial transparent homeotropic state exhibited a specular transmittance of 71.4%. As the applied voltage was increased, the specular transmittance decreased due to the electro-hydrodynamic effect. When the applied voltage was removed, the specular transmittance in the opaque state was 12.8%.
For switching from the focal-conic to the homeotropic state, a 100-Hz voltage wave was applied. The measured specular transmittance of the ion-doped CLC cell in the focal-conic state was 12.8%. As the applied voltage was increased, the specular transmittance increased because the applied field aligns the LC and dye molecules perpendicular to the substrates. When the applied voltage was removed, the device returned to the initial transparent homeotropic state with the specular transmittance of 71.4%. As shown in Fig. 6, both stable transparent and stable opaque states can be obtained with a temporarily applied voltages of 90 Vdc and a ± 90 V 100-Hz signal, respectively.
We compared the haze value of the reported ion-doped CLC cell with that of a bistable CLC cell driven with crossed patterned electrodes , as shown in Table 1. The cell parameters of the CLC cell driven with crossed patterned electrodes are as follows. Positive nematic LCs (E7, Δn: 0.223, Δε: 13.5, Merck) were mixed with 10 wt% of chiral dopant (S811, pitch: 1 μm), 6.2 wt% of UV curable monomer (RM257, Merck), 0.5 wt% of photo-initiator (Irgacure 651, BASF), and 1 wt% of black dichroic-dye (S-428, Mitsui). The width and gap of the patterned electrodes were 2.8 μm and 12 μm, respectively. The thickness of the LC layer was chosen to be 20 μm. The haze value in the opaque state of a CLC cell with patterned electrodes was quite low because there exist regions where LCs are not switched. On the other hand, because focal-conic domains were formed over the entire area of the reported cell, it can provide a haze value of 79.8%, which is approximately 12.7% higher than a CLC cell driven with patterned electrodes.
We have shown that we can fabricate an ion-doped CLC cell which is stable in both the homeotropic and focal-conic states. In other words, with the proper monomer concentration, we could develop a bistable ion-doped CLC cell. Figure 7 shows the photographs of an ion-doped CLC cell placed on a sheet of paper with a logo print. In the transparent state, we can see through the printed image clearly. In the opaque state, the printed image can be hidden completely and the cell provides a black color because of the dichroic-dye.
To check the stability over time, we measured the haze and specular transmittance of the ion-doped CLC cell hourly after the applied voltage was removed. Figure 8 shows that both states remained stable after 24 hours. Due to the aligning effect of the polymer structure and the stable focal-conic state, both the transparent and opaque states can be sustained without power consumption. Power is required only for switching between the two states.
We tested whether switching of the reported bistable light shutter between transparent and opaque states could be repeated multiple times without any degradation in device performance. As shown in Fig. 9, the specular transmittance in each state remained almost the same after 20 cycles of repeated switching.
The temporal switching behaviors of the fabricated ion-doped CLC cell are shown in Fig. 10. When a DC voltage of 90 V was applied to the fabricated cell for switching from the transparent to opaque state, the measured switching time was 114.9 ms. When a 100-Hz voltage wave of ± 90 V was applied to the fabricated cell for switching from the opaque to transparent state, the measured switching time was 15 ms. Because switching from the transparent to opaque state relies on the electro-hydrodynamic effect by ions, it is slower time than switching from the opaque to transparent state.
We demonstrated a bistable light shutter device using dye-doped CLCs with ionic dopants. The device is switchable between two stable states: homeotropic (transparent) and focal-conic (opaque) states. The reported cell relies on the electro-hydrodynamic effect for switching from the homeotropic to the focal-conic state to control light scattering and absorption simultaneously. The opaque focal-conic state can be used to hide objects behind a display panel or smart window and to present a black color. The reported cell is suitable for smart privacy window and display applications.
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2A1A05001067).
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