1. INTRODUCTION

Cholesteric liquid crystals are a great candidate for a polarization-independent light shutter because of their unique optical properties [1]. In the planar state they can reflect circularly polarized light, which has the same chirality as the cholesteric director configuration. With high voltage applied, the director is homeotropic, which theoretically provides a 100% transparent state. Yang et al. reported a cholesteric liquid crystal/polymer dispersion for haze-free light shutters [2]. The transmission is from 90% (the loss of light intensity in the on state is mainly due to reflections from the glass–air interfaces) to 1% (or 5% if turned off slowly) for the normal mode and 82% to 4% for the reverse mode. The turn-on time is 40 ms, while the turn-off time is 15 ms for the normal mode. The turn-on and turn-off times are approximately 5 ms for the reversed mode. Xu and Yang developed a dual frequency cholesteric light shutter [3]. The minimum transmittance of the stack of left and right cells in the planar texture is 8%. The transmittance of the stack in the homeotropic texture is 80% (the light loss is caused mainly by reflection from glass–air interfaces). They had made a shutter whose turn-off time is shorter than 40 ms. Hsiao et al. demonstrated a fast-switching bistable optical intensity modulator that has transmittance from about 90% to less than 10%, and the response time between 10% and 90% of the maximum and minimum transmittance difference is 10 to 20 ms [4]. Kim et al. proposed a cholesteric liquid crystal device with a three-terminal electrode structure that can be operated in both the dynamic and the bistable modes [5]. The planar state can be switched to the in-plane-field-induced state by applying an in-plane electric field. A fast turn-on time of less than 2 ms was achieved at an applied voltage of 45 V, while a fast relaxation time is less than 3 ms. However, the complex structure of this device is a drawback. In summary, cholesteric liquid crystals have been shown to provide polarization-independent devices with good optical properties, but sub-millisecond response times, as required for some applications, have not been reported.

Another approach to making a polarization-independent shutter device is to use polymer liquid crystal composite materials to have a switchable structure, such as holographic PDLCs and POLICRYPS structures. For example, De Sio et al. reported the first demonstration of a Bragg reflector utilizing the POLICRYPS fabrication technique. However, the reflection efficiency of the sample is quite modest ($\approx 16\%$) because $\mathrm{\Delta }n=0.04$ of the structure is small [6]. Ramsey et al. demonstrated that Bragg reflection gratings can be formed in this acrylate photopolymer system with reflection efficiencies of 52% with a peak notch at 1165 nm, and they found switching fields of the order of 7–8 MV/m and switching rise times of 64 μs with relaxation times of 6 ms [7]. Qi et al. investigated the performance of reflective holographic polymer dispersed liquid crystal (H-PDLC) displays as a function of sample thickness and curing laser light intensity [8]. Vincent et al. reported the results of the recording of H-PDLC reflection gratings while applying a shear stress parallel to the film plane, which could reduce the scatter when compared to random droplet directors and lead to a significant increase in the achievable diffraction efficiency (99%) [9]. Bunning et al. mentioned current performance levels reported for H-PDLC gratings as maximum diffraction efficiency $>70\%$, and switching speeds $<50\mathrm{\mu s}$ response and 150–200 μs (relaxation) [10]. However, a fast shutter with high transmission in the on state, low transmission in the off state, and fast response time has not yet been reported.

It is well known that, when the voltage is removed from a short pitch cholesteric device, the director quickly relaxes from a homeotropic state to a transient planar state, then goes through a multi-domain state, and finally reaches the equilibrium planar state, as shown below in Fig. 1 [11,12]. (The multi-domain state is sometimes called the “focal conic state” [1].)

 

Fig. 1. (a) Relaxation of cholesteric liquid crystal. (b) Reflection spectrum of cholesteric material from homeotropic state to planar state.

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Previous investigations of the time dependence of the reflection spectrum of cholesteric devices by Watson [14] found that soon after the removal of a high voltage that is holding the material in a homeotropic structure, there is a reflection peak at about 1000 nm wavelength. Then, there is time gap with no distinct reflection peak, and at about 10 ms a second peak develops at about 600 nm. From a theoretical viewpoint, that second peak is the reflection peak of the cholesteric material as expected from the formula ${\lambda }_{\mathrm{eq}}=n_{\mathrm{ave}}*P_o$, where $P_o$ is the equilibrium pitch of the cholesteric material. The reflection wavelength of the earlier peak is given by ${\lambda }_{\mathrm{tp}}=(K3/K2)*n_{\mathrm{ave}}*P_o$.

Typically, a cholesteric shutter uses the reflection wavelength associated with the equilibrium planar state, ${\lambda }_{\mathrm{eq}}$, However the relaxation time is long because of the long transition through the disordered multi-domain state. However, if we consider the method to operate it as a shutter for ${\lambda }_{\mathrm{tp}}$, it could be much faster, as the transition to that state is direct. In this paper we report a new cholesteric shutter that is based on this principle. By switching the director between a homeotropic state with high voltage applied and a transient planar state when voltage is removed, fast response time within sub-milliseconds was achieved. While one cholesteric cell using the ${\lambda }_{\mathrm{tp}}$ reflection peak would reflect only one of the circular polarization eigenmodes, a device made from a pair of cells can be used to provide a polarization-independent device.

2. EXPERIMENTS

A. Device Fabrication

A fast polarization-independent device was fabricated from two identical cells separated by a half-wave plate. Each cell used a high HTP chiral dopant, S5011 (4.6% by weight), and a high delta epsilon liquid crystal, MLC-2144, to have a transient planar state reflective peak in the visible wavelength range. Cells were assembled by patterned indium–tin oxide with PI 1211 coating and baked at 200°C for 1 h and assembled with 5 μm spacers. These cells were filled with the mixture above for sample cells and with pure E7 for intensity normalization of the optical data. For the sample cells, since we use only S5011 as the chiral dopant, we need a half-wave plate between two cells to make the second cell reflect circularly polarized light of handedness opposite that of the first. Index matching fluid was used to join the two cells. The pictures in Fig. 2 show the two-cell device in the top view and side view.

 

Fig. 2. Two-cell stack (a) top view and (b) side view.

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B. Dynamic Relaxing Process Measurement

The capacitance of the cells is directly related to director configuration. For example, the cell with positive dielectric constant liquid crystal in homeotropic state has the maximum capacitance, while a planar state has the minimum capacitance. This provides method to observe the director relaxing process by measurement of the time-dependent capacitance. The circuit shown in Fig. 3(a) was used for the capacitance measurement.

 

Fig. 3. (a) Capacitance measurement circuit and (b) the capacitance-time curve.

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From the voltage measured across the series capacitor, $C_{\mathrm{cap}}=10\mathrm{nF}$, and the total voltage, we calculated the capacitance of a cell $C_{\mathrm{cell}}$. First, with 100 V, 10,000 Hz voltage applied, we obtained a homeotropic capacitance $C_{\mathrm{cell}}$ of 6.39 nF. Then, we applied the same frequency but 2 V, and we measured the capacitance in this way and got $C_{\mathrm{cell}}=1.49\mathrm{nF}$. Here the evidence from our previous work shows that 2 V total and 1.74 V cross the cell has an insignificant effect on the director relaxation dynamics [13]. To determine the process of relaxing the director from the homeotropic to the planar state, periodic voltage was applied, with 100 V applied for 1 ms, and then 2 V for 1 ms. When it is in a homeotropic state, since the liquid crystal material MLC-2144 has positive delta epsilon, it gives maximum capacitance. When it goes to a transient planar state, the capacitance is the minimum value, because now the director is perpendicular to the electric field. Then, as the liquid crystal transforms into the multi-domain state, the capacitance is larger than the capacitance in the transient planar state, but still smaller than the capacitance in the homeotropic state. The T-C curve in Fig. 3(b) offers clear evidence that the director can quickly relax to the transient planar state from the homeotropic state.

C. Optics Measurement

The setup shown in Fig. 4 was used to measure the optical response of the device. Figure 4(a) is a sketch of the setup. A white-light source was set at 90 deg of the grating output direction. By grating wavelength selective deflection, we obtained a wavelength with 25 nm spectral width. Figure 4(b) shows the structure of the shutter (left) and reference cell used to normalize maximum intensity (right). The real setup is shown in Fig. 4(c).

 

Fig. 4. (a) Optics measurement sketch, (b) cell stack structure, and (c) photo of the setup.

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We tuned the grating to various wavelengths for our measurements. The spectral width of the light leaving the grating was about 25 nm.

3. RESULTS AND DISCUSSION

To normalize our optical data, we first tuned the grating to one of the test wavelengths and measured the transmission with the detector covered to find the “zero level.” Then, with the E7 device found its light transmission to find the “100%” level.

The optical response of the device consisting of two cells separated by a half-wave plate (as described in the paragraph on device fabrication) was measured. The voltage waveform and resulting optical response is shown in Fig. 5.

 

Fig. 5. (a) 1-ms-on, 1-ms-off driving form and (b) shutter transmission versus time.

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After each voltage pulse, when 0 V is applied, the director starts to relax from the homeotropic state to the transient planar state, causing the transmission to drop. Then it goes up, because the director field transforms to the multi-domain state. The multi-domain structure scatters light, especially for shorter wavelengths of light, lowering the transmission. After 1 ms, when the high voltage is reapplied by the next pulse, the director field transitions back to the homeotropic state, which is nearly transparent.

The 525 nm wavelength was used to explore the adjustable range of on and off times of this device using two cells. We first set the on time to be 1 ms, with off times of 0.4, 0.5, 0.6, 0.8, and 1 ms, as shown in Fig. 6(a). From Fig. 6(b), it is clear that if the off time is shorter than 0.5 ms, the director is still in the relaxing process between the homeotropic and transient planar states. When the off time is 0.5 ms, it is in the transient planar state. If the time is longer, the director starts to transition to the multi-domain state.

From experimental data, we see that if the director is on the way to relaxing from the homeotropic state to the transient planar state when we apply voltage, it is much easier for the director to return to the homeotropic state, as the green curve shows in Fig. 6, which is driven by 1 ms on and 0.4 ms off voltage.

 

Fig. 6. (a) 1 ms on with 0.4, 0.5, 0.6, 0.8, and 1 ms off driving form and (b) shutter transmission versus time.

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The above data has shown that the longer the device relaxes at 0 V, the more time it takes to return to the homeotropic state when the voltage is re-applied. We have simulated the director relaxation with time, and have made clear what is causing this effect.

The simulation method is based on our previous work [11–13].

The simulation parameters used were: $K11=13$, $K22=7$, $K33=1$; the equilibrium $\text{pitch}=187.5\mathrm{nm}$, (which means the transient planar pitch is around 375 nm with reflection peak at about 600 nm); the pretilt angle is 89 deg; and the simulation area is $3\mathrm{\mu m}\times 3\mathrm{\mu m}$ with grid points of 120 per micrometer.

In Fig. 7, the first three graphs are the relaxation states starting from the homeotropic state with [Fig. 7(a)] 1000 loops, [Fig. 7(b)] 2800 loops, and [Fig. 7(c)] 6400 loops. The red lines are used to describe directors. In Fig. 7(a), the director configuration has begun to relax from the homeotropic state toward the transient planar state. Figure 7(b) is the perfect transient planar state. If we relax director with longer time, we get Fig. 7(c), which is already in the multi-domain state.

The next step is to apply vertical high voltage to turn the director back to the homeotropic state. Figures 7(d)–7(f) show the director configuration after voltage was applied for 4000 loops. Figure 7(d) is the result by applying voltage in the director configuration of Fig. 7(a). Figure 7(e) is the result by applying voltage in the director configuration of Fig. 7(b). Figure 7(f) is the result by applying voltage in the director configuration of Fig. 7(c). Note that, the longer the relaxation from the homeotropic state, the more defects appear after a vertical field applied, which means that more time is needed to turn the director back to a perfect homeotropic state. Figures 7(g)–7(l) and 7(m)–7(r), respectively, show magnifications of the configurations for the areas marked with the larger black square and the smaller green square frames in Figs. 7(a)–7(f), to show the director more clearly.

We then investigated the effect of the voltage applied time, with the off time set at 1 ms. In Fig. 8, the results for voltage pulse lengths of 0.4, 0.5, 0.6, 0.8, and 1.0 ms are shown. From the graph, we can see that 0.4 ms on is too short to make the director go completely back to the homeotropic state. However, 0.5 ms on is long enough to cause the director to return to a high transmission level for this device with 80 V applied.

 

Fig. 7. (a) Before transient planar state, (b) perfect transient planar state, (c) after transient planar state, (d) applying voltage to turn director from state (a) back to homeotropic state, (e) applying voltage to turn director from state (b) back to homeotropic state, (f) applying voltage to turn director from state (c) back to homeotropic state. (g)–(l) Magnifications of director configurations in black frames of (a)–(f), and (m)–(r) magnifications of director configurations in green frames of (a)–(f).

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Fig. 8. (a) 1 ms off with 0.4, 0.5, 0.6, 0.8, and 1 ms on driving form and (b) shutter transmission versus time.

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We then tested a device consisting of a pair of two-cell devices (four cells) to improve the device contrast. Figure 9(a) is the waveforms used that have 1 ms on time and several off times. Figures 9(b)–9(d) show the transmission of the two device and the four-cell device for 625, 525, and 475 nm. It is obvious that the transmission of the four-cell stack provides a better dark state. From the graphs, we measured the delay and response times that are shown in Table 1.

Table 1. Device Transmission and Response Times

View Table

 

Fig. 9. (a) 1 ms on, 0.5, 0.6 ms off driving waveform, (b) shutter transmission versus time in 625 nm, (c) shutter transmission versus time in 525 nm, and (d) shutter transmission versus time in 475 nm.

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Here, we define:

As a demonstration of the device, we made an electric circuit of two green LED lights with the same brightness and frequency but different phase. One of the LED lights (bottom) phase matches perfectly with the dark state of the sample four-cell device, while the other LED light (top) is out of phase. Figure 10 shows pictures taken by the camera through the device, with it as close as possible to the front of the camera lens. The picture on the left was made by a four-cell device filled with E7 for normalization, while the picture at the right shows the image with the sample four-cell switching cholesteric device in front of camera. It is obvious that LED light in phase with the time of the transient planar state of the device is well blocked, while the light from the surroundings and from the other LED is only slighted attenuated.

 

Fig. 10. Transmission versus time of LED light in phase and out of phase, and real pictures of LED light.

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The device works well for applications where a short light blocking time is required, but there are other applications where a longer light blocking time is needed. Here we propose a driving voltage waveform to achieve long dark states by driving a stack of devices with different phases. We found that using a high voltage (110 V) can allow the on-state voltage pulse to be as low as 0.138 ms. The curves in Fig. 11(a) use that value of the on-state time and have a zero-voltage time of 0.69 ms.

As an example of how a cell with the optical response of the device shown in Fig. 11(a) can be used to block light for a long interval of time, we considered the case of stacking three of these devices whose driving waveforms are offset with respect to each other. Figure 11(b) shows the results of this approach using 550 nm light, where each cell has its drive waveform (voltage applied for 0.138 ms and removed for .69 ms) offset by .276 ms.

 

Fig. 11. (a) Transmission of a minimized two-cell stack versus time, and (b) method to achieve a long off state with a six-cell stack (three units).

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3. CONCLUSION

We successfully made a fast cholesteric shutter by switching the director between the homeotropic and transient planar states. The device shows high transmission in the homeotropic state and substantially reduced transmission in the transient planar state for unpolarized light. The response time of this shutter was measured as less than 0.3 ms. Further, we show a device design that can allow for an extended time interval for light blocking.