1. Introduction

Blue phase liquid crystal (LC) displays have attracted much attention owing to their short response times and no alignment layers. One of their shortcomings, however, is that the blue phase exists only over a small temperature range. Kikuchi et al. used polymers to stabilize the blue phase to increase the temperature range [1]. However, anchoring of the polymer requires the application of a high operating voltage to the blue phase LC cells. Coles and Yoshizawa utilized particular LCs to fabricate blue phase cells that are effective over a wide range of temperatures [2,3]. However, these LCs are not easily obtained. Most researchers in this field have utilized temperature stages to fix the temperature of the LC cells to maintain the blue phase. These temperature stages are, unfortunately, unfavorable for use in commercial LC devices as they are very large and bulky. Therefore, it is of great interest to researchers to develop blue phase LC cells that do not require temperature stages, polymers or specific LCs.

Buckypaper is a thin sheet that is comprised of an aggregate of carbon nanotubes, and is a near-perfect absorber in the visible regime [4]. Buckypaper is highly efficient in terms of generating and dissipating heat, and the heat generation and dissipation are positive proportion to its electrical resistivity and thermal conductivity, respectively [5,6]. Accordingly, buckypaper, with the assistance of environment, may act as a thermostat for use in temperature-sensitive devices, including optical cavities and blue phase LC cells.

A planar cholesteric liquid crystal (CLC) cell whose surface is covered with buckypaper is used to fabricate a voltage-induced optical attenuator. The experimental results reveal that the CLC cell exhibits a decrease (increase) in the intensity of the reflection band when a single-pulse voltage is applied (removed), and that the wavelength of the reflection band is unaffected by the application or removal of the voltage. The decrease (increase) in the intensity of the reflection band results from heating and absorption of the buckypaper (cooling of the environment). Application of the single-pulse voltage and then a lower DC voltage to the buckypaper can keep the temperature of the CLC cell constant. Using this method, the blue phase of a CLC cell can stably exist for more than an hour at room temperature.

2. Sample preparation and experimental setup

Figure 1 presents the configuration and experimental setup of the CLC cell with the buckypaper. The buckypaper is made from multi-walled carbon nanotubes by vacuum filtration, as described elsewhere [7,8]. The area, thickness and resistivity of the buckypaper are ~20 mm × 15 mm, ~25 μm and ~1.30 × 10⁻² Ω⋅cm, respectively. The CLCs are prepared by mixing right-handed chiral agents (CB15 from Merck) with nematic LCs (LCT-09-1475 from Merck, n_e = 1.58, n_o = 1.48 at 20°C). The mixing ratio of CB15: LCT-09-1475 in the CLC mixture is 43:57 by weight. The clearing point of the CLC mixture is ~28.3 °C. An empty cell is fabricated using two homogenously aligned glass substrates with an area of ~20 mm × 15 mm, and the two substrates are separated by 4.3μm-thick spacers. The empty cell is filled with the CLC mixture, and then sealed with polymer gel. Then, buckypaper is pasted onto one of the glass substrates using epoxy glue. The reflection spectra of the CLC cell with the buckypaper are obtained using a visible-light spectrometer (USB 2000 from Ocean Optics), a white light source and a Y-type reflection fiber. A DC power supply (2410 from Keithley) is used to apply voltages to the buckypaper. All of the spectral measurements are made at room temperature, i.e., 23 °C.

 

Fig. 1 Configuration and experimental setup of the CLC cell with buckypaper.

Download Full Size | PDF

3. Results and discussion

Figure 2(a) presents the dynamic reflection spectrum of the CLC cell with the buckypaper at a single-pulse voltage with an amplitude and duration of 5 V and 14 s, respectively. The reflection intensity of the spectral peak of the CLC cell is reduced to 10% of the maximum intensity at the voltage, as displayed in Fig. 2(a). The decrease in the reflection intensity is caused by the electrothermal effect of the carbon nanotubes [5]. When the voltage is applied to the buckypaper, the heat that it generates increases the temperature of the cell. As the temperature approaches the clear point of the CLCs, the heat causes a phase transition from the planar texture to an isotropic texture. Owing to this phase transition, the incident light passes through the CLC layer and is then absorbed by the black buckypaper. Therefore, the intensity of the reflection band is reduced at the single-pulse voltage. The wavelength of the reflection band remains constant as the reflection intensity decreases because the buckypaper heats up at a rate of ~12 °C/min when the single-pulse voltage is applied. This high heating rate accelerates the phase transition. A separate experiment reveals that when a planar CLC cell without the buckypaper is put on a heating stage, when a lower heating rate of ~10 °C/min is used, the reflection spectrum of the CLC cell is red-shifted and the reflection intensity is simultaneously reduced (data not shown). This result confirms that the fixed reflection wavelength under the applied single-pulse voltage arises from the high heating rate of the buckypaper. After 14 s, the buckypaper stops heating the CLC cell because the power supply stops applying the voltage to the buckypaper. When no voltage is applied, the isotropic texture becomes planar because the CLC cell is cooled by the environment via all the surfaces. Therefore, after 14 s, the intensity of the reflection band increases until the spectrum returns to its initial state [time (t) = 0 s]. The wavelength of the reflection band remains constant in the cooling period from t = 14 s to t = 55 s. This result is attributable to the presence of residue of the planar CLC molecules after heating. At t = 14 s, the intensity of the reflection band is not at its minimum value. Restated, at t = 14 s, the CLC cell reflects light of low intensity. Therefore, some of the LC molecules still exhibit the planar texture after 14 s. The residual planar CLC molecules should promote the transition of the isotropic LC molecules into the planar CLC molecules in the cooling period because the residual planar CLC molecules act as seed crystals in the epitaxial growth [9]. The pitch of the planar CLC molecules that had undergone the phase transition is the same as that of the residual planar CLC molecules. Accordingly, the wavelength of the reflection band remains constant throughout the cooling period. The inserts of Fig. 2(a) show the photographs of the CLC cell with the buckypaper at t = 0, 14 and 55 s. According to these photographs, the spectrum can return to its initial state after the removal of the single-pulse voltage. From Fig. 2(a), the CLC cell exhibits a decrease (increase) in the intensity of the reflection band when the single-pulse voltage is applied (removed), and it can be seen that the wavelength of the reflection band is unaffected by the application or removal of the voltage. Therefore, the CLC cell with the buckypaper is promising for use in voltage-induced optical attenuators.

 

Fig. 2 Dynamic reflection spectra of the CLC cell with buckypaper when single-pulse voltages of 5 V are applied for (a) 14 s, and (b) 18 s.

Download Full Size | PDF

The application of a single-pulse voltage with a longer duration of 18 s to the buckypaper eliminates the reflection band of the CLC cell, as presented in Fig. 2(b). In the cooling period from t = 18 s to t = 89 s, the reflection spectrum of the CLC cell exhibits two reflection peaks at λ = 550 nm and 575 nm at t = 64 s, possibly resulting from the competition among the effects of the anchoring force, the heat and the chiral molecules on LC alignment. These three factors affect the orientation of the LC molecules, so the CLC cell exhibits dual-color reflection at t = 64 s. The dual-color reflection of the CLC cell is not suitable for optical attenuators. Therefore, the voltage is applied to the buckypaper for a shorter period (14 s), as presented in Fig. 2(a).

In Fig. 2(a), the grayscale intensity of the reflection light cannot be controlled by the single-pulse voltage; this lack of control is undesirable for optical attenuators. Researchers are developing variable optical attenuators. As presented in Fig. 2(a), the applied voltage (environment) reduces (increases) the intensity of the reflection band. This result may be helpful in fabricating the variable optical attenuators. Figure 3 presents the dynamically detected wavelength-integrated intensity (I) of the CLC cell with the buckypaper under various voltages. This intensity, which is integrated over a range of wavelengths from 400 to 700 nm, is used because the CLC cell exhibits a broadband spectrum. A temperature sensor is used to measure the cell temperature at the applied voltages. In the period from t = 0 s to t = 13 s, the drop in I is caused by a single-pulse voltage of 5 V. After 13 s (at zero voltage), the increase in I arises from cooling by the environment. When a lower DC voltage of 3.0 V is used, after 21 s, both I and the cell temperature remain constant (26.2 °C). This result is attributed to the thermal equilibrium between the heating by the buckypaper and the cooling by the environment. To reduce the constant I, a single-pulse voltage of 5 V is applied to the buckypaper for 4 s and then a lower voltage of 3.4 V is applied. The single-pulse voltage breaks the thermal equilibrium and increases the cell temperature, and the lower DC voltage of 3.4 V keeps I and the cell temperature (27.1 °C) constant from t = 110 s to t = 185 s. After the power supply stops supplying the voltage of 3.4 V, I increases to its initial value at t = 0. The experimental results in Fig. 3 reveal that the two lower DC voltages (3.0 V and 3.4 V) and the environment are responsible for the constancy of I. Therefore, the CLC cell with the buckypaper can be used in reflection-type attenuators. As is estimated from Fig. 3, the time constant of the change in the cell temperature from 23 (26.2) °C to 26.2 (27.1) °C is around 20 (10) s. The time constant can be shortened by applying a high voltage to the buckypaper or using buckypaper with high electrical resistivity and high thermal conductivity. Efforts are underway in our laboratory to optimize the performance of the CLC cell with buckypaper, the results of which will be published in the near future.

 

Fig. 3 Dynamic detection of wavelength-integrated intensity (I) of the CLC cell with buckypaper at various voltages.

Download Full Size | PDF

Figures 4(a) and 4(b) show the thermal images of the bare buckypaper with an area of ~25 mm × 20 mm at zero applied voltage and an applied voltage of 5 V, respectively. The images are recorded by a thermal imager (Ti10 from Fluke), indicating that the buckypaper-based thermostat exhibits a wide temperature range from 23.3 °C to 87.3 °C. With a temperature higher than 87.3 °C, the bare buckypaper either exhibits a non-uniform temperature distribution in the sheet or incurs damage.

 

Fig. 4 Thermal images of bare buckypaper (a) at zero applied voltage, and (b) at applied voltage of 5.0 V. Temperatures in the marked locations with empty squares (☐) are 23.3 °C and 87.3 °C. The insert shows the bare buckypaper.

Download Full Size | PDF

4. Application

The experimental results in Fig. 3 reveal that the application of a single-pulse voltage of 5 V to the buckypaper can increase the cell temperature, and that the application of a lower DC voltage of 3.0 V or 3.4 V can keep the cell temperature constant. This result is useful for blue phase CLC cells. Figure 5(a) presents a blue phase CLC cell with the buckypaper at zero voltage at room temperature. The structure of the CLC cell is glass/CLC mixture (LCT-09-1475 LCs + 35 wt % S811 chiral agents)/glass/buckypaper, and the glass substrates, with an area of ~20 mm × 15 mm, are separated by two 12μm-thick plastic spacers. The area and thickness of the buckypaper are ~20 mm × 15 mm and ~25 μm, respectively. The blue phase exists in the temperature range of 27.1 °C to 32.0 °C. When no voltage is applied to the buckypaper, the LC cell appears black because the reflection band of the CLCs is in the ultraviolet regime and the buckypaper is a near-perfect absorber in the visible regime, as displayed in Fig. 5(a). A single-pulse voltage of 5.0 V is applied to the buckypaper for 13 s, to heat the CLCs. After the power supply is turned off, the environment starts cooling the CLC cell, reducing the cell temperature. During cooling, the application of a lower DC voltage of 2.5 V to the buckypaper can maintain the cell temperature at 28.1 °C. Therefore, the CLC cell appears blue when the lower voltage is applied, and the blue phase can stably exist for more than an hour at room temperature, as presented in Fig. 5(b). If the temperature of the environment changes after an hour, the blue phase can be maintained by modifying the single-pulse voltage and the DC voltage. The bottom portion of Fig. 5(b) shows the thermal image of the blue phase CLC cell at an applied voltage of 2.5 V. The thermal image reveals that the cell temperature is uniform over the whole cell when the DC voltage is applied to the buckypaper, and that the thermal changes occur in the mesophase. Uniformity of the cell temperature depends on the buckypaper. If a large area of buckypaper is uniform over the whole sheet, the blue phase can be achieved over large areas of the CLC cell. The blue phase LC cell with buckypaper does not require any temperature stage, polymer materials or particular LCs. Therefore, the buckypaper, with the assistance of the environment, can give rise to the appearance of the blue phase.

 

Fig. 5 Photographs and thermal images of the blue phase CLC cell with buckypaper (a) at zero applied voltage, and (b) at applied voltage of 2.5 V.

Download Full Size | PDF

4. Conclusion

This work utilizes a planar CLC cell with buckypaper to fabricate a voltage-induced optical attenuator with a constant reflection wavelength at different reflective intensities. The application (removal) of the single-pulse voltage to (from) the buckypaper gives rise to the decrease (increase) in the intensity of the reflection band. The variation in the reflection intensity exploits the fact that the buckypaper has high electrical resistivity and high thermal conductivity, and is a near-perfect absorber in the visible regime. Application of the single-pulse voltage and then the lower DC voltage to the buckypaper can maintain the constant temperature of the CLC cell because of the thermal equilibrium between the heating of the buckypaper and the cooling of the environment. This result indicates that the buckypaper, with the assistance of the environment, can act as a thermostat owing to its ability to keep the cell temperature constant. With this thermostat, the blue phase of CLCs can stably exist for more than an hour at room temperature, without a temperature stage, polymer materials or particular LCs.