1. Introduction

Liquid crystals (LCs) have been widely used for display applications. Their unique electro–optic properties also make them very attractive materials for photonic components, such as beam-steering devices, spatial light modulators, lasers, optical filters, switches, and optical nonlinear components [1]. Gratings based on LCs can play a major role in various optical systems requiring either periodic amplitude or phase modulation. Among them, LC phase gratings have the advantage of better tunability due to their higher index modulation. Various studies to improve their performance, such as diffraction efficiency, switching speed, switching voltage, and long-term stability of LC phase gratings, have been reported recently [2–13].

One of the most common fabrication methods for LC phase gratings is to build a periodic structure by the initial LC alignment patterned through either the mechanical method [2, 3] or photo-alignment technique [4, 5]. The diffraction profile can be controlled by applying an electric field. Another approach is utilizing the natural ability of LC materials with intrinsic diffraction properties, such as cholesteric and ferroelectric LCs [6, 7]. Switchable phase gratings using polymer-stabilized blue phase LCs exhibit submillisecond response time and very high diffraction efficiency although they suffer from a high operating voltage and noticeable hysteresis and require precise temperature control for device fabrication [8–10]. Recently, active studies on nematic LC-based phase gratings using interdigitated electrodes have been developed because of their outstanding features, such as simple fabrication, no diffraction at the off state, high diffraction efficiency, a large diffraction angle, and a low operating voltage [11–13].

In this paper, we report an electrically-switchable two-dimensional LC phase grating device with high haze in the translucent state for window display applications. This device consists of the top and bottom substrates with crossed interdigitated electrodes and vertically-aligned LCs sandwiched between the two substrates. This device exhibits a transparent state with high transparency and little dependence on the viewing direction. Moreover, a large spatial phase difference is induced regardless of the azimuth angle when an electric field is applied to the LC cell so that the LC cell can be switched to the translucent state with a high haze value (~84%) at a low operating voltage (7 V). In addition, we achieved submillisecond switching between the transparent and translucent states by employing the overdrive scheme and a vertical trigger pulse.

2. Principle

Figure 1 shows the device structures and on-state LC director configurations of one-dimensional (1-D) and two-dimensional (2-D) LC phase gratings. Initially, LC molecules are aligned perpendicular to the two substrates. For a 1-D grating cell, interdigitated electrodes are formed only on the bottom substrate, as shown in Fig. 1(a). This device has been used for in-plane switching of vertically aligned LCs with positive dielectric anisotropy [14, 15]. When an in-plane electric field is applied between the adjacent interdigitated electrodes, the LC molecules are tilted down in the opposite direction towards the center between interdigitated electrodes. At center positions above and between interdigitated electrodes, the LC molecules remain aligned perpendicular to the substrates. Therefore, a large spatial phase difference due to a refractive index change of the LC layer is induced along the horizontal direction, but there is no spatial phase difference along the vertical direction because there is no refractive index modulation in this direction [13]. In contrast, in a 2-D grating cell, interdigitated electrodes are formed on both substrates, and the interdigitated electrodes on each substrate are positioned at right angles to each other, as shown in Fig. 1(b). When an electric field is applied between the adjacent interdigitated electrodes on the top and bottom substrates, the LC molecules are tilted down along the electric field directions so that a large spatial phase difference is induced along the vertical and horizontal directions.

 

Fig. 1 Device structures and LC director configurations of (a) 1-D and (b) 2-D grating cells.

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To investigate the dependence of the spatial phase change on the azimuth angle, we calculated phase profiles of the output light resulted by the electric-field-induced LC director reorientation, as shown in Fig. 2. Numerical calculations were performed by using the finite element method and 2 × 2 extended Jones matrix method with the commercial software TechWiz LCD 3D (Sanayi System Company, Ltd., Korea). As expected, in a 1-D grating cell, a large spatial phase difference is induced along the horizontal direction, but there is no spatial phase difference along the vertical direction, as shown in Fig. 2(a). On the other hand, a 2-D grating cell shows a large spatial phase difference along both the vertical and horizontal directions because interdigitated electrodes are formed on both substrates. Interestingly, the 2-D grating cell shows double the spatial phase difference along the diagonal direction compared with that along the vertical or horizontal direction, as shown in Fig. 2(b), because more LC molecules are reoriented along the direction of the applied electric field thanks to interdigitated electrodes formed on both substrates. In other words, in this device, a large spatial phase difference is induced regardless of the azimuth angle when an electric field is applied to the LC cell. Therefore, when white light is incident to the LC cell, it is diffracted so that the LC cell can be switched to good translucent state thanks to a large spatial phase difference regardless of the azimuth angle.

 

Fig. 2 Calculated phase profiles of the output light along (a) vertical (V), horizontal (H), and (b) diagonal (D) directions.

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3. Experimental results and discussion

To evaluate the electro–optical characteristics of the device, we fabricated an LC cell employing the structure with crossed interdigitated electrodes shown in Fig. 1. The width of the interdigitated electrodes on each substrate was 2.8 μm, and the gap between them was 4 μm. To align LCs vertically, a thin polyimide layer was spin-coated onto each substrate and baked at 220 °C for 1 h. The cell was assembled using silica spacers with a diameter of 20 μm. Finally, LCs with positive dielectric anisotropy were injected into the cell via capillary action. The dielectric and optical anisotropies of the LC (Merck E7) used in the fabrication were 13.5 and 0.223, respectively.

To confirm the dependence of diffraction characteristics on the azimuth angle in the fabricated LC cells, we measured the diffraction efficiency for the zeroth order as the applied voltage was increased, as shown in Fig. 3. Here, we focused on how much the incident light was transferred from the zeroth order to the higher orders. A linearly polarized He-Ne laser beam (λ = 543.5 nm) was used for this experiment. The far-field intensity of the zeroth order was detected with a photo-diode located 30 cm away from the LC cell.

 

Fig. 3 (a) Measured voltage-dependent diffraction efficiency for the zeroth order of the vertical (V), horizontal (H), and diagonal (D) polarization. Diffraction patterns of (b) 1-D and (c) 2-D grating cells.

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In a 1-D grating cell, the light intensity of the zeroth order is largely reduced for the horizontal polarization because most of the laser energy goes to the higher orders. However, there is little change in the intensity of the zeroth order for the vertical polarization because there is no refractive index modulation along this direction. Although a periodic refractive index distribution along the direction perpendicular to interdigitated electrodes exists owing to the periodically interdigitated ITO electrodes, the refractive index modulation induced by these transparent electrodes is tiny, so most of the incident light energy is transmitted unaffected by the cell. As expected, the diffraction characteristics of a 1-D grating cell are largely dependent on the polarization direction, as shown in Fig. 3. This is why a 1-D diffraction grating cell exhibits a low haze value in the translucent state.

In contrast, in a 2-D grating device, most laser energy goes to the higher orders so that the intensity of the zeroth order is largely reduced irrespective of the polarization direction. Moreover, when a laser beam whose polarization is diagonal to the crossed interdigitated electrodes is incident to a 2-D grating cell, a strong diffraction effect is observed owing to a spatial phase difference much larger than that of a 1-D grating cell. We can see clearly that the diffraction energy is well transferred from the zeroth order to the higher orders regardless of the polarization direction, as shown in Fig. 3(c). Therefore, we can expect that a 2-D grating cell with crossed interdigitated electrodes may be switched to an excellent translucent state thanks to a large spatial phase difference regardless of the azimuth angle.

To demonstrate the optical performance of the fabricated LC cells, we measured the total transmittance, specular transmittance, and haze using a haze meter (HM-65W, Murakami Color Research Laboratory). The specular [diffuse] transmittance T_s [T_d] 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. 4. The total transmittance T_t is the sum of the specular transmittance T_s and the diffuse transmittance T_d. The haze H can be calculated as H = T_d/T_t.

 

Fig. 4 Experimental setup for evaluation of the optical performance of the fabricated cells.

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We compared haze values of a 1-D grating and a 2-D grating cell, as shown in Fig. 5. The 1-D grating cell showed a haze of 56.9% at an applied voltage of 7 V, whereas the 2-D grating cell with crossed interdigitated electrodes showed a haze of 83.8% at the same applied voltage. The haze value of the 2-D grating cell with crossed interdigitated electrodes was 47.3% higher than that of the 1-D grating cell. It is comparable to that of previously reported LC light shutters based on light scattering. Recently, a light shutter using polymer-stabilized LCs with crossed patterned electrodes was reported for switching between the transparent and translucent (or opaque) states [16]. Not only the scattering effect due to the polymer structure but also the diffraction effect due to the refractive index change of the LC layer contribute to the high haze in the translucent state of this device. In contrast, the LC cell reported in this paper does not contain any polymer matrices, so haze in the translucent state is primarily dependent on diffraction of white incident light by the electric-field-induced periodic continuous LC profile. Therefore, to generate a large spatial phase difference with little dependence on the azimuth angle, a relatively large cell gap is required to reduce the crosstalk between the electric fields generated by the top and bottom electrodes.

 

Fig. 5 The measured haze values of the fabricated 1-D and 2-D grating cells as functions of the applied voltage.

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Figure 6 shows the dependence of the electro–optical characteristics on the cell gap of LC cells. When the cell gap was 10 μm, the measured total transmittance, specular transmittance, and haze of the 2-D grating cell in the transparent state were 77.6%, 76.9%, and 0.9%, respectively, whereas those of the 2-D grating cell in the translucent state (7 V) were 76.7%, 24.1%, and 68.6%, respectively. When the cell gap was increased from 10 μm to 20 μm, the haze in the translucent state increased by 22.2%. When the cell gap was 20 μm, the measured total transmittance, specular transmittance, and haze of the 2-D grating cell in the transparent state were 75.9%, 75.3%, and 0.9%, respectively, whereas those of the 2-D grating cell in the translucent state (7 V) were 77.3%, 12.5%, and 83.8%, respectively. These results show that we should fabricate an LC cell with a sufficiently large cell gap to achieve a good translucent state owing to the strong diffraction effect without electric field distortion.

 

Fig. 6 Measured (a) total transmittance (T_t), specular transmittance (T_s), and (b) haze of the fabricated 2-D grating cells as functions of the applied voltage with the cell gap as a parameter.

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We captured images of a 2-D grating cell by placing it on a printed image. Viewing angle dependence of the LC cell with crossed interdigitated electrodes in the transparent and translucent states are shown in Fig. 7. As shown in Fig. 7, we can view the clear background image in the transparent state like a window because the cell has high transmittance thanks to an LC layer without a polymer structure. In the translucent state, an LC cell with crossed interdigitated electrodes can hide the background view by the strong diffraction effect due to a large spatial phase difference regardless of the azimuth angle. Moreover, the device exhibited wide viewing angle characteristics in both the transparent and translucent states, as shown in Fig. 7.

 

Fig. 7 Viewing angle dependence of the fabricated 2-D grating cell with a cell gap of 20 μm.

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In addition to optical performance, such as high transparency in the transparent state and high haze in the translucent state, a short response time is one of the major requirements for window display applications. We investigated the dynamic switching behavior of the fabricated LC cells, as shown in Fig. 8. To measure the response time of the fabricated cells, 7 V was applied to LC cells. The turn-on and turn-off times of the 1-D grating cell were 6.55 ms and 10.97 ms, respectively, whereas those of the 2-D grating cell were 7.36 ms and 22.23 ms, respectively, as shown in Fig. 8(a). In a 1-D grating cell, the LC molecules were tilted down in the opposite direction towards the center between interdigitated electrodes, as shown in Fig. 1(a). At center positions above and between interdigitated electrodes, the LC molecules remained vertically aligned, and there was no change in the polar angle of the LC director. Therefore, the LC molecules were 2-D confined, resulting in a relatively short response time despite a large cell gap of 20 μm [15, 17–19]. On the other hand, in the 2-D grating cell, the confinement effect was significantly decreased because of the crosstalk between the two electric fields generated by interdigitated electrodes on each substrate, as shown in Fig. 1(b), which resulted in a relatively slow response time of the 2-D grating cell.

 

Fig. 8 (a) Temporal switching behavior of the fabricated 2-D grating cell driven with (b) the overdrive and a vertical trigger pulse.

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To achieve a fast response in a 2-D grating device, we can employ the overdrive scheme [20] and a vertical trigger pulse [21–25], as shown in Fig. 8(b). Initially, the cell is in the transparent state. We applied an in-plane voltage of 15 V higher than the voltage of 7 V corresponding to the translucent state for a short duration, and then the voltage corresponding to the translucent state was applied to the LC cell. For switching from the translucent state to the transparent state, a vertical trigger pulse of 20 V was applied between the top and bottom interdigitated electrodes for 1 ms. The measured turn-on and turn-off times of the 2-D grating cell using the overdrive and a trigger pulse were 0.43 ms and 0.51 ms, respectively, as shown in Fig. 8(a). The switching process using the overdrive and a vertical trigger pulse was forcibly controlled by applying an electric field so that submillisecond switching between transparent and translucent states could be achieved.

4. Conclusion

We investigated electro–optical characteristics of a vertically-aligned LC cell with crossed interdigitated electrodes for window display applications. The device can be switched between the transparent and translucent states by applying an electric field without polymer stabilization. The device exhibited outstanding features, such as a low operating voltage, high transparency and wide viewing angle characteristics in the transparent state. Moreover, the device exhibited high haze (83.8%) by the strong diffraction effect thanks to a large spatial phase difference with little dependence on the azimuth angle. In addition, we achieved submillisecond switching between transparent and translucent states by employing the overdrive scheme and a vertical trigger pulse. We believe that this device could be a potential candidate for window display applications.