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An electrically switchable diffraction grating (ESDG) based on cholesteric liquid crystal (CLC) filled into the cell with slit electrodes is demonstrated in this study. On one hand, with low voltage, the ESDG has high second order diffraction efficiency because of the alternating planar and fingerprint textures. With high voltage, on the other hand, the ESDG has high first order diffraction efficiency because of the alternating planar and homeotropic textures. The first and second order diffraction efficiencies of ESDG are electrically swapped. The maximum diffraction efficiency of the ESDG is approximately 32% at each grating mode.
© 2014 Optical Society of America
1. Introduction
In recent years, interest in cholesteric liquid crystal (CLC) gratings has emerged for electro-optical devices, such as optical modulators [1–3], diffraction gratings, and displays, because of their electrical tunability [4–8]. When a low voltage is applied to the cell, the CLC in the planar texture transforms to the fingerprint (FP) texture that diffracts the incident light [9–12]. The second order diffraction efficiency of the FP grating (FPG) is higher than the first order diffraction efficiency, but the diffraction angle varies with voltage. Furthermore, owing to the non-uniform FP texture and the associated scattering effect, the diffraction efficiency of the FPG at each order is low [10–15]. Therefore, the potential applications of FPG are limited. On the contrary, the diffraction angle of nematic phase grating (NPG) is always fixed, and its diffraction efficiency is high. Notably, the first order diffraction efficiency of NPG is higher than the second order diffraction efficiency. NPG can be fabricated by filling the nematic into a cell with slit electrodes.
In this paper, we demonstrate an electrically switchable diffraction grating (ESDG) that can be operated in FPG or NPG mode. The maximum diffraction efficiencies (MDEs) of ESDG can be swapped between the first and second diffraction orders. Furthermore, the slit electrode structure significantly enhances the second order diffraction efficiency of the grating, which has not been observed in the conventional FPG, thereby increasing the applicability. The fabricated ESDG can be used as beam splitter and light switch grating. The electro-optical properties and possible mechanisms of the demonstrated ESDG are also discussed in this paper.
2. Experiment
The CLC mixture used in this study is composed of nematic E7 (Merck) and chiral agent CB15 (Merck). E7 has a positive dielectric anisotropic at 1 kHz, and refractive indices of ne = 1.75 and no = 1.52 for extraordinary and ordinary waves, respectively. The concentration of CB15 in the CLC mixture is 0.68 wt%, and therefore the ratio between the cell thickness and the pitch length of CLC is approximately 0.9. The CLC is injected into the empty cell with slit electrodes. The top substrate has a whole electrode, whereas the bottom substrate has slit electrodes (Fig. 1). Both the slit width and slit gap are w. The top and bottom substrates are coated with homogeneous polyimide and are rubbed antiparallel along the slit electrode direction. A polarized optical microscope (POM) is employed to observe the liquid crystal (LC) texture of the cell. The diffraction characteristics of the cell are measured by a He-Ne laser with a wavelength of 632.8 nm followed by a polarizer. The transmission axis of the polarizer is parallel to the rubbing direction of the cell. The diffracted intensity is recorded using a photodetector connected to a multi-meter. The signal is then fed to a computer for analysis. The nth order diffraction efficiency, ηn, of the grating is defined as ηn = (In/Io) x 100%, where In is the nth order diffraction intensity, and Io is the intensity of the incident beam without passing through any optical device.
3. Results and discussion
Figure 2(a) shows the first and second order diffraction efficiencies of FPG. FPG is formed by applying a small voltage to the CLC cell fabricated by an antiparallel-rubbed cell with whole electrodes on both the top and the bottom substrates. The cell approaches MDE with an application of 2.4 V because of the formed FP texture [Fig. 2(a)]. However, the diffraction efficiency of FPG is low, and the second order MDE of FPG is higher than the first order MDE. When the applied voltage is high, the diffraction patterns disappear because of the formed homeotropic texture. The first and second order diffraction efficiencies of NPG based on slit electrodes are illustrated in Fig. 2(b). The nematic LC used is E7. Notably, the NPG has MDE at 1.7 V, and the first order MDE is far above the second order MDE.
Fig. 2 (a) First and second order diffraction efficiencies of the FPG at various applied voltages. The cell thickness is 18 μm and the CLC pitch length is 20 μm; (b) First and second order diffraction efficiencies of the NPG with slit electrodes at various applied voltages. The cell thickness is 18 μm and the slit width is 20 20 μm.
Figure 2 shows that FPG and NPG have different diffraction properties. In this paper, we propose an ESDG by filling the CLC into the slit electrode cell. The MDEs of ESDG can be electrically swapped between the first and the second orders. The operation principle of the ESDG is shown in Fig. 3. As illustrated in Fig. 3(a), without voltage, the CLC is in the planar state, and the incident light cannot be diffracted. However, when a low voltage is applied to the cell, the CLCs in the electrode area are transformed to the FP state; the CLCs in the non-slit electrode area are still maintained in the planar texture [Fig. 3(b)]. CLCs also appeared planar and FP textures indicate that the diffraction of ESDG comes as a result of the summation of planar grating (PG) and FPG. Due to the created FPG, the second order MDE is higher than the first order MDE of ESDG. When a high voltage is applied to the cell, the CLCs in the electrode area are transformed to the homeotropic state; the CLCs in the non-slit electrode area are maintained in the planar texture. The diffraction intensity of ESDG is the summation of PG and NPG. Therefore, the first order MDE is higher than the second order MDE of ESDG, as shown in Fig. 3(c). Notably, the created PG provides a background diffraction for the proposed ESDG.
Fig. 3 Schematic demonstration of the CLC textures in the ESDG cell: (a) without voltage, (b) with low voltage, and (c) with high voltage.
The first and second order MDEs of the ESDG are measured at various slit widths to investigate the effects of PG at the non-slit electrode area on the diffraction of the ESDG. When the applied voltage is low (1.6 V), the FP texture is formed, the second order MDE is higher than the first order MDE, and the difference between second and first order MDEs reaches maxima when the slit width is approximately 20 μm [Fig. 4(a)]. When the applied voltage is high (8 V), the FP texture is transformed into the homeotropic texture, and the first order MDE is higher than the second order MDE of ESDG. When the slit electrode is 20 μm wide, the difference between the first and second order MDEs still reaches maxima. The results indicate that the MDEs of ESDG between the first and second orders are interchangeable. When the applied voltage is low (1.6 V), the second and first order MDEs are ~32% and ~2%, respectively. When the applied voltage is high (8 V), the second and first order MDEs are ~9% and ~32%, respectively.
Fig. 4 First and second order MDEs of the ESDG at various slit electrode widths. (a) 1.6 V is applied to the ESDG and (b) 8 V is applied to the ESDG. The cell thickness is 16 μm and the slit width is 20 μm.
POM images of the ESDG at an applied voltage of 0, 1.6, and 8 V are shown in Fig. 5. In Fig. 5(a), without voltage, CLC is in the planar state. When the applied voltage is 1.6 V, the FP structure and the planar texture are formed at the slit electrode area (part B) and at the non-electrode area (part A), respectively, as illustrated in Fig. 5(b). Given the fringing electric field caused by the slit electrodes (part B), a light stripe exists at the middle of part A. When the applied voltage is 8 V, the homeotropic state and the planar texture are formed at the slit electrode area (part B) and at the non-electrode area (part A), respectively, as shown in Fig. 5(c).
Fig. 5 POM images of the ESDG at (a) 0, (b) 1.6, and (c) 8 V. The cell thickness is 16 μm and the slit width is 20 μm.
Figure 6 shows diffraction patterns of the NPG, FPG, and ESDG, respectively. The diffraction pattern of Fig. 6(a) is attributed to NPG; that is, the phase difference between the horizontally- and homeotropically-aligned LCs, whereas that of Fig. 6(b) is attributed to FPG. Placing the CLC into the slit electrode-patterned cell creates the ESDG, and the diffraction pattern arises from the superposition of different types of gratings. Figures 6(c)–6(e) show diffraction patterns of the ESDG at 0, 1.6, and 8 V, respectively. Without voltage, the incident beam cannot be diffracted, as illustrated in Fig. 6(c). When a low voltage (1.6 V) is applied to the cell, PG and FPG are formed, and the second order diffraction efficiency of ESDG is higher than the first order diffraction efficiency. PG and NPG are formed as the voltage is increased to 8 V, and the first order diffraction efficiency of ESDG becomes greater than the second order diffraction efficiency. Notably, the transformation times of the ESDG operation modes from Figs. 6(c) to 6(d), Figs. 6(d) to 6(e), and Figs. 6(e) to 6(d) are approximately 15, 5, and 30 s, respectively.
Fig. 6 Diffraction patterns of (a) the NPG formed by filling the nematic into the antiparallel-rubbed cell with slit electrode and (b) the FPG formed by filling the CLC into the antiparallel-rubbed cell with whole electrodes on both the top and bottom substrates. The applied voltage in (a) and (b) is 2 V. Diffraction patterns of the ESDG at (a) 0, (b) 1.6, and (c) 8 V.
4. Conclusions
This study demonstrates an ESDG based on CLC filled into a cell with slit electrodes. The results reveal that the first and second order MDEs of ESDG are electrically swapped. The formed slit electrodes significantly enhance the second diffraction efficiency of ESDG, thereby increasing the potential applications of conventional FPG. When the applied voltage is low, the first and second order MDEs of the ESDG are 2% and 32%, respectively. When the applied voltage is high, the first and second order MDEs of the ESDG are 32% and 9%, respectively. Further studies on the electro-optical effects and applications of ESDG are currently being undertaken.
Acknowledgments
This work was financially supported by the National Science Council of the Republic of China, Taiwan (Contract Nos. NSC 102-2112-M-110-007-MY3, NSC 101-2112-M-110-012-MY3, and NSC 101-2112-M-018-002-MY3).
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