本論文主要探討在固定液晶層厚度條件下對其進行分層結構，以了解其對圓孔型電極液晶透鏡光電性能之影響，該透鏡是由上下玻璃基板與薄蓋玻片所構成之雙液晶層結構液晶盒，研究針對不同液晶層厚度比例進行實驗，包括雙電壓操作、透鏡焦距以及影像能力的評估。
液晶分層透鏡是利用圓孔型電極於兩液晶層中產生非均勻對稱電場以轉動液晶分子使其液晶分子分佈滿足適當的折射係數梯度分佈而具有調制入射光波前的能力達到匯聚或發散光束的效果。
實驗結果顯示在相同電壓下的非均勻電場分佈中，較厚的液晶層可使電場的平均水平分量下降，結果表明上層75 μm與下層25 μm搭配之液晶分層透鏡，明顯降低了抑制不連續線產生的垂直場臨界電壓值，同時增加了透鏡的調焦範圍，其成像的品質也相較於其他厚度比例的液晶分層透鏡優異。

SUMMARY
In this thesis, electro-optical performance of hole-patterned electrode liquid crystal lenses with a constant liquid crystal layer affected by variously subdivided layers was investigated. As a result, the experiment found that under the same non-uniform electric field distribution, the thicker liquid crystal layer reduces the average horizontal component of the electric field. As a result, it obviously reduces the threshold voltages of vertical electric field to effectively prevent disclination line occurrence when the LC lens with an upper layer of 75 μm and a lower layer of 25 μm. Meanwhile, the tunable focuses achieve larger increase and better imaging performance than others.

Key words: hole-patterned electrode, subdivided layers of liquid crystal lens, dual driving voltage, disclination line

INTRODUCTION
Although the stacked structure of liquid crystal layers brings many improvements to the liquid crystal lens, it also causes additional defects to lens, that is, the disclination lines at the short focal length. Due to the upper LC layer closing to the hole-patterned electrode, there is a larger electric field gradient. If we extend the upper LC layer to the bottom of lens for a short thickness, the average horizontal components of the electric field in the layer will be reduced to a certain extent. By restraining part of LC molecules from reversed rotating, we can prevent the occurrence of disclination lines, and the voltage controlling the vertical field also be operated with a lower value.
In this study, three type of LCLs fabricated with subdivided LC layers are demonstrated and evaluated their optical performance.
 
EXPERIMENTAL MATERIALS

Fig. 1 Cross-section view of the subdivided layers in a hole-patterned liquid crystal lens structure.

Figure 1 shows the cross-section view of the subdivided layers in a hole-patterned liquid crystal lens, which is composed of top glass substrate (0.55 mm thickness) and bottom glass substrate (1.1 mm thickness) with whole ITO films and the second substrate (0.55 mm thickness) is a hole-patterned (radius 3 mm) with aluminum film. There is a cover slip between upper and lower LC layers (filled with E7 LCs, purchased from ECHO CHEMICAL CO.). In this experiment, we fabricate three type of LCLs subdivided LC layers of a constant liquid crystal thickness (100μm). LCL (50-50) is the lens with 50 μm upper and lower layers. LCL (75-25) is the lens with 75 μm upper layer and 25 μm upper. The last one which is LCL (25-75) with 25 μm upper layer and 75 μm upper.

RESULT AND DISCUSSION
Figure 2 is shown comparisons of interference patterns at the beginning of disclination lines for three type of above LC lenses when we control the driving voltage V2 at 80 Vrms. As a result, we observe that a small area of disclination line starting to appear in the center of LCL(25-75) and LCL(50-50) when V1 operated to a value lower than the threshold voltage. This area will slowly spread as time goes on and eventually extend to both sides of the lens edge. However, there is the smaller average horizontal component of the electric field in the LCL(75-25), the threshold voltage of V1 can be operated at a lower value than the other two structures, as shown in Fig.2 (c).

Fig. 2 Comparisons of interference patterns at the beginning of disclination lines for three type of LC lenses when V2 at 80 Vrms. (a) LCL (25-75) with V1 at 30 Vrms (b) LCL (50-50) with V1 at 5 Vrms (c) LCL (75-25) with V1 at 0 Vrms.

Finally, various experimental parameters to affect the image performance of LCLs are evaluated which results are shown in Fig. 3. We control the three type lenses at the same focal length of 7cm. Black and white line pair imaging, it is obvious that the edge of LCL (75-25) is sharper than LCL (50-50) and single LC layer lens.

Fig. 3 Observation of imaging performance in three types of lenses (a) LCL (50-50) applied voltage V2=80 Vrms and V1=15 Vrms (b) LCL (75-25) applied voltage V2=80 Vrms and V1=0 Vrms (c) LCL with single LC layer applied voltage V2=80 Vrms and V1=10 Vrms (d) glass lens.
 
CONCLUSION
This study has demonstrated the stacked structure liquid crystal lens with thicker upper LC layer effectively reducing the average horizontal component of the electric field. It obviously reduces the threshold voltage of vertical field and restrain disclination line at the same time. The proposed LCL (75-25) with 40 Vrms operated by V2 obtained the focal length reduction effectively and good imaging quality is also maintained compared to another two LCLs.

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