浮接電極應用於改善液晶微透鏡之不連續線研究

Investigation of Using Floating Electrode to Prevent Disclination Lines on Liquid Crystal Microlens

孫崇璿(Tsung-Hsuan Sun)

指導教授 ： 蔡永傑

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201800460

摘要

本論文使用一般普遍使用的圓形蝕刻電極操作液晶，使液晶折射率分布形成拋物線型進而得到聚焦效果，而由於圓孔蝕刻電極的結構因素，液晶導軸方向不一致所產生的不連續線會導致成像品質及反應速率的下降，我們利用一定厚度的絕緣層來減少電場、電壓變化太劇烈的情況，使液晶往相同方向轉動，以降低不連續線的出現，並評估不同厚度對液晶微透鏡的影響，得知絕緣層厚度越厚，會影響孔徑邊緣至孔徑中心的水平電場及折射率分布變化量，進而改變最短焦距的大小及操作電壓。而將孔徑放大，因為孔徑中心到邊緣的電場、電壓變化量跟著變大，使液晶轉軸變的更容易往反方向轉動，因此需要更厚的絕緣層來平緩，也因此，相同液晶層厚度下，孔徑變大反而會使焦距變長，且反應速率、操作電壓也跟著上升。
利用這樣的方法雖然能有效的降低不連續線的出現，但這樣的作法會不僅使焦距變長，且操作電壓會跟著上升，因此本論文提出利用浮接電極來改善這樣的問題，將浮接電極置於圓孔蝕刻電極上方，能夠增加液晶層中的垂直電場，降低水平電場的影響，抑制不連續線的出現。藉由浮接電極的電容偶和特性，浮接電壓會隨與圓形蝕刻電極間絕緣層的厚度影響，在超過特定厚度下，液晶微透鏡在操作電壓範圍內不會有不連續線的出現，且有比單純使用絕緣層時更低的操作電壓及較短的焦距。
但只使用浮接電極的情況，會使液晶透鏡的相位分布從孔徑邊緣向中心縮減的問題，且隨電壓越高越嚴重。藉由模擬軟體，我們實際觀察到液晶層的液晶導軸分布，並發現這樣的問題在較薄的液晶層厚度下有所改善，可以使在圓形蝕刻電極下的液晶導軸整體角度下降，並改善在高電壓下相位分布的不對稱。

關鍵字

液晶透鏡 ； 微透鏡 ； 參數優化 ； 不連續線 ； 圓形電極

並列摘要

In this thesis, we use general hole-patterned electrode for our liquid crystal micro-lens and make the refractive index of the liquid crystal parabolic to get the focusing ability. However, based on the structure of the hole-patterned electrode, the disclination lines may occur, because of the direction of the liquid crystal may rotate in a different way and it will result in the aberration, slow response time, and poor quality of the image.
In the first part of this thesis, we use insulator, which has been the most common method used in hole-patterned electrode, to prevent disclination lines. Through computer simulations using software 3D Techwiz, we investigate the interaction between inverse domain and the effect of insulator. We use certain thickness of the insulator to reduce the effect of the horizontal electric field, which is responsible for the occurrence of the disclination lines, around the edge of the aperture to make the direction of the liquid crystal rotate in the same way. After the analysis of the different thickness of the insulator, we know that the thicker insulator will smooth the phase profile of the refractive index from the edge to the center in the aperture, which increases the focal length and operation voltage. Moreover, for the liquid crystal micro-lens in larger aperture size, the disclination lines is much easier to occur because the difference of the horizontal electric field and the voltage is larger. Therefore, it needs thicker insulator to prevent liquid crystal from rotating in different way. On the other hand, a thicker insulator will lead to the longer focal length and larger operation voltage.
In order to improve the problem of using insulator, we propose and investigate the use of a floating electrode on top of the hole-pattern electrode. Instead of using insulator, which decreases the both horizontal and vertical electric fields in liquid crystal layer, floating electrode can help increase the vertical electric field in liquid crystal layer and hence reduces the effect of the horizontal electric field. This means floating electrode can help suppress the disclination lines in a different way. By the characteristic of capacitive coupling, the floating voltage depends on the insulator between floating electrode and hole-patterned electrode. Over certain thickness of the insulator, the disclination lines can be totally suppressed in the range of applied voltage. Because there is no insulator between hole-patterned electrode and liquid crystal layer, the applied voltage can be much lower. Hence, we can get a LC microlens with lower operation voltage and shorter focal length.
However, without insulator between hole-patterned electrode and liquid crystal layer, the profile of LC lens shrinks from the edge of aperture. It affects the actual focal length and the profile shrinks more at higher applied voltage. Through computer simulations, we found that one possible way to improve the shrinking problem may be by the use of a thinner liquid crystal layer, which results in reduction of the director angle around edge of aperture and hence helps improve the asymmetrical profile at higher applied voltage.

並列關鍵字

liquid crystal lens ； microlens ； parameter optimization ； hole-patterend

參考文獻

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