臺灣博碩士論文加值系統

摘要

本論文藉由觀察自我相位調制產生的繞射環研究在向列型液晶(BL009)摻雜偶氮染料(D2)的轉動效應。自我相位調制為入射雷射光改變了樣品的折射率，折射率改變影響雷射本身的相位，因此在遠場形成繞射環，繞射環的圈數N正比於樣品在照射中心的折射率改變量，而折射率改變量越大即對應到樣品在照射區域中心的相為偏移。
實驗分單光子(綠光雷射)及雙光子(紅光及綠光雷射)進行，單光子驗結果顯示，由入射綠光引致的相為偏移為入射綠光光強 (IG)的函數。當入射綠光光強IG=0~0.7 W/cm2、IG=0.7~1.1 W/cm2 與IG>1.1W/cm2，在這三個區間，會有不同現象產生，我們可以觀察到中心處的最大相位偏移先變大(IG=0~0.7 W/cm2)，再變小直到幾乎為0(IG=1.1 W/cm2)，之後又變大的現象(IG<3.1 W/cm2)。我們認為偶氮染料吸收綠光，並從trans→cis態的過程中，會使液晶朝與光場垂直的方向轉動，這樣轉動所貢獻的力矩為負力矩 ，其值與trans→cis態的分子數有關。偶氮染料吸收紅光與熱，並從cis→trans態，會使液晶朝與光場平行的方向轉動，力矩為正力矩 ，其值與cis→trans的分子數有關，而總力矩為 與 的疊加，總力矩值越大會使液晶轉動的角度越大。在IG=0~0.7 W/cm2時，偶氮染料從trans→cis態的分子數較cis→trans態多，即 ，總力矩為負力矩且隨著光強增強而變大，因此繞射環圈數隨著光強增強而變多，在IG=0.7~1.1 W/cm2時，因為cis→trans態的數目逐漸增多，即 逐漸增強，總力矩隨著光強增強而變小(此時力矩仍為負力矩)，因此繞射環圈數隨著光強增強而變少，在IG>1.1W/cm2時，cis→trans態施予液晶的力矩 已經大於trans→cis態施予液晶的力矩 ，總力矩變為一正力矩且隨著光強增強而變大，因此繞射環圈數又再變大。在光強IG>3.1 W/cm2時，因為偶氮染料分子快速的從trans→cis態，使得在照射區域中心處的溫度上升超過相變點，在此處的液晶成為各方均向態，因此在遠場可以觀察到一Fraunhofer diffraction。
在液晶摻雜偶氮染料照射綠光時，同時入射紅光(雙光子效應)，紅光加速cis→tans態的轉換，因此增強正力矩 ，若正力矩 大於負 ，液晶與偶氮染料分子會朝與光場平行的方向轉動。

Abstract

In this thesis, we study the photo-induced reorientation of liquid crystals doped with azo dyes (D2) by observing the diffraction patterns resulting from self-phase modulation. Self-phase modulation results from that the pump laser induces the change of refractive index of the sample, which influences the phase of the pump laser itself, and then causes a diffraction pattern in the far field. The number of diffraction ring is proportional to the change of refractive index in the central part of illuminated region. The more change of refractive index is, the larger phase shift is resulted in the central part of illuminated region.
Experiments consist of two parts; the first part is performed using a green laser, and the second part is performed with pumping a red and a green laser simultaneously. The experimental results of the first part show that the change of refractive index of the sample caused by a pump green laser is a function of the intensity of the green laser (IG). At intensities of green laser IG =0~ 0.7 W/cm2, IG=0.7~1.1 W/cm2 and IG>1.1W/cm2, we observe different phenomena in these three intensity intervals. Initially, the maximum phase shift increases (IG =0~ 0.7 W/cm2), and then decreases almost to zero at IG=1.1 W/cm2, and finally, increases again with IG being less than 3.1 W/cm2. The azo dye transits from trans to cis by absorbing green light. The transition causes the liquid crystals tend to reorient to the direction perpendicular to the polarization of pump light. Such a transition contributes to a negative torque . The torque is proportional to the number of dyes transiting from trans to cis. The azo dye transits from cis to trans by absorbing red light and heat. The transition causes the liquid crystals tend to reorient to the direction parallel to the polarization of pump light. This transition contributes to a positive torque . The torque is proportional to the number of dyes transiting from cis to trans. The total torque is summation of these two torques. The greater the total torque is, the larger the reoriented angle of liquid crystals is. At IG=0~0.7 W/cm2, the number of transition of trans to cis is larger than that of cis to trans which represents . The total torque is negative and increases with increasing intensity of pump light, so the number of diffraction ring increases with increasing intensity of the pump light. At IG=0.7~1.1 W/cm2, because more dyes transit from cis to trans, is larger than that at IG=0~0.7 W/cm2. The total torque is negative and decreases with increasing intensity of pump light. The number of diffraction ring decreases with increasing intensity of pump light. At IG>1.1W/cm2, more cis-state isomers transit to trans isomer by absorbing heat, and lead to be larger than . The total torque is positive and increases with increasing intensity of pump light. The number of diffraction ring becomes more. Illuminating only green light on the liquid crystals doped with azo dye with IG>3.1 W/cm2, the transition from trans to cis is so fast which causes the temperature in the center of illumination is higher than the nematic-isotropic temperature. The liquid crystals is isotropic in the center of illumination, and a Fraunhofer diffraction in the far field is observed.
Illuminating a red laser and a green laser simultaneously on liquid crystals doped with azo dye, the red light speeds up the transition of dyes from cis to trans and causes the larger positive torque . If the positive torque is larger than negative torque , the total torque is positive and then the liquid crystals and azo-dye molecules tend to reorient to the direction parallel to the polarization of pump light.

目錄
中文摘要…………………………………………………………… I
英文摘要…………………………………………………………… IV
誌謝………………………………………………………………... VI
目錄………………………………………………………………… VII
表目錄................................................ IX
圖目錄……………………………………………………………… IX

第一章 緒論
§1-1 前言…………………………………………………………… 1
§1-2 液晶導論……………………………………………………… 3
§1-3 液晶物理……………………………………………………… 9
第二章 相關理論
§2-1 光引致的染料分子轉動…………………………………… 18
§2-2 光引致Freedericksz Transition (OFT)…..…………… 27
§2-3 液晶中的自我相位調節機制 (Self-phase modulation)……. 28
第三章 樣品配製與實驗設置
§3-1 樣品配製…………………………………………………….. 31
§3-2 實驗設置…………………………………………………….. 39
第四章 結果與討論
§4-1觀察Diode pumped Solid State (DPSS)雷射光 (532 nm)以45 ∘入
射樣品所產生單光子自我相位調制(self-phase modulation)
現象………………………………..…………………….... 42
§ 4-2用DPSS雷射 (532 nm)以45 o入射樣品，同時用氬氪離子 (Ar+
and Kr+)雷射 (649 nm)以5 o入射樣品時觀察雙光自我相位調制
效應 …………………………………………………....……… 52
第五章 結論與未來展望
§ 5-1 結論…………………………………………………………. 61
§ 5-2 未來展望......................................... 63
參考文獻…………………………………………………………... 64

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