摘 要

　　膽固醇液晶之分子排列具有一維的週期性結構，並且其周期的單位是在光波長之尺度，所以我們可視之為”一維的光子晶體”。光在光子晶體中的波形，類似於電子在晶體內分布模式。所以，膽固醇液晶具有光子晶體能隙特性，並且在能隙邊緣的波長，其在液晶元件之傳播如同一共振腔結構。所以當添加雷射染料於平行結構的膽固醇液晶樣品中，作為粒子數反轉的工作物質，在適當的激發下，染料之放射光譜與膽固醇液晶的反射波段能隙邊緣適當的重疊，則可在能隙邊緣的波長位置得到加強性增益的效果，而沿螺旋軸的方向激發雷射光。

　　而在本論文中，除了製作出膽固醇液晶雷射元件之外，更藉由研究各種對膽固醇液晶反射波段調控的方法，而得到在相同元件下，不同輸出波長的雷射。實驗上分別使用了光控及電控方法。

　　其中光控的部分是利用添加一種可光調親手性偶氮材料Azo-B，這種染料分子在UV光的照射下，具有trans態轉cis態的特性，而影響膽固醇液晶的螺距大小，使得膽固醇液晶的反射波段往短波長移動。實驗上更首先使用了添加兩種雷射染料的方法，使膽固醇液晶反射波段移動的波長位置皆包含於雷射染料的放射光譜內，所以在適當的激發下，利用UV光的照射時間調控膽固醇液晶雷射輸出波長，其可調控的範圍可達100nm，並為一可回覆的元件，且具實用潛力。(這一部分的研究成果已經發表於期刊Appl. Phys. Lett.86,16120 (2005))

　　而電控方面，則首先利用了負型液晶摻雜少量正型液晶調配成膽固醇液晶材料。在外加直流電壓下，由於樣品內部產生電致流體動力運動，使得膽固醇液晶反射波段有藍移的現象，並且隨外加直流電壓的增加，反射波段移動的波長範圍越大。目前實驗上激發出的雷射波長調控範圍雖然約為10nm，但是除了可回覆的特點外，更具有定頻維持的優勢。還可藉由控制頻率的大小，來作為雷射輸出的開關。所以在應用上更為合適。

　　除此之外，實驗上還觀測摻雜高分子聚合物的膽固醇液晶雷射元件對雷射輸出的現象，由於膽固醇液晶受到高分子聚合物邊界拉拔作用的影響，導致膽固醇液晶分子間螺旋軸方向有不同角度的分布，所以在適當的激發之下，可以同時觀察到不同波長位置的雷射輸出現象。

　　總之，只要利用一激發雷射，再配合具有可見光區大範圍調變能力的膽固醇液晶裝置，即可得到不同波長的雷射輸出。因此作為雷射輸出的應用，本研究非常具有實用的價值。

Abstract

　Cholesteric liquid crystal (CLC) aligned in a planar texture which has periodic dielectric structure with periodicity in the range of optical wavelengths is considered as an one-dimensional (1-D) photonic crystal (PC). Like a semiconductor, the PC has a band gap. The group velocity at the photonic band edges is real and tends towards zero. Thus, CLC doped with a laser dye is expected to lase at the edges of the band gap, in which the density of states of light exhibit a narrow singularity. This thesis studies CLC lasers with optically and electrically tunable capabilities.

　For optically tunable CLC lasers, (experiment Ⅰ), planar CLCs doped with two laser dyes are the lasing cells. Adding photo-tunable chiral material (Azo-B) with proper pumping allows the CLCs to be lased at the photonic band gap edges with a tuning range over 100nm in visible region (from 563 to 667 nm). Tuning is achieved by irradiating the sample with a UV light to transform the chiral Azo-B material undergoing trans-cis isomerization in the CLC films, which, in turn, changes the pitch. Additionally, the tuning is reversible. These characteristics make the device have great application potentials. (This part of work has been published in Appl. Phys. Letter. 86, 16120(2005)).

　For the experiment with electrical tenability, (experiment Ⅱ), a dye-doped planar CLC having a negative dielectropy is used as the lasing device. The sample is doped with a small amount of a positive dielectric LC. The reflection band of such a dye-doped CLC sample is blue-shift under the application of a dc voltage. Thus, the lasing wavelength is blue-shift. The tuning of this laser is even more stable than that of CLC lasers in experiment Ⅰ and can be reversible. Moreover, the lasing can be switched on/off by controlling the frequency of the applying voltage.

　Finally in experiment Ⅲ, we also observe the lasing behavior of dye-doped polymer dispersed cholesteric liquid crystals cell (PDCLC). PDCLC films are prepared using polymerization-induced phase separation (PIPS). The lasing occurs at the selective reflection band edges and numerous lasing peaks appear simultaneously on the top of the emission band.

參考文獻

[1] P. G. de Gennes and J. Prost, “The Physics of Liquid Crystals”, 2nded.,
Clarendon Press, Oxford (1993).
[2] L. M. Blinov and V. G. Chigrinov, “Electrooptic Effects in Liquid Crystal
Materials”, Springer-Verlag Publishing Co., New York (1994).
[3] B. Bahadur, “Liquid Crystals：Applications and Uses”, 1, World Scientific,
Singapore (1990).
[4] F. Reintzer, Monatsh. Chem. 9, 421 (1888).
[5] Letter from F. Reintzer to O. Lehmann, reported by H. Kelker, Mol. Cryst.
Liq. Cryst. 21, 1 (1973).
[6] H.-S. Kitzerow, C. Bahr,”Chirality in Liquid Crystals”, Springer, New York
(2001)
[7] Andrew J. Lovinger, Karl R. Amundson and Don D. Davis, Chem. Mater.
6, 1726 (1994).
[8] Grant R. Fowles,“Introduction to Modern Optics”, 2nd ed., University of
Utah, New York (1975).
[9] 朱自強, 王仕璠, 蘇顯渝,“現代光學教程 ”, 四川大學出版社, 成都 (1990).
[10] A. Yariv, “Quantum Electronics”, John Wiley & Sons Press, New York (1989).
[11] Iam-Choon Khoo,“Liquid Crystals-Physical Properties and Nonlinear
Optical Phenomena”, John Wiley �t Sons Press, New York (1995).
[12] 1.P. G. de Gennes and J. Prost,“The Physics of Liquid Crystals”, 2nd ed.,
Clarendon Press, Oxford (1993).
[13] H. Kozawaguchi and M. Wada, Mol. Crys. Liq. Crys. 45, 55 (1978).
[14] P. G. de Gennes, Sol. State Commun. 6, 163 (1968).
[15] R. B. Meyer, Appl. Phys. Lett. 12, 281 (1968).
[16] W. Helfrich, Phys. Rev. Lett. 23, 372 (1969).
[17] L. M. Blinov and V. G. Chigrinov, “Electrooptic Effects in Liquid Crystal
Materials”, Springer-Verlag, New York (1994).
[18] Jonathan P. Dowling, J. Appl. Phys. 75, 1896 (1994).
[19] V. I. Koop, Opt. Lett. 23, 1707 (1998).
[20] A. Munoz F, Opt. Lett. 26, 804 (2001).
[21] L. S. Goldberg and J. M. Schnur, U.S. patent 3, 771,065 (1973).
[22] Heino Finkelmann, Adv. Matter. 14, 1069 (2001).
[23] Jui-Hsiang Liu and Hung-Yu Wang, J. Appl. Polym. Sci. 91, 789 (2004).
[24] Christian Ruslim and Kunihiro Ichimura, J. Phys. Chem. B 104, 6529 (2000).
[25] H.-K. Lee, A. Kanazawa, T. Shiono, T. Ikeda, T. Fujisawa, M. Aizawa, and B.
Lee, Chem. Mater. 10, 1402 (1998).
[26] P. V. Shibaev, V. I. Kopp, and A. Z. Genack, J. Phys. Chem. B, 107, 6961-
6964 (2003).
[27] R. Williams, J, Chem. Phys. 39, 384 (1963)
[28] W. Helfrich, J. Chem. Phys. 51, 4092 (1969)
[29] D.Demus, J.Goodby, G.W.Gray, H.-W.Spiess,V.Vill, ”Handbook of Liquid
Crystals”, Wiley-VCH, New York (1998), Chap.9.