二倍頻是一種利用非線性光學原理而產生的技術，透過吸收兩個相同單一頻率的光子產生一個倍頻的光子。在進似電偶極下，這一種機制必須在非中心對稱的介質中發生，如：液體與液體、空氣與液體的介面及各式的絕緣體或半導體的表面。二倍頻最主要的貢獻在於分子吸附在介面上，我們使用具有類似親水性及親油性的發色團單分子：DR1、Rhodamine 6G、Pwi-39及NCTU-a，利用改變入射光的偏振方向與收集不同偏振方向二倍頻訊號的關係，求得分子在介面的排列角度。由溶液的濃度與二倍頻訊號的關係，求得分子在介面的覆蓋率。並在外加電場下，研究其吸附在介面的程度。

Optical second harmonic generation (SHG) is a nonlinear process that converts two photons of frequency w to a single photon of frequency 2w. In the electricdipole approximation, this process requires a noncentrosymmetric medium. For thin films present at the interface between two centrosymmetric media－such as the interface between two liquids, the interface between a liquid and air, or for the surface of a variety of insulators and semiconductors－only the molecules which participate in the asymmetry of the interface contribute to the SHG. Surface SGH from molecules which exhibit a large nonlinear optical response at a specific wavelength can be used to measure the relative surface coverage of molecules at the interface as a function of the external electrochemical parameters. The polarization dependence of the surface SHG can be used to estimate the average molecular orientation of chromophores at the liquid/liquid electrochemical interface. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed chromophores with respect to the interface.

Chapter 1. Introduction / 1
1.1 Review of optical second-harmonic generation / 1
1.2 SHG measurements for adsorption and surface coverage / 2
1.3 SHG measurements for molecular orientation / 7
1.4 Optical surface SHG at liquid and liquid interface / 8
1.5 Other surface SHG experiments and recently application / 11
Chapter 2. Theory / 12
2.1 To model the electromagnetic properties of surface / 12
2.2 Fresnel coefficients / 20
2.3 Phenomenological treatment of surface SHG / 24
2.4 Molecular orientation calculation / 29
2.5 Organic thin films at liquid and liquid interface / 39
Chapter 3. Experimental considerations and data analysis / 45
3.1 Experimental setup / 45
3.2 Sample / 50
3.3 Surface coverage and angular distribution / 53
3.4 Effects of electric field in the SSHG / 64
Chapter 4. Conclusion / 72
Appendix A / 76
Reference /77

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