應用摻鈦藍寶石雷射觀測單一及混合液體之非線性光學性質研究

Study of Nonlinear Optical Properties of Single and Binary Liquids with a Ti:sapphire Laser

蔡忠翰(Chung-Han Tsai)

指導教授 ： 湯兆崙

國立中正大學/理學院/物理學系/碩士(2012年)

摘要

本論文旨在探討簡單透明液體在飛秒脈衝下的非線性光學現象。我們以中心波長817nm、重複率81.95MHz、脈衝時寬21 fs(FWHM)的摻鈦藍寶石(Ti:Al2O3) 雷射為光源，以Z-scan技術觀測簡單透明液體(如CS2、烷基類、苯基類、水及DMSO等)、及其混合溶液(甲醇摻CCl4)的非線性光學性質變化。我們從Maxwells equations出發寫下準單色光電場與三階極化率的關係，進而推導出線性與非線性的Beer’s law，再配合Raman induced Kerr effect來解釋透明液體分子的非線性吸收來自stimulated Raman scattering造成分子內振動形成熱源，再加上高重複率脈衝造成跨脈衝累積效應，最後在液體中形成溫度梯度而產生negative thermal lens effect，而此thermal lens effect會隨曝光時間拉長而快速增強後趨於平緩。在混合溶液中，我們發現不同濃度比例會造成非線性效應的增強或減弱，且其非線性效應隨時間變化與單一樣品不同，其原因在於不同液體其熱擴散速度不同而形成液體發生濃度梯度分布(即Soret effect)，而此濃度分布所造成的密度分布就造成了溶液的非線性折射率改變。

關鍵字

飛秒雷射 ； 非線性光學 ； 混合液體 ； 摻鈦藍寶石雷射

並列摘要

We report on nonlinear absorptive and refractive properties of simple transparent liquids methanol, carbon tetrachloride and their mixtures using the Z-scan technique with a Ti:sapphire laser. This laser delivers pulses with a central wavelength of 817nm, a repetition rate of 82MHz, a pulse width (FWHM) of 30fs and a maximal average power of 300mW.
Thermal lensing effect was observed for all liquids and was found to increase with the pulse train width and gradually turn steady on the time scale of the characteristic time of thermal diffusivity. In liquid mixtures, the interactions between the component liquids would enhance or weaken the thermal lensing effect, which is predominantly contributed by temperature gradient induced concentration gradient, namely the Soret effect.

並列關鍵字

Ti:sapphire laser ； nonlinear optical properties ； binary liquids

參考文獻

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