飛秒脈衝雷射引發簡單分子透明液體熱透鏡效應之研究

Thermal Lensing Effect of Transparent Simple Molecular Liquids Studied with Femtosecond Laser Pulses

李乙氣(Yi-Ci Li)

指導教授 ： 魏台輝

國立中正大學/理學院/物理學系/博士(2012年)

摘要

使用Z-scan技術與800 nm的28飛秒脈衝雷(重複率為82 MHz)，我們調查甲醇，水，和二硫化碳之非線性折射。我們的報告驗證這些透明樣品(不具備線性與非線性吸收)的熱透鏡效應。此外，我們以受激拉曼散射造成的分子運動(分子內的振動)以及此運動之鬆弛解釋透明樣品獲得雷射光能的機制。並且定量計算跨脈衝累積效應造成之溫度升高引起的折射率變化。
使用電子控制快門，我們將連續輸出之脈衝截斷為脈衝串。脈衝串時間寬度與相鄰兩脈衝串間距皆長於熱擴散時間常數τth。脈衝串傳播經過樣品後，我們採取了在不同時間的測量(曝光時間τexp，相對於每個脈衝串的前緣)。結果顯示，在所有的開孔Z-scan曲線中，焦點附近皆沒有可辨別之現象。閉孔Z-scan曲線則顯示負透鏡效應之結果。我們使用不同的入射光功率，調查閉孔Z-scan曲線之峰谷差(DTp-v)。結果顯示，當τexp < τth時，峰谷差迅速增加。當τexp超過5毫秒(大於τth到達一定程度以上)，峰谷差逐漸變為緩慢增加，並最終趨於穩定不再變化。
在理論模型中，我們引入了一個參數 ImK(3) (拉曼極化率虛部，imaginary part of Raman susceptibility)，這使我們能夠定量計算透明液體的拉曼增益(GR(r)，Raman gain)。另一方面，我們引入dW，其表示分子受到受激拉曼散射過程以外的影響而得到的能量。根據惠更斯-菲涅耳原理(Huygens-Fresnel principle)，我們模擬閉孔Z-scan曲線，並取得了理論DTp-v值。適當的挑選 ImK(3) 和 dW，我們可以將實驗與理論之Z-scan曲線匹配。因此，我們可以估算不同的透明液體，與超短脈衝雷射相互作用之拉曼增益，並解釋簡單分子透明液體獲得雷射光能的機制。

關鍵字

受激拉曼散射 ； 飛秒脈衝 ； 透明液體 ； 熱透鏡效應 ； 簡單分子 ； Z掃描

並列摘要

Using the Z-scan technique with an 800 nm 28 femtosecond laser running at 82 MHz, we investigated the negative nonlinear refraction of methanol, water, and carbon disulfide. We report our verification of the thermal lensing effect in these transparent (both linearly and nonlinearly) samples. In addition, we explain this effect in terms of the relaxation of stimulated Raman excited motions (intra-molecular vibrations) and pulse-to-pulse accumulative temperature raise induced refractive index change.
Using an electronic controlled shutter, we interrupted the continuously output pulses into trains with both the train-width and train-to-train separation considerably longer than the thermal diffusivity time constant τth. After the pulse train radiated upon the sample, we took measurements at different times (exposure times, τexp) relative to the leading edge of each train. The transmitted signals were recorded with respect to the sample position as open and closed aperture Z-scan curves. In all the open aperture Z-scan curves, no discernable structure at the vicinity of the focal point were observed. Therefore, the magnitude of the lensing effect is represented by the peak-to-valley difference (DTp-v) in a closed aperture Z-scan curve. Using several incident laser power, DTp-v was investigated with respect to τexp. For τexp < τth, DTp-v rapidly increases with τexp. After τexp exceeds 5 ms (larger than τth to some extent), the variation of DTp-v with τexp gradually turns slow and, eventually, steady.
In our model, we introduce a parameter ImK(3) which enables us tocalculate the Raman gain GR(r) of transparent liquids. On the other hand, we introduce another parameter dW which denotes the work done on nuclei by the effects other than stimulated Raman scattering process. We simulate each closed aperture Z-scan curve according to Huygens-Fresnel principle and obtain a theoretical DTp-v. With some carefully chosen ImK(3) and dW, we can match theoretical DTp-v to the experimental one. We can therefore estimate the Raman gain of different transparent liquids interacting with ultrashort laser pulses.

並列關鍵字

stimulated Raman scattering ； femtosecond laser pulse ； transparent liquid ； thermal lensing effect ； simple molecular ； Z-scan

參考文獻

Chapter 1

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