82 MHz飛秒雷射脈衝在二溴乙烷與二氯乙烷混合溶液中造成的熱透鏡效應研究

Study of Thermal Lensing Effects Induced in C2H4Br2-C2H4Cl2 Mixture by 82 MHz Femtosecond Laser Pulses

宋展志(CHAN-CHIH SUNG)

指導教授 ： 魏台輝

國立中正大學/理學院/物理學系/碩士(2016年)

摘要

以波長為815 nm、時寬為21 femtosecond (fs)、重複率為82 MHz的鈦藍寶石脈衝雷射(Ti:Al2O3)為光源，我們對不同質量分率(mass fraction)的二氯乙烷與二溴乙烷混合溶液(二氯乙烷-溴乙烷)進行Z-scan實驗量測，藉以探討混合溶液的熱透鏡效應，並且釐清熱擴散與質量擴散對熱透鏡效應的貢獻。
過去我們曾以同樣的光源分別對二氯乙烷與二溴乙烷等二種簡單液體進行Z-scan實驗量測，結果發現單光子吸收對樣品增溫的貢獻大於雙光子吸收與受激光散射。單光子吸收後的非輻射性鬆弛造成空間中不均勻的樣品增溫，進而造成溫度梯度；接下來，溫度梯度在樣品中驅動一熱聲波而造成空間中不均勻的密度改變。對屬於流體靜力學系統(hydrostatic systems)的單一成分樣品而言，其熱力學座標為壓力、密度以及溫度，其中獨立者有二。藉著取密度與溫度為獨立的熱力學座標，我們將折射率的改變表達成delta n(密度，溫度)。
在本論文中，當我們以二氯乙烷-二溴乙烷為樣品時，因為樣品為二元溶液，因此熱力學座標除包括壓力、密度以及溫度外，另包括二元成分的質量分率wA 與wB 以及化學位能uA與uB。以上”prime”用以表是我們所調查的樣品是二元系統；wA 與wB分別代表A和B成分的質量分率，其中wA=1wB。因為多了四個熱力學座標(其中獨立者有二：wA與uA)，我們可以將折射率改變寫成delta n(溫度，密度，wA與uA))。今因支配wA變化的方程式可以寫成wA，Dθ和Dmd的函數，其中Dθ與Dmd分為二元溶液的熱擴散與質量擴散係數，當我們用Z-scan量測二氯乙烷-二溴乙烷的n之後，可進一步決定Dθ、Dmd以及索雷特係數ST(等同於Dθ/Dmd)之值(包括正負號)。
在本實驗之前，光外差探測-熱擴散致瑞利散射(OHD-TDFRS)技術最常被用來探討熱擴散與質量擴散，取其有高敏感度的優點。由於OHD-TDFRS是利用兩道相干的雷射光作為寫入光源(pump)造成光柵，再用另以一道雷射光作為偵測光源，相較之下，Z-scan技術雖然敏感度較低但仍足夠調查熱擴散與質量擴散效應，而且它有操作簡單的優勢。此外，當我們在本研究中使用815 nm的雷射脈衝為光源時，因為二氯乙烷與二溴乙烷都具有吸光的能力，所以我們不需要額外加入惰性染料到二氯乙烷-二溴乙烷中以加強它對雷射光的吸收；相反地，當大部份OHD-TDFRS量測都以可見連續光為光源時，研究者無可避免地需要在樣品中加入惰性染料以增強樣品的吸收。

關鍵字

飛秒 ； 混合溶液

並列摘要

Using the Z-scan technique with 815 nm 21 femtosecond (fs) TEM00 mode laser pulses delivered at a repetition rate of 82 MHz, we investigate the thermal lensing effect of C2H4Br2-C2H4Cl2 mixture, a binary system, at various mass fractions.
Before this work, we have explored the thermal lensing effect induced in the constituent components C2H4Br2 and C2H4Cl2 using the same experimental technique. As a results, we verified that linear absorption dominates over two-photon absorption and stimulated light scattering in heating of both samples. Relaxation subsequent to this linear absorption causes a spatially non-uniform temperature rise and hence the gradient of temperature Afterwards, gradient of temperature drives in the samples an acoustic wave, namely thermal acoustic wave, propagating in the lateral direction at a speed of v in the order of 1000 m/s. This wave renders a spatially non-uniform sample density change When both samples are hydrostatic systems with P (the pressure), temperature and density change as the thermodynamic coordinates, two of them are independent. Selecting temperature and density as the independent coordinates, we represent the refraction change by delta n(density,temperature). Since density change becomes noticeable after the acoustic wave propagates across the beam cross-section in the transit-time τac (define as w0/v with w0 denoting the beam radius at the waist) estimated in 10 ns order, an 21 fs-pulse with its duration greatly shorter than τac does not experience the thermal lensing effect induced by itself. However, it experiences the thermal lensing effect induced by pulses prior to it because the pulse-to-pulse separation of τpp=12.2 ns ((82 MHz)1) is comparable with ac but greatly shorter than the thermal diffusivity times τth in the sub-ms to ms order.
When C2H4Br2-C2H4Cl2 mixture is a binary hydrostatic system, it contains P, temperature, density, uA, uB, wA and wB as the thermodynamic coordinates. Here the primes relates the properties to the binary system. u and w denote the chemical potential and mass fraction specified by the subscripts A and B. Here A and B relate to C2H4Br2 and C2H4Cl2, respectively. Since wB equals 1wA, there are four independent thermodynamic coordinates among the above-mentioned five. Taking P, temperature, density, uA, uB, wA and wB as the independent coordinates, we represent delta n as delta n(density,temperature, wA) This suggests that we incorporate thermal and mass diffusions into the mechanisms of thermal lensing effect. When the change of wA is governed by function of wA,D and Dmd, where D and Dmd represent the coefficients of thermal diffusion and mass diffusion, respectively, the goal of this thesis is to measure the signs and magnitudes of D、Dmd and the Soret coefficient ST (define as D/Dmd).
Before this study, the optical heterodyne detection scheme embedded thermal diffusion forced Rayleigh scattering (OHD-TDFRS) technique has been used to explore thermal and mass diffusions in various binary systems, because it has the advantage of higher sensitivity. In comparison with OHD-TDFRS technique, Z-scan technique has the advantage of ease of operation and the disadvantage of higher sensitivity to convection. Besides, use of the 815 nm laser pulses in this study, in contrast to visible continuous light commonly used in OHD-TDFRS measurements, unnecessitates adding inert dyes into C2H4Br2-C2H4Cl2 to strengthen the absorption. This avoids the undesired artifacts caused by the dyes.

並列關鍵字

Z-scan

參考文獻

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