有機溶液的暫態熱透鏡效應研究

本論文以二倍頻、seeding-injection、Q-Switched Nd:YAG Laser為光源，透過Z-scan技術調查SiNc-toluene溶液中光吸收所引起的熱透鏡效應。數年前，本實驗室曾在以上雷射系統內的seeder 尚可工作時透過 Z-Scan 技術調查過該樣品的熱透鏡效應。由於 seeder 目前無法工作，雷射無法輸出單縱模脈衝，進而光功率(power)隨時間的變化不再是高斯分佈。在此情況下我們不能精確地掌握雷射脈衝的光強(intensity)對時間的分佈情形，這使得定量計算熱透鏡效應變成一件困難的事情。在本論文中，我們用焦距為f = 10 cm的平凸透鏡將532 nm的TEM00模態雷射光斑聚焦，使其在束腰處的半徑為19 m (HW1/e2M)；另外我們也將示波器量到的光功率隨時間分佈函數近似為高斯函數，然後對實驗結果進行定量的擬合。當我們進行定量的擬合時，必須嚴謹地以數值方法求出熱致聲波方程式的解，藉以算出溶劑的溫度與密度隨時間與空間變化的情形，然後將此溫度與密度帶入熱折射率函數，用以計算熱折率(thermal refractive index)，進而擬合Z-scan實驗曲線。若我們將對熱致聲波方程式作穩態近似(steady state approximation，或稱instantaneous expansion approximation)時，所得熱折射率並不能成功擬合結果。據此，我們結論實驗所觀察到的熱透鏡效應屬於暫態而非穩態，而其原因在於脈衝時寬並非遠大於thermal transit time (熱致聲波傳遍光斑的時間)。透過本論文的實驗與理論，我們所得到的參數與多年前得到者不盡相同，可能原因之一固然是我們不能精確掌握光功率隨時間變化的情形，但我們也不能排除樣品差異的因素。

關鍵字

Z軸掃描技術；脈衝光能檢測器；暫態熱透鏡效應；五能級模型

並列摘要

In this study, we use Z-scan technique with a frequency doubled, seeding injection, Q-Switched Nd:YAG laser to survey photo absorption-induced thermal lensing effect in a SiNc-toluene solution. Several years ago, ex-graduate students of our laboratory had been finished this experiment using Z-scan technique when the seeder in of the laser system worked properly. After the seeder was damaged some time ago, the laser has not export pulses with single longitudinal mode and hence a temporally Gaussian distribution of intensity. In stead, it has output laser pulses with multi longitudinal modes and hence temporal distribution of intensity varying from pulse to pulse. This hinders us from monitoring the intensity of laser pulses in time regime precisely. Under this circumstance, quantitative study of thermal lensing effect becomes very difficult. In this study, we focus, using a plane-convex lens with a focal length of 10 cm, the 532 nm TEM00 mode laser beam to the waist of radius 19 m half-width at e2 maximum (HW e2M). In addition, we approximate the temporal dependence of power, measured by an oscilloscope, as from which we derive the corresponding intensity to fit the experimental results quantitatively. To fulfill the theoretical fitting of the experimental results, we have to strictly solve the thermal acoustic wave equation by numerical methods to obtain the temperature and density of the solvent as a function of time and space. Afterwards, we substitute these temperature and density into thermal refractive index function to fit the Z-Scan curve. It is worth noting that when we make a steady state approximation, referred to as instantaneous expansion approximation, of the thermal acoustic wave equation, the simulated Z-scan curve deviates from the experimental one greatly and shows an exaggerated negative lensing effect. This indicates that the experimentally observed thermal lensing effect is in the transient regime but not in the steady state regime. This is understandable because the laser pulse width is shorter or comparable with the thermal transit time (the time for a thermal acoustic wave to propagate across the beam radius which is 20 ns). Via the theoretical and experimental study in thesis, the parameters we acquired differs from the corresponding ones we obtained a few years ago. One possible reason is that we can’t master the power distribution of laser pulses in time regime precisely. The other possible reason is that the samples are different somehow.