研究飛秒雷射對奈米粒子的捕捉及噴發動態學的新手法

Abstract

脈衝雷射捕捉技術最近變得越來越重要。特別的是，飛秒脈衝雷射捕捉實驗中有許多有趣的現象。其中一個有趣的現象是被捕捉的奈米粒子會從雷射焦點向外有方向性的噴發且噴發方向由雷射光的極化方向決定。我們認為這現象與飛秒脈衝雷射的捕捉動力學中粒子在脈衝中的捕捉及放鬆相關。在這篇論文中，我們提出兩個新的方法來研究飛秒雷射對奈米粒子的捕捉及噴發動態學。 在第一個主題中，我們改變兩飛秒雷射脈衝之間的時間間隔來控制捕捉力量隨時間的變化。結果發現奈米粒子的噴發頻率在十皮秒的時間間隔內會快速減少。這表示有一種比擴散還要快的鬆弛過程存在於飛秒雷射捕捉的動態學中。我們假設原因是奈米粒子的動量被干擾後快速鬆弛減弱。這個結果顯示這種動量快速鬆弛減弱過程對於脈衝雷射捕捉很重要，也給對於如何有效捕捉一些提示。 在第二個主題中，我們用兩種不同的雷射來捕捉粒子及使粒子噴發。我們用高脈衝頻率的雷射或是連續雷射來捕捉粒子，再用高峰值能量飛秒脈衝雷射來使奈米粒子噴發。我們揭示高峰值能量脈衝雷射對於噴發是必要的。峰值能量能夠改變粒子噴發的門檻。此外，我們能用這種方法來估計雷射有效捕捉範圍。這個結果顯示飛秒脈衝雷射的有效捕捉範圍比連續雷射的有效捕捉範圍更大。我們認為這是飛秒脈衝雷射捕捉效率高的原因之一。 我們成功地用新的方法研究奈米粒子的捕捉及噴發動態學。我們認為這些方法在未來會成為雷射捕捉研究者有利的工具。

Laser trapping technique is becoming more and more important recently. Especially, many interesting phenomena are observed in optical trapping with femtosecond pulses. An example is the ejection of trapped nanoparticles from the focal spot toward directions that are decided by the polarization of trapping beam. We think such phenomena are closely related to the trapping dynamics by femtosecond pulses, which repeats impulsive trapping and slower releasing of particles between pulses. In this thesis, we proposed two new methods for investigating femtosecond laser trapping and ejection dynamics of nanoparticles. In the first topic, we controlled temporal evolution of trapping force by femtosecond double pulse train method with changing time interval between trains. The frequencies of ejection showed ultrafast decay within ten picoseconds. It meant a changing of particle status that is much faster than the diffusion exists in the dynamics of femtosecond pulse trapping. We assumed this is due to a disturbance of the motion of inertia on nanoparticles. This finding indicates that such an ultrafast process has important role for femtosecond laser trapping and also gives some hints for the effective trapping. In the second topic, we separated the role of trapping and ejection of particles into two different lasers. We trapped particles using highly repetitive pulse laser or cw laser and ejected out particles from the focal spot using high peak power femtosecond pulses. We revealed that high peak power pulses are necessary for ejection of particles. Furthermore, we could estimate the effective trapping area by applying this method. The result indicated that the effective trapping area of femtosecond laser is larger than that of continuous-wave laser. We assumed this is a reason for the efficient trapping of femtosecond laser. We successfully revealed trapping and ejection dynamics of nanoparticles by a new approach. We think these methods will become very powerful tool for researchers of laser trapping in future.