應用飛秒雷射脈衝列捕捉奈米粒子的機制

Abstract

最近的一些實驗結果指出在某些情形之下，短脈衝列(short pulse train)能比連續波雷射更有效率地捕捉微小粒子。經由一個波寬為10^-13秒和頻率為80MHz的脈衝列幾乎可以穩定地捕捉粒子。為了去探討其捕捉機制，我們利用數值模擬去展示脈衝列與連續波雷射的捕捉過程。我們也個別對脈衝列與連續波雷射提出了簡易的近似方法。 連續波雷射的捕捉中，我們發現粒子的運動是基於熱運動的關係。捕捉機制包含了慢與快兩種不同的時間常數。我們認為這是由捕捉力與擾動而造成的機制。 脈衝列的捕捉中，在脈衝波波寬間，粒子的運動是可以忽略的，只考慮由雷射所產生的速度變化。而其速度的改變會使粒子在雷射關閉的時候往焦點移動。在我們模擬中，脈衝列和連續波雷射的捕捉效率幾乎是相同的。我們認為這是因為我們的捕捉力忽略了非線性。

Recent experiments show that a short pulse train can trap small particles more efficiently than CW laser in some cases. It is interesting to note that a stable trapping is realized by a pulse train with duration of second and the repetition of MHz, laser of off almost (99.999%) all the time. In order to discuss the trapping mechanism, we performed numerical simulation of the trapping process both in the cases of pulse train and CW laser. We also proposed simple approximations for CW trapping and pulse train trapping, respectively. In the case of CW trapping, we found that the motion induced by CW laser is attributed to the motion with the thermal velocity ( plus thermal fluctuation). The trapping mechanism consists of two mechanism having slow and fast time constants. We consider that they are attributed the motion due to the optical force and due to the fluctuation. In the case of pulse train, the particle motion during the pulse duration (~ s) is negligibly small, and only the change of velocity is induced by a pulse. This induced velocity gives rise to the motion toward the focus spot in the intervals of pulses (~ s) while the laser is off. Trapping efficiency obtained in our simulation with pulse train is almost the same as the CW case. We consider that this is because we ignore the nonlinearity in the optical force.