Trapping mechanism of nanoparticles by femto-second laser pulse train

• Main Authors: Shao, Chien-Hao, 邵乾豪
• Other Authors: 寺西慶哲
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/62188408880570248715

碩士 === 國立交通大學 === 物理研究所 === 103 === 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.