連續波及飛秒脈衝雷射應用於微米至奈米大小球狀物質的光學捕捉

Abstract

微奈米結構有機物或聚合物的光學捕捉提供光學調控分子排列以及聚焦雷射產生動態變化的資訊。這篇論文含蓋了兩個方向，分別是藉由聚焦連續波雷射光學捕捉引發微米大小液晶液珠內的分子排列以及飛秒雷射照射在50奈米大小的聚苯乙烯球所產生的動態變化。 我們探討在聚焦連續波雷射照射下液晶液珠內的分子重排而重排發生自內部並遍及整個液珠，且相轉變的門檻能量隨液珠大小而改變。我們認為當焦點體積內的光學重組克服液珠的介面錨定效應便會產生重排。藉由探討電場對液晶液珠的作用能以及引入液珠介面錨定效應的概念，我們成功建立了可解釋隨液珠大小而產生不同變化的模型。 飛秒脈衝雷射照射在50奈米大小聚苯乙烯球的動態變化呈現出極化方向控制的兩個相反方向並交互變換的散射且與雷射極化方向相互垂直。由於光電場和極化介電球誘導偶極矩之間的相互作用，我們重新討論作用在介電奈米球體上的勞倫茲力用以解釋新現象的發生。我們考慮高數值孔徑物鏡產生的軸向及橫向電場導出軸向電場是散射力及時間變化力橫向組成的原因並且控制雷利粒子散射的方向。透過這些發現，我們提供了聚焦超快雷射脈衝雷利粒子動態光學捕捉很重要的資訊。

Optical trapping of organic or polymeric micro- and nano-structures provides information on how their molecular configuration is optically manipulated or how their dynamical motions are controlled by the focused laser beam. This thesis covers both ideas of optical trapping-induced molecular configuration in micro-sized individual 4’-n-pentyl-4-cyanobiphenyl liquid crystalline droplets by tightly focused continues wave laser beam and dynamical motions of 50-nm-sized polystyrene beads by femtosecond pulsed laser beam. We explore the reconfiguration in liquid crystal droplet under the irradiation of tightly focused cw laser beam. We show that reconfiguration took place throughout the inside of droplet, and the threshold power of the phase transition depends on droplet size. We propose that the droplet reconfiguration takes place when optical reorientation at the focal volume overcomes the droplet-liquid interfacial anchoring effect. By considering interaction energy of liquid crystal droplet by electric field and introducing liquid crystal droplet interfacial anchoring effect, we succeeded in building a model which can indicate the dynamic change depending on size. The dynamic motions of the 50 nm-sized polystyrene beads impinged by femtosecond pulsed laser beams show the polarization-controlled scattering in two opposite directions, in alternating manner, perpendicular to the laser polarization. To understand this new phenomenon we reconsider Lorentz force acting on a dielectric spherical nanoparticle due to interactions between light electric field and induced dipole moment of the polarizable dielectric sphere. We take axial and lateral electric fields into account produced by high numerical aperture objective lens. The axial electric field is responsible for lateral components of the scattering and temporal forces, and hence, controls the scattering directions of the Rayleigh particles. These findings provide important information about the dynamic optical trapping of the Rayleigh particles by highly focused ultrashort laser pulses.