雙光子共振增強雷射光鑷捕捉奈米粒子效率之實驗及理論探討

Abstract

雖然利用光鑷操控微小粒子已經發展許久，但此方法對於奈米尺度粒子的操控仍是一大挑戰，其中針對極化率小的奈米粒子更是如此。為了解決這個問題，我博士班的工作即專注在探討增強光鑷對介電奈米物質操控力的方法，並且提出以雙光子共振為基礎的方法。在本論文中，我們將展示與定量光強度與波長對雙光子共振光鑷的影響，並且，我們將透過光阱中捕捉的奈米粒子數量來對光鑷強度進行評估，而非過往文獻中以單一粒子來進行評估。論文之結論如下：（1）利用波長為染料摻雜聚合物粒子吸收峰兩倍的皮秒脈衝雷射，我們首次展現了雙光子共振能增強光鑷對粒子的捕捉能力；（2）造成光鑷捕捉染料摻雜聚合物粒子能力增強的原因，部分為雷射之光場與聚合物粒子之摻雜染料間的有效折射率受雙光子共振而增強；（3）此論文所發現光鑷增強的獨特光譜特性，可以透過雙光子共振之分散態的極化率實部與勞侖茲態的極化率虛部進行解釋。我們認為這裡提出的研究結果不但有助於對雙光子共振增強光鑷機制的了解，也提供了對奈米粒子操控、誘導聚集以及透過摻雜染料光譜特性不同而進行奈米粒子分類的可能性。

Investigation of the confinement of nanoparticles with an optical field is important to advance our understanding of the crystallization, nucleation and growth of nanomaterials. Compared to metallic nanoparticles made of gold or silver, the polarizability of dielectric nanoparticles of comparable dimension is much smaller, and hence, optical trapping of dielectric nanoparticles remains challenging. Toward this end, this Ph.D. work focuses on the exploration of a novel mechanism, two-photon resonance, to enhance optical trapping of dielectric nanoparticles. In particular, we not only demonstrate an enhanced optical confinement of nanoparticles with a picosecond (ps) pulsed, near-infrared (NIR) laser, but also characterize its dependence on the intensity and wavelength of the trapping laser. Distinct from most of preceding researches that address mainly the trapping of single nanoparticle, this research concerns an increased number of nanoparticles confined near the laser focus through the use of laser trapping. In specific, we demonstrate for the first time that (1) two-photon resonance increases the capability to confine numerous dye-doped polymeric nanoparticles through the employment of a ps-pulsed, NIR laser with its half-wavelength particularly falling within the absorption band of the dopant, (2) the enhanced confinement of doped nanoparticles is facilitated in part through an increased effective refractive index resulting from two-photon resonance between the optical field of laser and the dopant of nanobead, and (3) the distinctive spectral dependence of the enhanced confinement of doped nanoparticles is explained with the dispersively shaped real polarizability and the Lorentzianly shaped imaginary polarizability in the vicinity of two-photon resonance. We envisage that our approach not only advances the understanding of a novel mechanism of optical confinement mediated by two-photon resonance but also would arise new applications such as controlling the confinement (or aggregation) of nanomaterials and sorting of nanoparticles based on their spectral properties.