奈米懸浮液中的光學自聚焦效應及半整數角動量旋渦光在自聚焦光折變晶體行為之研究

Optical self-focusing effect in nano-suspension and half-charge vortex light beam in a self-focusing photo refractive crystal

陳致融(Chih-Rong Chen)

指導教授 ： 石明豐

國立臺灣大學/理學院/物理研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02723

摘要

本論文討論在奈米懸浮液中的光學非線性效應以及旋渦光在自聚焦晶體內的行為。在非線性光學中，物質的折射率隨著入射光的強度而改變時，物質會呈現凸(凹)透鏡的效應，影響入射光的相位分佈進而改變其傳播的行為，稱做光學的自聚(散)焦效應。自聚焦效應的物理成因有很多，可以來自在晶體內產生的高階電偶極、介質吸收入射光產生溫度的上升…等。
我們發展了一套光學量測技術來測量奈米懸浮液中的光學非線性效應，一道強的幫浦光入射樣本，而另一道探測光用干涉儀來量測相位，幫浦光局部的加熱樣本形成一個溫度場，奈米粒子被溫度梯度場驅動而重新分布，這個現象稱做Soret效應或稱做熱泳，熱泳廣泛的發生及應用在有溫差的混合物中，例如在海洋的對流、星球內部的流動、長晶、分離原油…等。這套高靈敏度的系統可以量測溫差約0.06 ℃時奈米粒子濃度以及溫度的改變，而在本文的研究中，我們發現在聚苯乙烯(Polystyrene)奈米小球的吸收會隨著奈米球的直徑減小而增加，當奈米球小於30奈米的時候這個效應會明顯的加熱樣本。
在奈米懸浮液的非線性之研究當中，我們設計了一種全新的操控方式來控制奈米粒子的分佈。用光驅動奈米粒子有三個機制：一個是光的吸收而局部加熱樣本，形成熱泳；一個是光場的梯度造成粒子不均勻的偶極力而驅動奈米粒子，光鉗；第三種是奈米粒子藉由散射入射光而得到動量的散射力。前兩種機制已經發展許久，甚至在適當的條件下可以形成空間光孤子(optical spatial soliton)，但是藉由散射力操控奈米粒子的研究卻很少。本文中我們實現了用光的散射力推動奈米粒子，藉由奈米粒子與溶液的黏滯力推動出流場分布，最後藉由流場重新影響奈米粒子的濃度分布，形成光的自聚焦效應。我們觀察到這個自聚焦的光會達到一個穩定態，代表由光推動出來的流場而建立的折射率分布是穩定不隨時間變化的，也就是說，光產生波導，而後波導又回頭影響入射光，達到一個自洽解。在入射光是200 mW的條件下，折射率增加 ，光推動出來的流速大約30 μm/s，而當入射光增強到1.6 W時，我們觀察到自聚焦效應過強變得不穩定，使自聚焦的光分裂成很多顆光孤子，這是高階光孤子的效應(high order solitons)。然而這個非線性的動態過程目前還不了解，是未來可以繼續研究的方向之一。
環型旋渦(vortex)是自然界普遍存在的有趣現象，由於對光的操控與分析非常成熟，旋渦光(vortex light beam)成為非常適合研究旋渦傳波特性的一個方式。本文中，我們研究半整數角動量的旋渦光在自聚焦晶體中的傳播行為，因為其特殊的螺旋相位以及強烈的軸向不穩定性(Azimuthal Instability)，在晶體中半整數旋渦光分裂成三顆光斑，而後隨著光斑與非線性晶體的交互作用，其中兩個相位相近的光斑會合併再一起，實驗觀測與理論結果相當吻合。

關鍵字

自聚焦效應 ； 奈米懸浮液 ； 旋渦光

並列摘要

This dissertation presents the research on optical self-focusing effect in nano-suspension and the half-charge vortex light beam in a self-focusing photorefractive crystal. In nonlinear optics, the dependence of the refractive index of a media on the input light intensity gives rise to the media focusing or defocusing the input light beam. This nonlinearity may come from different physical origins. In this research, we seek for a novel physical interaction among light, nanoparticles, and flow to realize the optical self-focusing effect in the nano-suspension.
In the nano-suspension, solvent (or solute) absorbs the light causing temperature gradient and it drives nanoparticles to migrate to the region with higher or lower temperature. This phenomena is photophoresis, or Soret effect. In this dissertation, we establish a new method to investigate the optical nonlinear effect in the nano-suspension. In this method, a pump laser beam is to induce the temperature gradient causing the nanoparticles to move, and a weak probe laser beam is used to measure the phase shift at the pump beam center by using a very sensitive Mach-Zehnder interferometer. Due to the high sensitivity of the interferometer, we can measure the thermal effect and the Soret coefficient when the temperature change is very low, as low as 0.06 ℃.
We present a new way to concentrate the distribution of nanoparticles by a light beam. This redistribution nanoparticles then can cause self-focusing effect for the light beam itself. The nanoparticles are driven by the laser beam via Rayleigh scattering and drag the surrounding water molecules to move forward with the particles and then create a flow. Due to the scattering loss, the velocity of nanoparticles decrease along the propagation of the light beam causing nanoparticles accumulated longitudinally in the channel of the light beam. This behavior is similar to a highway “traffic jam”. In this scenario, we observe stable self-focusing effect in the nano-suspension. When the input power is of 200 mW, the refractive index increases by and the velocity of laser-driven flow is about 30 μm/s. As the input power increasing to 1.6 W, the self-focusing nonlinearity is too strong causing the light beam unstable and breaks the light beam into many filaments. This behavior is similar to high order soliton effect.
Vortex is an interesting phenomenon appearing in many branches of physics. Due to the simplicity of experiment, optical vortex light beam is a suitable tool to study the properties of vortex in nonlinear systems. In this dissertation, we study the half-charge vortex light beam in a self-focusing medium. We observed the half-charge vortex light beam is unstable in propagation and breaks up into three filaments due to the azimuthal instability. After the three filaments are formed, they then interact according to their relative phases and relative distances. In the end, the two filaments with closer phases fuse into one filament. This observation is in good agreement with numerical simulation and perturbation analysis.

並列關鍵字

self-focusing effect ； nano-suspension ； two-phase flow

參考文獻

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