利用光的散射力在奈米懸浮液中產生非線性效應

Abstract

本實驗在探討雷射光在奈米懸浮液中產生的非線性效應，我們將波長532nm的綠光雷射聚焦在裝有聚苯乙烯(Polystyrene)奈米懸浮液中，利用光的散射力及奈米球粒子與溶液間黏滯力推動奈米粒子形成流場，觀察光的自聚焦效應，並用波長632nm的氦氖雷射架設干涉儀，由干涉條紋的偏移可大略推估該區域折射率的變化。 為了更了解內部的物理機制，我們做了以下幾個實驗，首先我們在玻璃槽容器的不同位置進行實驗，觀察在靠近容器邊界流場對稱性被破壞時的現象，試著找出自聚焦效應較佳的條件。由於流體在不同溫度下會有不同的黏滯係數(Viscosity)，一般流體黏滯係數會隨溫度升高而下降，就水而言在10°C時的黏滯係數約是30°C時的1.6倍，因此我們改變奈米懸浮液的溫度，從9°C改變到40°C，觀察不同溫度下對雷射光自聚焦效應造成的影響。 雷射光在奈米懸浮液中傳播時會自然的繞射，我們改變不同的入射光束和功率大小，找出適合的條件使自聚焦效應平衡繞射現象，讓光束在出射面和入射面時的大小相同，即空間光孤子現象(Optical Spatial Soliton)，綜合上述實驗，我們發現在距離容器底部1.0mm處自聚焦效果清楚穩定，在改變溫度的實驗下在29.8°C時比起9.8°C自聚焦效應變化劇烈，而在入射光束半高寬13µm，功率約300mW的條件下得到空間光孤子的現象，綜合以上結果嘗試推論奈米懸浮液內部理論模型。

This thesis presents the research on nonlinear optical effect in nanosuspensions. A continuous wave laser (wavelength at 532nm) is launched into a cuvette filled with a suspension of Polystyrene nanoparticles. Optical scattering force and viscosity will drive a flow in the nanoparticle suspension. The flow and the light beam can concentrate the nanoparticles and self-focus the laser beam. We use the He-Ne laser (wavelength at 632nm) to obtain the local refractive index change by interference. In order to understand the physical mechanism, we conduct several experiments. We launch the laser beam at different positions of the cuvette and observe the phenomena near the boundary when the flow symmetry is destroyed. We try to find better conditions for self-focusing. The viscosity of water decreases as temperature rises, as it is about 1.6 times at 10°C than at 30°C. We conduct the experiment from 9°C to 40°C, and observe how the temperature affects the results. Laser beam has a natural tendency to diffracts as it propagates. We change different beam waists and power to find the suitable conditions in which self-focusing balances diffraction, in which the beam diameter remains invariant during propagation, in other word, spatial solitons form. By these experiment results, we come up a hypothesis to explain the formation of the nanoparticle-concentration self-focusing mechanism.