單一金奈米球非線性散射之機制

Abstract

不久之前，單一金奈米粒子的飽和及可壓制散射特性剛剛才被發現。飽和散射指的是散射率到某一個光強度時會開始減弱；而可壓製散射則是我們可以用一道光來壓制另一道不同波長光的散射率。這兩個現象都是以連續波雷射觀察到的。藉著這兩種非線性現象，無光致褪色的超解析顯微術已經被實現。儘管我們觀察到這樣的現象與局域表面電漿子共振有很強的波長相關性，其物理機制卻尚未被完全了解。在本論文中，我們回顧過去在飽和及可壓制散射的實驗結果，接著引入 模型以描述這個單一金奈米粒子非線性散射的特性。我們做了一系列不同波長(405~700nm)、大小(直徑20~80nm)及偏振的散射實驗與此模型擬合。這些實驗結果顯示我們觀察到的光學非線性由局域表面電漿子共振所增強，且其大小與其非線性程度有很大的正相關性。我們沒有觀察到可見的偏振相關性。在考慮有可能的微觀非線性來源後，單一金奈米粒子由連續波雷射觀察到的飽和及可壓制散射唯一可能的來源是熱光效應，也就是金的晶格溫度造成的 。

Previously, saturable and suppressible scattering of single gold nanosphere have been reported. Saturable scattering means the decreasing reflectance as intensity of incident laser reached certain threshold; suppressible scattering means that one laser beam can suppress the reflectance of another laser with different wavelength. Both phenomena are based on continuous wave laser. Based on these nonlinearities ,non-bleaching superresolution microscopies are realized. Although it has been shown that saturation intensities depend on the wavelengths and localized surface plasmon resonance band, the mechanism of the nonlinear scattering is still unclear. In this thesis, we first summarize our previous experimental results of saturable and suppressible scattering. In small particle limit model is introduced to fit with experimental results of nonlinear scattering by single gold nanosphere. We have performed extensive wavelength (405~700nm), sizes (diameter 20~80nm), and polarization studies to characterize nonlinear scattering in a single nanoparticle. The results show the optical nonlinearity is enhanced by localized surface plasmon resonance, and the size is positively related to the nonlinearity behavior. No polarization dependence was observed. After extensive discussion of the possible candidates for microscopic mechanism, saturable and suppressible of gold nanosphere by continuous wave laser are attributed to thermo-optical effect, i.e. hot lattice of gold.