包覆於三氧化二鎵殼層裡的奈米金顆粒結構之近場電漿子增強效應之研究

Abstract

在本論文中，我們使用有限時域差分法與近場探針顯微術探討於三氧化二鎵殼層中奈米金顆粒球間電漿子近場電磁場之間隙增強效應現象。於理論模擬部分，我們先以二維有限時域差分法模擬此結構之電磁場強度分佈，探討其隨著結構改變如顆粒大小、間隙長短、殼層寬度及顆粒形狀等等之變化趨勢。透過模擬結果，我們可以發現當顆粒間隙距離減小時，間隙增強現象隨之增強。不僅如此，隨奈米金顆粒結構尺度增大，因另一維度的間隙局限增長而使間隙增強現象增強。相較包覆於空氣裡的金顆粒對結構，披覆於三氧化二鎵之金顆粒對因侷限於較高折射係數的殼層腔故具有較高的近場間隙增強因子值。此外，我們使用三維有限時域差分法分析此奈米豆莢結構之電磁場強度，發現侷限於間隙之電磁場因繞射現象隨著遠離間隙結構而快速發散，以致於豆莢殼層外之電磁場強度呈現低局域性分佈。然而，當激發波長越接近響應波長時，殼層表面電磁場強度變強且分佈之局域性提升。於實驗部分，我們分別使用探針顯微儀與近場光學顯微儀量測奈米豆莢之結構與其近場光學特性。原子力顯微儀與靜電力式顯微儀之量測結果獲得此三氧化二鎵殼層中奈米金顆粒球大小約20nm至90nm、顆粒間之間隙約20nm至100nm、殼層厚度約為100nm至300nm。當我們使用不同波段之光源激發奈米豆莢結構，於近場光學影像實驗結果發現此樣品之局域電磁場強度在綠光激發下較紅光與藍光激發下來得大，顯示奈米豆莢構具有明顯之波長選擇性。此外，比較於殼層表面近場電磁場強度分佈之局域性與波長關係之模擬結果，可知奈米豆莢結構之響應波長應座落於綠光波段區間。

We study near-field plasmonic enhancement of the golden nanoparticles embedded in Ga2O3 shell by using finite difference time domain method (FDTD) and near-field scanning probe microscopes. In two-dimensional FDTD simulation, we explore the variations of field distributions with different sizes and shapes of metallic particles, gap distances between particles and the width of the Ga2O3 shell. We observe that plasmonic gap enhancement increases with either the decrease of gap distances or the increase of particles’ sizes. In addition, near-field gap-enhanced factor for the structure embedded in the Ga2O3 shell is larger than that embedded in the air due to higher refractive index of Ga2O3. We use three-dimensional FDTD method to obtain the field distributions of the whole structure with different incident wavelengths. Gap enhancement is reduced as the field propagates through the Ga2O3 shell due to diffraction. Besides, higher field intensity and enhancement are observed at the interface of Ga2O3 and the air as the incident wavelength is near to the resonant wavelength of the structure. In experiments, we use the scanning probe microscopes and near-field optical microscopes to obtain the structure parameters and near-field optical images of the sample. Observing images obtained by the atomic force microscope and the electrostatic force microscope, we acquired the diameters of golden nanoparticles are from 20nm to 90nm, the gap distances are from 20nm to 100nm and the shellwidths are from 100nm to 300nm. Images seen in near-field scanning optical microscope with different launched light sources show higher field intensity and enhancement are observed as the light source is green, showing the structure is wavelength selective. Additionally, the resonant wavelength of this structure is at the regime of 520–570 nm based on the results obtained by 3D simulations.