金領結型奈米天線之非線性散射現象及其機制探究

我們曾使用波長在電漿共振峰內的高強度雷射照射單一金奈米球，並觀測到非線性散射現象。該現象已經被應用在超解析顯微術的領域，利用類似於受激輻射耗散 (stimulated emission depletion, STED) 或飽和激發 (saturation excitation, SAX) 的實驗架設，就能夠實現以不會光致褪色(photo-bleaching)的散射訊號做為對比的超解析顯微術。波長相關性實驗也指出，當入射波長越接近共振峰時，觀察到非線性效應所需的照射強度也就越低，故非線性散射現象與侷域表面電漿子共振現象 (localized surface plasmon resonance, LSPR) 高度相關，但詳細的原理仍尚未被解明。LSPR 導致的最廣為人知的結果包含了增加散射、吸收截面積 (cross section) 以及在近場範圍內的巨大場提升強度 (field enhancement)。這兩項效應皆有可能是非線性散射背後機制的主導因子，但在金奈米球的實驗中並無法輕易調控這兩項因子的強度，故無法用以決定主導因子。因此，在本研究中，金領結型奈米天線 (gold bowtie nanoantenna) 被用以區分兩項因子造成的效應，由於對稱性的破壞，金領結型奈米天線具有兩種正交的共振模態，分別對應到平行或垂直領結軸的偏振方向，這兩個共振模態可以同時擁有類似大小的截面積和差距極大的場提升強度。截面積與場提升強度都可藉由時域有限差分法 (finite difference time domain, FDTD) 計算得出數值。將觀察到非線性散射所需的雷射照射強度的變化與上述兩項因子的變化做比較，可排除場提升強度做為主導因子的可能性，並可給出背後機制相關的洞見以及對未來找尋新的非線性散射材料或結構有莫大的幫助。

關鍵字

金領結型奈米天線；電漿子學；非線性散射；截面積；場提升強度；時域有限差分法

並列摘要

Nonlinear scattering has been observed in a single gold nanosphere (GNS) under high-intensity laser illumination with wavelength inside the plasmonic resonance band. It has been already applied to novel non-bleaching super-resolution microscopy based on scattering, with STED-like and SAX-like setup. Previous spectral analyses show nonlinear scattering is highly related to localized surface plasmon resonance (LSPR) in the gold nanoparticle. When the applied wavelength is closer to the resonance peak, the nonlinearity becomes observable in lower excitation intensity. However, the underlying mechanism is not fully unterstood. LSPR in metallic nanoparticles is known for enhancing both the scattering/absorption cross section and the large field enhancement in near-field. Both factors are plausible to account for the mechanism of nonlinear scattering. It is not simple to manipulate cross section and field enhancement in a single GNS due to its spherical symmetry, so the dominant factor can not been determined with nanosphere experiment only. In this study, gold bowtie nanoantennae (GNA) are used to distinguish the effect of these factors. Because of the symmetry breaking, there are two orthogonal resonance modes in GNAs with polarization either parallel or perpendicular to the bowtie axis. These two modes can give comparable cross section but significantly distinct field enhancement. The magnitude of cross section and field enhancement can be calculated by finite difference time domain method. By comparing the change of the intensity threshold, when nonlinearity becomes observable, with the change in cross section and field enhancement, one can determine the dominating factor. The result eliminates the possibility of field enhancement being dominant, giving insights of the underlying mechanism related to absorption cross section. The findings would help searching for more materials or structures for nonlinear scattering.

並列關鍵字

Gold Bowtie Nanoantenna ； Plasmonics ； Nonlinear Scattering ； Cross Section ； Field Enhancement ； Finite Difference Time Domain

參考文獻

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金領結型奈米天線之非線性散射現象及其機制探究

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