圓形偏極光照射下金奈米粒子之力學行為研究

Abstract

本文研究桿型(nanorod)、球型、長型及扁型迴轉橢圓體(spheroid)之金奈米粒子在線性偏極化光或圓形偏極光照射下的力學行為，依據Maxwell電磁理論作為基礎，使用多重多極中心展開法作為模擬計算，並利用Maxwell應力張量分析金奈粒子上電磁場對其所產生之應力。將應力沿著奈米粒子做表面積分可得到光力，若將應力乘上力臂再沿著表面積分後可得光力矩。 若波源為線性偏極化光，分析結果發現長橢球金奈米粒子之光力矩具有兩種模態，當波長介於短軸表面電漿共振(TSPR)與長軸表面電漿共振(LSPR)之間，所表現之垂直模態(perpendicular mode)，以及當波長長於LSPR時之平行模態(parallel mode)，而當波長為LSPR時光力矩恰為零，為該兩種模態之轉捩點(turning point)。長軸與線性偏極化之夾角決定光力矩的大小。 當金奈米粒子受右旋圓形偏極光照射時，光力矩可造成其連續旋轉。若入射方向沿著迴轉橢圓體之旋轉軸(axis of revolution)時，光力矩的成因僅為光子被金奈米粒子吸收所形成的角動量轉換。因此光力矩與吸收功率的比值正比於波長，與形狀無關，即桿型、球型、長型及扁型迴轉橢圓體之結果皆一樣。而沿其他方向入射時，光力矩除了因光子被吸收而生成外，光子因散射而造成的角動量改變也會提供金奈米粒子額外的光力矩。此光力矩與波長及入射角有關。以長型迴轉橢圓體為例，當圓形偏極光沿短軸入射時，光力矩最大值之波長發生於LSPR，該LSPR會隨著細長比增加而產生紅位移現象。另外我們也考慮在水中金奈米粒子因旋轉而產生的黏滯阻力，以探討金奈米桿(gold nanorod)或金奈米線(gold nanowire)之最大值光力矩、最高轉速與波長的關係。

The main purpose of this thesis is to study the mechanical responses of gold nanoparticles (nanorod, nanowire as well as spherical, prolate and oblate nanospheroid) irradiated by linearly polarized (LP) or circularly polarized (CP) light. Based on Maxwell’s equations, the multiple-multipole (MMP) method was used in investigate the optical force and optical torque in terms of Maxwell stress tensor produced by the electromagnetic field on gold nanoparticles. If a gold nanorod/wire or prolate nanospheroid is irradiated by linearly polarized light, simulation results show that there are two alignment modes caused by the optical torque; one is perpendicular mode and the other parallel mode. The wavelength-range of the former is between the longitudinal surface plasmon resonance (LSPR) and transverse surface plasmon resonance (TSPR), whereas the wavelength of the latter is longer than LSPR. The turning point between the two modes is at LSPR with a null optical torque. If these gold nanoparticles are irradiated by a CP light, the optical torque will continuously drive them to rotate. The optical torque is contributed by the absorbed photons and the scattered one. Through absorption, the angular momentums of the absorbed photons are transferred to the irradiated gold nanoparticle. In addition, the difference of the total angular momentum between the scattered photons and incident ones also contributes the optical torque. If the incident CP plane wave is along the axis of revolution of these nanoparticles, the contribution of the scattering part is null. Therefore, only the absorption part contributes the optical torque; the ratio of optical torque to absorbing power is proportional to wavelength. This phenomenon is independent of the shape and size; it is the same for nanorod/nanowire as well as spherical, prolate and oblate nanospheroids. Moreover, we also considered the viscous torque to discuss the optimal wavelength to obtain highest rotation speed