Department of Physics, National Tsing-Hua University, Hsinchu30013, Taiwan.
Nano Lett. 2010 Feb 10;10(2):632-7. doi: 10.1021/nl903693v.
We apply a nanomanipulation technique to assemble pairs of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying tip-to-tip separation (25-125 nm). These pairs are immobilized onto indium tin oxide coated silica substrates and studied as plasmonic dimers by polarization-selective total internal reflection (TIR) microscopy and spectroscopy. We confirm that the plasmon coupling modes with the scattering polarization along the incident light direction result from the transverse-magnetic-polarized incident light, which induces two near-field-coupled dipole moments oriented normal to the air-substrate interface. In such cases, both in-phase (antibonding) and antiphase (bonding) plasmon coupling modes can be directly observed with the incident light wave vector perpendicular and parallel to the dimer axis, respectively. The observation of antiphase plasmon coupling modes ("dark" plasmons) is made possible by the unique polarization nature of the TIR-generated evanescent field. Furthermore, with decreasing nanocrystal separation, the plasmon coupling modes shift to shorter wavelengths for the incident light perpendicular to the dimer axis, whereas relatively large red shifts of the plasmonic coupling modes are found for the parallel incident light.
我们采用纳米操纵技术，沿着八面体金纳米晶体（边长 150nm）的长轴将其一对一对地组装起来，其尖端到尖端的间距（25-125nm）各不相同。这些对晶体被固定在氧化铟锡涂覆的二氧化硅衬底上，并通过偏振选择性全内反射（TIR）显微镜和光谱学研究其等离子体二聚体的性质。我们证实，散射偏振方向与入射光方向一致的等离子体耦合模式是由横向磁偏振入射光引起的，该模式会诱导两个近场耦合的偶极矩，其方向垂直于空气-衬底界面。在这种情况下，当入射光的波矢分别垂直和平行于二聚体轴时，可以直接观察到同相（反键）和反相（成键）的等离子体耦合模式。通过 TIR 产生的消逝场的独特偏振性质，使得可以观察到反相等离子体耦合模式（“暗”等离子体）。此外，随着纳米晶体间距的减小，对于垂直于二聚体轴的入射光，等离子体耦合模式会向较短的波长移动，而对于平行的入射光，则会发现较大的等离子体耦合模式红移。
