Controlling the excitation of radially polarized conical plasmons in plasmonic tips in liquids
Controlling the excitation of radially polarized conical plasmons in plasmonic tips in liquids
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控制液体中等离激元尖端中径向偏振圆锥形等离激元的激发
DOI：
10.1039/c6ra09341h
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发表时间：
2016
期刊：
影响因子：
3.9
通讯作者：
T. Pertsch
中科院分区：
文献类型：
--
作者：
B. N. Tugchin;N. Janunts;M. Steinert;K. Dietrich;D. Sivun;S. Ramachandran;K. V. Nerkararyan;A. Tünnermann;T. Pertsch
等离子体尖端具有等离子体和扫描探针显微镜（SPM）的优点，利用径向极化的锥形等离子体并在其顶端产生热点。等离子体尖端是锥形的、全金属涂层的涡纤维，具有m形折射率曲线。旋涡光纤允许径向偏振模式传播在一个很长的距离与高模态纯度。当光纤模式到达锥形区域时，它在金属-电介质外界面处共振激发等离子体模式。本文从理论和实验两方面研究了等离子体针尖在液体中的行为。通过绝热削尖涡旋光纤，径向偏振模式从直径为115 μm的光纤被限制到直径为1.42 μm的锥形部分，成为波导模式。在这个区域，等离子体模式被激发，从而到达直径为200nm的顶点。我们的计算表明，等离子体激元耦合效率的提高是由于两个相互竞争的过程：相互作用区域的显著增加和场在金属中的穿透深度的轻微减少。通过选择一种允许或禁止相位匹配的液体，我们证明了等离子体耦合效率可以增加或消失。由于润湿效应，一个锥形的液体层在尖端形成一个额外的波导，并允许光纤模式与液体层的共振耦合。
Having virtues from plasmons and scanning probe microscopy (SPM), plasmonic tips employ radially polarized conical plasmons and create hot-spots at their apexes. Plasmonic tips are tapered and fully metal-coated vortex fibers that have M-shaped refractive index profiles. Vortex fibers allow the radially polarized mode to propagate over a long distance with high modal purity. When the fiber mode reaches the tapered region, it resonantly excites the plasmon mode at a metal-dielectric outer interface. In this paper, we study the plasmonic tip's behavior in liquids both theoretically and experimentally. By adiabatically tapering the vortex fiber, the radially polarized mode gets confined from the fiber with a diameter of 115 μm down to the tapered part with a diameter of 1.42 μm as a waveguide mode. In this region, the plasmon mode gets excited thus reaches the apex with a diameter of 200 nm. Our calculations show that the plasmon coupling efficiency increases in liquids due to two competing processes: a significant increase of the interaction region and slight decrease of the penetration depth of fields in metal. By choosing a liquid that either allows or forbids the phase-matching, we demonstrate that the plasmon coupling efficiency can increase or vanish. Due to the wetting effect, a tapered liquid-layer forms over the tip like an additional waveguide and allows resonant coupling of fiber modes to the liquid layer.
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