Quasi-linearly polarized hybrid modes in tapered and metal-coated tips with circular apertures: understanding the functionality of aperture tips
Quasi-linearly polarized hybrid modes in tapered and metal-coated tips with circular apertures: understanding the functionality of aperture tips
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DOI：
10.1088/1367-2630/aa6feb
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发表时间：
2017-06
影响因子：
3.3
通讯作者：
Bayarjargal N Tugchin;N. Janunts;M. Steinert;Kay Dietrich;E. Kley;A. Tünnermann;T. Pertsch
中科院分区：
文献类型：
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作者：
Bayarjargal N Tugchin;N. Janunts;M. Steinert;Kay Dietrich;E. Kley;A. Tünnermann;T. Pertsch
在这项研究中，我们通过分析和实验研究了准线性偏振（LP）、混合、等离子体和光子模式在扫描近场光学显微镜中使用孔径尖端进行光学检测和激发中的作用。孔径尖端是锥形的金属涂层光纤，其顶端有小圆孔。在孔径尖端中，存在在金属包层与内部介电纤维的界面处结合的等离激元模式，以及在介电纤维芯中折射率增加的区域中引导的光子模式。基本光子模式虽然受到自由空间高斯光束的激发，但会经历截止并转变为倏逝模式。朝向尖端孔径，光子模式也变得比等离子体模式损耗更大，并且由于吸收和反射而导致的功率衰减预计至少为 10−9。相比之下，基本等离子体模式没有截止，因此可以一直到达尖端孔径。由于两种模式通过芯半径低于 600 nm 的锥体传播的非绝热性，因此模式之间会发生耦合。等离子体模式的传输效率（包括耦合效率和传播损耗）预计约为 10−6，比光子模式至少大 3 个数量级。朝向尖端孔径，光子模式的纵向场变得比横向场更强，而横向场对于等离子体模式始终占主导地位。通过实验，我们获得了尖端孔径处近场的偏振分辨图像，并与基本准 LP 等离子体和光子模式的 x 和 y 分量进行了比较。结果表明，孔径近场的 x 和 y 分量的图案和强度比均与基本等离子体模式的匹配。因此，我们得出结论，只有等离子体模式到达尖端孔径，从而控制尖端孔径之外的近场相互作用。我们的结论对于所有孔径尖端仍然有效，无论主要影响孔径尖端总传输效率的包覆金属类型如何。
In this study, we investigate analytically and experimentally the roles of quasi-linearly polarized (LP), hybrid, plasmonic and photonic modes in optical detection and excitation with aperture tips in scanning near-field optical microscopy. Aperture tips are tapered and metal-coated optical fibers where small circular apertures are made at the apex. In aperture tips, there exist plasmonic modes that are bound at the interface of the metal cladding to the inner dielectric fiber and photonic modes that are guided in the area of the increased index in the dielectric fiber core. The fundamental photonic mode, although excited by the free-space Gaussian beam, experiences cutoff and turns into an evanescent mode. The photonic mode also becomes lossier than the plasmonic mode toward the tip aperture, and its power decay due to absorption and reflection is expected to be at least 10−9. In contrast, the fundamental plasmonic mode has no cutoff and thus reaches all the way to the tip aperture. Due to the non-adiabaticity of both modes’ propagations through the taper below a core radius of 600 nm, there occurs coupling between the modes. The transmission efficiency of the plasmonic mode, including the coupling efficiency and the propagation loss, is expected to be about 10−6 that is at least 3 orders of magnitude larger than that of the photonic mode. Toward the tip aperture, the longitudinal field of the photonic mode becomes stronger than the transverse ones while the transverse fields always dominate for the plasmonic mode. Experimentally, we obtain polarization resolved images of the near-field at the tip aperture and compare with the x- and y-components of the fundamental quasi-LP plasmonic and photonic modes. The results show that not only the pattern but also the intensity ratios of the x- and y-components of the aperture near-field match with that of the fundamental plasmonic mode. Consequently, we conclude that only the plasmonic mode reaches the tip aperture and thus governs the near-field interaction outside the tip aperture. Our conclusion remains valid for all aperture tips regardless of the cladding metal type that mainly influences the total transmission efficiency of the aperture tip.