Department of Chemistry, University ofWashington, Seattle, Washington 98195, USA.
Nano Lett. 2010 Feb 10;10(2):592-6. doi: 10.1021/nl903574a.
Focusing light to subwavelength dimensions has been a long-standing desire in optics but has remained challenging, even with new strategies based on near-field effects, polaritons, and metamaterials. The adiabatic propagation of surface plasmon polaritons (SPP) on a conical taper as proposed theoretically has recently emerged as particularly promising to obtain a nanoconfined light source at the tip. Employing grating-coupling of SPPs onto gold tips, we demonstrate plasmonic nanofocusing into a localized excitation of approximately 20 nm in size and investigate its near- and far-field behavior. For cone angles of approximately 10-20 degrees , the breakdown of the adiabatic propagation conditions is found to be localized at or near the apex region with approximately 10 nm radius. Despite an asymmetric side-on SPP excitation, the apex far-field emission with axial polarization characteristics representing a radially symmetric SPP mode in the nanofocus confirms that the conical tip acts as an effective mode filter with only the fundamental radially symmetric TM mode (m = 0) propagating to the apex. We demonstrate the use of these tips as a source for nearly background-free scattering-type scanning near-field optical microscopy (s-SNOM).
将光聚焦到亚波长尺寸一直是光学领域的长期愿望，但即使采用基于近场效应、极化激元和超材料的新策略，这仍然具有挑战性。最近，理论上提出的在锥形渐缩管上的表面等离激元（SPP）的绝热传播，作为在尖端获得纳米限制光源的方法，特别有前景。我们采用 SPP 的光栅耦合到金尖端，演示了等离子体纳米聚焦成大约 20nm 大小的局域激发，并研究了其近场和远场行为。对于大约 10-20 度的圆锥角，发现绝热传播条件的破坏局域在或靠近尖端区域，半径大约为 10nm。尽管存在非对称的侧向 SPP 激发，但具有轴向偏振特性的尖端远场发射，代表在纳米焦点中的径向对称 SPP 模式，证实了锥形尖端作为有效的模式滤波器，只有基本的径向对称 TM 模式（m=0）传播到尖端。我们展示了这些尖端作为用于几乎无背景散射型扫描近场光学显微镜（s-SNOM）的光源的用途。
