Coupling between optical antenna resonances is a powerful way to control the distribution of light in nanoscale systems. When the strength of coupling is fine-tuned against resonance loss, a critical coupling condition is often met, where energy can be efficiently directed between the system’s components. In this work, we use this concept to nanofocus optical energy into the 50 nm gap of a waveguide resonator, which on its own cannot be excited by external illumination. Light couples to the waveguide antenna via Fano interference with a bar antenna dimer. As a composite antenna, the shifting of the dimer relative to the waveguide resonator enables the precise tuning of their mutual coupling. We find a critical coupling condition where light is maximally focused into the waveguide’s gap corresponding to unity coupling cooperativity. Our interpretation of critical-coupling-induced nanofocusing is supported by the simultaneous maximization of both second and third harmonic generation at the critical condition.

中文翻译：

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临界耦合纳米间隙波导谐振器中的纳米聚焦


光学天线谐振之间的耦合是控制纳米级系统中光分布的有效方法。当耦合强度针对谐振损耗进行微调时，通常会满足临界耦合条件，其中能量可以在系统组件之间有效地引导。在这项工作中，我们利用这一概念将光能纳米聚焦到波导谐振器的 50 nm 间隙中，该谐振器本身无法被外部照明激发。光通过 Fano 干扰棒状天线二聚器耦合到波导天线。作为复合天线，二聚体相对于波导谐振器的移动能够精确调谐它们的相互耦合。我们发现了一个临界耦合条件，其中光最大程度地聚焦到与单位耦合协同性相对应的波导间隙中。我们对临界耦合引起的纳米聚焦的解释得到了临界条件下二次和三次谐波同时产生最大化的支持。