Nanofocusing in Critically Coupled Nanogap Waveguide Resonators

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.