This paper by X. Cui et al. presents an experimental realization and theoretical analysis of side-coupled nanobeam cavities, which support three resonant supermodes. The cavities are created by incorporating rows of holes in sub-micrometer-wide silicon waveguides positioned close to each other. It is shown that the resonant frequencies can be precisely controlled by varying the transversal separation between the nanobeams and introducing the longitudinal shifts between the positions of holes along the individual nanobeams. This tuning mechanism works efficiently since the supermodes extend over the three coupled nanobeams, and are thereby sensitive to their relative positions. Importantly, experimental measurements demonstrate that the modes have high quality factors approaching 80,000, while being tightly confined within few-micrometer-long nanobeam sections. The theoretical modelling predicts that the optimization of structures and post-fabrication tuning can be employed to compensate for possible fabrication inaccuracies. This suggests a path for achieving strong enhancement of four-wave-mixing by precisely aligning the pump, signal, and idler wavelengths with the three resonances in coupled nanobeam cavities.
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