Determination of Nonlinear Optical Efficiencies of Ultrahigh‑Q Photonic Crystal Nanocavities with Structural Imperfections

Ultrahigh-quality (Q) photonic nanocavities offer a great possibility for the enhancement of nonlinear optical efficiency at low-input light due to the long light–matter interaction in their tiny spaces. However, the determination of nonlinear optical efficiency in ultrahigh-Q nanocavities remains unclear, which may hinder the understanding of nonlinear optics and extension to various applications of unusual light generation and all-optical processing in miniaturized optical devices. Here, we determine experimentally and theoretically the second-harmonic generation (SHG) efficiency in ultrahigh-Q photonic-crystal nanocavities. It is found that the random nanometer-scale imperfections in the fabricated cavity cause a large uncertainty in the estimation of SHG efficiency. Reliable SHG efficiencies are obtained through statistical analysis of the imperfections and reduction of the uncertainty in the measurement. Finally, we achieve a record-high SHG efficiency of 4000%/W in the cavity with an ultrahigh-Q factor of 7.1 × 10⁵, which is in good agreement with theory. These results provide a new step toward the further enhancement of various nonlinear optical effects in nanophotonics.